Effects Of Protein Level And Lys/Met On Performance, Nutrient Digestibility And Rumen Development For Weaned Calves

Weaned calves were used in a series of trials to study the effect of protein levels in starter feeds on growth performance, digestive physiology and rumen development, and the effects of lysine to methionine ratios in starter feeds with low protein levels on growth performance, nutrient digestibility and nitrogen metabolism.Experiment 1 was designed to study the effect of protein levels in starter feeds on growth performance, digestive physiology and rumen development in weaned calves. Fifteen male Holstein calves were fed a starter feed containing 16.22 (low protein), 20.21 (medium protein) or 24.30% crude protein (high protein) respectively to examine the effect of dietary protein on growth performance, serum parameters, nutrient metabolism and rumen development. Restricted amounts of starter feeds were fed to calves from 8 to 16 wk of age. Live weights, growth parameters and concentrations of blood metabolites were determined every two weeks. Three calves were selected from each group and two digestion trials were conducted at 10 and 13 wk. The rumen fluid was taken 3 h after feeding at 12 wk of age and then euthanized at 16 wk of age to investigate rumen fermentation, digesition and enzymes activity. All calves were euthanized at 16 wk of age and rumen tissues were sampled for measurements of the development of ruminal epithelium. The results showed that, although no statistical differences were detected among treatments (P>0.05), the weight gain of calves fed low protein starter feeds was lower than that of the other calves. The serum urea nitrogen of calves fed high protein starter feeds was higher than that of the other calves. Calves on low protein starter feeds had lower apparent digestibility of the nutrient except crude protein at 10 wk of age. The apparent digestibility of nutrients except dry matter at 13 wk of age tended to increase as the crude protein content in starter feeds increased. Fecal nitrogen, urinary nitrogen, absorbed nitrogen, retained nitrogen and total utilization of nitrogen increased linearly with the increasing dietary protein levels. The calves in group HP had a lower ruminal pH at 12 wk of age, had a higher ruminal pH at 16 wk of age. Ruminal concentrations of ammonia nitrogen and volatile fatty acids in calves fed starter feeds with HP were higher than those in the other calves. Theα-amylase activity was lower in calves fed MP starter feed than that fed the other starter feeds. The protease and cellulases activities tended to increase as the crude protein content increased. And they were higher in 16wk than in 12wk in the same treatment. Dietary protein levels had no effects on the development of the rumen .Experiment 2 was designed to examine the effects of the lysine to methionine ratio in diets on growth performance, nutrient digestibility and metabolism in weaned calves. Twenty four weaned calves were randomly divided into four groups and fed with a control starter diet with a crude protein level of 19.64% DM, or a test diet (15.22 crude protein% DM) with a lysine to methionine ratio of 2.5:1, 3.1:1 or 3.7:1 for 8 wk. The calves were weighed and blood samples were collected on the initial and final day of the trial. Four calves were selected from each group and digestion trials were conducted from the third week of the trial by total feces collection method. The result indicated that the gain of calves in group 3.1﹕1 was 834 g/d, which was 10.21% higher than that of calves in control group numerically (P﹥0.05). Serum urea nitrogen, arginine and valine concentrations in calves fed the test diet were lower than those in control calves (P﹤0.05). But it did not differ among test groups. The ratio of lysine to methionine had no influence on nutrient digestibility (P﹥0.05). But the utilization and apparent biological value of nitrogen in group 3.1﹕1 and group 3.7﹕1 were higher than group 2.5﹕1 and control group numerically(P﹥0.05).In conclusion, 20.21% protein was found to be the optimal to promote growth and development of the calves. In addition, the possibility of optimizing the lysine to methionine ratio at low protein levels was discussed and the calves fed the low protein diet with a lysine to methionine ratio of 3.1:1 had improved growth performance, and increase operating factor of protein.