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Abstract
Grass-legume pastures rely on biologically fixed nitrogen (N) and thus are often N deficient in the spring. To overcome this period of N deficiency, researchers in the United Kingdom (UK), New Zealand (NZ), and recently the United States (US), have developed programs of early season N applications to clover/grass pastures. However, the effect of such early season N application on clover N fixation and N fertilizer N use efficiency of clover/grass swards is not known in the climate of the northeast US. A three-year study was conducted in Pennsylvania on a white clover (Trifolium repens)/orchardgrass (Dactyls glomerata) sward, fertilized with three rates of 15N (22.4, 44.8, and 89.6 kg N ha−1) and cut at three target sward harvest heights (TSHH) (15, 22.5, and 30 cm). The objective of the study was to determine the effects of N fertilization and TSHH on the fraction of clover N derived from fixation (FNDF), the total clover N derived from fixation (TNDF), and fertilizer N use efficiency (FNUE) of the combined sward. Over the three years of the study, full season FNDF in the clover was about 0.90 and the TNDF was about 135 kg N ha−1. The TSHH decreased TNDF during April and May, but this effect disappeared by the end of September. Nitrogen fertilizer use efficiency at the two lowest fertilization rates was greater than 0.50, but decreased to 0.36 at the highest N fertilization rate. The data indicates that, at a 44.6-kg N ha−1 rate and a 15-cm TSHH, the TNDF and the FNUE will be maximized.