Frost‐seeding a legume into an established stand of winter wheat (Triticum aestivum L.) or interseeding a legume into a small grain at planting has potential to provide the benefits of a legume green manure while still allowing for the harvest of a revenue‐producing crop. Field studies were conducted at three Michigan locations to quantify N accumulation by alfalfa (Medicago sativa L.) and red clover (Trifolium pratense L.) frost‐seeded into winter wheat or interseeded with oat (A vena sativa L.) and to evaluate the response of a subsequent corn (Zea mays L.) crop to legume and fertilizer N. Cropping sequences included corn following either wheat, wheat frost‐seeded with a legume, oat, or oat interseeded with a legume. Corn was planted either no‐till following wheat or conventionally (moldboard plow) following oat. Frost‐seeding and interseeding alfalfa and red clover had no effect on small grain yield, and stands of alfalfa and red clover were adequate (>13 plants/sq ft) even though N fertilizer had been applied to the small grains. Nitrogen accumulation did not differ among alfalfa or red clover cultivars, and averaged 80, 50, and 116 lb N/acre for alfalfa; and 96, 99, and 176 lb N/acre for red clover frost‐seeded into winter wheat at the three locations. When interseeded with oat, alfalfa contained an average of 44 lb N/acre and red clover contained an average of 33 lb N/acre prior to fall plowing. Corn response to the frost‐seeded legumes differed among locations due primarily to differences in precipitation during the weeks just prior to and following corn planting. When soil water was adequate, corn grain yields following the small grain seeded with a legume were 4 to 62% greater than following the small grain without the legume. With below‐normal precipitation following corn planting, corn grain yields in the legume systems were reduced by 3 to 27%, primarily due to delayed and reduced emergence. Fertilizer replacement values based on grain yield ranged from 0 to 49 lb N/acre for alfalfa and from 0 to 113 lb N/acre for red clover. Response of corn to the preceding legume differed by year, location, and seeding method.

Legume cover crops can supply all or most of the N required by a subsequent crop if legume biomass is of sufficient quantity and N mineralization is approximately synchronous with crop demand. Three 2‐yr crop rotation cycles were conducted on a Lamoine silt loam (fine, illitic, nonacid, frigid Aeric Epiaquept) soil in Maine to (i) evaluate biomass and N accumulation of alfalfa (Medicago sativa L.), winter rye (Secale cereale L.), and hairy vetch (Vicia villosa Roth subsp. villosa) plus winter rye cover crops; (ii) determine sweet corn (Zea mays L.) response to legume and fertilizer N sources in a barley (Hordeum vulgare L.)–sweet corn rotation; and (iii) assess the accuracy of the presidedress soil nitrate test (PSNT) and leaf chlorophyll N test (LCNT) for distinguishing N‐responsive and nonresponsive sweet corn. Both legumes accumulated more N than rye grown alone, although total biomass was similar. Sweet corn following rye always exhibited a linear response to N fertilizer (up to 156 kg N ha−1), but generally exhibited no response to added N following either alfalfa or hairy vetch plus winter rye (VR). Both PSNT and LCNT were 75% accurate in identifying plots responsive to additional fertilizer N. The legume cover crops grown were able to replace all or nearly all of the N fertilizer required by a subsequent sweet corn crop, with fertilizer replacement values (FRVs) of 58 to 156 kg N ha−1 in a short‐season environment. These cover crops are a viable alternative source of N, greatly reducing or eliminating the need for N fertilizer.

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Production of pOhlto (Solanum tuberosum L.) in rotation with :m N 2 -fixing legume requires better information on the N contributi,>n from the legume and on management of N fertilizer following a lEgume. Objectives of this rt~search were to: (i) quantify N accumulathn by legumes grown as either green manure or hay; and (ii) evalw.te the vine and tuber yield response of a subsequent potato crop to legume and fertilizer N. We established rotations at two Michig:ln locations in 1987 on McBride sandy loam (coarse-loamy, mixt,d, frigid Alfie Fmgiothods) and Oshtemo sandy loam (coarse-loamy, mixed, mesic Typic Ha11ludalfs). First-year crops included: alfalfa (Medicago slltivll L.), birdsfoot trefoil (Lotus corniculatus L.), and red clover (Trifolium prlltense L.) grown as both green manure (0 harvests) and hay (two Oil' three seeding-year harvests); non-dormant 'Nitro' alfalfa hay; sweetclover (Melilotus spp.) and hairy vetch (Vicill villosll Roth) green manures; com (Zell mll)'S L.); fallow; a:~d potato. The second-year crop was potato, fertilized with 0, 75, 1~:0, or 225 kg N ha-1 • PBowdown N yield [PDN = fall(herbage + roo(IN + spring herbage N) of legumes ranged from 33 (fall-seeded hairy vetch) to 238 kg N Jha· 1 (sweetclover), with the PDN yield of alfatra, red clover, sweetclover, nnd spring-seeded hairy vetch genera:lly EXceeding 150 kg ha-1 • Seeding-year harvest of legumes tended to have little effect on PDN yield, compared to a green manure crop of the same species. Potato vine dry matter and N content late in the seas~n were 61 to 100 and 75 to 145% higher, respectively, following :legumes than following non-legumes, but total and marketable toter yields were not a:ffected by rotation at either location. Fertilizer N rate increased total, null'ketable, and cull tuber yield at MRF, with optimum N rates of about 120 and 170 kg ha-1 for marketable and total tuber yield, respectively. Nitrogen rate had no effect on tut,er yield at KBS, suggestinl! that moisture, not available N, limited tli.ber yield. The difference in vegetative and tuber yield responses to rotation indicates that legume N eventually became available to the

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