Effects of dietary protein levels on growth, nutrient apparent digestibility, nitrogen emission and serum biochemical indices of small-tailed lambs of Han sheep

This experiment was conducted to investigate the effects of different dietary protein levels on growth, apparent digestibility, nitrogen emission and serum biochemical indices of male lambs of small-tailed Han sheep to a provide theoretical basis for low protein scientific feeding of small-tailed Han lambs. Thirty-six 3-month-old male lambs with similar body weight (18.05 ± 1.36) kg and good health were randomly divided into three groups with 12 lambs in each group. Group Ⅰ was fed a diet with a low level of protein (12.00%), group II was fed a diet with a medium level of protein (13.40%), group Ⅲ was fed a diet with a high level of protein (14.80%), and the dietary metabolizable energy level of each experimental group was the same (10.50 MJ·kg⁻¹). The pre-trial period lasted for 14 days, and the trial period lasted for 90 days. The results demonstrated that: 1) dietary protein intake had significant effects on organic matter (OMI) and crude protein intake (CPI), average daily gain (ADG) and feed to gain (F/G) of fattening lambs (P < 0.05). The OMI of fattening lambs in group Ⅰ was significantly higher than that in group Ⅲ (P < 0.05). CPI, ADG and F/G of fattening lambs in group Ⅲ were significantly higher than those in groups Ⅰ and Ⅱ (P < 0.05). 2) dietary protein intake had significant effects on the apparent digestibility of dry matter (DM), organic matter (OM), crude protein (CP), neutral detergent fiber (NDF) and acid detergent fiber (ADF) of fattening lambs (P < 0.05). Apparent digestibility of DM, OM and ADF of fattening lambs in groups Ⅱ and Ⅲ were significantly higher than those in group Ⅰ (P < 0.05). The apparent digestibility of CP and NDF in groups Ⅱ and Ⅲ was significantly higher than that in group Ⅰ (P < 0.05). 3) dietary protein intake had significant effects on nitrogen intake, excretion of fecal nitrogen, urinary nitrogen and total nitrogen, and nitrogen retention of fattening lambs (P < 0.05). Nitrogen intake, urinary nitrogen excretion and total nitrogen excretion of faltering lambs in group Ⅲ were significantly higher than those in groups Ⅰ and Ⅱ (P < 0.05). Fecal nitrogen excretion in group Ⅲ was significantly higher than that in group Ⅰ (P < 0.05). Nitrogen retention in groups Ⅱ and Ⅲ was significantly higher than that in group Ⅰ (P < 0.05). 4) dietary protein intake had significant effects on the contents of urinary nitrogen, glucose, alanine aminotransferase, total bilirubin, creatinine, low density lipoprotein and non-esterified fatty acid in serum of fattening lambs (P < 0.05). The content of blood and urine nitrogen in group Ⅲ was significantly higher than that in groups Ⅰ and Ⅱ (P < 0.05). The glucose content in group Ⅲ was significantly higher than that in group Ⅰ (P < 0.05). The contents of alanine aminotransferase and total bilirubin in fattening lambs in group Ⅰ was significantly higher than those in group Ⅲ (P < 0.05). The contents of creatinine and low density lipoprotein in groups Ⅰ and Ⅱ were significantly higher than those in group Ⅲ (P < 0.05). The content of non-esterified fatty acids in group Ⅰ was significantly higher than that in group Ⅱ (P < 0.05), and in group Ⅱ was significantly higher than that in group Ⅲ (P < 0.05). In conclusion, the experimental results demonstrated that reducing the protein content of feed can improve the utilization rate of protein and reduce nitrogen emissions. The optimal dietary protein content of feed for fattening male lambs of 3-month-old small-tailed Han lambs is 12.00%.