By: Teo Spengler
Yellow sweetclover (may be spelled as two words), also called ribbed melilot, is neither a true clover nor especially sweet. It is a legume plant with the scientific name Mililotus officianalis, and sometimes used as food for livestock. Read on for more information on why yellow sweetclover is considered a weed in some areas and tips on yellow sweetclover management.
So what is yellow sweetclover? A forage crop? Or is yellow sweetclover a weed? That all depends on your perspective. The biennial plant is a legume that grows to 6 feet (2 m.) tall and is topped by bright yellow flowers. It has coarse stems and the leaves are toothed.
Yellow sweetclover is not a native plant to this country but was imported from Europe and Asia. It is used as food livestock and as hay when it is young. After the plant flowers, it becomes stemmy, which makes it problematic as hay. An even more serious problem with sweetclover is the fact that it contains the toxin coumarin. This gives the legume a bitter taste.
Yellow sweetclover becomes more toxic when heated or spoiled. If eaten in this stage, it reduces an animal’s blood-clotting ability and can be lethal. That’s why controlling yellow sweetclover is important.
In many areas, yellow sweetclover is considered a weed. That’s because it spreads rapidly and often grows where it isn’t wanted, like open fields, roadways and other disturbed sites. The seeds can remain viable for 30 years or more.
There are many beneficial yellow sweetclover uses, however. This plant provides food for wildlife and also nectar for honeybees. It is also a nitrogen-fixing plant used as a cover crop and, as mentioned, works as feed for livestock.
That being said, the low-level toxicants contained in the plant can be dangerous for animals, both livestock and wildlife. Feeding on moldy yellow sweetclover can cause a fatal hemorrhaging disorder.
Yellow sweetclover plants are drought tolerant and exceptionally cold tolerant. They propagate by seeds and produce a lot of them. If you are interested in controlling yellow sweetclover, it’s best to act before the yellow flowers bloom.
Remove the plants early, before seeds are formed. This is the key to yellow sweetclover management. How to remove them? Hand pulling works well, if you don’t have acres to deal with. Mowing also works for larger areas, and controlled burns can help with controlling yellow sweetclover.
What about controlling yellow sweetclover when it is mature? At this stage, you’ll have to remove the seeds. That is more difficult since the seeds are tough and durable. They resist soil fumigation as well as solarization.
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In this guide to Minnesota forage legumes, you’ll find features, uses and varieties for several common species, including various clovers, birdsfoot trefoil, crown vetch, cicer milkvetch and alfalfa.
To identify individual legume species, you can use leaf traits.
Legume leaves are usually compound, meaning they have more than one leaflet per leaf, and often have large stipules. The leaves are borne on petioles, which are attached to stems (Figure 1).
Though leaves of clovers and alfalfa typically have three leaflets per leaf, they sometimes have four or five.
The frequency of four or more leaflets per leaf is influenced by both the plant’s genetic makeup and the growing environment. Because four-leaf clovers occur relatively infrequently, they’re said in folklore to impart good luck. Some legumes have 10 or more leaflets.
Four arrangements, or organizations, of leaflets occur in the leaves of legume species commonly grown in the north-central region of the United States:
Palmately trifoliolate: Red, white, alsike and kura clover. These legumes are called true clovers.
Pinnately trifoliolate: Alfalfa and sweetclover.
Odd pinnate: Birdsfoot trefoil, crownvetch and cicer milkvetch.
Even pinnate with tendrils: Hairy vetch.
Legume flowers usually are showy and colorful. These features enhance the plants’ ability to attract its insect pollinators, including many species of native bees as well as the European honeybee.
Legume flower parts are the standard (also called the banner), wings and keel (Figure 2). The keel surrounds the male and female sexual parts.
Legume flowers are arranged in groups called inflorescences (Figure 3). The most common legume inflorescences are the:
Head: In red, white, alsike and kura clover.
Raceme: Alfalfa, sweetclover and cicer milkvetch.
Umbel: Birdsfoot trefoil and crownvetch.
A head typically will contain many flowers, while racemes and umbels contain few.
After pollination, legume seeds develop in pods. Pods can contain several seeds, as in birdsfoot trefoil and alfalfa, or only one seed, as in kura clover and sweetclover. When pods are dried, they can shatter and seed is dispersed.
Forage legumes are usually tap-rooted plants with fine secondary roots produced from the tap root. These secondary roots are usually nodulated by nitrogen-fixing bacteria. Figures 4 and 5 illustrates this for birdsfoot trefoil and red clover.
A very large tap root gives legumes such as alfalfa, kura clover and sweetclover greater drought tolerance than other forage legumes. In contrast, the more fibrous, shallow root systems of other legumes, such as white and alsike clover, reduce their drought resistance.
Stolons are horizontal aboveground stems (Figure 6). Rhizomes are horizontal belowground stems.
Both allow for vegetative reproduction without seeds, as new stems and roots can arise from nodes on stolons and rhizomes. This enhances plant persistence while creating more root sites for nodule growth.
Stolons are found in white clover, while rhizomes are found in kura clover, cicer milkvetch and crownvetch. Legumes with rhizomes are among the most persistent species.
Kura clover (Trifolium ambiguum Bieb.) is a relatively low-growing, spreading perennial with excellent potential for grazing. It’s also called Caucasian, Pellett’s or honey clover.
Kura clover is native to the Caucasus region of Europe and named for the Kura River in the country of Georgia. It was introduced into the United States in about 1910, but remained little known until the 1940s.
That’s when Frank Pellett, impressed with its potential for honey production and its desirable agronomic traits, wrote about it in the American Bee Journal. However, interest didn’t increase until recently because of inadequate seed supplies of improved varieties and the unavailability of appropriate nitrogen-fixing rhizobium.
The Soil Conservation Service and the University of Kentucky released the first-named U.S. variety, Rhizo, in 1990. Very persistent, it’s survived more than 15 years of continuous grazing in St. Paul and 12 years mixed with different grasses in Arlington, Wis.
Newer varieties include Everlast, Endura and Cossack. Endura has survived 22 years in mixture with tall fescue and smooth bromegrass in rotationally grazed pastures near Lancaster, Wis.
Kura clover has a deep branching taproot and rhizomes (horizontal belowground stems), which enable it to vigorously spread (Figure 6). Its crowns can be two inches below the soil surface. Individual plants can increase through rhizome growth by about one foot per year with no competition, and less with grass competition.
By fall of the seeding year, kura clover can have significant root and rhizome growth. A five-year-old stand can produce more than three tons per acre of belowground biomass (30 percent roots, 45 percent rhizomes, 25 percent crowns).
Leaflets are usually trifoliolate, oblong and watermarked, with considerable variation in leaf characteristics in a population. Leaves with four or five leaflets have been observed. Leaflets and stems are not hairy, but leaflet margins are acutely serrated at the edges.
Leaf size varies considerably with growing conditions during the season, spanning from 1 to 3 inches long and 1/4 to 2 inches wide.
Kura clover has excellent tolerance to many stressful management and climatic factors. It has no major disease problems and is productive in diverse environments. Kura clover has greater persistence under rotational and continuous grazing, and frequent cutting, than any commonly grown legume.
Following two to four cuts per year for three years in southern Minnesota, kura clover had plant populations greater than 90 percent (alfalfa and other legumes were 50 percent or less). Its persistence is due in part to the extensive underground root, crown and rhizome system that’s a site for considerable carbohydrate storage.
For legumes like alfalfa and red clover, frequent harvesting depletes root carbohydrate concentration, but concentration of carbohydrates in kura clover’s belowground structures is only minimally affected.
Kura clover is very winter-hardy. It goes dormant in the early fall in response to the short day length and low temperatures. It’s very resistant to injury from freezing and thawing, persisting more than 20 years and surviving extreme winter conditions in Minnesota and Wisconsin, while all other legumes died.
Kura clover has excellent tolerance to drought, although it’ll become dormant during extended dry periods and yield less than alfalfa (but similar to other clovers and birdsfoot trefoil). Although herbage growth reduces during drought, kura clover resumes growth following replenishment of soil moisture.
It can also often withstand poorly drained soils, survive flooding and survive on sites with a high water table. In Australia, kura clover plants had an 80 percent survival rate when flooded up to 40 days.
Kura clover is best-suited as a grazing crop because of its prostrate growth habit and very leafy, high-moisture forage. However, its first growth in the spring that contains an elongated stem can be harvested for hay or haylage. Subsequent regrowth will be leaves supported by petioles originating from crowns and rhizomes.
Consequently, for most of the season, kura clover is very leafy and high in feeding value. Its forage quality often exceeds the quality of other commonly grown legumes. Greatest yields occur in the spring yields are less in summer and fall. Forage yields range from two to six tons per acre, with an average of about four tons per acre being most likely.
Mixtures of grass and kura clover can produce quality forage suitable for lactating dairy cows or to support high weight gains of lambs or steers. Kura clover tolerates rotational grazing at intervals of 14 to 28 days or, though not recommended, continuous grazing.
Under stressful grazing or clipping, plants adapt by shortening leaf petioles, resulting in leaves closer to the soil surface. Unlike red clover, breeding ewes can safely graze kura clover because it doesn’t contain phytoestrogens.
Because of very low fiber content, high protein and potential for bloat, plant kura clover in mixture with perennial grasses. Mixtures should be 30 to 60 percent grass. Kura clover can successfully be established with most commonly used perennial grasses.
Base your choice of grass species on productivity and persistence of the grass at a site, and on management preference for any particular grass. Long-term grazing trials in Minnesota and Wisconsin show that grass selection influences the yield and grass composition of mixtures (Figure 8).
Mixtures of well-nodulated kura clover with grass have yielded as much as grass fertilized with up to 300 pounds of nitrogen per acre. Establishing kura clover with the noncompetitive birdsfoot trefoil enhances seedling-year yield, first-year yield and kura clover establishment. Over time, kura clover population increases while birdsfoot trefoil population declines.
Establishing kura clover is more challenging than most other forage legumes. It has less seedling vigor than white clover and birdsfoot trefoil. Seedlings are fragile and develop slowly.
However, resources spent to establish kura clover are an investment that provides years of returns. Forage production and stand density in the seeding year will usually be low but, as it’s been said, “Kura clover sleeps in the first year, creeps in the second year and leaps in the third year.”
Because of its lack of seedling vigor, it’s essential to minimize competition with weeds or companion crops at establishment. To achieve the greatest kura clover establishment yields, use an herbicide for weed control in the seeding year. If you plant companion crops to reduce soil erosion, harvest them for forage at vegetative stages to reduce competition.
Kura clover has also been successfully seeded into pastures with no-till strategies, provided that existing grasses are suppressed with herbicides such as glyphosate. No-till approaches reduce soil erosion and can reduce annual weed pressure.
Kura clover has been successfully established by transplanting portions of the underground rhizomes. Transplanted rhizomes with one or more nodes can form new roots and crowns and spread within a field if soil moisture and fertility are adequate.
Large fields have been established using a potato digger to extract rhizomes and crowns followed by spreading of the plant material onto a newly tilled field using a manure spreader and incorporating by disking.
Biological nitrogen fixation develops more slowly for kura clover than for other legumes.
Improve establishment and seeding year yields by applying small amounts of nitrogen fertilizer in the seeding year on coarse soils low in nitrogen (20 pounds of nitrogen per acre at establishment and again at first harvest, approximately 60 days later). However, nitrogen fertilization can sometimes also increase weed competition if weeds aren’t controlled.
Because of its prostrate and spreading growth habit, kura clover has potential for use in soil stabilization projects. Its dense rhizome mat holds soil and prevents erosion.
With adequate suppression and seeding technology, kura clover can also be managed as a living mulch in corn with little or no reduction in grain or silage yield.
Kura clover supplies almost all the nitrogen required, provides permanent ground cover to reduce soil and nutrient runoff and recovers to full production of pasture the following season. However, when soybean was no-till seeded into a suppressed kura clover living mulch, soybean yields were reduced when compared to soybean seeded into kura clover killed by tillage.
Kura clover flowers in response to long days in the spring.
Initial regrowth is upright stems supporting one or two large, fragrant, pink-white flowers. If the initial regrowth isn’t cut and allowed to mature, seed is produced in July or August.
Producing kura clover seed is more difficult than for many other legume forage species. Growers should consider a field’s previous crop history. Seed purity is an issue because legumes previously on the site, such as red clover, produce a similarly sized seed as kura. This makes them difficult to separate.
Seed production fields are seeded in rows, or broadcast, at approximately one-quarter of the forage seeding rate. A firm seedbed allowing a uniform seeding depth of 1/4 to 1/2 inch is desirable. Kura clover is generally established using herbicides, without a companion crop, after the initial spring weed flush, and before late July.
Soil fertility requirements for seed production are the same as for forage production a soil test prior to establishment and every two years during production is recommended. After establishment, rogueing or spot-spraying to eliminate problem perennial weeds such as thistles may be necessary.
Pollinating insects, especially bees, are important for seed production. One honeybee hive per acre is recommended for seed production, but you may need more bees if there’s competition for pollinators from other crops.
Successful harvesting requires prompt, timely and careful action. Swath fields when the majority of stems have turned brown, and combine when the crop is dry or direct-combine in the field after using a chemical desiccant.
To maximize seed yield, careful combine calibration is required because the seed is difficult to thresh. Kura clover seed fields may become sod-bound after several years due to the large root mass and rhizome production. Decline in seed production may follow, which would necessitate taking the field out of production.
An additional use associated with seed production is honey production. Kura clover flowers are highly scented with a shallow corolla, keeping its high sugar content nectar readily available to bees. If kura clover is allowed to flower, you can manage kura clover pastures or seed production fields to produce honey as an additional source of income.
The biennial yellow sweet clover takes two years to produce a flowering plant. The first year, the yellow sweet clover grows in a rosette. After a vernalisation period it produces a shoot and flowers. It can produce up to 2.5 tons of dry matter and can grow up to 24 inches. If conditions are favorable, it can reach up to 8 feet in the second year. Below ground, its tap root can extend down 5 feet by the end of spring. White clover does not produce as much biomass as yellow. It is taller and stemmier plant that is better for soil building.
White Sweetclover is more commonly grown as an beneficial crop because bees and other pollinators are quite fond of the blossoms. It matures about two weeks later than Yellow Sweetclover. It is also grown as nitrogen producing green manure crop. Sweet clover has a poor feed value and is not recommended for livestock feed or forage.
The Michigan State University Extension states…
Sweet clover has historically been used for grazing or as an emergency feed. But Kim Cassida, Michigan State University Extension forage specialist does not recommend it because it can be toxic to livestock. “The toxic factor is dicumarol, which is formed from coumarin only when the sweet clover is spoiled by certain kinds of mold. Poisoning is more likely from hay than from silage/haylage and it has been reported in cattle, sheep, pigs and horses. Poisoning has occasionally occurred while grazing. Sweet clover forage does not have to be visibly moldy to be toxic. Most varieties of sweet clover hay, haylage and silage should only be fed with great caution and when no other options are available. A few varieties of yellow (‘Norgold’) and white (‘Cumino,’ ‘Denta,’ ‘Polara’) sweet clover have been bred for low coumarin content and thus safer forage use, but these varieties are not as widely available as those typically used as cover crops.” The Merck Veterinary Manual sweet clover entry is a good source of details on sweet clover poisoning.
Winter hardy, best for fall sowing.
Botanical description: This description covers characteristics that may be relevant to fire ecology and is not meant for identification. Keys for identification are available (e.g., [82,98,101,191,235,267,268]).
Aboveground description: Yellow and white sweetclover have very similar growth habits and morphology [11,117]. In their vegetative state, yellow and white sweetclover are difficult to distinguish [98,110]. The most obvious distinction between the two species is flower color, which is yellow for yellow sweetclover and white for white sweetclover [101,107,201,268]. When flowers are dry, however, both may appear cream colored .
Sweetclover is a biennial [126,127,158,191], although in rare instances annual and perennial growth forms were observed. At the Matanuska Research Farm in south-central Alaska, sweetclover seed from mid-latitudes and from Alaskan roadsides produced plants that flowered in the first year. Artificial light experiments revealed that annual growth was triggered by long light periods and a lack of darkness [124,125]. In London, Ontario, some sweetclover plants clipped "rigorously" during the growing season flowered in the 3rd growing season. All plants died after flowering (Cavers unpublished data cited in ). Studies by Smith  showed that annual and biennial white sweetclover growth is controlled by a single gene, which can be altered through a single mutation.
In the 1st year of growth, sweetclover produces a single stem with many branches. Near the end of the 1st growing season, nutrients are allocated below ground to the taproot, and root crown buds are formed. In the 2nd year of growth, sweetclover stem number increases, plants are much larger, and flowers and seeds are produced (review by ). More details about phenology and the characteristics of 1st and 2nd year plants are provided below in the Belowground description and Seasonal development sections.
Sweetclover is generally an erect, freely branched plant up to 10 feet (3 m) tall [82,235]. Stems are coarse with alternate, 3-pinnate leaves and axillary flowers [93,158,282]. Leaflets are small, 0.4 to 1 inch (1-2.5 cm), and the pea-like, perfect flowers occur in 30- to 70-flowered racemes that measure 1.5 to 4.7 inches (4-12 cm) long [101,158,191,241]. Legumes are up to 4 mm long, scarcely dehiscent, and typically produce just 1 seed, but may produce 2 [82,98,169,191].
Although yellow and white sweetclover are more alike than different, the following morphological differences are common:
Sweetclover growth form and morphology are variable, not only because many different cultivars, forms, and ecotypes were introduced in North America but also because growth characteristics can be influenced by environmental conditions. Plant height increases with increasing day length, and low temperatures can limit flower production (review by ). For descriptions of some sweetclover cultivars, see the following references: [219,273,285].
Belowground description: Sweetclover produces a taproot with secondary fibrous roots and bacterial nodules [11,101,268]. Taproots are semiwoody, and lateral roots can be extensive (reviews by [45,254]). Lateral roots may extend 6 to 8 inches (15-20 cm) from the taproot (review by ). In experimental fields in Columbus, Ohio, 1st-year sweetclover taproots penetrated over 4 feet (1.2 m) deep. Second-year plants had roots up to 5.5 feet (1.7 m) deep in July. Bacterial nodules occurred on roots as deep as 4 feet (1.2 m). Increased root length between these 1st- and 2nd-year plants may not reflect plant age but site differences. Most sweetclover plants attained their maximum root length in the 1st year. Root systems were shallower in dry than moist soils .
Bare  reports that yellow sweetclover roots are generally shorter but spread farther than white sweetclover roots. First-year white sweetclover dug from a field in Columbus, Ohio, in early May produced taproots that averaged 9.3 inches (23.6 cm) long and 0.3 inches (0.8 cm) in diameter . On a sandy site near Central City, Nebraska, researchers described the root system of a 1st-year white sweetclover plant that was 3 feet (1 m) tall with 15 stems. Near the root crown, the taproot measured 1.5 inches (4 cm) in diameter. Diameter tapered to about 1 cm by about 1.5 feet (0.5 m) deep and remained that size to the maximum taproot depth of 5 feet (1.5 m). Most lateral roots also reached 5 feet (1.5 m) deep, but rarely did they extend more than 2 feet (0.6 m) from the taproot. Sublateral roots were abundant, and generally the soil up to 5 feet (1.5 m) deep beneath white sweetclover was "well filled with roots" .
Sweetclover flowers April to October throughout North America [11,52,82,117,158,191,235,241,292]. Generally yellow sweetclover flowers 1 to 3 weeks earlier than white sweetclover [39,55,220,229,254,267,289]. The flowering rate in individual white sweetclover racemes is usually about twice as fast as that for yellow sweetclover , which may explain why Willard  suggests that yellow sweetclover flowering is more uniform than that of white sweetclover. Moisture conditions, elevation, and likely other site factors can affect flowering. Following a dry spring and an especially dry June in Ohio, sweetclover flowered for a second time in mid- to late July. There were several moderate rains in July . In ponderosa pine forests in north-central Arizona, the flowering period for yellow sweetclover was much shorter at a site that was 600 feet (180 m) higher than its comparison site .
Sweetclover green-up, fruit development, and seed maturation were described in a few locations. In north-central Arizona, vegetative growth of yellow sweetclover began in mid-April. Vegetative growth was delayed 2 to 3 weeks at a high-elevation site compared to a low-elevation site. Mature yellow sweetclover fruits were present by late August at the low-elevation site, and fruit maturation continued into November at the high-elevation site . In southern Ontario, there were usually ripe yellow sweetclover seeds by late July, but it was early August before ripe white sweetclover seeds occurred. Often sweetclover seeds remained on the plant through the winter (Rempel unpublished data cited in ).
Root crown buds and carbohydrate storage: In late summer and early fall, 1st-year sweetclover plants increase their taproot size, root crown bud number and size, and underground carbohydrate storage. In fields near Ames, Iowa, stems are 90% or more of the total weight of 2- to 3-month-old white sweetclover. By late September of the 1st growing season, roots provide up to 80% of total plant weight . In fields in Wisconsin, the largest increases in root size and carbohydrate storage for 1st-year sweetclover occurred between 18 September and 18 October . Additional field studies conducted in Ames, Iowa showed that 1st-year sweetclover taproot weight and root crown bud abundance increased from late summer to early fall. When particularly "vigorous" 1st-year yellow sweetclover plants were excavated on 20 August, taproots averaged 2.6 g, and there were 2.2 root crown buds/plant. Taproot weight averaged 11.7 g, and there were 29.4 crown buds/plant on 20 November. Greenhouse experiments revealed that increases in taproot size and root crown bud production occurred with decreasing photoperiod . In experimental fields in Columbus, Ohio, sweetclover produced large root crown buds in August. Buds became larger and more numerous until November . At the Agronomy Research Center in West Lafayette, Indiana, researchers found that total nonstructural root carbohydrates (TNCs) for 1st-year yellow sweetclover were highest from November to December. TNCs were lowest in May after the emergence of 2nd-year plants . For more on how these changes in root development and storage may affect management of sweetclover, see Control.
Sweetclover reproduces from seed. Cases of vegetative sprouting after damage have been reported, but are rare (see Vegetative regeneration for more information).
Fertilization: After reviewing published studies and conducting original studies on many yellow and white sweetclover strains and varieties, Kirk and Stevenson  concluded that self fertilization does occur naturally for some white sweetclover strains or varieties, but seed production from naturally self-fertilized yellow sweetclover is rare. Studies that followed the Kirk and Stevenson review (published in 1931) generally support their conclusions. In the greenhouse, researchers used genetic markers to determine that cross-fertilization in annual and biennial white sweetclover populations averaged 67% and 58%, respectively . When yellow sweetclover flowers were artificially self pollinated, fruit set ranged from 0 to 69.2% and averaged 19.3% . In a field experiment along the Rio Grande in Albuquerque, New Mexico, fruit set was low when yellow sweetclover racemes were protected from insect visitors. Fruit set by protected racemes (6%) was significantly (P Seed production: High levels of seed production are reported for sweetclover (, Rempel and Cavers unpublished data cited in ), and available reports indicate that white sweetclover generally produces more seed than yellow sweetclover (Rempel and Cavers unpublished data cited in ). However, seed production estimates using calculations that assume all flowers produce fruits and all fruits produce 1 seed can largely overestimate production . Methods used to determine seed production in the following studies were not reported. In London, Ontario, large white sweetclover growing in open conditions produced 200,000 to 350,000 seeds/plant. Large yellow sweetclover growing under similar conditions rarely produced more than 100,000 seeds/plant (Rempel and Cavers unpublished data cited in ). In North Dakota, an average-sized white sweetclover with 5 stems produced 14,235 seeds .
Klemow  found that estimates of seed production were exaggerated when estimation calculations did not factor in flower abortion and empty fruits. In a white sweetclover population ecology study in an abandoned rock quarry in Syracuse, New York, Klemow [126,127] estimated that white sweetclover produced an average of 4,380 fruits/plant and 11,640 fruits/plant in sparsely vegetated (cover 8%) and densely vegetated (cover 41%) sites, respectively. About 80% of fruits contained a seed, so the average seed production per plant was 3,530 on the sparse site and 9,710 on the dense site. Fruit and seed production estimates, however, assumed that all flower buds counted in August would develop into flowers, form fruits, and produce seed. A later visit to these sites showed that flower buds were often aborted or failed to produce fruit. When abortion and empty fruits were factored into calculations, the most and least seed produced by white sweetclover over the 5-year study period was 5,000 and 171 seeds/plant, respectively [126,127].
Sweetclover seed production is often reduced if plants are damaged, grow on infertile sites, or if insect visitation is limited by weather, insectivorous birds, or other means. In Ontario, Canada, researchers observed that damaged sweetclover plants or plants in extremely infertile soils sometimes produced less than 100 seeds (Rempel and Cavers unpublished data cited in ). After many field observations and studies, Coe and Martin  found that seed production was greater in dense than in sparse sweetclover stands, reduced in drought conditions, and lower when cloudy, rainy weather limited insect visitation. In Arlington, Virginia, a 3-foot (0.9 m)-tall sweetclover produced 196 racemes, and racemes produced an average of 20.4 fruits each. This plant grew in a stand density of 4 sweetclover/ft². When plants were protected from insects, racemes averaged 0.63 fruits each . Along the Rio Grande in Albuquerque, New Mexico, yellow sweetclover fruit set increased with increasing distance from cliff swallow colonies. When plants were within 660 feet (200 m) of the colonies, fruit set was reduced by about half. Once cliff swallow chicks fledged, however, the relationship between fruit set and colony proximity was lost. Cliff swallows were consuming insects that visited yellow sweetclover. Researchers estimated that a cliff swallow colony of 150 nests could consume over 500,000 insects per day, based on an average insect size of 5 mm .
Seed dispersal: Because sweetclover lacks appendages for wind dispersal, most seed falls near the parent plant (review by ), but observations and experiments indicate that long-distance dispersal by animals and water is possible. Long-distance dispersal may also occur through the transport of contaminated seed or animal feed (, review by ).
Water: Several sources suggest that sweetclover seed is dispersed by water. Based on plant distributions along waterways in Alaska [37,225], Montana , Arizona , and New Jersey , sweetclover seed dispersal by water seems likely. Experiments conducted in London, Ontario, showed that over 65% of white sweetclover seeds were still floating after 15 minutes in violently agitated water (unpublished experiments described in ).
Animal: Observations and experiments leave little doubt that sweetclover seed is transported by animals. In the Intermountain West, sweetclover spread along cattle trails was reported , and in the Missouri Ozarks, sweetclover was restricted to horse trails . While collecting sweetclover seeds for later experiments, students found that sweetclover seeds with fruit layers attached were transported on human clothing (unpublished experiments described in ).
Experiments show that viable sweetclover seed can be recovered from animal feces. When white-tailed deer pellets were collected from mixed-deciduous forests in Ithaca, New York, a maximum of 13 white sweetclover seeds germinated/pellet group . Three white sweetclover seeds were collected from crops of mourning doves, and 1 seed germinated. A seed recovered from the gizzard did not germinate . When calves, horses, sheep, hogs, and chickens were fed a known quantity of white sweetclover seed, 17.7%, 10%, 17.1%, 11%, and 0% of the seed germinated from collected feces, respectively. When recovered seeds were treated with sulfuric acid, germination rates increased by 40% or more, indicating that a large portion of white sweetclover seeds were still hard after passing through these animals (see Germination for more about hard sweetclover seed). Five percent of white sweetclover seeds that remained inside calves for up to 48 hours germinated. Ten percent of seeds recovered after 48 to 80 hours inside calves germinated. Sweetclover seeds may also be transported in partially composted manure. Two percent of white sweetclover seeds germinated after 2 months of burial in manure .
Although wind dispersal is relatively unimportant for sweetclover, a study found that the weight of white sweetclover seeds depended on time of production. Fifty seeds produced early in the growing season and late in the growing season averaged 78.9 mg and 59.3 mg, respectively . Whether or not lighter seeds could be dispersed longer distances is unknown, and germination percentages were not reported.
Seed banking: Studies clearly indicate that sweetclover produces a seed bank however, estimates of the longevity of seed in the soil vary from >2.5 [126,260] to 81 years (review by ). Sweetclover seeds have germinated after 81 years of storage (Becquerel 1934 cited in ), but field studies involving the recovery and germination of buried seed over time are lacking.
Sweetclover produces a percentage of hard seeds (see Germination) that germinate only after scarification. Hard seeds likely make up the majority of the seed bank [115,233]. When white sweetclover seed was buried in pots on an abandoned rock quarry near Syracuse, New York, 43% and 26% of the seed on sparsely and densely vegetated sites, respectively, failed to germinate but was still viable a year later . In other studies, sweetclover emerged from soil samples although plants were absent from the aboveground community [29,198].
Storage conditions: Sweetclover seeds have survived and germinated after decades in storage, but storage conditions were rarely described, making it difficult to assess their relevance to field situations (Becquerel 1934 cited in , Ewart 1908 cited in , Munn 1954 cited in , Crocker 1938 cited in ). Thirty percent of white sweetclover seeds germinated after 19 years in an unheated shed in Twin Falls, Idaho. Maximum and minimum temperatures for Twin Falls can be 105 °F (41 °C) and -26 °F (-32 °C), and in the shed, temperatures were slightly higher . After 81 years of storage in unknown conditions, 0.6% of white sweetclover seed was viable (Crocker 1938 cited in ).
Field conditions: Field studies suggest that sweetclover seed remains viable after 14 to 17 years in the soil but may survive over 50 years in the soil. Several researchers indicate that sweetclover can be abundant even after "several years" without mature plants on a site (, review by ). After 5 years of underwater storage in Prosser, Washington, a small proportion of white sweetclover seed germinated, but 42% of seeds were still firm . In North Dakota, sweetclover remained viable in the soil for at least 14 years. Sweetclover was planted and allowed to produce seed on 2 agricultural plots. In the following years, plots were cultivated and planted to other crops. Sweetclover seedlings emerged almost every spring for 14 years, even though 1st-year plants were killed each year . On experimental plots at the University of Saskatchewan, white sweetclover seed survived 17 years in the soil. Crop history records and the distribution and quantity of seed led the researcher to conclude that white sweetclover germinated from soil-stored seed, not dispersed seed . In another field study, a researcher visited several areas where circumstances would indicate long-lived, soil-stored seed. In Copenhagen, Denmark, a pork market that was built in 1910 was torn down in 1961. Some archaeological digging occurred, and by 1963, yellow sweetclover was growing on site. Because wind-dispersal is unlikely, the researcher speculated that yellow sweetclover germinated from soil-stored seed .
Germination: Sweetclover generally produces both readily germinable and water-impermeable or "hard" seeds. Percentages of hard seed produced vary. Of the sweetclover seeds collected from the Royal Botanic Gardens in Cambridge, England, 32% of yellow sweetclover and 85% of white sweetclover seeds were hard . Over 90% of white sweetclover seeds collected in July and August from roads and grasslands near Leuven, Belgium, were hard . The seed collected from sweetclover that had germinated from seed stored in the soil for up to 14 years in North Dakota was nearly 100% hard . Factors controlling the proportion of hard seed produced were not described in the available literature (2010).
Germination of hard sweetclover seeds can be encouraged by heat treatments and fluctuating temperatures around freezing. Light is not required for sweetclover germination, and high temperatures (95 °F (35 °C)) discourage germination . One study found that germination of white sweetclover seeds was significantly lower (P=0.0069) in the field (6.7%) than in the laboratory (11.8%) , suggesting that germination results from greenhouse studies may not be fully realized in field conditions.
Heat: Researchers report that fire can stimulate germination of soil-stored sweetclover seed ([35,128], review by ). In the laboratory, heat treatments have increased the germination of hard sweetclover seed. After soaking hard white sweetclover seeds in 180 °F (80 °C) water, most became permeable to water (Martin 1922 cited in ). Dry heat treatments of 150 °F (66 °C) for 5 days produced a maximum germination increase of 10% for hard sweetclover seed. One minute at 220 °F (100 °C) produced only a 2.5% increase in hard seed germination, but 4 minutes at 220 °F (100 °) significantly (P Chilling: Alternating temperatures that include near freezing temperatures may increase germination of hard sweetclover seed more than constant freezing or chilling temperatures [155,260]. After conducting several experiments, Martin  found that moisture content did not affect softening or germination of hard seed but that 2 or more months of alternating temperatures around freezing produced high germination percentages. Seeds buried outdoors from October to late April at 1 to 3 inches (2.5-7.5 cm) deep germinated better than seeds buried deeper, where minmum temperatures and temperature fluctuations were reduced . After conducting field and laboratory experiments on white sweetclover seed collected in Leuven, Belgium, researchers concluded that chilling and exposure to alternating temperatures increased germination . Fluctuating cold temperatures may not be sufficient for germination of all hard sweetclover seed, however. In experimental field plots at the University of Saskatchewan, nearly all white sweetclover seeds required scarification to germinate even after 17 years in the soil .
Other vegetation: Presence of other vegetation may affect sweetclover germination. In northern Arizona, yellow sweetclover formed dense stands on sites lacking bunchgrass cover, but as bunchgrasses increased yellow sweetclover decreased. Experiments revealed that live foliage extracts from Arizona fescue (Festuca arizonica) and mountain muhly (Muhlenbergia montana) significantly reduced the germination percentage, germination rate, and initial root development of yellow sweetclover (P Seedling establishment and plant growth: Sweetclover seedling establishment and growth are generally best in moist and moderated conditions. Several studies indicate that sweetclover utilizes other vegetation as nurse plants for successful establishment. Yellow sweetclover seedlings are considered more "vigorous" than white sweetclover seedlings (review by ), but reasons for this claim were not provided.
Emergence timing: Although sweetclover seedlings can emerge throughout the growing season, emergence peaks in the spring ([126,127], review by ). In southern Ontario, sweetclover seedlings emerge throughout the year, but emergence is greatest in March and April. Another smaller peak in emergence occurs in September or October. With 3 or more days above freezing, winter emergence can occur. Survival rates are best for seedlings emerging from late March to early May. Early emergence can compress the sweetclover life cycle into one year: Sweetclover seedlings that emerged in February or March on the gravel bars of the Thames River typically flowered in their first year (review by ).
Burial and moisture: Moisture is important for initial sweetclover establishment (125,124, review by ), and planting guides suggest seeding sweetclover 0.5 inch (1.3 cm) deep in "heavy soils with good moisture", and 1 inch (2.5 cm) or deeper in "light soils" or low-moisture conditions (review by ). Experiments conducted on white sweetclover seed collected from China's Qubqi Desert also indicate that deeper burial may improve survival of seedlings in low-moisture conditions. Generally, establishment in sand was better from seeds buried more deeply (1.2 inches (30 mm) under a low-moisture regime and more shallowly 0.2 inch (5 mm) under a high-moisture regime. Seeds buried 2 inches (5 cm) deep generally failed to establish. Because light is not required for germination, researchers speculated that anoxic conditions at this depth may explain poor establishment. In a companion experiment, researchers found that white sweetclover seeds submerged for 6 days produced radicles that were deformed or missing growing tips .
Site conditions: Observations indicate that sweetclover seedling density, arrangement, and survival vary with site conditions. High seedling densities are common in riparian areas. Along the Nenana River floodplain in Alaska, white sweetclover seedling density ranged from 407 to 1,307 seedlings/2 m² plot. Within the plots, seedlings were highly clumped. There were often more than 60 white sweetclover seedlings/400 cm² . Following winter flooding on the dry surface zone of the Hassayampa River in central Arizona, sweetclover seedling establishment was high. Establishment timing and high sweetclover seedling cover prevented establishment of saltcedar (Tamarix spp.) in the area .
In Ontario, sweetclover seedling density and survival varied by soil type. On fertile clay or loam soils, sweetclover seedlings were typically clumped and the majority did not survive. In the greenhouse, more than 75% of sweetclover seedlings died within 5 months when they occurred at a density of 600 seedlings/m². On gravelly, sandy, or stony soils, sweetclover seedling mortality was high fewer than 5% of seedlings survived more than 4 months. On steep eroded banks and frequently disturbed sites, sweetclover seedling emergence was low but subsequent mortality was also low, so that these inhospitable sites generally supported a few large plants that typically produced abundant flowers (review by ).
Disturbances and neighboring vegetation: Sweetclover is often associated with disturbed, open sites, but some studies show that sweetclover establishment may be improved when other vegetation is present. Sweetclover seedling survival increased with gap size in a Kentucky bluegrass (Poa pratensis)-dominated old field in southwestern Michigan. Although sweetclover germination was high in the 0.5, 1, 2, and 3 cm diameter gaps cut into the sod, early seedling survival was significantly lower in small gaps than in large gaps (P ≤ 0.01). The study area burned several weeks before this field experiment, so light levels were high in all intact plots. Researchers suggested that species removal may have lead to the loss of safe sites and made conditions too harsh for successful establishment . In an abandoned rock quarry in Syracuse, New York, there were up to 15.5 white sweetclover seedlings/0.25 m² on a sparsely vegetated site and 116 white sweetclover seedlings/0.25 m² on a densely vegetated site. Water-holding capacity was greater on the dense than the sparse site. In a year that was very warm and dry during the initial seedling establishment stage, the presence of vegetation seemed to improve survival of white sweetclover seedlings. However, when 5 years of data were pooled, survival of white sweetclover was not significantly different between densely and sparsely vegetated sites. Establishment and survival of white sweetclover on these sites in a normal and a dry year are shown in the table below [126,127].
Throughout its nonnative range, sweetclover is described on open, disturbed sites that include roadsides, railways, fields, and waterways [10,37,48,98,151,188].
Climate: The wide distribution of sweetclover implies wide climatic tolerance. Moisture is important for sweetclover seedling establishment, but once established, plants tolerate extremely dry conditions. In the fall, contractile roots pull sweetclover root crowns beneath the soil surface ( ≥ 2 inches (5 cm)), protecting plants from freezing temperatures (review by ). Yellow sweetclover is considered more heat and drought tolerant than white sweetclover (reviews by [220,254]). Although yellow sweetclover has also been described as more cold hardy than white sweetclover (review by ), current distributions suggest this may not be true (see General Distribution). In Alaska, sweetclover occupies habitats with extreme weather. In Ketchikan, annual precipitation averages 160 inches (3,940 mm) and temperatures average 45 °F (7.2 °C). In interior Alaska, annual precipitation can be as low as 6 inches (170 mm), and the average annual temperature can be as low as 26 °F (-3.3 °C) . During growth chamber experiments, researchers found that 1- to 4-week-old yellow sweetclover seedling survival was high at 21 °F (-6 °C). Survival was much lower at 18 °F (-8 °C) .
|Elevation: Range of elevations reported for sweetclover in western North America|
|1,600 to 8,500 feet (488-2,591 m) yellow sweetclover occurs about 330 feet (100 m) above and below white sweetclover |
|California||Below 4,920 feet (1,500 m) |
|Colorado||Yellow sweetclover: 4,000 to 7,500 feet (1,220-2,290 m) white sweetclover: 4,500 to 7,500 feet (1,370-2,290 m) |
|Hawaii||White sweetclover: 15 to 4,400 feet (5-1,340 m) |
|Nevada||Yellow sweetclover: 2,300 to 6,300 feet (700-1,900 m) white sweetclover 1,200 to 6,500 feet (370-1,980 m) |
|New Mexico||4,000 to 8,000 feet (1,200-2,400 m) |
|Utah||Yellow sweetclover 4,000 to 8,010 feet (1,220-2,440 m) white sweetclover 3,490 to 7,000 feet (1,065-2,135 m) |
|British Columbia (southeast)||Good growth from 5,910 to 7,320 feet (1,800-2,230 m) poor growth above 7,970 feet (2,430 m) |
Soils: Sweetclover grows on a variety of alkaline or slightly acidic soils ([33,37], review by ). Very low nutrient levels and fine- and coarse-textured soils are tolerated ([37,167,245,279], review by ). Several reviews indicate that yellow sweetclover tolerates nutrient-poor and dry soils better than white sweetclover [49,89,219,254].
Sweetclover occupies a variety of soil types and textures but growth and productivity can vary by soil type and region. Residents of Fort Smith near Canada's Wood Buffalo National Park reported that sweetclover expanded its range on fine-textured soils but nevertheless was primarily restricted to disturbed sites . In meadows in Michigan's Oakland County, white sweetclover was "plentiful" on sites with "considerable clay" . In southwestern North Dakota, yellow sweetclover occupied a variety of habitats with textural classes ranging from loams to clays and pH ranging from 7.9 to 8.8 . A review reports that sweetclover is most productive on silt loams to clay loams with neutral to alkaline pH . Wasser  reports a minimum pH tolerance of 5.5 for yellow sweetclover. Seeding of yellow sweetclover was successful on a South Dakota rangeland where soils were up to 65% clay . On riverbanks in Quebec, white sweetclover was most common on alkaline, sandy soils with very rapid drainage and low to high degrees of stoniness . On Alaska rivers and roads, white sweetclover density was lower on cobbly than on sandy surfaces .
Many studies report an association between white sweetclover and calcareous soils. White sweetclover was especially common on calcareous soils in Michigan , the northeastern United States and southeastern Canada , the Gulf and Atlantic Coasts , and eastern Texas . During a study conducted in Canada, researchers found that white sweetclover plants grown from seeds collected on calcareous soils grew well only on calcareous soils. White sweetclover plants grown from seed collected on acidic soils grew well on acidic and calcareous soils but grew best on calcareous soils .
Salinity: Sweetclover tolerates moderate salinity [119,257]. A review reports that salinity levels of 0.2 to 0.4% or 2 to 4 ppt (3-5 mS/cm) are tolerated (review by ). In Alberta, Saskatchewan, and Manitoba, white sweetclover occurred on soils where salt crystals were visible on the surface [17,50]. Yellow sweetclover was reported in a marsh near Lincoln, Nebraska, where the salinity averaged 0.2%  white sweetclover grew along the South River in Anne Arundel County, Maryland, where salinity levels ranged from 0.2 to 3.6% .
Moisture and flooding: Sweetclover is common in riparian areas and typically tolerates short-duration flooding early in the growing season [11,238]. A review indicates that white sweetclover is slightly more flood tolerant than yellow sweetclover. In southern Ontario, white sweetclover is occasional along rivers with several weeks of winter and spring flooding (review by ). Along Alaska's Nenana River, white sweetclover survived shallow flooding that lasted only a few days . In southern Idaho, Rosentreter  reported that yellow and white sweetclover abundance increased in periodically flooded stream banks, but yellow sweetclover is typically killed by high water during the growing season (review by ). During field experiments in London, Ontario, fewer than 10% of sweetclover plants survived 5 days of immersion in the Thames River when the temperature was 68 °F (20 °C) (Weekes and Cavers unpublished data cited in ).
Generally sweetclover is an early to mid-seral species common on open, disturbed sites. Sweetclover rarely persists in dense shade and often appears early in the succession of recently disturbed or bare sites. It is important to note that year-to-year sweetclover cover can vary a lot "boom" growth years are common . In South Dakota, times when areas are covered with white and/or yellow sweetclover flowers are described as "sweetclover years" . In big sagebrush/grasslands in central Montana, researchers reported 10% to 12% cover of yellow sweetclover in one year and less than 1% cover the next . Large fluctuations in sweetclover cover make interpretation of seral change along a chronosequence difficult.
Shade: Most reviews and studies indicate that sweetclover grows best in full sun or partial shade. A review of Upper Midwest habitats indicates that sweetclover is most frequent in open, disturbed upland prairies, savannas, and dunes (review by ). Other reviews report that sweetclover is less "vigorous" and produces fewer seeds in shade than in full sun  however, shade tolerance may be greater in hot, dry climates .
Most studies and observations indicate that although sweetclover is common on open sites, some degree of shade tolerance also exists. At the Mammoth campground in Yellowstone National Park, yellow sweetclover was positively associated with open canopy conditions (P Bare site succession: Bare soil is rapidly colonized by sweetclover, but rarely does sweetclover persist as a dominant. On calcareous soils deposited during construction in central Germany, white sweetclover dominated (60-75% cover) in the 2nd and 4th years of succession. White sweetclover populations collapsed in the 5th year, but in the 7th and 10th years of succession, white sweetclover cover exceeded 10% . In Plzen, Czech Republic, white sweetclover dominated a nutrient-poor site 6 years after bare soil was left by a human-caused disturbance. White sweetclover did not dominate in any other year . Yellow sweetclover production was greatest 4 years after disturbance in a sagebrush habitat in northwestern Colorado. After all vegetation and the top 2 inches (5 cm) of soil were removed and the remaining 14 inches (35 cm) of soil was mixed, yellow sweetclover production was 1 g/m² in first postdisturbance year, 13 g/m² in the 2nd, 5 g/m² in the 3rd, 32 g/m² in the 4th, and less than 1 g/m² in the 5th, 6th, and 7th years . Near Duluth, Minnesota, yellow sweetclover appeared 4 years after bare sand was deposited in a high-water year. Persistence beyond this time was not reported . On fly-ash mine pits in Tennessee, the importance of white sweetclover was greatest on 8-year-old pits when 6-month, 3-year, and 8-year-old pits were compared. Fly ash that is deposited into the pits is "essentially sterile", free of seeds and other reproductive plant material . Sweetclover occurred in the early succession of sand flats in eastern New York created by deposition of material dredged from the Hudson Estuary channel. Dredging began in 1929, and species composition was first evaluated in 1935 . In vacant lots in Montreal, Quebec, white sweetclover-dominated sites had a large amount (10.8-26.5%) of bare ground .
Floodplain succession: On floodplains, sweetclover is common in early- and mid-seral stages of succession. In southwestern North Dakota, yellow sweetclover occurs in early-seral eastern cottonwood-Rocky Mountain juniper (P. deltoides-Juniperus scopulorum) stands, and mid-seral eastern cottonwood-green ash (Fraxinus pennsylvanica) stands on more stable floodplains . In eastern cottonwood stands along the Missouri River in southeastern South Dakota, white sweetclover cover generally decreased with increasing eastern cottonwood stand age. In stands estimated to be 10, 14, 23, 35, and 55 years old, yellow sweetclover cover averaged 8%, 12%, 2%, 2%, and 0%, respectively .
Old field succession: Abandoned agricultural fields are common sweetclover habitat. Typically sweetclover abundance is lowest in the most successionally advanced old fields. Likely the composition and density of associated vegetation affects sweetclover persistence. In the Black Forest of central Colorado, yellow sweetclover occurred on a 4-year-old field but not on 1-, 9-, or 22-year-old fields . When a previously cultivated site at the Fermi National Accelerator Laboratory in Illinois was seeded with native tallgrass prairie species, white sweetclover frequency was greater after 1 to 6 years after seeding than 12 years after seeding . At the WW Kellogg Biological Station in Kalamazoo County, Michigan, large white sweetclover patches occurred in fields last plowed 10 to 16 years previously . When different-aged stands were sampled along a chronosequence of old field to deciduous forest in southwestern Ohio, sweetclover was present but not common on 2-, 10-, and 50-year-old fields. Sweetclover did not occur in stands 90 or 200 years old. Two-year-old fields were dominated by red clover (Trifolium pratense) 10-year-old stands were dominated by Canada goldenrod (Solidago canadensis) and meadow fescue (Schedonorus pratensis) 50-year-old stands were dominated by Canada goldenrod and had 30% cover of white ash (F. americana) and black cherry (Prunus serotina). Stands 90 years and older were dominated by deciduous forest species .
Disturbances: Throughout its nonnative range, sweetclover is described on disturbed sites [10,89,201,267,282,298]. Generally sweetclover occurs on recently disturbed sites, but without further disturbance, sweetclover fades from the community. In mixed- and shortgrass prairie near Cheyenne, Wyoming, yellow sweetclover occurred and was sometimes frequent on sites disturbed about 3 to 25 years earlier. On undisturbed sites or sites disturbed more than 25 years ago, yellow sweetclover was rare or absent .
In prairies, sweetclover often occurs on soil mounds created by wildlife. In the prairie potholes of Montana, North Dakota, South Dakota, and western Minnesota, sweetclover occurred on earth mounds created by pocket gophers and badgers . In Billings, North Dakota, yellow sweetclover occurred in all 4 active prairie dog towns visited . On Cayler Prairie Preserve in Dickinson County, Iowa, sweetclover was consistently associated with excavation mounds created by badgers that were hunting ground squirrels. Sweetclover was uncommon in undisturbed tallgrass prairie . In Wisconsin prairies, soil mounds created by ants or other animals are the first establishment site for white sweetclover .
Roads and waterways are common sweetclover habitats and have been important to sweetclover spread (see Introduction and spread). In Alaska, sweetclover was restricted to roadsides and floodplains and did not occur in roadless areas . In the Northern Rocky Mountains, yellow sweetclover occurred in disturbed areas (roads, ditch banks, or logged sites) above and below timberline but did not occur in little-disturbed or undisturbed vegetation . In mixed-grass prairie and open ponderosa pine woodlands in Wind Cave National Park, South Dakota, logistical regression analyses revealed that sweetclover was associated with roads and trails. Yellow sweetclover was also common in prairie dog towns, and white sweetclover was most common on low-elevation, recently burned sites . In the wet to mesic Chiwaukee tallgrass prairie in Wisconsin, density of white sweetclover was significantly greater on past disturbed than undisturbed transects . In the late 1990s, researchers surveyed 1,940 miles (3,120 km) of county and state roads in western Adirondack Park, New York. White sweetclover occurred at more than 100 sites in the survey area .
Grazing: In the studies that evaluated sweetclover on grazed and ungrazed sites, typically cover on grazed sites was similar to or greater than cover on ungrazed sites. Yellow sweetclover persisted on grazed and ungrazed sites during drought conditions in semiarid grassland in north-central Arizona. Moderate- and high-impact, short-duration grazing rotations were evaluated. During the 8-year study, precipitation levels for the 8 months prior to July were 2.5 to 12 inches (62.4-312.8 mm) lower than the 20-year average . In western Colorado, yellow sweetclover cover within long-term (41-51 years) ungulate exclosures in sagebrush and mountain shrubland did not differ much from cover outside the exclosures . When grazed and ungrazed portions of Fults Prairie, Illinois, were compared, yellow sweetclover was absent from the ungrazed portion and had 36% frequency on the grazed portion. Researchers suggested that decreased abundance of climax vegetation on grazed sites increased the success of yellow sweetclover and other “weedy” species . While aboveground cover of yellow sweetclover may not differ on grazed and ungrazed sites, seed density differed in soils collected from grazed and ungrazed mixed-grass prairie sites in western North Dakota. Fewer than 5 yellow sweetclover seedlings emerged from soil samples taken from grazed sites, and more than 20 seedlings emerged from soil samples from ungrazed sites .
For information about sweetclover and fire-disturbed sites, see the discussion on Long-term fire effects and postfire succession.
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