Interseeded Forage Legume Potential as Winter Ground Cover, Nitrogen Source, and Competitor

Legume winter ground covers may contribute to row crop systems by providing protection from wind and precipitation and by releasing N to the subsequent row crop, but to attain effective size, these legumes must begin growth well before the winter. A 3‐yr study was conducted to evaluate the performance of four commonly used forage legumes interseeded into corn (Zea mays L.). Alfalfa (Medicago sativa L.), yellow sweetclover (Melilotus officinalis Lam.), red clover (Trifolium pratense L.), and alsike clover (T. hybridum L.) were seeded in a mix consisting of equal seed numbers (pure live seed) from each species. Seeding dates were (i) at corn planting and (ii) at last cultivation. Control treatments were nonseeded. Two levels of weed control also were evaluated. In addition to the factorial experiment, the four species were seeded separately in other corn plots. The herbicide EPTC was also evaluated for use in interseeding. In drier than normal seasons (summer of 1984 and spring of 1985), soil moisture was significantly depleted under interseedings; this depletion was not observed in seasons of normal precipitation. Legume stands and ground cover reflected moisture and light conditions during establishment. In the 2 yr that rows were oriented east‐and‐west, stands conformed to a skewed modal distribution across the inter‐row. Alfalfa and sweetclover usually were better established and produced more cover than either red or alsike clover. Corn yields were not significantly reduced by interseeding per se, but they were reduced when interseeding at the early date interrupted weed control. In treatments that used the herbicide, corn yields were greater than where herbicide was not used, but yields were still significantly below those of the nonseeded control. Research Question Row crop producers around the USA are examining methods of protecting the soil between harvest and planting, and winter cover crops are one alternative receiving interest. Cover crops, by sequestering soil N, also reduce leaching of N below the rooting zone, and legume covers can fix additional N that may be used by row crops in the rotation. Farmers considering the practice need to know: appropriate cover crop species for their area; the most economical and effective methods for establishing the cover crop; and how to manage the cover crop to avoid competition with the row crop. This project was designed to answer those questions relative to use of forage legumes as cover crops following corn under mid western conditions. Literature Summary Research on cover crops in corn has often found that practices that favored cover crops (e.g., wide rows, earlier seeding dates) decreased corn yield. Corn yield can also be decreased by competition from weeds associated with cover cropping. While weed control is a real issue, weed effects on the row crop should be understood as a question separate from effects of the cover crop itself. Study Description The study was conducted over 3 yr on a farm in central Iowa. Soils were loam, clay loam, and silty clay loam mollisols of recent glacial origin. The core experiment evaluated the interseeding into corn of a mixture of four common forage legumes: alfalfa, yellow sweetclover, red clover, and alsike clover. The mix was seeded between the rows of corn at one of two dates: at corn planting (early May) or at the time of last cultivation (around 1 July). Check plots were left unseeded. In order to differentiate cover crop effects from weed effects, weeds were controlled by rototiller and by hand, as. appropriate, either (i) only up to the date of interseeding or (ii) throughout the experiment. The two factors, seeding date and weeding level, were combined as shown in Table . In a separate experiment, the four legume species were seeded individually at the early date in order to compare species performance. In a third experiment, plots were seeded at the early date with the species mixture and treated with the herbicide EPTC (Eptam™). These plots were compared with plots receiving mechanical weed control and with nonweeded plots. Applied Questions What are the best forage legumes for winter cover crops in corn? Alfalfa and sweetclover generally performed better than red clover and alsike clover in terms of stand establishment. Alsike and red clover established better in the one instance when light and moisture competition were at a minimum. What is the most economical use of this winter cover crop seed? When corn rows ran east‐west, poorest establishment tended to be at the edges of the inter‐row zone. This corresponds to the locations receiving the most mid‐day sunlight in summer. In corn with east‐west rows, the more shaded central portions of the inter‐row zone are probably more hospitable for seedling cover crops. Optimum seeding date depended on weather and soil condition in this study. Does a forage legume winter cover crop interseeded into corn reduce corn yields? Corn yields were not reduced by interseeding either at corn planting or at the last cultivation (Fig. ). The exception (early seeding 1984) occurred when corn was replanted into already‐established cover crops. In fact, relative to the weeded, unseeded check treatment, interseeding was sometimes associated with slightly greater yields. For plots seeded at the early date, however, lack of weed control did reduce corn yield. Mechanical control was more successful in this regard than was the EPTC treatment. While cover crop interseeding did not reduce corn yields, the legumes did draw on soil moisture, at least in the top foot of soil. Cover seeded at the early date consistently withdrew more soil moisture than the cover crop seeded at last cultivation of the corn. Core experiment factorial treatment design. Weed control level Seeding method Only through seeding date (“Wo”) Throughout the year (“WV”) At planting (“Si”) S1W0 S1W1 At last cultivation (“S2”) S2W0 S2W1 No seeding (“S0”) S0w0 (weeded through last cultivation date) S0Wi Corn yields over 3 yr. (Vertical lines represent LSD005 values and apply within years. LSD on herbicide treatment applies to early‐seeded treatments only.)