Effect Of Dietary Energy Level On Growth, Digestion And Metabolism, Ruminal Environment In Heifers Aged 7 To 10 Months

This study was conducted using heifers aged from 7 to 10 months as trial animals and different levels of dietary energy for lactation as treatments. The regular method was utilized to evaluate the effect of dietary energy level on the growth proformance and serum indices of heifers, measured the parameters of ruminal fluid collected by vacuum technique, and analysed the differences of digestion and metabolism of nutrients among the treatments using the method of digestion and metabolism trial, finally, we also analysed the changes of rumen microflora induced by the dietary energy level with high-throughput sequencing technology. According to the results, we drew a conclusion about the appropriate dietary net energy level for lactation for the heifers aged 7-10 months. The three experiments were described as follows. Experiment 1: Effects of diets with different energy levels on growth performance and serum indices in Chinese Holstein Heifers aged 7 to 10 monthsThe aim of this study was to investigate the effect of total mixes rations(TMR) difference in energy level on growth performance and serum indices in Chinese Holstein heifers aged 7 to 10 months. Twenty four heifers with the age of about 200.2±3.14 days were randomly divided into 3 dietary treatments with 8 heifers each: metabolic energy of TMR is 8.87(L treatment), 9.83(M treatment), and 10.88(H treatment) MJ/kg, respectively. The amount of feed for each treatment is the same calculated based on the 2.6% of average weight of M treatment, and the exact feed intake of each heifer was recorded daily. The experiment lasted 90 days. Growth performance, including body weight, withers height, body length, heart girth, abdominal girth, hip width, nipple length and body condition scoring were determined and blood samples were collected on 210, 240, 270 and 300 days of age before feeding in the morning for analysis. The results showed that: the average daily gain of heifers was significantly among treatments(P<0.05) from d 210 to 300(0.69, 0.81, 0.96 kg/d for treatment L, M and H, respectively). On d 240, the left posterior nipple of heifers in treatment L was significantly higher than that of treatment H(P<0.05), while on 240 and 270 days of age, right posterior nipple of heifers in both treatment L and M were significantly higher than that of treatment H(P<0.05). The concentration of serum triglyceride(TG) of heifers in both treatment L and M were significantly(P<0.05) lower than that of treatment H on d 240. The concentration of serum estradiol(E2) of heifers in treatment L was significantly higher than that of treatment H(P<0.05) on 300 d. Taking ADG and serum indexs into consideration, the shape and development of mammary gland of heifers could be maintained when the ME content of TMR is 9.83 MJ/kg DM. Experiment 2: Effects of different dietary energy levels on the digestion and metabolism of nutritent and rumen fermentation of Chinese Holstein heifers aged 7 to 10 monthsThe study was performed to investigate the effects of different dietary energy levels on the digestion and metabolism of nutrient and rumen fermentation of heifers with age of 7 to 10 months. The trial animals and treatments were as same as the experiment 1 with 90 experimental days, and four heifers aged 262~270d and 292~300d were randomly selected from each treatment. The feed intake per heifer each day, faeces, urine and other regular parameters were measured. Prior to morning feeding of the day 210, 240, 270 and 300, ruminal fluid was collected under vacuum to measure the fermentation parameters. The results indicated that the apparent digestibility of dry matter, organic matter and ether extract increased as the energy level improved(P<0.05); the metabolic energy level had a significantly negative effect on the apparent digestibility of acid detergent fiber(P<0.05); the rate of digestible energy and metabolic energy increased as the energy level advanced(P<0.05). The apparent digestibility of nitrogen and retained nitrogen/nitrogen intake in treatment H was significantly higher than other two treatments(P<0.05), retained nitrogen/digestible nitrogen in treatment L was higher than treatment M and H from 262 to 270d; however, retained nitrogen/digestible nitrogen increased as the energy level advanced from 292 to 300d(P<0.01). On 240 d, ruminal pH in treatment H had the higher level than treatment L and M(P<0.05), concentration of pentanoic acid and isovaleric acid in treatment M was superior compared with treatment H(P<0.05), and the total of volatile fatty acid had not been affected by the dietary energy(P>0.05). Therefore, we could conclude that the dietary energy level had a significant effect on the nutrient digestion and metabolism, but did not exert effect on the rumen fermentation. Experiment 3: Effects of dietary energy level on rumen microbial of 7- to 10-month-old heifersThe study was aimed to examine the effects of dietary energy level on rumen microflora of 7 to 10 months heifers. The experiment design was the same as experiment 1 and lasted for 90 days. Prior to morning feeding on 240, 270 and 300 d, rumen fluid was collected from four heifers readomly selected from each treatment to measure the variation of microbial community by high-throughput sequencing technology. The results showed that on day 270, Ace and Chao1 indexes decreased with dietary energy level increased(P<0.1), the content of Firmicutes in M treatment was significantly higher than H treatment(P<0.05) and the Actinobacteria content decreased significantly with the dietary energy increased(P<0.05). Through top3 advanced bacterial constitutional diagram of Actinobacteria on 270 d, Bifidobacteriales showed a decreasing trend in the high dietary energy level, Actinomycetales and Coriobacteriales showed an opposite trend. Principal component analysis presented that microbial diversity was in a stable situation on 300 d. In conclusion, the dietary energy had an effect on the rumial microflora.In conclusion, compared with the dietary energy level of 8.87 MJ/kg(L treatment) and 10.88 MJ/kg(H treatment), the treatment with 9.83 MJ/kg(M treatment) could not only maintain the average daily gains of heifers(0.81 kg/d) aged 7 to 10 months, but also benefit for the development of mammary gland, healthy status, body dimensions and digestion and absorption of nutrient.