424 Botanical Composition and Quality of Beef Cattle Diets on a Burned Versus Unburned Foothills Rangeland

Abstract

Abstract Current management paradigms suggest deferring grazing rangeland for two years after a wildfire to avoid additional stress on native plant species, but there is limited research supporting these recommendations. Therefore, this experiment was conducted within and adjacent to a burn area of an August 2020 wildfire to evaluate the differences in diet quality, botanical composition, and foraging efficiency of beef cattle grazing both burned and unburned rangelands. A randomized complete block design with a 2×2 factorial arrangement of treatments contrasting burn versus unburned and June versus September grazing periods were used. Pastures (4 ha) within the burn area were grazed in either June 2021 or deferred to September 2021. Grazing occurred for two days with 20 cow/calf pairs in June and 16 cow/calf pairs in September. Cattle diet composition and masticate samples were collected during 20-minute bite-count periods using six ruminally cannulated cows in each pasture prior to and after the two-day grazing periods. Cannulated cows grazed a pasture adjacent to the burn area to compare diet quality and composition between burned and unburned rangeland. Diet composition displayed a burn treatment by season of use interaction (P < 0.01) with greater forb consumption in burned than unburned in June (P < 0.01), but not different in September (P = 0.58). Additionally, both bites per minute and grams per minute displayed a treatment by season of use interaction (P < 0.01). Specifically, cattle grazing unburned pastures in June tended to take more bites per minute (P = 0.07) than in burned pastures. In contrast, the opposite was observed with September diets where cattle took more bites per minute in burned pastures than unburned pastures (P < 0.01). Grams per minute did not differ between September treatments (P = 0.26), but cattle did consume more grams per minute in unburned pastures in June (P < 0.01). The ash content of diets tended to display a treatment by season of use interaction (P = 0.09) with dietary ash tending to be greater in burned pastures in June (P = 0.07), but not differing in September (P = 0.54). Both dietary NDF and CP displayed a treatment by season interaction (P < 0.01), where crude protein was greater and NDF was lower for June burned pastures than unburned pastures (P < 0.01) but did not differ in September (P ≥ 0.27). In summary, diet quality was improved for grazing during the growing season after wildfire. The differences in diet quality may be related to the increase in forb consumption in post-fire vegetation communities.