Initial Nutritive Value and Utilization Affect Apparent Diet Quality of Grazed Forage

Abstract

An on‐farm study was conducted to measure the effect of forage nutrive value and utilization on diet quality selected by cattle grazing rotationally stocked pastures. In New York 20 paddocks on three farms were grazed by lactating dairy cows or heifers, and in West Virginia 47 paddocks on four farms were grazed by lactating beef cows and calves or yearlings. Most pastures were grazed for one to three days. For each nutritive value component ‐ crude protein (CP), acid detergent fiber (ADF), neutral detergent fiber (NDF), non structural carbohydrates (NSC), and total digestible nutrients (TDN) ‐ apparent diet quality was calculated as the components calculated mass in the pregrazing forage mass minus the calculated mass in the postgrazing forage mass divided by the forage mass disappearing during grazing. Forage utilization was calculated as pregrazing forage mass minus postgrazing forage mass divided by pregrazing forage mass. Cattle grazed selectively increasing CP, NSC, and TDN and decreasing ADF and NDF in the apparent diet compared to the pregrazing forage. Initial pasture nutritive value had the major effect on apparent diet quality. Forage utilization modified apparent intake by reducing the magnitude of selective grazing.