DIETARY-PROTEIN DEGRADABILITY EFFECTS ON PLASMA AND MILK UREA NITROGEN AND MILK NONPROTEIN NITROGEN IN HOLSTEIN COWS

Fifteen multiparous, midlactation Holstein cows were used in a 5 x 5 Greco-Latin square to investigate the effects of various amounts of ruminally degradable and undegradable protein on plasma urea and milk urea N and NPN constituents. Diets were designed to provide the following percentage of degradable and undegradable protein relative to NRC requirements: diet A, 80, 80%; diet B, 100, 100%; diet C, 120, 80%; diet D, 100, 120%; and diet E, 120, 120%. All cows were fed for ad libitum intake a TMR consisting of 47% corn silage, 18% hay-crop silage, and 35% concentrate (DM basis). Concentrate consisted of various amounts of dry com plus soybean, fish, and blood meals to alter the degradability of protein. In situ measurements were used to determine grain protein degradability. Plasma urea N was increased across diets A to E, respectively: 8.2, 14.8, 16.5, 17.8, and 20.7 mg/dl. The within-cow coefficient of correlation between plasma and milk urea N was .88. The intake of undegradable protein elevated plasma and milk urea N to a similar extent as intake of degradable protein. Multiple regression analysis across all diets revealed a negative relationship of energy intake on plasma urea N. Diet significantly influenced milk true protein yield for diets A to E, respectively: .73, .80, .73, .78, and .81 kg/d. Milk true protein yield was influenced by milk yield, undegradable protein, and energy intake. Milk NPN concentration increased across diets A to E, respectively: 28.7, 33.9, 35.6, 36.8, and 39.8 mg/dl. The increase in milk NPN concentration across diets was attributed largely to an increase in the urea component of milk NPN.