The influence of rest and deferment on beef cattle diet quality and composition after an August wildfire on perennial bunchgrass rangeland in southwestern Montana

Abstract

Current management paradigms suggest deferring burned rangeland for two years after a fire. However, there has been little research into how forage quality changes within the first two years after a fire and how it affects livestock diet quality, botanical composition, and foraging efficiency. Study 1 evaluated the differences between unburned and burned pastures during the first year after a fire, and Study 2 evaluated the differences in rested and unrested pastures. Pastures were arranged in a randomized complete block design contrasting June versus September, burn versus unburn (Study 1), and one-year rest versus no rest (Study 2). Pastures were grazed in either June or September by cow-calf pairs for two days at a time. Cattle diet composition and masticate samples were collected during 20-minte bite-count periods using six ruminally cannulated cows in each pasture prior to and after two-day grazing periods. Composition of diets upon pasture entry differed among treatments (P < 0.01) with June diets having greater proportions of forbs than September diets. In addition, June diets with no deferment had greater forb composition than June diets with one year deferment (P < 0.01). Crude protein in June diets was substantially greater than that in September diets (P < 0.01) and within the June grazing period, highest for the non-deferred treatment (P < 0.01). June diets had lower neutral detergent fiber (NDF) concentrations than September diets (P < 0.01) and within the June grazing treatments, the first year after burning was lowest in NDF content (P < 0.01). There were no differences in ash concentration averaging 16.46% across all treatments and years (P ? 0.07). There was no effect of grazing treatment on grams per bite (P ? 0.34) averaging 1.36 grams per bite for pre- and post-grazing treatments. Season of use and timing of grazing following a wildfire impacts cattle diet composition and quality, with the greatest impact in the spring following a wildfire. Cattle diets in the burned sites were high quality (higher CP, lower NDF) during the growing season immediately following a wildfire.