Influence of Dietary Protein Concentration and Degradability on Milk Production, Composition, and Ruminal Protein Metabolism

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Abstract
Four groups of cows in early lactation, each group containing one mature cow and three cows in first lactation, were in a 4 .times. 4 Latin-square arrangement of treatments for us to study influences of altering quantity of undegraded dietary crude protein and quantity of crude protein on milk production and composition. Diets supplemented with protein sources were (1) soybean meal positive control (22.7% crude protein), (2) whole cottonseed-corn gluten meal (14.7% crude protein), (3) extruded whole soybean (14.5% crude protein), or (4) soybean meal supplemented (15.7% crude protein). Concentrate and sorghum silage were fed in a ratio of 62:38 dry matter. Dry matter intake was not influenced by dietary crude protein concentration or source. Cows consuming diet 2 produced less milk, milk protein, total solids, and solids-not-fat than cows receiving diets 1 and 4. Efficiency of conversion of dietary crude protein to milk protein was highest for cows receiving diet 3 and lowest for diet 1. Trial 2, a 4 .times. 4 Latin-square trial for collection of abomasal digesta, had four ruminal and abomasal cannulated steers and the four diets from the lactation trial; the trial was to determine the influence of source and concentrations of protein on quantity of protein reaching the abomasum daily. Crude protein intake by steers fed diet 1 was greater than for the other three diets. Percentages recovery of dietary crude protein were 122 and 130 for treatments 2 and 3, intermediate for treatment 4 (107%), and lowest for diet 1 (88.0%); crude protein digestibility in the total tract was highest for steers receiving diet 1.