Evaluating the “added nitrogen interaction”; effect in forage grasses

Abstract

Abstract In northeastern United States, intensive grazing systems utilizing cool‐season grasses are being increasingly adopted by dairy fanners as a way of reducing operating costs. Since nitrogen (N) fertilization is a major factor in cool‐season grass production in northeastern United States, the accurate determination of N‐use efficiency is necessary in determining N fluxes in intensive grazing systems. When using isotopic N, the added N interaction (ANI) or ‘priming’ effect can greatly affect the calculation of N‐use efficiency of crops and forages. The objective of this paper is to evaluate the magnitude of ANI as it relates to determination of N‐use efficiency of forage grasses in northeastern United States. Data from experiments on the effects of soils, environment, and N fertilization on the yield and N‐use efficiency of various forage grasses was used in this study. In these experiments multiple rates of isotopic ammonium nitrate and ammonium sulfate were applied to cool‐and warm‐season grasses. Nitrogen‐use efficiency was calculated using the isotopic and difference methods, and ANI was determined as the difference in N uptake as determined by these two methods. In experiments in which the N source was ammonium sulfate, ANI averaged 24.2 and 38.5% of the applied N on cool‐and warm‐season grasses, respectively. The ANI averaged 7.4% of the applied N on cool‐season grass where the N source was ammonium nitrate. The ANI was always greater in the spring growth of cool‐season grasses than it was in the fall growth. However, the seasonal difference was greater in the case where the N source was ammonium sulfate than it was where the N source was ammonium nitrate.