Effect of Dietary Protein Degradability on Cows in Early Lactation

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Abstract
Five completely mixed isocaloric diets were formulated with ratio of 50:50 roughage:concentrate (dry matter) to vary in protein degradability. Three diets at 14% crude protein included 10.8% soybean meal (diet 1), 8.0% corn gluten meal plus 0.5% urea (diet 2), or 10.6% corn gluten meal (diet 3) as the protein source. Diet 4 was 17% crude protein with soybean meal and diet 5, 12.6% protein with urea. Protein disappearance after 24 h nylon bag incubation in rumen fistulated steers was 54, 49, 43 and 58% for diets 1-4. Diets were fed to 45 Holstein cows averaging 36 (.+-. 14) days postpartum with a 2 wk covariate and a 6 wk experimental period. Cows were fed diet 1 during the covariate period. Treatment means adjusted by covariance for the variable and days postpartum, for average daily milk yield, milk fat percent, milk protein percent, milk lactose percent and solids-corrected milk were 30.8, 2.95, 3.07, 5.08, and 28.5; 32.9, 2.40, 2.96, 5.20, and 26.2; 34.2, 2.71, 2.88, 5.18, and 27.8; 36.7, 2.81, 3.09, 5.10, and 28.5; 31.7, 2.43, 2.93, 5.24 and 25.4 for diets 1-5. Decreasing dietary protein degradability at a 14% crude protein increased milk production, decreased milk protein percent and had a quadratic effect on milk fat percent. Increasing dietary protein percent raised milk yields, milk protein percent and solids-corrected milk yield.