Animal age and sex effects on diets of grazing cattle

The effects of animal age and sex on chemical and botanical composition of diets of cattle grazing native rangelands were evaluated in a 2-year study. Samples were collected monthly from June through October using esophageall y cannulated suckling calves, yearling heifers, mature cows, and mature ste ers. Dietary crude protein and digestibility differed among animal classes, but these differences varied over time. These 2 diet quality indicators di d not vary in the same manner over time for all animal classes, Dietary cru de protein varied from a low of 7.2% for steers in August 1994 to a high of 14.3% for heifers in June 1993, In vitro digestibility varied from a low o f 50.7% for cows in October 1993 to a high of 74.3% for calves in June 1993 , Botanical composition of diets varied with animal class and sampling date with interactions among these. Cool-season grasses accounted for an averag e of 70% of the diet with a range of 33 to 90%, Shrubs varied from 1 to 61% of the diet. Differences in chemical composition among age and sex classes of cattle grazing native rangeland during the growing season may be partia lly related to differences in botanical composition of diets. Animals used to obtain diet samples should, therefore, be of similar physiological state and age as animals being monitored for performance.