Within-season grass herbage crude-protein- and nitrate-N concentrations asaffected by rates and seasonal distribution of fertilizer nitrogen in a high yearly rainfall climate

An orchardgrass study in which three rates of N (100, 200 and 400 kg ha(-1) ) each distributed in 1/0/0/0, 0.75/0.25/0/0, 0.50/0.25/0.25/0 and 0.25/0.2 5/0.25/0.25 proportions prior to four cut intervals examined crude-protein- N and nitrate-N concentrations in grass herbage at each cut in three trials . Crude-protein-N concentration frequently increased to a greater degree an d in a different pattern (based on cut) than yield as the rate of N applica tion increased. This showed that crude-protein-N by itself cannot be used a s a method for determining the N sufficiency status of grass. Both rate and distribution of fertilizer N strongly influenced plant nitrate-N concentra tion; the degree of change varied considerably among cuts and trials. Plant nitrate-N concentration in the control did not correspond to yield respons iveness to N application, making it a poor indicator of the plant's need fo r fertilizer applications. Residual effects of N applications on plant nitr ate-N were noted into the last cut of the season from a single spring appli cation. The effect of N rate and distribution, then, was a function of imme diate and residual effects of the applications. There was some evidence tha t N present in the soil in nitrate-N form enhanced the potential for high n itrate-N in the plant. Plant nitrate-N concentrations accounted for up to 2 9% of the total N in the plant with concentrations greater than 4000 mg N k g(-1) at the highest N application rates. Plant nitrate-N did not exceed 10 00 mg N kg(-1), a concentration considered safe for ruminants, when 75 kg N ha(-1) or less ammonium nitrate was applied as a single application prior to a growth interval for all cuts. Since grass protein- and nitrate-N conce ntrations respond differently than yield to N applications, a specific comb ination of rate and distribution of fertilizer will not necessarily produce maximum herbage quantity and quality simultaneously.