Craig Sheaffer, Nancy Ehlke, Jake Jungers, University of Minnesota, St. Paul
Beginning in the early 1990’s, the University of Minnesota had a significant program to study kura clover (Trifolium ambiguum L.) and its use in Midwest grazing systems. Kura clover appears to be an ideal pasture legume. It has yield similar to red clover but greater persistence under weather extremes than any legume. Its forage is leafy and has a high nutritive value. In addition, if allowed to flower, it’s sweet smell earned it the name of honey clover. Alas, after decades of agronomic and plant breeding research, its adoption by producers has been disappointing. Seed availability, seed cost, and poor establishment have been identified as limiting factors. The renewed interest caused by kura’s use as a living mulch for corn production has faded because of challenges with managing kura competition and its effect on grain yield. Following is an historic account of research in Minnesota and a discussion of important traits of kura clover.
Seed availability and cost have been a major limiting factor to the use of kura clover. Seed available is often VNS (variety not stated) and in some years no seed is available. We attempted to work with growers in Roseau, MN to develop a kura clover seed industry but seed production was unprofitable. There were two issues, the first relates to declining seed yields as stands age. For example, in research plots, Rhizo had first production year yields of 480 lb/acre; however, they declined to 184, and 126 lb/acre in years 2 and 3, respectively. As previously described, kura clover prioritizes asexual reproduction via rhizomes and not seed production. The second seed production issue is poor seed harvest from the field by combines because of challenges of threshing/separating of the small seed from flower parts and foliage.
An alternative approach is to establish kura clover by transplanting rhizomes with one or more nodes from an existing field and incorporating them within a tilled field. A potato digger was used to extract rhizomes and crowns that were then spread onto a newly tilled field using a manure spreader and incorporated by disking. While this method of establishment has been successful, it has not been commercially adopted.
The optimum soil pH for kura clover establishment and yield is between 6 and 7; therefore, on acid soils liming was beneficial to establishment. We found a 52% increase in yield as soil pH increases from 4.9 to 6.5 and an increase in nodulation. Kura clover yield also responded to fertilization of nutrients such as potassium and phosphorus.
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| Kura clover in full flower the year after seeding. |
Kura clover traits
Persistence
Kura clover is a remarkably persistent forage legume. Plants remain today from initial experiments in the early 1990’s in Minnesota. Kura clover has survived extreme winter weather including lethal air temperatures, ice sheeting, and flooding. It becomes dormant during drought but rapidly recovers when normal precipitation occurs. It can tolerate continuous grazing and compaction due to hoof traffic. The secret of Kura’s persistence is its belowground morphology. It has been identified as a “guerilla” or “clonal” species which invests considerable energy resources in developing underground stems (rhizomes) that form new secondary plants (Figure 1). |
| Figure 1. A kura clover plant showing foliage, roots, and rhizomes. |
By spreading to new areas in a field, guerilla plants can utilize nutrient and water resources from a wider area than stationary tap rooted plants like alfalfa. We found that combined below ground biomass (root, crown, and rhizome) yields within the top 12 inch of soil to be over 1 ton/acre in the seeding year and almost 3 ton/acre in 8-year-old stands (Table 1). In these 8-year-old old stands, rhizomes and secondary crowns produced from rhizomes comprised about 45% of the below ground biomass. Because of its spreading growth habit kura clover provides significant ground cover with the potential to have from 100-150 plants per ft2 (Figure 2).
Table 1. Forage yield, total below ground biomass (TBB), rhizome biomass and crown number in the spring following three years of continuous and rotational grazing at 14 and 28 days. The 28-day treatment was also grazed late in the fall.
Kura clover provides the best yield potential when rotationally grazed at a 28 day interval but will survive more frequent and continuous grazing. With continuous grazing, kura clover demonstrates phenotypic plasticity. In this adaptation to excessive defoliation, kura clover varies the amount of photosynthetic energy sent to various plant organs and reduces leaf size and petiole length; therefore, small leaves of kura clover hug the soil surface to escape defoliation.
Table 2. Kura clover leafiness, nutritive value, and lamb performance when grazing kura clover compared to birdsfoot trefoil. Values are an average of three years of grazing.
Table 1. Forage yield, total below ground biomass (TBB), rhizome biomass and crown number in the spring following three years of continuous and rotational grazing at 14 and 28 days. The 28-day treatment was also grazed late in the fall.
| Grazing treatments |
Yield | TBB | Rhizomes | Crowns |
|---|---|---|---|---|
| ton/a | lb/a | lb/a | no/ft2 | |
| Continuous | 2.6 | 4900 | 2456 | 148 |
| 14-day | 3.6 | 4232 | 1870 | 113 |
| 28-day | 4.5 | 5769 | 3050 | 147 |
| 28-day + fall | 4.3 | 5000 | 2000 | 118 |
Source: Peterson et al., 1994. Agronomy J. 86: 655-667.
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| Figure 2. Stand density (ground cover) of sod-seeded alfalfa, kura clover, and red clover over stand age. Source: Cuomo et al. Agron J. 95: 1591-1594. |
Kura clover provides the best yield potential when rotationally grazed at a 28 day interval but will survive more frequent and continuous grazing. With continuous grazing, kura clover demonstrates phenotypic plasticity. In this adaptation to excessive defoliation, kura clover varies the amount of photosynthetic energy sent to various plant organs and reduces leaf size and petiole length; therefore, small leaves of kura clover hug the soil surface to escape defoliation.
Nutritive value and yield
We found kura clover was very leafy and high in nutritive value resulting in good average sheep daily gains, long-term carrying capacity, and seasonal gain per acre (Table 2). High leafiness % is due to Kura clover only flowering in the spring with no upright stems later in the season. Our work was confirmed by Albrecht in Wisconsin who reported steer average daily gains and gain/acre of 2.7 lb/day and 916 lb/acre/year, respectively, for kura-grass mixtures.Table 2. Kura clover leafiness, nutritive value, and lamb performance when grazing kura clover compared to birdsfoot trefoil. Values are an average of three years of grazing.
| Trait | Kura clover | Birdsfoot trefoil |
|---|---|---|
| Leafiness | 97% | 81% |
| Nutritive value | ||
| Crude protein | 25% | 22% |
| Digestibility | 83% | 72% |
| NDF | 31% | 33% |
| Lamb performance | ||
| Avg. daily gain | 0.50 lb | 0.43 lb. |
| Grazing | 1664 days/ac | 1460 days/ac |
| Season gain | 783 lb/ac | 628 lb/ac |
Source: Sheaffer et al., 1992 Agron. J. 84: 176-180.
Herbage yield, under good growing conditions is from 3 to 5 ton/acre which is similar to red clover but about 20% less than alfalfa under similar growing conditions. The greatest kura clover yields occur in June when upright flowering stems develop.
Minimizing bloat
Kura’s nutrient rich and leafy foliage has caused frothy legume bloat. Therefore, its mixture with perennial grasses is advised. We evaluated timothy, reed canarygrass, Kentucky bluegrass, and orchardgrass as components of binary mixtures with kura clover and found that the best grass to use in mixture depends on the environment. Orchardgrass was competitive and comprised 60-75% of mixtures, whereas, reed canargygrass comprised 40-50% of the mixtures. Timothy and Kentucky bluegrass were less competitive and comprised about 40% of the mixture. In addition to reducing the incidence of bloat, mixtures have potential to provide higher yield than kura clover monocultures as well as to facilitate drying should hay be made. Kura clover forage has a high moisture content (90% or more) that is challenging to dry.Biological N fixation
Established stands of kura clover fix about 150 lb N per acre. Kura clover derives about 50 % of its N derived from fixation and is able to also scavenge soil N. However, the development of nodulation is a slow process relative to other clovers and can take as long as 50 days. Therefore, seeding year N fixation is low (20 lb/acre), and on some soils kura establishment benefits by N fertilization in the seeding year. Kura clover requires a very specific strain of Rhizobium that does not nodulate other clovers and is not naturally found in Midwestern soils. We sampled soils in long-term Kura stands in the Northern U.S. and identified and released a new strain that improved nodulation, biological nitrogen fixation, and establishment. Today, kura clover inoculum is availability through E-Commerce Biologics (https://ecommbio.com/).Variety development
Much of our initial research was conducted using the variety Rhizo, and later with Endura and Cossack. Generally, Rhizo had lower herbage and rhizome yields than the other varieties. The University of Minnesota released the population NF-93, based on superior seedling vigor; however, because of seed production difficulties it was never marketed.Seed availability and cost have been a major limiting factor to the use of kura clover. Seed available is often VNS (variety not stated) and in some years no seed is available. We attempted to work with growers in Roseau, MN to develop a kura clover seed industry but seed production was unprofitable. There were two issues, the first relates to declining seed yields as stands age. For example, in research plots, Rhizo had first production year yields of 480 lb/acre; however, they declined to 184, and 126 lb/acre in years 2 and 3, respectively. As previously described, kura clover prioritizes asexual reproduction via rhizomes and not seed production. The second seed production issue is poor seed harvest from the field by combines because of challenges of threshing/separating of the small seed from flower parts and foliage.
Establishment
The phrase “Kura sleeps, creeps, and then leaps” was coined by USDA-ARS scientist G.C. Marten and describes the initial slow establishment followed by significant spreading in subsequent years. This is illustrated in Figure 1. The initial slow growth is related to small seed size, its slow development of nitrogen fixation, and its allocation of nutrients to roots and rhizomes at the expense of foliage growth. Therefore, best establishment occurs when competition from companion crops, and weeds is minimized. With adequate rainfall, we found late summer seedlings were more successful than spring seedings because of reduced weed competition. Kura clover has been successfully established by drilling or broadcasting into conventionally tilled seedbeds and no-till seeded into suppressed pastures, but establishment is more challenging than for red clover and alfalfa.An alternative approach is to establish kura clover by transplanting rhizomes with one or more nodes from an existing field and incorporating them within a tilled field. A potato digger was used to extract rhizomes and crowns that were then spread onto a newly tilled field using a manure spreader and incorporated by disking. While this method of establishment has been successful, it has not been commercially adopted.
The optimum soil pH for kura clover establishment and yield is between 6 and 7; therefore, on acid soils liming was beneficial to establishment. We found a 52% increase in yield as soil pH increases from 4.9 to 6.5 and an increase in nodulation. Kura clover yield also responded to fertilization of nutrients such as potassium and phosphorus.
Kura clover living mulch
Kura clover has been evaluated as a living mulch when intercropped with corn and soybeans. The proposed system has been tested in a continuous corn/soy rotation, and also with alternating years of intercropping where kura clover monocultures are used for forage or seed production. In the system, established stands of kura clover are suppressed by herbicides to reduce its competition, the corn is seeded and later harvested, and kura clover regrows following corn harvest. In the fall and following year, Kura can be grazed by livestock. Both no-till and zone tillage systems have been evaluated. The system has potential to reduce N fertilizer use, provide continuous erosion protection, suppress annual weeds, increase soil health, and reduce tillage. This living mulch system has been successfully used with little or no reduction in corn grain or silage yield; however, because of inconsistency of suppression of kura clover, grain yields have sometimes been disappointing, due to competition for water. Weather conditions during the growing season can have a significant impact on the success of the system. Despite over two decades of research on refinement of the kura living mulch system, it does not appear to have significant adoption by farmers in Minnesota. This is because it best fits within a livestock system for grazing in alternate years, and it is a dynamic system requiring significant management and specialized tillage equipment. The system's outcome is inconsistent because of the additional water use, particularly on soils with low water-holding capacity. Living mulch systems might be best suited for more sensitive environments and those at high risk for water quality challenges. For more discussion of living mulch systems see:- Kura clover living mulch provides opportunity for high corn yield with lower N input
- Maine performance in a kura clover living mulch under drought conditions
Summary
Kura clover is a unique forage legume with an uncertain future. It is a ‘guerilla” species that allocates significant amounts of nutrients below ground resulting in outstanding persistence. Seed shortages and establishment challenges have limited its adoption by farmers. A team of scientists including Gordon Marten and Michael Russelle of the USDA-ARS; and Craig Sheaffer, Paul Peterson, Phillipe Seguin, Peter Graham, Lee DeHaan, and Nancy Ehlke, of the University of Minnesota conducted pioneering research on kura clover. John Baker, USDA-ARS, has led investigation on kura clover living mulch systems. A more complete history of kura clover also known as Caucasian, and honey clover as well as citations supporting this article are shown at: Forage legumes: Clovers, birdsfoot trefoil, cicer milkvetch, crownvetch and alfalfa.
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