THE INFLUENCE OF DIETARY ENERGY-SOURCE AND DIETARY-PROTEIN LEVEL ON MILK PROTEIN-CONCENTRATION FROM DAIRY-COWS

To investigate the effects of energy source and protein level of diets on milk protein content, 12 multiparous Holstein-Friesian cows were u sed in a 4 X 4 Latin square change-over experiment with 4-week periods . Four diets were offered, with ad libitum silage as proportionately 0 .40 of the diet, and the remaining 0.60 as one of four concentrates, t wo based on barley and two on molassed sugar-beet pulp. Two protein le vels were achieved by altering the amounts of digestible undegraded pr otein in the concentrates, with all diets formulated to supply equal q uantities of rumen degradable protein. There was no effect of diet on dry-matter intakes. Both starch and high dietary protein levels signif icantly increased milk protein concentration (P < 0.05), but had no ef fects on milk fat and lactose concentrations,. Mean milk yields were s ignificantly higher (P < 0.05) with increased dietary protein. Dietary protein significantly affected the yields of milk protein (P < 0.01) and lactose (P < 0.05) but not that of fat Urinary allantoin excretion was significantly greater with both high protein (P < 0.05) and starc h-based diets (P < 0.05). No significant interaction effects were foun d. It is concluded that dietary effects were due largely to difference s in supply of rumen degradable protein; increases in milk protein con centration were therefore brought about by increasing the protein supp ly to the animal.