Switchgrass nitrogen fertility response and nutrient cycling in a hay system

Nitrogen (N) requirements for switchgrass (Panicum virgatum L.) forage production in the Mid-south are largely unknown. This study determined switchgrass forage yield, quality, N removal, and soil fertility response in a two-forage harvest (pre-anthesis) system after split applications of 0, 84, 168, 252, and 336 kg N ha(-1) yr(-1) from 2014 to 2016. Controls were represented by no fertilizer and a 0 N rate with supplemental P, K, Mg, and S. For all experimental years, yield did not benefit from supplemental P, K, Mg, and S, suggesting N was the limiting nutrient. Yield also did not differ between the 252 and the 336 kg N ha(-1) yr(-1) rates (9.2 and 9.4 Mg ha(-1), respectively), indicating no further growth beyond the 252 kg N ha(-1) threshold in a two-cut forage system. At greater N fertility rates (336 kg N ha(-1)), the majority of the soil nutrient concentrations were reduced compared with the 252 kg N ha(-1) rate, owing to greater nutrient removal in harvested plant tissue. Tissue N removal was greatest for the highest N application, albeit not different from the 252 kg N ha(-1) rate; N use efficiency had an inverse relationship with N removal, peaking during the lowest N application (84 kg N ha(-1)). Forage quality varied, but, in general, digestibility improved under greater N fertility. Quantifying trends in forage yield and quality based on N fertility are useful for predicting break-even prices per fertilizer response.