Diurnal Variation in Milk and Plasma Urea Nitrogen in Holstein and Jersey Cows in Response to Degradable Dietary Protein and Added Fat

Abstract Four Holstein and four Jersey cows fitted with ruminal and duodenal cannulas were used in two 4 × 4 Latin squares to investigate the effects of varying protein degradability and supplemental fat on diurnal changes in plasma and milk urea N. Dietary dry matter contained 16.2% crude protein with two concentrations of ruminally undegradable protein (RUP) that were obtained by substituting blood meal for a portion of soybean meal. Treatments were 1) 29% RUP and 0% added fat, 2) 29% RUP and 2.7% added fat (Ca soaps of fatty acids), 3) 41% RUP and 0% added fat, and 4) 41% RUP and 2.7% added fat. Dry matter of the total mixed diet fed at 1000 and 1400 h consisted of 30% corn silage, 29% alfalfa haylage, and 41% concentrate. Ruminal ammonia, plasma urea N, and milk urea N were measured every 4 h over a 24-h period. Dry matter intake was depressed 6.7% by added fat. Ruminal ammonia was 25 to 45% lower when the 41% RUP diets were fed. Overall, the concentration of plasma urea N and milk components were not influenced by diet. However, milk urea N was higher in Holsteins than in Jerseys. Both plasma and milk urea N increased within 2 h after the 1000-h feeding followed by a decline at 6 h after the 1400-h feeding. In this short-term study, fat supplementation had no effect on milk production or yields of milk components. The inclusion of blood meal, however, increased the yields of milk components. Plasma and milk urea N did not differ among dietary treatments but varied throughout the day in relation to the time of feeding.