Nitrogen Replacement Values of Kura Clover and Birdsfoot Trefoil in Mixtures with Cool‐Season Grasses

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Abstract
Kura clover (Trifolium ambiguum M. Bieb.) is a perennial legume that, when grown in mixture with grass, may improve forage yield while providing biologically fixed N. Kura clover's value as an alternative to N fertilization of cool‐season grasses for improved yield is unknown. Fertilizer N replacement values (FNRVs) of kura clover and birdsfoot trefoil (Lotus corniculatus L.), in terms of total season forage yield, were compared when grown in binary mixtures with Kentucky bluegrass (Poa pratensis L.), smooth bromegrass (Bromus inermis Leyss.), or orchardgrass (Dactylis glomerata L.). Separate experiments for each grass were conducted from 1994 through 1996 near Arlington and Lancaster, WI. Treatments in each experiment included six levels of N fertilizer on grass monocultures and two non‐N‐fertilized binary mixtures of grass with either legume. The FNRV for either kura clover or birdsfoot trefoil correlated positively with total season legume yield. When averaged across locations, kura clover–smooth bromegrass mixtures had annual FNRVs that ranged from 74 to 325 kg N ha−1 while those of birdsfoot trefoil–smooth bromegrass mixtures ranged from 147 to 295 kg N ha−1. When averaged across years and locations, FNRVs for kura clover and birdsfoot trefoil were 251 and 269 kg N ha−1 in orchardgrass, respectively. Grass yields in mixtures were similar to grass monocultures receiving no fertilizer N. Because kura clover outyielded birdsfoot trefoil in all 1996 mixtures, we expect the FNRV of kura clover to exceed that of birdsfoot trefoil in the long term.

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