Seasonal Diurnal Variation in Composition of Cow Diets

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Abstract
Seasonal diets between fall 1977 and spring 1979 were collected in morning and evening from esophageally fistulated cows in the Post Oak Savannah of east-central Texas [USA]. Chemical content of diurnal diets were similar within a season except CP was higher in evening collections as compared to morning collections during fall. Cows appeared to select for energy in vitro digestible organic matter over CP. Botanical composition of morning and evening diets differed only during summer. Since the cows spent more time during hot summer afternoons in the shade of woody plants, less grass and more forbs, vines and woody vegetation were selected. Research based on morning diet collections only might result in biased samples. Recognition of nutritional and environmental stresses with potential to alter grazing behavior of animals is critical for accurate sampling of livestock diets.