Abstract: This study was conducted to investigate the effects of coated methionine (CM) and coated folic acid (CFA) supplementation and their interactions on growth performance, nutrients apparent digestibility, energy and nitrogen metabolism, and synthesis of rumen microbial protein in finishing Doper×Thin-tailed Han F₂ crossbred male lambs. Forty Doper×Thin-tailed Han F₂ crossbred male lambs with similar body weight ((30.08 ±1.34) kg) and good condition were arranged into 4 groups in a randomized design with a 2×2 factorial arrangement. One factor was supplemented CFA (0 mg,CFA-or 4 mg·(kg·d)^-1,CFA+) in the diet according to the body weight of the experimental animals,and the other factor was supplemented CM (0 g,CM-or 0.1 g·(kg·d)^-1,CM+). The feeding experiment lasted for 85 days with 15 d of adaptation and followed by 70 d of trial period. A digestibility trial was performed 10 days before the end of the trial period. The results showed that:1) Average daily weight gain and feed conversion ratio were significantly improved during 1-30 d with CFA supplementation in the ration of finishing lambs(P<0.05); There was a significant effect of CM supplementation on increasing feed conversion ratio (P=0.036), but no significant effect on average daily weight gain (P>0.05) during 1-30 d;Average daily weight gain and feed conversion ratio were significantly increased during 31-60 d and 1-60 d with CFA or CM supplementation in the ration for finishing Doper×Thin-tailed Han F₂ crossbred male lambs (P<0.05). The significant interaction between CFA and CM supplementation was found in feed conversion ratio during 1-60 d (P=0.007). 2) The apparent digestibilities of dry matter, organic matter, neutral detergent fiber and acid detergent fiber were significantly increased by adding CFA in the ration (P<0.05), the apparent digestibility of organic matter was tended to increase with CM added in the ration (P=0.066), and the apparent digestibility of dry matter was tended to increased (P=0.095). There was no significant interaction effect on the apparent digestibility of the nutrients (P>0.05). 3) The addition of CFA or CM significantly decreased urinary energy loss, enhanced digestibility and metabolizability of energy (P<0.05). The apparent digestibility of nitrogen and the deposited nitrogen were significantly increased (P<0.05) by significantly reducing fecal nitrogen, urinary nitrogen and total excretion nitrogen (P<0.05). A significant interaction effect between CFA and CM addition was found in reduction of nitrogen excretion and elevation of nitrogen digestibility (P<0.05). 4) The addition of CFA or CM significantly increased the urinary excretion of purine derivatives and the synthesis of rumen microbial protein (P<0.05). The results showed that the addition of CM or CFA improved the digestibility of dry matter and organic matter, enhanced digestibility and metabolizable ratio of total energy, increased apparent digestibility of nitrogen and nitrogen deposition, thus promoted the growth of Doper×Thin-tailed Han F₂ crossbred male lambs.

Key words: coated methionine, coated folic acid, nutrient digestibility, growth performance, Doper×Thin-tailed Han F₂ crossbred male lambs