Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 1, 2002
Publication Date: October 8, 2002
Interpretive Summary: Dietary protein supplies the protein used by the cow both by stimulating growth of the microbes in the rumen (the first compartment of the cow's stomach) and by providing protein that is digested directly by the cow. High energy diets stimulate protein formation by rumen microbes and aid in the cow's protein metabolism. Overfeeding protein results in excessive nitrogen excretion, especially in the urine, the form that is the most polluting. Overfeeding energy as grain can depress rumen pH, digestion and milk fat production, and cause health problems for the cow. To identify optimal levels of dietary protein and energy, 63 Holstein cows were fed nine diets (three levels of protein, added as soybean meal, at each of three levels of energy, obtained by replacing forage with corn grain). A blend of alfalfa plus corn silages, was fed at 75, 63 and 50% of the diet. Feed intake and production of milk, protein and fat were lower on 15.1% than on 16.7 and 18.4% protein diets. Compared to 16.7% dietary protein, the only effect of feeding 18.4% protein was to increase urinary nitrogen excretion and to reduce protein efficiency. Increasing dietary energy (reducing silage and adding corn grain) increased feed efficiency, yield of milk, protein and milk sugar, and reduced urinary nitrogen excretion. This research indicates that there is considerable wastage of dietary protein on dairy farms in the U.S. because many farmers commonly feed their cows diets with more than 18% protein. Also, farmers can improve protein efficiency by feeding as much grain energy as the health of the cow permits. This means that dairy farmers could substantially reduce nitrogen pollution of the environment by minimizing protein and maximizing energy in the diets they feed their milking cows.
Technical Abstract: Forty-five multiparous and 18 primiparous Holstein cows averaging 41 kg/d of milk were fed three levels of crude protein (CP), each at three levels of neutral detergent fiber (NDF), to identify optimal dietary CP and energy. Cows were blocked by lactation and days in milk into seven groups of nine and randomly assigned to an incomplete 9x9 Latin square trial with four, 4 week periods (total 16 weeks). Diets were formulated from alfalfa and corn silages, high moisture corn, soybean meal, and minerals and vitamins. Forage was 60% alfalfa and 40% corn silage; NDF contents of 36, 32, and 28% were obtained by feeding 75, 63, and 50% forage dry matter (DM). Dietary CP contents of 15.1, 16.7, and 18.4% were obtained by replacing high moisture corn with soybean meal. Production was measured over the last 2-weeks of each period. Spot fecal and urine samples were collected from 36 cows to estimate N excretion using fecal indigestible ADF and urinary creatinine as markers. There were no interactions (P ¿ 0.21) between dietary CP and NDF for any trait; thus, effects of CP were not confounded by NDF or vice versa. Intake of DM and fat yield were lower on 15.1% CP than at higher CP. There were linear increases in milk urea and urinary N excretion, and linear decreases in N efficiency, with increasing dietary CP. Increasing CP from 15.1 to 18.4% reduced milk N from 31 to 25% of dietary N, increased urinary N from 23 to 35% of dietary N, and reduced fecal N from 45 to 41% of dietary N. Decreasing dietary NDF gave linear increases in BW gain, milk yield, yield of protein, true protein, lactose, and SNF, and milk/DM intake and milk N/N intake, and linear decreases in milk urea. However, fat yield was lower on 28% than 32% dietary NDF. Reducing dietary NDF from 36 to 28% increased purine derivative excretion by 19%, suggesting that microbial protein supply was increased. Increasing CP by adding soybean meal to the diet of cows averaging 34 kg/d of milk increased intake and fat yield but depressed N efficiency. Increasing dietary energy (by reducing forage) improved milk yield and efficiency and decreased excretion of environmentally labile urinary N.