Effects of low-protein diet supplemented with tryptophan and leucine on growth performance, digestibility, blood metabolites of Holstein calves

The objective of this study was to evaluate the effects of low-protein diet supplemented with tryptophan and leucine on growth performance and blood parameters of preweaning calves. Seventy-five Holstein male calves with similar body weight (42 ± 2.35 kg) were randomly assigned to 5 treatments. The five treatments were showed as follows: positive control (HP) group (22 % CP), negative control (LP) group (19 % CP), L group (19 % CP + 1.15 % Leu), T group (19 % CP + 0.26 % Trp) and LT group (19 % CP + 1.15 % Leu + 0.26 % Trp). The results showed that:1) The average daily gain (ADG) of calves in LP group was significantly lower than that in HP, L and LT group. And, there was no difference on ADG among HP, L and LT group. The F/G value in L group was significantly lower than othergroups, and there was no difference among other groups. 2) During the trial period from day 10 to day 34, fecal scores and diarrhea rates LP group was significantly higher than other groups.3) Serum uric acid (UA) and blood urea nitrogen (BUN) content were increased in calves fed low protein diets. Supplementation of tryptophan and leucine in low protein diets significantly reduced UA and BUN content and serum restored to normal concentration(P < 0.05). Serum total protein (TP) and albumin (ALB) contents of calves fed low protein diets were reduced. Supplementation of tryptophan and leucine in low protein diets significantly increased TP and ALB contents and restored to normal concentration(P < 0.05). 4) In blood hormone parameters, serum growth hormone (GH) and IGF-1 concentration were significantly reduced in calves fed a low-protein diet resulting in slow growth(P < 0.05). Supplementation of leucine and tryptophan in the low-protein diet restored normal concentration of these two hormones and promoted growth. 5) In serum antioxidation capacity and immunity function, serum super oxide dismutase (SOD), Catalas (CAT), interferon-γ(IFN-γ), interleukin-2 (IL-2) and immunoglobulin M (IgM) content were decreased in calves fed low protein diets, which easily led to decreased immunity and inflammation. Supplementation of tryptophan and leucine in low protein diet significantly increased serum SOD, CAT, IFN-γ, IL-2 and IgM contents (P < 0.05) and alleviated the risk of inflammation. 6) The concentration of histidine (His) in LP were significantly lower than those in HP (P < 0.05). Compared with LP, the contents of His, leucine (Leu), threonine (Thr), total essential amino acids and tryptophan (Trp) in L and LT increased significantly (P < 0.05). SerumTtrp content in T and LT were significantly increased compared with group LP (P < 0.05). In summary, compared with HP (22 % CP), LP (19 % CP) reduced the daily weight gain of calves and had a negative effect on the immunity and antioxidant capacity of calves. The supplement of leucine (1.15 %) and tryptophan (0.26 %) in low protein diet could increase the daily weight gain, improve immunity and antioxidant capacity, and reduce inflammatory factors. Our finding highlights the importance of tryptophan and leucine supplementation in low-protein diets to optimize calf health and growth.