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Effect of Excess Dietary Protein on Feed Intake and Nitrogen Metabolism in Steers¹

Michigan State University, East Lansing 48824

Abstract

Growing Holstein steers were fed four rations containing from 10 to 40% protein. Soybean meal and isolated soy protein concentrates were used as supplemental protein sources. Extensive ruminal bypass of the soy protein was anticipated and this study was designed to elucidate the role of high protein availability (e.g., amino acids absorbed from the small intestine substantially above the animal's amino acid requirements) on feed intake in steers. Steer dry matter intake for the two high (32.5 and 40,0%) protein rations was depressed primarily during days 2 and 3 with a subsequent recovery to initial intake levels between days 5 to 10. Overall, plasma amino acid levels were not markedly influenced by dietary protein level and hence in this study an "amino-static" control of feed intake in steers could not be demonstrated. Steers fed the two high protein rations had very high ruminal ammonia levels and elevated plasma urea nitrogen concentrations. It was concluded that, although for all rations the protein requirements of the steers were met, extensive degradation of the dietary protein negated the experimental design of achieving extensive ruminal protein bypass and an excessive absorption of amino acids from the small intestine. The steers, despite ruminal ammonia levels in excess of 100 mg/100 ml rumen liquor, did not exhibit any signs of ammonia toxicity. Apparently excessive levels of ruminal ammonia arising from breakdown of performed protein can be tolerated by ruminants when their amino acid requirements are met.

¹ Michigan Agricultural Experiment Station Journal Article No. 7448

² Ruminant Nutrition Laboratory, Department of Animal Husbandry

³ Present-address: Tennessee State University, Nashville 37203