If legume cover crops are to be an effective, environmentally sound N source for corn (Zea mays L.), there must be a synchrony between legume N release and corn demand. A field study was conducted in Wisconsin during 1991 and 1992 that measured the release of legume N throughout the growing season using mesh bags, and compared resultant levels of soil mineral N following legume incorporation to those following fertilizer N applied at the recommended rate (179 kg N ha−1) and a control (no cover crop, no fertilizer) in a conventional tillage (CT) system. Corn N uptake during the growing season was also measured to determine if legume N could meet uptake demands. Hairy vetch (Vicia villosa Roth) and red clover (Trifolium pratense L.) residues decomposed rapidly, releasing half of their N within 4 wk after burial, while very little N was released after 10 wk (corresponding to corn silking). Soil tests indicated an increase in mineral N levels corresponding to legume N release, similar to those following an application of 179 kg ha−1 fertilizer N, occurring before the period of rapid N uptake by corn. Mean corn grain yields of 11.25 Mg ha−1 in 1991 and 10.89 Mg ha−1 in 1992 following the legumes were similar to those produced with 179 kg ha−1 fertilizer N, indicating that, in addition to releasing N in synchrony with the uptake pattern of corn, legumes released N in adequate amounts for corn production. Finally, postharvest levels of potentially leachable soil NO3‐N following the legumes were similar to or less than when following fertilizer. Therefore, legume cover crops can be an effective N source for corn in the Upper Midwest.

The use of legume cover crops as green manures is a means of reducing the amount of N fertilizer used in corn (Zea mays L.) production, but has not been adequately tested in the upper Midwest. A field study was conducted from 1989 to 1993 near Arlington, WI, to determine the value of legume cover crops in a 2‐yr oat (Avena sativa L.)/legume‐corn rotation. The objectives of this study were to identify the most productive legume species and planting method to be used with oat, to measure the effect on subsequent corn grain yield, and to evaluate the economic viability of oat/legume‐corn rotations. Corn yields following the legumes were compared with those of corn following oat without a legume, using six rates of N fertilizer to estimate the fertilizer replacement value of the legumes. Maximum seeding year N yields (tops and roots) were produced by medium red clover (Trifolium pratense L.) companion seeded with oat, and hairy vetch (Vicia villosa Roth) seeded after oat harvest. Averaged over years, these two cover crop options yielded 118 lb N/acre in the seeding year. Mean corn grain yields (bushels per acre) following these two legumes were 163 for red clover and 167 for hairy vetch. Estimated fertilizer replacement values of these legumes ranged from 65 to 103 lb N/acre. Gross margins (2‐yr rotation mean) of these oat/legume‐corn rotations were $166/acre using red clover and $153/acre using hairy vetch and were similar to those obtained with an oat‐corn and continuous corn rotation where corn was grown with 160 lb fertilizer N. These results indicate a great potential for reducing fertilizer N inputs in corn production by using legume cover crops, without economic penalty for producers who adopt the practice. Research Question Use of legume cover crops as green manures is one way to reduce the amount of N fertilizer inputs for corn (grain) production. Little information is available on the potential role of legume cover crops in cash grain systems of the Midwest. Most recent cover crop research has been conducted in the southern USA where the growing season is longer, different legume species are used, and oat are not commonly grown. The objectives of this study were to identify the most productive legume species and seeding method to be used with oat, to measure the effect of these legumes on the grain yield of a subsequent corn crop, and to evaluate the economic viability of oat/legume‐corn rotations relative to oat/no legume‐corn and continuous corn rotations grown with fertilizer N. Literature Summary Seeding year productivity of forage legumes grown in annual rotation with corn has been evaluated to a limited extent in the upper Midwest. This work, however, has focused on legumes used as annual forages, managed for maximum forage production in the seeding year where only the regrowth following final harvest is available as an N source for corn. Most cash grain systems have no use for forage, making this approach unappealing. A few studies have evaluated unharvested legumes as an N source for a subsequent cr...

Use of forage legumes as cover crops in the year prior to corn (Zea mays L.) is one way to reduce the amount of N fertilizer used in corn production. This study evaluated several forage legumes for dry matter (DM) production during the seeding year when established in grain rotations. Field studies were conducted near Arlington and Marshfield, WI, in 1989 and 1990. Treatments consisted of nine forage legumes: medium and mammoth red clover (Trifolium pratense L.), dormant and nondormant alfalfa (Medicago sativa L.), alsike clover (T. hybridum L.), ladino clover (T. repens L.), hairy vetch (Vicia villosa Roth), and two cultivars of yellow sweetclover [Melilotus officinalis (L.) Lam.]. Five establishment treatments tested were: seeded in the spring, companion‐seeded with oat (Avena sativa L.), interseeded into corn at last cultivation, and seeded following harvest of either canning pea (Pisum sativum L.), or oat for grain. In general, legume DM yields were highest in establishment periods which offered the longest growing season with the least amount of competition from the primary crop. Hairy vetch, the only annual legume used in the study, produced the highest yields in each of the five establishment periods, as high as 9.76 Mg ha−1 when spring seeded. The climbiing nature of this legume, however, makes it an unwise choice for companion seeding with small grains, a situation where the true clover species are best adapted. Dry soil conditions following pea or oat harvest often limited legume germination, seedling growth, and ultimately DM yield. Interseeding of legume cover crops into corn at last cultivation resulted in low seeding‐year DM yields. From the results of this study, we conclude that several legume‐establishment period combinations could be used successfully to produce N‐containing biomass.

Core Ideas Radish as a cover crop does not supply nitrogen to the subsequent corn crop. Radish can result in neutral, negative, and positive effects on corn yield. Radish can have substantial nitrogen uptake in the fall, but effects on spring soil nitrogen are variable. Radish (Raphanus sativus L.) grown as a cover crop can accumulate a significant amount of N when planted by late summer. However, it remains unclear if the N in the radish biomass can supply N to a subsequent corn (Zea mays L.) crop. The objectives of this project were to: (i) measure radish growth and N uptake, (ii) determine the effect of radish growth on plant available N content in soil throughout the subsequent growing season, and (iii) determine the effects of radish on corn yields and response to N fertilizer. This study was conducted across nine site‐years in northeastern and southern Wisconsin, with radish planted mid‐August following winter wheat (Triticum aestivum L.) harvest. The experimental design was a randomized complete block, split plot design, with cover crop as the whole plot factor and N rate as the split plot factor. Radish N uptake was ranged from 19.7 to 202 kg ha−1 across all site‐years. The effect of radish on in‐season plant available N (PAN) content differed across growing seasons, with radish both increasing and decreasing PAN. The ANOVA and regression analysis showed mostly neutral effects of radish on corn yield, although corn yield increases and decreases following radish occurred. This research supports the use of radish as a trap crop for fall N, as environmentally meaningful yields of N were contained in plant biomass, but also demonstrates that radish has no N fertilizer replacement value to the subsequent crop.

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Sunn hemp (Crotalaria juncea L.) is recommended as a warm season cover crop in the Midwest due to its ability to produce high levels of biomass and fix atmospheric nitrogen. It can also be grown in biculture with other cover crops to enhance overall ecosystem services. Two field experiments were conducted over four growing seasons (2014 – 2017) in Wisconsin on a forest derived Fox silt loam (Fine-loamy, mixed, mesic Typic Hapludalfs) under organic certification to determine the effect of planting date on sunn hemp dry matter yield, N and C addition and to determine the effect of species ratio in a biculture with sorghum-sudan [Sorghum bicolor (L.) Moench] on the same output variables as well as seed cost and the related per unit cost of production. Planting dates significantly affected all biomass yield variables, which declined linearly from the initial date and appear related to growing degree accumulation. Regression analysis revealed a biomass yield decline of 1.3% per day (8.9% week-1) in relative yield, and 0.90 Mg day-1 (0.61 Mg week-1) in actual yield. In biculture, sunn hemp grown in a planting ratio of 50:50 with sorghum-sudan maximized N addition through nitrogen fixation and added N from dry matter, without a significant difference in the dry matter recorded. Analysis of seed cost data revealed that as the ratio of sunn hemp in the planting mixture decreased, the cost per hectare decreased. The cost of production per unit of DM, N, C and CO2 equivalent at this planting ratio were 7.08 $ Mg-1, 0.57 $ kg-1, 17.51 $ t-1 and 4.78 $ t-1 respectively. In pure culture, early planting dates (June 15th to July 15th) are recommended for sunn hemp, and in biculture, a planting ratio of 50:50 with sorghum-sudan could serve Midwestern producers well by reducing per unit cost of biomass production.

Crops like corn and soybean occupy vast area in the Midwest, USA. When land is left fallow after the harvest of these crops, a number of degradation factors operate and bring about soil erosion, nutrient loss, decreased soil organic carbon, reduced biological activity and increase in weed biomass. Integrating cover crops (CCs) into this system would build benefits that the very system lacks. There are various CCs available, but leguminous CCs allows for reduced application of fertilizer nitrogen and builds the soil fixed atmospheric nitrogen. Winter CCs are restricted in the Midwest because of the short planting window which greatly minimizes the biomass accumulation. Warm season CCs would serve well here. Sunn hemp is one such tropical CC that grows well in temperate conditions too, without producing seeds. It comes with many benefits -including decreased soil erosion, improved soil organic carbon, increase in soil fixed nitrogen, higher biomass that adds organic matter and N to the soil, reduced weed density and weed biomass. The timing and method of termination influences the residue management. Going by the benefits it adds, sunn hemp is a viable warm season CC that can be grown in the Midwest and has great potential in fallows, prevented plant acres, areas of crop failure (planted and failed) and also in areas after the harvest of the short season small grains or processing crops. However, intensive research on sunn hemp is needed in the Midwest which is discussed.

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