Effects of dietary protein on milk, rumen, and blood parameters in dairy cattle fed low fiber diets

Eighteen multiparous and 9 primiparous Holstein cows were used to determine the effects of a 13 and 23% crude protein concentrate on milk fat depression during early lactation. Beginning on d 22 postpartum, cows were fed a high fiber diet (27% acid detergent fiber) for 3 wk and then switched to a low fiber diet (9 to 10% acid detergent fiber) for 6 wk. Crude protein percentages calculated from dry matter consumption were 13.5 and 17.9% during the high fiber period and 12.7 and 22.3% during the low fiber period. Daily milk and fat yields for both primiparous and multiparous cows were greater for the high protein treatment. The magnitude of decline in milk fat percentage (from high to low fiber) was greater for the low protein treatment, as determined by nonlinear regression. The high protein treatment was more effective in reducing the severity of milk fat depression in primiparous cows than in multiparous cows. Dietary crude protein had no effect on milk protein or solids-not-fat percentages, rumen volatile fatty acid molar proportions, or serum acetate concentration. The mechanism by which the high protein ration minimized the fat depression response to low fiber rations by primiparous cows is unknown.