Whole-season grass response to and recovery of nitrogen applied at various rates and distributions in a high rainfall environment

Abstract

High rates of nitrogen (N) fertilizer are often used on perennial grass in the coastal region of British Columbia and the Pacific Northwest, but there is little information on optimum rates for abundant high-quality yields and on their environmental implications. A field trial was conducted in each of 3 yr to determine the effect of rates and distributions of N on whole-season herbage yield and quality, and pre- and post-season extractable inorganic N in the soil. Yearly rates were 100, 200 and 400 kg N ha−1 applied to each of four cuts in the following distributions: 1.00/0/0/0, 0.50/0.25/0.25/0 and 0.25/0.25/0.25/0.25. Whole-season yield was increased by increasing rates of N in all three trials, but the increase varied from 17% (Trial 1) to 127% (Trial 3). Distributing the N uniformly through the season resulted in only a 5% increase in yield compared with applying all of the N at the beginning of the season. Rate of N had a substantial effect on average herbage crude protein and nitrate concentrations, but the distribution effect was greater on herbage N constituents than on yield. Increasing rates of N consistently increased average herbage nitrate concentrations, and crude protein in two of three trials. Applying all of the N at the beginning of the season increased average herbage crude protein and nitrate concentrations more than distributing it evenly through the season. Extractable inorganic N in the soil at the end of the season increased only at 400 kg N ha−1 rate and was not affected by distribution. Although distribution pattern influenced herbage yield and nitrogen concentrations, distribution did not influence total herbage N uptake or recovery in herbage plus soil. Rates and distributions of N on grass influenced herbage yield and quality, and soil extractable inorganic N in different ways; therefore, compromises in N management are required to optimize forage yield and quality, and soil nitrate concentrations. Autumn soil inorganic N testing may be useful as feedback information for fertilizer recommendations in the subsequent season. Spring soil inorganic N testing was a poor predictor of crop response to fertilizer in the high rainfall environment of the study. Key words: Nitrogen, plant crude protein, plant nitrate, forage yield, forage quality, apparent N recovery, residual soil N, soil N test