Residual nitrogen effects on soil, forage, and steer gain

Nitrogen fertilization is a common practice on introduced grass pastures established on marginal farmland in the Southern Great Plains. The efficiency of N fertilizer use on pastures and concern about nitrate movement into substrata prompted this study of residual N effects following fertilization. The study was conducted on Old World bluestem (Bothriochloa ischaemum L.) pastures on Pratt soil (sandy, mixed thermic Psammentic Haplustalfs) in western Oklahoma where the 57-year average annual precipitation is 566 mm yr(-1). Herbage production and steer gains were quantified over 3 summer grazing seasons on paddocks fertilized annually with 0, 34, 68, or 102 kg N ha(-1) yr(-1) during the preceding 5 years. Peak standing ungrazed herbage yields mere 2- to 4-fold greater in paddocks fertilized the preceding 5 years and were linearly related to the total N applied the previous 5 gears, Steer weight gain responded linearly to N with an average of 0.63 kg gain over 3 years per kg N applied over the preceding 5 years. No differences (P > 0.05) in soil nitrate concentrations to a depth of 2.8 m were measured among the N rate treatments. Overall, substantial effects of residual N mere measured in both herbage mass and steer weight gain for 3 years following 5 years of N fertilization.