Effects of dietary undegradable crude protein level on total non-structural carbohydrate (TNC) digestibility, and milk yield and composition of dairy cows

This trial was designed to investigate whether increasing the amount of ruminal undegradable crude protein (CP) content of rations given to high producing dairy cows, affected total non-structural carbohydrate TNC digestibility and milk production. A total of 36 Israeli-Holstein multiparous dairy cows were assigned to three treatments according to parity number, calving date, milk yield during the previous lactation and body weight (BW) 2 days after calving. Diets were formulated to contain (on a DM basis) 45% forage, 16.5% CP, 1.7 Mcal/kg net energy lactation (NE), and differed in dietary CP undegradability, as follows: (1) low undegradable CP (LUDCP)-UDCP, 36% of dietary CP; (2) medium UDCP (MUDCP)-UDCP, 41% of dietary CP; and (3) high UDCP (HUDCP)-UDCP, 46% of dietary CP. Dietary UDCP was increased by reducing concentrations of soybean meal and urea and increasing those of fish meal and gluten meal. The cows were maintained in the trial for 3 months. Average dry matter intakes of cows on LUDCP, MUDCP and HUDCP treatments were 25.0, 23.3 and 23.3 kg/day, respectively (P > 0.1). Treatment averages of milk yield (kg/day), milk fat, protein and lactose (%) were 43.3, 3.01, 2.97 and 4.48, respectively, and no significant differences between diets were observed. Apparent digestibilities of dietary CP and TNC were increased by UDCP supplementation (P < 0.02 and P < 0.0001, respectively). Differences in body condition score (BCS) between the beginning and the end of the experiment were -0.48, -0.17 and +0.03 units, respectively, in the cows fed the LUDCP, MUDCP and HUDCP rations (P < 0.005). A linear equation was fitted to describe the changes in BCS of the cows given the various treatments; its slope differed significantly among treatments (P < 0.0001). Plasma glucose concentrations were 51.7, 58.4 and 58.5 mg/100 ml, respectively, in the cows fed the LUDCP, MUDCP and HUDCP rations (Contrast < 0.02). It was concluded that by increasing dietary UDCP up to 7.3%, starch digestion in the small intestine and dietary energy utilization efficiency of milking cows on high-grain rations were significantly improved. (C) 2002 Elsevier Science BV All rights reserved.