Nitrogen Fertilizer Application Effects on Switchgrass Herbage Mass, Nutritive Value and Nutrient Removal

Abstract

Switchgrass (Panicum virgatum L.) can provide summer forage when forage production from cool‐season grasses is limited. This experiment was conducted over two growing seasons (2014 and 2015) to determine nitrogen (N) fertilizer application rate and time of harvest effect on switchgrass herbage mass, nutritive value, and nutrient removal. Treatments consisted of a split‐plot arrangement of five N rates (0, 45, 80, 135, and 180 kg ha−1) applied annually as main plots and harvest time (harvested in July for forage or October for biomass) as subplots in a randomized complete block design with three replications. Nitrogen rate × time of harvest interaction had no effect on herbage mass or nutritive value. Increasing N application rates increased herbage mass, crude protein, in vitro dry matter digestibility, and neutral detergent fiber concentrations, but not acid detergent fiber. A quadratic function described the relationship between herbage mass and N rate, with maximum yield occurring at 151 kg N ha−1. The economic optimum N rates ranged from 90 to 120 kg N ha−1 and depend on switchgrass hay price and N fertilizer costs. Generally, herbage mass was greater for switchgrass harvested for biomass, but nutritive value and nutrient removal were greater when switchgrass was harvested for forage. Switchgrass herbage mass and nutrient removal were greater in 2014 than in 2015. Our findings showed that N fertilizer rates for switchgrass under rain‐fed conditions need to be adjusted on the basis of expected herbage mass produced, hay value, and prior year N removal rates.