Title: Effects of fire and nitrogen addition on forage quality of Aristida purpurea Authors
Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 18, 2013
Publication Date: N/A
Interpretive Summary: Forage quality of purple threeawn one growing season following fire treatment showed marked improvement compared to non-burned plots, whereas nitrogen addition effects were not as substantial. In terms of potential effects on animal production and health, post-fire purple threeawn forage quality is comparable to that of many commonly utilized rangeland forages. Of particular interest was the fact that fire reduced silica content. High silica content is a recognized problem in purple threeawn and ours was the first evidence we are aware of indicating fire can reduce silica content. Justifying nitrogen addition as a prerequisite treatment to grazing would be difficult based on our results and the costs of nitrogen. Decisions about post-fire grazing management to reduce purple threeawn density should be based on a number of factors including timing and intensity, in situ animal preference and performance, and plant community response to integrated management tools. Our data include changes in forage quality factors throughout the growing season for fire and nitrogen addition but, due to the avoidance of purple threeawn following seed head emergence, early season grazing would be essential in order to delay flowering. Prescribed fire shows strong potential as a prerequisite treatment for increasing the suitability of purple threeawn as a forage species.
Technical Abstract: Purple threeawn (Aristida purpurea Nutt.) is a native perennial bunchgrass with limited forage value that dominates sites with disturbed soils and persists with continued severe grazing. Fire and nitrogen addition have been used to reduce threeawn and may increase grazing utilization of threeawn by livestock. We evaluated effects of fire, spring urea nitrogen addition, and phenological stage on purple threeawn forage quality one year post-fire on two similar sites in southeastern Montana during the 2011 (site 1) and 2012 (site 2) growing seasons. Season of fire (no fire, summer fire, fall fire) and rate of nitrogen addition (0, 46, 80 kg N·ha-1) were arranged in a completely randomized, fully factorial design. Samples were collected at five phenological stages throughout each growing season. Forage quality was assessed using nutrient analyses of crude protein (CP), net energy (NEm), and total digestible nutrients; anti-quality analyses of neutral detergent fiber (NDF), acid detergent fiber, and silica; in vitro fermentation for organic matter disappearance (IVOMD) and NDF disappearance; and gas production (asymptotic gas production, fractional rate of gas production, lag time, and average fermentation rate). In vegetative stages, summer and fall fire increased CP from 6.2% to 12.1 and 13.0%, and NDF decreased from 72.1% to 69.4 and 68.2%. Summer and fall fire reduce silica content from 7.0% to 4.1 and 4.3%. Purple threeawn IVOMD increased by 14.0 and 13.0% following summer and fall fire compared to non-burned plots. Nitrogen addition increased CP from 7.5% to 8.0 and 8.4% with 46 and 80 kg N·ha-1. In vitro fermentation and gas production variables did not change due to nitrogen addition. Fire generally improved forage quality to a greater extent than did nitrogen addition. Results indicate that fire shows a strong potential as a prerequisite treatment for improving the suitability of purple threeawn as a forage species.