12Background: Fat is the main substance that provides energy to animals. However, 13 the use of fat in twin Hu lambs has not been investigated. Thirty pairs of male twin 14 lambs were examined to investigate the effects of dietary fat on the metabolism of 15 energy and nitrogen, ruminal fermentation, and microbial communities. The twins are 16 randomly allotted to two groups (high fat: HF, normal fat: NF). Two diets of equal 17 protein and different fat levels. The metabolism test was made at 50-60 days of age. 18 Nine pairs of twin lambs are slaughtered randomly, and the rumen fluid is collected at 19 60 days of age. 20Results: The initial body weight (BW) in the HF group did not differ from that of NF 21 group (P > 0.05), but the final BW was tended to higher than that of NF group (0.05 < 22 P < 0.1). The digestive energy (DE), metabolism energy (ME), DE/ME in the HF group 2 23 tend to be higher than those in the NF group (0.05 < P < 0.1). Ruminal ammonia 24 nitrogen (NH 3 -N) and the proportion of total volatile fatty acids (TVFA) are higher than 25 that in the NF group (P < 0.05). A high throughput sequencing analysis reveals that 26 there were no differences between the two groups in terms of the richness estimates and 27 diversity indices (P > 0.05). The Proteobacteria and Fibrobacteres phyla were higher 28 than that in NF group (P<0.05). 29 Conclusions: This study demonstrated that high fat diet before weaning can affect the 30 abundance of several groups of rumen bacteria in rumen, such as significantly 31 increasing phyla Proteobacteria and Fibrobacteres, and genera of Succinivibrio, 32 Alloprevotella, and Saccharofermentans, but significantly decreasing genera of 33 Clostridium IV, Dialister, Roseburia, and Butyrivibrio. And high fat diet improved the 34 performance of lambs at weight gain, energy utilization, and had effect on VFA 35 composition but no effects on serum enzymes and serum hormone.36 37 42 plane of nutrition pre-weaning have higher starter intakes and average daily gain during 43 the weaning period [3]. 44Fat is an essential nutrient for young animals. It is the main supplier and storage 3 45 of energy in animals. Fat has been used to feed cattle and sheep for a long time [4][5][6]. 46 Raeth-Knight [7] suggested that high fat diets can lead to rapid growth, which allows 47 heifers to reach breeding size earlier with lower production costs. Other researchers 48 suggested that fat may be the result of the dietary fat, fat type, additive amounts and 49 interactions. 50The influence of dietary fat vary among studies, which could be associated with 51 species of animals, type and concentrations of fat and dietary composition [8,9]. 52 However, little information can be found on the effects of dietary fat on twin Hu lambs, 53 a local breed in China and the ewes lamb twice a year with 2-3 lambs in most cases per 54 year. With the similar genetic background in twin lambs, twin Hu lambs is an ideal 55 model to use in the present study. 56 Dietary fat shapes rumen microbiota, most of the ...