Cattle diets from yearlong and rotational grazing at two stocking densities on the Texas Experimental Ranch

Abstract

Five paddocks of a heavily stocked (3.3 AUM/ha) rotational grazing (RG) system at two stocking densities, two paddocks simulated 14 (RG-14) and three paddocks simulated 42 (RG-42) paddocks per herd, and a moderately stocked (2.02 AUM/ha) continuously grazed (MC) paddock were studied at the Texas Experimental Ranch to evaluate their effects on diet selection. The dietary percentages were determined for major plant species, classes, and plant parts. In vitro digestibility of organic matter (IVOMD), crude protein (CP), and organic matter (OM) were also determined. Precipitation was 14% below the 22-year average and irregularly distributed through the 2-year period from 1982 to 1984. Texas wintergrass (Stipa leucotricha Trin. Rupr.), sideoats grama (Bouteloua curtlpendula Michx. Torr.), and other warm-season grasses were the major dietary components. The percentage of cool-season grass and warm-season grass did not differ significantly (P < 0.05) under RG and MC, but there were more forbs in diets from MC than RG, during last five trials. Cows on RG selected slightly more sideoats grama than on MC. Grasses accounted for 92% of diets in both RG-14 and RG-42, and 89% in MC. Cattle on RG shifted from cool-season grasses to warm-season grasses earlier in the spring than those on MC. The variation in diet selection between paddocks within stocking densities on RG was greater than the difference between stocking densities. Seasonal variation was the major factor affecting botanical and chemical composition of diet. IVOMD, CP, and OM did not differ significantly between RG and MC; however, CP was significantly different between simulated RG-14 and RG-42 paddocks per herd (8.3 vs 9.2%, respectively). IVOMD and CP were highest in June 1983, May 1984, and August 1984. These high values were associated with rapid plant growth and high live-dead ratio. During periods of active plant growth, diet quality tended to decrease from the beginning to the end of the grazing period; however, during dormant periods there was little indication of diet sensitivity to grazing pressure.