Variation in nutritive quality and mineral content of grazed and clipped forage from native range in southwest Idaho1

Changes in nutrient content of grazed and clipped forages during the growing season were evaluated over a 2-yr period via samples from native range in southwestern Idaho. Mature, ruminally fistulated beef cows were used from April to September (yr 1; n = 4) and May to October (yr 2, n = 3) to collect masticated diet samples every 2 to 4 wk. Clipped samples were also collected concurrent with grazed samples. Attempts were made to clip forage similar to what the cows had been observed to be consuming; however, cattle selected (P < 0.05) a diet higher in nutritive quality than what was clipped for some nutrients. In yr 1, forage CP was greater (P = 0.05) in grazed than clipped samples in July, whereas in yr 2 grazed CP tended (P = 0.06) to be greater in August and was greater (P < 0.05) in September than clipped samples. Nutrient content of samples was adequate to meet lactating beef cow needs (NRC requirements) for CP and TDN until mid to late summer and adequate for K but deficient for P, Na, Ca, Mg, and S throughout the grazing season. Micronutrients Fe, Mn, and Mo were adequate during the grazing season, whereas Cu and Zn were deficient. Based on the variation in nutrient content by month, we report that forage quality is highly variable throughout the grazing season. Furthermore, clipped sample nutrient content is generally similar to grazed sample content and provides an adequate prediction of consumed forage quality.