Effect of amount of undegradable crude protein in the diets of high-yielding dairy cows on energy balance and reproduction

This trial was designed to investigate effects of the amount of dietary undegradable crude protein (CP) and the ratio of degradable organic matter to degradable CP in the diets of high-yielding dairy cows on milk yield, energy balance and fertility during the first three months postpartum. Forty-five Israeli Holstein cows were assigned to three dietary treatments at 2 d postpartum. Dietary treatments were (1) 16.7% CP, of which 32% was undegradable (control); (2) 16.7% CP, of which 38% was undegradable; and (3) 18.0% CP, of which 35% was undegradable. Corn gluten meal (CGM) was added to diets 2 and 3 in order to increase undegradability. Cows were fed in groups, and diets were offered as a total mixed ration. Intakes of dry matter (DMI) and CP (kg/d) were 23.8 and 3.975, 21.4 and 3.574, and 20.4 and 3.672, for cows on diets 1, 2 and 3, respectively. No significant differences among the three treatments were found for milk yield, milk protein concentration, milk fat, or protein and lactose yields. Control cows lost 35.0 kg of body weight (BW) during the first 27.2 d postpartum, and then started gaining weight. At approximately day 70 the control cows attained their calving weight. Cows on diets 2 and 3 lost 55 kg of BW during 45 d postpartum and, by the end of the trial, these cows still had not attained their calving weight. Change in body condition score (BCS) during the experimental period revealed the same trends. Mean concentrations (mg/100 ml) of ammonia N in ruminal fluids and concentrations of plasma urea N were 14.0 and 20.3, 12.7 and 21.2, and 14.7 and 21.3 for cows on diets 1, 2 and 3, respectively. No significant differences among treatments were detected for any reproduction parameter. It was concluded that the differences between control cows and cows on diets 2 and 3 in DMI, BW and BCS during the first period postpartum cannot be attributed to the level of dietary undegradable CP only but to a more specific effect of CGM.  2000 Elsevier Science B.V. All rights reserved.