Effect And Mechanism Of Different Energy Level On Growth,Digestion And Metabolism, Ruminal Environment In Heifers Aged4to6Months

This study was conducted to investigate the effects of different dietary net energy for lactation level on growth and development, serum indices, digestion and metabolism of nutrients, and ruminal environment of heifers aged4to6months. Body weight, body size and serum indices were determined to study the the effects of different energy level on growth performance of heifers. Nutrition digestion and metabolism and rumen fermentation were measured to study the diet digestible ability. Rumen microflora was analyzed by16S rDNA and real time polymerase chain reaction molecular technology to study the variation of microbial community. The present study included3trails which described as follows.Experiment1aimed to study the the effects of different dietary energy level on growth performance and serum indices of heifers aged4to6months. A total of32Chinese Holstein heifers aged about98days were randomly divided into4groups. Each group was subjected to one of the following4diets with6.24,7.04,7.53, and7.85MJ/kg of net energy for lactation on a dry matter basis for82days. Growth performance was determined and blood samples were collected at98,120,150, and180days of age prior to morning feeding. The results showed as follows. During98to180days of age, the average daily gains which were0.64,0.75,0.78, and0.84kg/d, respectively, for the4groups, did not differ (P>0.05), but during151to180days of age, the heifers of group D had a greater average daily gain than those of group A (P<0.05). At180days of age, the body length of heifers of group D was significantly increased than that of group A (P<0.05). The heifers of group D had a greater body condition scores than those of group A (P<0.01). The net energy levels of7.04and7.53MJ/kg improved the feed conversion ratio (P<0.05). Nipple lengths did not differ among treatments (P>0.05). During98to180days of age, heifers fed high energy diets (groups C and D) had increased serum cholesterol content than group A (P<0.05); serum low density lipoprotein content tended to increase with the increasing energy level (P<0.10), whereas serum urea nitrogen content had the opposite tendency (p<0.10). Heifers fed the7.04MJ/kg diet had the highest serum contents of estradiol and leptin, and those fed the7.85MJ/kg diet had the lowest serum contents of insulin and insulin-like growth factor-I.Experiment2was performed to investigate the effects of different dietary energy level on nutrition digestion and metabolism rules of heifers aged4to6months. The experiment design was the same as experiment1. Four heifers aged151-157days and181-187days were randomly selected from each group to conduct, collected samples and measured the apparent digestibility, and the utilization rate of energy and nitrogen. The results showed that:The apparent digestibility of gross energy increased first and then decreased (P<0.10). The net energy level has a significantly effected on the apparent digestibility of dry matter, neutral detergent fiber and acid detergent fiber, digestible energy and metabolic energy (P<0.05); When heifers aged180days, the retained nitrogen/nitrogen intake and retained nitrogen/digestible nitrogen increased first and decreased later with the increasing amount of dietary net energy level (P<0.10), and the value of group C reached the peak.Experiment3aimed to examine the effects of dietary energy level on rumen fermentation and rumen microbial of4-to6-month-old heifers. The experiment design was the same as experiment1. Prior to morning feeding of the second days after monthly weighing, rumen fluid was collected to measure the fermentation parameters, the variation of microbial community and quantity in181-day-old heifers, and the changes with the days of age. Four heifers were randomly selected in each group. Heifers fed with increasing net energy level diet had a significantly increased ruminal propionate ratio (P<0.05), reduced proportion of acetic acid and isobutyric acid (P<0.05), and a significantly reduced ratio of acetate to propionate (P<0.01), but there was no significant effect on the concentration of total volatile fatty acid (P>0.05). Therefore, dietary net energy level can significantly affect the rumen acetate, propionate, isobutyrate and the ratio of acetate to propionate. Through denaturing gradient gelelectrophoresis and real time polymerase chain reaction analysis, in the same age, and the four paralle samples, rumen microflora with high similarity, and rumen microbial diversity and quantity tends to steady state.In conclusion, the dietary net energy at7.53MJ/kg (C) can maintain the average daily gains of heifers (0.78kg/d) with no adverse effects on body dimensions and mammary development and have the overall nutrition balance of heifers and digestion and metabolism levels.