265. Influence of Timing of Grazing During the Growing Season on Forage Utilization, Foraging Efficiency, Botanical Composition, and Diet Quality in Semi-arid Rangelands.

Abstract

Current grazing management recommendations for foothill bunchgrass rangelands include deferring grazing during the growing season to allow forage recovery from defoliation on an every-other-year or a two out of every-three-year basis. Although research supports that deferment during the growing season is fundamental to maintaining sustainable grazing systems, there is limited information on how grazing behavior changes throughout the growing season. Therefore, our objectives were to evaluate the impacts of grazing timing during the growing season on diet quality, botanical composition of diet, and foraging behavior of beef cattle grazing on semi-arid, bunchgrass-dominated rangelands. In 2024 and 2025, we utilized sixteen ruminally-cannulated cattle in a complete randomized block design with 16 four-hectare pastures (four blocks, each containing four pastures). Four grazing treatments were randomly assigned within each block: 1) early season (mid-May to early June), 2) mid-season (early June to late June), 3) late season (late June to mid-July), and 4) ungrazed control pastures. At the initiation of each grazing treatment, six ruminally-evacuated cannulated cows were observed grazing for 20 minutes to collect bite counts by species. Rumen diet samples were collected from the grazing bouts to evaluate foraging behavior, diet composition, and diet quality. Pastures were clipped to assess forage production by functional group and potential regrowth. For foraging behavior, grams consumed per minute and bites per minute displayed a treatment × year interaction (P ≤ 0.03). Where grams consumed per minute displayed a quadratic pattern in year two and bites consumed per minute displayed a quadratic pattern in year one (P = 0.02) and a linear pattern in year two (P = 0.01). Grams/bite differed among treatments (P = 0.04), linearly increasing as the grazing season progressed (P < 0.01). In both years, grass composition of the diet decreased linearly (P < 0.01) as the growing season progressed. Conversely, forb composition of the diet increased linearly (P < 0.01) both years. Dietary crude protein displayed a treatment × year interaction, where it linearly decreased in year one (P < 0.01) and quadratically decreased in year two (P < 0.01). Relative preference in the diet for both grass and forbs displayed treatment × year interactions (P < 0.01), where preference for grass linearly decreased both years (P < 0.01), and relative preference for forbs linearly increased in year one (P < 0.01). In summary, grazing behavior and diet composition differed across time throughout the growing season. This suggests that as the season progresses, cattle alter their foraging strategy to optimize nutrient intake. Grass-to-forb ratio of the diet, diet quality, and foraging behavior, as a function of grazing timing within the growing season, reflect the changing foraging strategies as the growing season progresses.