Legume Cover Crops as a Nitrogen Source for Corn in an Oat-Corn Rotation

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 QuestionUse 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 SummarySeeding 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 crop, but have used a target crop other than corn, evaluated only one response year, or measured the fertilizer replacement value without analyzing the overall rotation within an economic framework. Study DescriptionThe study was conducted from 1989 to 1993 at the University of Wisconsin Agricultural Research Station near Arlington, WI, on a Plano silt loam. The effect of forage legumes used as green manures for a subsequent corn crop was evaluated in a 2-yr oat/legume-corn rotation. Five legumes, nondormant alfalfa, dormant alfalfa, medium red clover, yellow sweetclover, and hairy vetch were both companion seeded with oat and seeded after oat harvest (sequentially seeded). Total (tops and roots) N accumulation was measured at the end of the seeding year. Corn was grown following the oat/legume combinations with no added N, following oat with no legume and six rates (0–200 lb/acre) of added N, and following corn with the same rates of added N. Grain yield was used to determine the effect of legumes on the subsequent corn crop. An economic analysis was conducted on the basis of gross margin (gross return — variable cost), using costs for all inputs used and crop yields. Rotations in the analysis included oat/companion seeded red clover-corn, oat/sequentially seeded hairy vetch-corn, oat/no legume-corn with 160 lb fertilizer N/acre, and continuous corn with 160 lb fertilizer N/acre. Applied QuestionWhat is the best legume/oat system to use for corn? Medium red clover companion seeded with oat, and hairy vetch seeded after oat harvest were the most productive legumes as indicated by seeding year N yield (Table 1). Mean N yield of these two legumes was 118 lb/acre. Nitrogen yield of other legumes was limited by insect damage (companion seeded alfalfa), mechanical damage during oat harvest and slow subsequent regrowth (companion seeded sweetclover and hairy vetch), and, except for hairy vetch, the short growing season for the sequentially seeded legumes. The effect of establishment method, companion vs. sequentially seeding on legume N accumulation was significant in all years, but inconsistent. Legumes had a significant positive effect on corn yield every year, and were 123% of yield of the no legume-no fertilizer control when averaged over years (Table 1). Mean grain yield following the most productive legumes, companion seeded red clover and sequentially seeded hairy vetch, were 123% of the no legume-no fertilizer control. There was no clear relationship between legume N accumulation and subsequent corn yield (Table 1). On medium-textured soils with higher organic matter content, crop rotation may contribute substantially to the positive effect of a previous legume on corn yield, masking N effects. Now do productive oat-corn rotations using legumes compare economically to oat-corn and continuous corn rotations using N fertilizer? When averaged over the rotation, both oat/companion seeded red clover-corn and oat/sequentially seeded hairy vetch-corn rotations performed similarly to oat/no legume-corn and continuous corn rotations grown with the recommended rate of N (Table 2). The oat-corn rotation using red clover produced a greater gross margin than hairy vetch because of lower seeding costs, both seed and the additional field operations required for sequential seeding. The oat/companion seeded red clover rotation also produced a slightly greater gross margin than the oat/no legume-corn rotation using fertilizer N. Because the only difference in inputs between these two rotations was red clover seed vs. N fertilizer, it appears that clover was a competitive substitute for fertilizer in this rotation. Gross margins of the legume rotations including red clover and hairy vetch were similar to those continuous corn grown with 160 lb N/acre. Lower gross returns for legume rotations were offset by lower variable costs, resulting in similar gross margins. The competitive economic performance of the legume containing rotations compared with the nonlegume rotation and continuous corn should make this low input alternative attractive to many producers. Table 1Mean legume seeding year N accumulation and subsequent corn grain yield with no added N fertilizer. Legume/establishment methodLegume N accumulationCorn, grain yieldlb/acre†bu/acre‡Companion seeded Nondormant alfalfa 56156 Dormant alfalfa 49141 Red clover128163 Sweetclover 47148 H&y vetch 79161Sequentially seeded Nondormant alfalfa 47163 Dormant alfalfa 42153 Red clover 50149 Sweetclover 67154 Hairy vetch108167No legume-no N fertilizer134Legume Mean 67156Contrasts Companion vs. sequentially seeded* Legume vs. no legume***,**Significant at the 0.05 and 0.01 levels of probability, respectively. †Includes tops and roots. ‡15% moisture. Table 2Mean economic performance of oat/companion seeded red clover-corn (0 + RC-C), oat/sequentially seeded hairy vetch-corn (0 + HV-C), oat/no legume-cornll60 lb N (O-C), and continuous cornll60 lb N (C-C) rotations from 1989 to 1993. Rotation Item†O + RC-CO + HV-C0-CC-C---------------- $/acre ---------------A. Variable costs Oat phase 47.58 67.36 28.28 Corn phase135.21140.35156.42162.93 Rotation mean: 91.40103.86 92.35162.93B. Gross returns Oat phase144.45135.09135.09 Corn phase370.01379.09374.55329.15 Rotation mean:257.23257.09254.82329.15C. Gross margin‡165.83153.20162.47166.22†Prices used in analysis: corn, $2.27/bu; oat, $1.17/bu; straw, $45/ton; N fertilizer, $0.12/1b N. ‡Gross margin = mean gross return — mean variable costs.