RED CLOVER IN THE "TWENTY-FIRST" CENTURY
Red clover (Trifolium pratense L.) is one of the leading forage legumes in the U.S., Canada, and northern and eastern Europe. Red clover has maintained a prominent roll in animal agriculture in the more temperate regions of the world due, in part to high seedling vigor, ease of establishment, rapid growth, excellent soil improvement characteristics, high forage quality and reasonably high yield. Records would indicate that it was first cultivated in Europe in the third and fourth centuries, recorded as being in Italy by 1550, in England by 1645, and in the United States by 1663 (Taylor and Quesenberry, 1996). The early settlers brought red clover to the US as it was an excellent feed source for their animals. As settlement moved throughout the nation red clover moved with it. Being adapted to temperate climates, red clover was and is used most prominently in the northeastern and northcentral regions of the US, and southeastern Canada for hay, silage, grazing and soil improvement. Use of red clover declined substantially beginning in the 1950s as alfalfa (Medicago sativa L.) became more prominent forage for hay and silage. Nationwide, red clover and grass mixtures declined from approximately 15 million to 12 million acres between 1950 and 1970, and based on seed disappearance they probably declined to about 11 million acres by 1987. However, with the increase amount of grazing over the past decade the use of red clover has increased (Taylor and Smith, 1995).
I would like to briefly review some of the progress made over the past five decades, report on the results of some current research, and speculate what one might expect to see (or what might be needed) in red clover improvement and use in this century.
IMPROVEMENT IN RED CLOVER OVER THE PAST FIVE DECADES
In Wisconsin and other parts of the U.S. red clover has long been considered to be a short-lived perennial. Environmental stress and root rot diseases contribute to the lack of persistence of red clover (Taylor and Smith, 1979; Smith and Kretschmer, 1989; Venuto et al., 1992). Of the root rot causing pathogens of red clover, organisms of the Fusarium species have been isolated most consistently from diseased roots (Kilpatrick et al., 1954; Chi, 1965; Velde, 1980; Leath, 1985). The foliar disease, northern anthracnose, caused by Aureobasidum caulivora (Kirchn.) W.B. Cooke, is endemic and causes severe forage loss during the first growth of the season. Therefore, resistance to these pathogens should lead to improved persistence and yield. Selection for plant longevity in older stands of red clover and the incorporation of genes for resistance to root rot pathogens are the most practical methods of improving persistence of red clover (Smith, 1983; Venuto et al., 1992).
Earlier (Smith, 1998) I reported to you on the improvement the USDA - Agricultural Research Service - University of Wisconsin project has made in red clover through selection for persistence and disease resistance since the 1950s. Let us regress a minute and review that information. The result of repeated selection over that period for persistence and disease resistance has been the release of better cultivars: Lakeland released in 1953, Arlington in 1973 (Smith, et al., 1973) and Marathon in 1987 (Smith, 1994). Both forage yield and resistance to root rot and northern anthracnose has been improved (Figure 1). Resistance to the two diseases, root rot and northern anthracnose, has steadily increased with the development of new germplasm. Two to three cycles of selection for resistance to northern anthracnose was applied during each cycle of selection for persistence. On the other hand, only one cycle of selection for root rot occurred after the third cycle of selection for persistence. Earlier improvement in root rot resistance was the result of natural selection for resistance with the selection of healthy, persistent plants. Total forage yield increased over the cycles of selection due, in part, to selection for disease resistance and to increased persistence. There was no difference in forage yield (1.9 Tons Dry Matter per Acre) among the populations developed over the four decades in the second year of production (Figure 2). The differences among the populations occurs in the third and fourth year of production with Marathon and Wis Exp producing excellent forage yields in these latter years of production. Selection in exiting 3- and 4-year old stands of yield trials has been effective in improving persistence and to some extent root rot resistance. Similar results for forage yield have been observed in the red clover varieties developed by private companies (Figure 3).
Red clover cultivars on the market today are superior to those available two decades ago. Stand longevity, forage field, and disease resistance has been improved. Current germplasm being developed by the public and private programs appear to be even superior to those on the market today. So you might expect to see better, more persistent cultivars in the future. In our USDA/ARS - University of Wisconsin program we have the next generation of germplasm ready to be released as either germplasm or cultivars (Figures 2 and 3 - Wis Exp). Other germplasm lines with resistance to the root-knot nematode, Aphanomyces, Fusarium and Mycoleptodiscus are ready for release.
EVALUATION OF RED CLOVER CULTIVARS WITH GRASSES
Red clover is most often grown with companion grass for hay, silage or grazing. However, little information is available on the performance of cultivars in competition with grass species. Smith and Sharpee (1992) suggested that the relative performance of red clover cultivars may not be similar when grown in monoculture vs. in competition with grass.
The object of this research was to evaluate current, persistent red clover cultivars when grown with a grass companion using two harvest management systems. The research was conducted in cooperation with Dan Wiersma, Assist. Superintendent, Marshfield Agricultural Research Station and Mike Mlynarek, Superintendent Ashland Agricultural Research Station, College of Agricultural and Life Sciences, University of Wisconsin. The research has been reported at the 1999 Annual Meetings of Crop Science Society of America in Salt Lake City, UT in Nov. 1999 (Wiersma et al., 1999).
Eleven cultivars or experimental strains (Entries 1 through 11) were evaluated with or without bromegrass (Bromus inermis) at Arlington, WI or timothy (Phleum pratense) at Marshfield and Ashland, WI. The entries were subjected to two harvest management systems (2-cut or 3-cut per year) at Arlington and Marshfield and one (2-cut) at Ashland, WI. Seeding rates were 6, 3, and 3 lbs per acre for clover, timothy, and bromegrass, respectively. Plots were harvested when red clover reached 20% bloom for the 3-cut harvest system and 40% for the 2-cut system. To determine botanical distribution in the clover-grass mixtures, two 40 x 40 cm samples per plot were taken just prior to harvest. Yield of clover and grass was determined as % clover or grass times total plot dry weight. Plots were established in May, 1996 and data collected in 1997 (2nd year), 1998 (3rd year), and 1999 (4th year).
As expected, the yield of clover grown without grass was, in general, greater than when grown with grass, although the total grass-clover yields was greater. Both at Arlington and Marshfield the 3-cut system produced more red clover whether grown with or without grass than the 2-cut system (Figure 4.). However, the 2-cut system produced more red clover than the 3-cut system in the fourth year of productions at all locations. The decline in red clover production over time was less drastic in the 2-cut system.
The performance of red clover cultivars was influenced by the location, the cutting system, and the presence or absence of grass. In general, the performance of the cultivars grown without grass did not predict the corresponding response when grown with grass. Of the 15 possible comparisons of the performance of the cultivars with and without grass only one was significant (Table 1: at Arlington under the 3-cut system in the 3rd year). In fact, this significant correlation, -0.64, was in the opposite direction than expected. If performance in pure stands (without grass) is going to predict performance with grass the correlation should be positive and high (greater that 0.62 but less than 1.00). Examples of this lack of correlation between cultivar yields when grown with and without grass can be observed in Figures 5, 6, and 7.
From these results it would appear that red clover cultivar performance in the absence of grass competition was not indicative of their performance when grown with grass competition. Future selection of red clover cultivars should be done in the management system that are similar to the system used by the growers.
RED CLOVER OF THE FUTURE
It has been long recognized that red clover is an excellent forage legume for hay, silage, and grazing. As indicated earlier, the use of red clover in the US has declined over the past 100 years from about 30 million acres in the early 1900s to about 11 million by 1987. This decline in use has primarily been due to low nitrogen fertilizer prices, an increase use of alfalfa, a decrease in legume-grain row crop rotations, and the short-life of the plants. However, since the 1980s the use of red clover has stabilized, if not increased somewhat. This has been due to the development of improved, persistent, disease resistant cultivars; the recognition of its value in grazing conditions; and an increased recognition of its importance in crop rotations and soil conservation. Recent research has identified chemical compounds in red clover, some of which are important in protein utilization, others in animal reproductive processes, and still others as pharmaceutical products. Therefore, the use of red clover will continue to occupy a position of major importance in US agriculture.
Improvement in Agronomic Characteristics
Disease resistance and persistence: Even though current cultivars have good disease resistance and are relatively persistent, continued improvement in these attributes is needed. Lack of resistance to the soil-borne Fusarium pathogens is one of the leading causes of stand failure in red clover. Some improvement has been made in this direction, but further, concentrated efforts are needed and should be rewarded with even a greater degree of persistence in the crop.
Crown position and root structure: Unlike alfalfa and some other forage legumes where the crown of the growing plant is positioned just below the soil surface, the crown of red clover is positioned at the soil surface. This exposes it to a greater degree of environmental stresses such as temperature, disease organisms, physical abuse, etc. Developing red clover cultivars where the crown is position just below the soil surface would give greater protection against these stresses and undoubtedly lead to increased persistence. Initially red clover produces a primary tap root, but as it is exposed to diseases and other stresses it gives way to a secondary, branching system that develops reasonably close to the soil surface. Types need to be identified that have a disease tolerant tap root structure with an extensive secondary system. Rhizomes, underground stems, have not been observed in the red clover species, however, related species do have such structures. Efforts need to be extended to interspecifically transfer, through molecular or other techniques, this desirable attribute. Incorporation of the rhizome structure would enhance persistence and improve drought tolerance in the species.
Grazing tolerance and grass competition: Red clover is being used extensively for grazing and most often is grown with a companion grass whether grazed or used for hay or silage. Very little effort has been expended to develop red clover cultivars that are adapted to these conditions. As mentioned earlier in this report the performance of red clover cultivars in monoculture is not indicative of their performance when grown with grass companion crops. It is presumed that this would be true whether used for grazing or otherwise. A major component of any future red clover genetic and breeding projects should include the development of germplasm tolerant to these uses.
Chemical Attributes and Quality Improvement
Protein utilization and protection: Papadopoulos and McKersie (1983) reported that red clover has 90% less proteolysis than alfalfa during ensiling. Also, Albrecht and Muck (1991) report ensiled red clover yields 30 to 40 % less non protein nitrogen (NPN) than ensiled alfalfa. If proteolysis and NPN formation can be reduced, then forage protein utilization by the ruminant animal should be improved. Jones et al. (1995) report that this effect in red clover is the result of the polyphenol oxidase (PPO) enzyme system in red clover that inhibits activity of the plant's proteases in the silo. Among the forage legumes this PPO enzyme system is unique to red clover. This important attribute in red clover should be exploited by either genetically increasing its activity in red clover or genetically transferring the attribute to alfalfa and other legumes or both.
Phytoestrogens: Red clover contains high levels of phytoestrogens which have been reported to interfere with normal reproduction in sheep. Although no documented reports have been recorded for dairy cows it remains that this may present a problem, especially for grazing cows. The oestrogenic compounds are isoflavones which include formononetin, diadzein, genistein, and biochanin A (Cox and Braden, 1974). Formononetin (7-hydroxy-4'-methoxyisoflavone) is recognized as the most important of these oestrogenic compounds in red clover (Morley et al., 1966, 1968). Selection for lower levels of formononetin in red clover has been effective (McDonald et al.,1994). The authors report improved fertility of ewes grazing the low formononetin red clover. Since red clover is being used extensively in grazing conditions, red clover cultivars with low levels formononetin need to be developed. Contrastingly, dietary compounds, Promensil, with high estrogenic activity are being market as a supplement in human diets to combat PMS and osteoporosis.
Pectic polysaccharides and forage quality: While protein content in most forage legumes is quite high, the full potential of the protein, especially the leaves, is not realized due to rapid degradation rates and conversion to ammonia in the rumen. Numerous schemes to alter the proteins of forage legumes range from chemical to physical treatments to actually genetically altering the plant to produce proteins more resistant to degradation (Broderick et al., 1991). An alternate strategy to improve utilization of plant protein would be to match the rapidly degraded protein with an equally rapidly degraded carbohydrate (RDC) source such that energy would not be limiting to rumen microbes (Stokes et al., 1991). This assures that the plant protein would be efficiently converted to microbial biomass benefiting the ruminant. Pectic polysaccharides (commonly known as pectins) represent a potential source of RDC , and a significant portion of the total structural polysaccharides in forage legume cell walls are pectic materials. Thus, increasing the cell wall pectic polysaccharide (pectin) content of forage legumes would improve utilization of their nutritional potential. Earlier research has shown that genetic variability exists in red clover for cell wall pectin concentration (Hatfield and Smith, 1995). Divergent populations of red clover and other forage legume germplasm with a range of cell wall pectin concentration are needed to assess the actual impact of increased plant pectin on forage quality and animal performance.
Red clover cultivars on the market today are superior to those available two decades ago. It is expected that you will see better, more persisten cultivars in the future. The next generation of red clover germplasm developed in the USDA/ARS - University of Wisconsin program is ready to be released as either germplasm o rcultivars. However, future selection of red clover cultivars should be done in the management system that are similar to the system used by the growers. The prominence of red clover in US agriculture should remain high, if not increase, as you see new cultivars and uses of the crop. Because of its unique qualities of high seedling vigor, excellent forage quality, ease of establishment, competitiveness with grass and importance to soil conservation you should see red clover continuing to play a major roll in world agriculture.
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 Research Geneticist (Retired) and Professor of Agronomy, USDA-ARS, US Dairy Forage Research Center, 1925 Linden Dr., Madison, WI. USA. 53706