Old World Bluestem Responses to Nitrogen Fertilization

Old World bluestem (Bothriochloa ischaemum L.) is being extensively seeded on marginal farmland in the Southern Plains. This field study was conducted in western Oklahoma to develop guidelines for efficient N fertilization of this perennial, warmseason grass established on soil depleted in plant-available N by 80 to 90 years of cultivation and erosion. Ammonium nitrate at 0, 35, 70, and 105 kg N ha-1 yr-1 was broadcast on Old World bluestem stands for 5 years on Woodward sandy loam (Typic Ustocrept) and for 3 years on Pratt loamy sand (Psammentic Haplustalf). Forage production averaged 800 kg ha-1 yr-1 without N fertilization. An average of 30 kg of forage was produced per kg N applied at rates of 35 and 70 kg N ha-1 yr-1. The 105 kg N ha-1 yr-1 treatment produced substantially more forage than the 70 kg N ha-1 yr-1 treatment only in years with above-average precipitation or favorable precipitation distribution. Partial die-out of Old World bluestem occurred one year; N fertilization increased die-out on Pratt loamy sand. Application of 70 kg N ha-1 in April was more effective in forage production and fertilizer N use in some years than split applications of 35 kg N ha-1 in April and June. Accumulation of N in forage over 5 years, plus residual N in stem bases, roots, and the surface 0.1 m of the Woodward sandy loam was 1,040 and 1,350 kg N ha-1 for the 0 N and 70 kg N ha-1 yr-1 treatments, respectively. The difference of these values, 310 kg N ha-1, suggests that most of the 350 kg N ha-1 applied over 5 years was accounted for; however, the standard error of difference between the means was large (50 kg N ha-1).