Effects of Different Degradation Properties of Dietary Protein in the Rumen on the Weight Gain and Nitrogen Balance of Calves at a Low or Medium Level of Protein Intake

Abstract

Based on the presupposition that the susceptibility of feed proteins to rumen-bypass increases in the order of A, B and C when their properties of degradation in the rumen are classified into the above three types, this study aims to examine the effects of differ-ences in the degradation properties of protein in concentrate diets containing CP at a 14% or 12% level on the growth and the nitrogen (N) balance of 3 to 6 month old Holstein male calves. Two growth trials were conducted under the condition of ad libitum feeding of a concentrate diet and rice straw, and by allotting two types of diets containing the same level of CP to two groups of seven calves each in one trial. Comparisons were made between A-and BC-type diets containing CP at a 14% level in trial 1, and at a 12% level in trial 2, but no significant difference was detected in body weight gain, feed per gain, and CP consumption per gain between the types of diet at both CP levels. The N balance trial was executed in a 4×4 Latin square design by allotting four diets similar to those used in growth trials to four calves initially weighing an average 114 kg. Daily rations which consisted of 88% concentrate diet and 12% rice straw were restricted to 3% of body weight. As a result, the proportion of N excreted into feces was significantly higher for the A-type diet with CP at a 14% level (P<0.05), while that of N excreted into urine was significantly higher for the BC-type diet with CP at a 12% level (P<0.05). There was no significant difference in the proportion of retained N to N intake among the four diets, but the proportion of retained N to apparently-absorbed N was significantly lower for the A-type diet with CP at a 14% level than for the two diets with CP at a 12% level (P<0.05). Also it tended to be higher for the BC-type diet than for the A-type diet at the level of 14% CP. Concentrations of serum urea-N (SUN) were higher at the 14% level of CP than at the 12% level not only in the N balance trial but also in the growth trial. Comparing SUN concentrations at the same CP level, the trend was for the BC-type diet to be higher than the A-type diet at a 14% level of CP, but there was rather a small tendency for the A-type diet at a 12% level of CP to be slightly higher. In the N balance trial, in which concentrations of plasma free amino acids were determined, a significant increase in leucine (P<0.05) as well as an obvious decrease in lysine were detected when BC-type diets were fed at both CP levels.