Abstract: 【Objective】This study was conducted to investigate the effects of dietary guanidinoacetic acid (GAA) and betaine (BT) on growth performance, rumen fermentation and blood metabolites in lambs, so as to provide a support for applying GAA and BT on ruminants.【Methods】Forty-eight 3-month-old Dorper× Thin-tailed Han F₁ hybrid male lambs with similar body weight (22.03 ± 1.3 kg; mean ± SD) were randomly divided into 4 groups with a 2×2 factorial arrangement, and the experimental factors were GAA (0 or 900 mg/kg) and BT (0 or 5 g/day). Lambs in four groups were control group, GAA group, BT group and GAA+BT group, respectively. The feeding experiment lasted for 75 days with 15 days of adaptation. On the first day of the formal experiment period, the BW before morning feeding was weighed as the initial BW, and each lamb were recorded accurately the amount of feed and residual of weight every day; at the end of the trial period, blood samples were collected from the jugular vein, and final body weight (FBW) was weighed before morning feeding to calculate average daily gain (ADG), average daily feed intake (ADFI) and feed efficiency (FE). Blood serum was used for the determination of creatine metabolism indexes, homocysteine and lactate content. The rumen fluid was collected from all lambs at 3 hours after feeding at the end of the experiment, which were used to measured rumen fermentation parameters, digestive enzyme activity and functional microorganisms.【Result】The results showed as follows: (1) Compared to the control group, the addition of GAA in the diet did not significantly increase daily weight gain (ADG) (P>0.05), while the addition of BT increased ADG significantly (P<0.05). The addition of GAA or BT in the diet increased feed efficiency (FE) significantly (P<0.05), and GAA and BT had significant interaction on ADG and FE (P<0.05). (2) The rumen pH was decreased by addition of GAA or BT (P<0.05), and the concentration of propionic acid and total volatile fatty acid were increased by GAA or BT (P<0.05). There was no significant interaction between GAA and BT on rumen fermentation in lambs (P>0.05). (3) Addition of GAA or BT increased the populations of R.flavefaciens, F.succinogenes, B.fibrisolvens, P.ruminicola (P<0.05), and decreased the populations of total protozoa (P<0.05). Addition of BT decreased the populations of total methanogens and had a tendency to increase the populations of Rb.Amylophilus (P<0.05, P=0.098). GAA had a tendency to decrease the populations of total methanogens (P=0.088). Addition of GAA and BT had no significant interaction effect on ruminal main functional microflora in lambs (P>0.05). (4) The activity of protease was increased by addition of GAA and BT (P<0.05). Addition of BT increased carboxymethyl-cellulase, xylanase and α-amylase activities (P<0.05)，and had a tendency to increase cellobiase activity(P=0.075). GAA increased pectase activity (P<0.05), and had a tendency to increase α-amylase activitiy (P=0.056). The significant interaction effect between GAA and BT was observed on pectase (P<0.05). (5) Addition of GAA or BT increased the content of creatine in blood serum (P<0.05). Addition of GAA had a tendency to increase the concentration of homocysteine (Hcy) and to decrease L-arginine-glycine amidinotransferase (AGAT) (P=0.053, P=0.056). BT decreased the content of Hcy in blood serum, and had a tendency to increase creatine kinase (P=0.063). GAA and BT had no interactive significant effects on blood metabolites (P>0.05).【Conclusion】In conclusion, addition of GAA or BT promoted rumen fermentation, increased the populations of functional rumen microflora and microbial enzyme activity, elevated serum creatine levels, and decreased the content of homocysteine. However, the combination addition of GAA and BT did not further increase the performance of lambs when compared with the addition of GAA or BT alone.

Key words: guanidinoacetic acid, betaine, lambs, growth performance, rumen fermentation