Long-Term Residual Effects of Nitrogen Fertilization on Western Wheatgrass

Abstract

Nitrogen (N) fertilization can be an effective way of increasing forage production. The question is how much does N fertilization increase forage yield of western wheatgrass (Agropyron smithii) when there is not a shift in species composition as occurs when N is applied to a native range site. The objectives of this research were to determine the residual effects of a single application of (1) 6 geometric rates of N and phosphorus (P) on forage yield, in vitro dry matter digestibility (IVDMD), crude protein (CP), and phosphorus (P) concentration (conc) of western wheatgrass grown near Sidney, Mont. during a 10-year period. Ammonium nitrate was applied at 0, 40, 80, 160, 320, and 640 kg N/ha in March 1973 and triple super phosphate at 45 kg P/ha on split plots during August 1975. A single application of N increased forage yield by 0.0, 0.0, 0.95, 0.35, 0.0, 1.16, 0.52, and 1.41 kg/ha per kg of N applied the 1st, 2nd, 3rd, 4th, 5th, 6th, 7th, and 10th year sampled, respectively, regardless of N rate. Nitrogen fertilization increased the accumulative forage (P<0.01) and CP (P<0.01) yield over the 8 harvest-years by 4.35 and 0.87 kg/ha per kg of N applied. Nitrogen fertilization increased the average forage IVDMD by 0.1 percentage units (P<0.05) and decreased P conc by 0.03 percentage units per 100 kg N/ha applied (P<0.01). Application of 45 kg P/ha in 1975 increased the P conc of the forage an average of 0.04 percentage units each year, increased forage yield only the 10th year by 150 kg/ha, and had no effect on IVDMD or CP. This study also showed that long-term observations are necessary to measure the residual effects of fertilization.