Effects of Different Levels of Protein Supplements in the Diet of Early-Weaned Yaks on Growth Performance, Intestinal Development, and Immune Response to Tuberculosis

This study was conducted to determine the effects of different levels of crude protein (CP) supplements to the diet of early-weaned yaks on their growth performance, intestinal development, and immune response. Forty 3-month-old weaned yaks were selected and assigned to four dietary groups (Control, 17, 19 and 21% CP). Dietary CP supplements had a significant effect on average daily gain (ADG), crypt depth (CD) (duodenum, jejunum and ileum), villous height (VH) (duodenum, jejunum and ileum) and CD/VH (jejunum and ileum). Average daily gain, CD (duodenum, jejunum and ileum) and VH (ileum) showed quadratic increases as the dietary CP increased, whereas CD/VH (jejunum and ileum) ratios showed quadratic decreases. Blood urea nitrogen (BUN), glucose (GLU), immunoglobulin G (IgG), IgM, interleukin-1 (IL-1), IL-2, tumor necrosis factor (TNF)-α, and interferon (IFN)-γ concentrations increased significantly, whereas albumin (ALB), alanine aminotransferase (ALT) and aspartate aminotransferase (AST) decreased significantly with dietary CP supplements. Dietary CP supplements significantly increased the concentrations of IL-6, TNF-α, IFN-γ and the nuclear factor of activated T cell transcription factor (NFAT) for gene expression. As the dietary CP supplements increased, IL-6, IFN-γ and NF-AT gene expression showed quadratic increases. These results showed that the appropriate dietary CP supplementation improved the growth performance and intestinal development of earlyweaned yaks and thus that the CP supplements were beneficial and enhanced the humoral immunity response of yaks.