Effects Of CP/DE Ratios On Growth Performance And Nutrients Digestion In Different Physiological Stages Of Heifers

Heifer is the base of dairy cows and the rearing stage of heifers is a key link to dairy cattle production.Proper rearing of heifers can not only prolong the production life, improve milk yield ,reduce rearing costs, but also benefit the long-term development of dairy industry.In the research, heifers on two physiological stages (3- to 5-month-old and 8- to 10-month-old) were used to study the effects of different ratios of dietary CP to digestible energy on growth performance and nutrients digestion.The objective of the study was provided basic parameters for rearing Chinese Holstein heifers.In Experiment 1,thirty weaned Chinese Holstein calves of 70d old and 85.18±11.59kg of BW,were divided into 3 groups in a randomized experiment design. Each group was randomly assigned to one of three rations with a dietary CP to digestible energy(CP:DE)ratio of 56.3 (Treatment A), 57.2 (Treatment B) or 60.9 (Treatment C) g CP per Mcal DE. The rations were fed for 90 d and the amount of the ration offered to each heifer was adjusted every 10 days to allow weight gains of approximate 0.80 kg/d.The results showed that Average daily gain (ADG) did not differ among the treatments (0.83, 0.80 and 0.81 kg/d for Treatment A, B and C, respectively)(P>0.05).The teat length of heifers were not significant differences among the treatments. Feed efficiency were not significantly different among the treatments (4.29, 4.24 and 3.96 Treatment A, B and C, respectively)(P>0.05).The apparent digestibility of CP increased with increasing CP:DE ratios and total average apparent digestibility of CP were65.87%,67.68% and 69.90% for A,B,C respectively(P<0.05).After three months, the apparent digestibility of P became steady. The other Nutrient's apparent digestibility(such as DM,OM,GE,ADF) were not significant difference. The concentration of plasma urea N increased with increasing CP:DE ratios(P<0.05). No significant effect(P>0.05) was found on the parameters of E2, P4, PRL,GH,IGF-1 by the treatments. The results of feed efficiency,digestibility of nutrients and ADG indicated that the diet with a CP:DE ratio of 56.3 g CP per Mcal DE could meet needs of 3- to 5-month-old China Holstein calves gaining 0.80 kg/d.In Experiment 2, eighteen 7-month-old Chinese Holstein heifers (234±21.9kg of BW) were divided into 3 groups in a randomized experiment design. Each group was randomly assigned to one of three treatment rations with a CP:DE ratios of 37.9 (Treatment D),44.4 (Treatment E) or 50.5 (Treatment F) g of CP per Mcal DE. Treatment period was 90 d and the amount of the ration offered to each heifer was adjusted every 10 days to allow weight gains of approximate 0.90 kg/d.The results showed that ADG did not differ among the treatments ,0.90,0.89,0.86 kg/d for the Treatment A,B,C, respectively),(P>0.05) and No significant effect(P>0.05) was found on the teat length by the treatments. The apparent digestibility of CP increases with increasing CP:DE ratios(P<0.05)and The apparent digestibility of the nutrients increases with age. The increased CP:DE ratios were accompanied by increasing concentration of plasma urea N and ruminlalAmmonia nitrogen .The treatments were not significant effect on the the parameters of E2, P4, PRL,GH,IGF-1(P>0.05). The. Results indicated that the diet of CP:DE ratios was 37.9 g of CP per Mcal DE,could meet needs of the 8-to-10 month-old China Holstein calves gaining 0.90 kg/d.On the whole,by the growth experiment and digestion experiment ,This study systematically evaluedthe effect of different CP:DE ratios on ADG, the apparent digestibility of the nutrients and some biochemical parameters of the Chinese Holstein heifers at the allometric growth stages . Two diets with varying CP:DE ratios could meet the heifer's needs at corresponding stages. It is concluded that, after nine months of the feeding trials, the apparent digestibility of nutrients in Chinese Holstein heifers became steady, indicating that the proportion of the stomach and digestive function of the heifers basically achieve the adult ruminant level. The results provided a theoretic basis to the practice.