Effect of Rumen-Protected Lysine Supplementation on Growth Performance, Blood Metabolites, Rumen Fermentation and Bacterial Community on Feedlot Yaks Offered Corn-Based Diets

Feedlots rely on corn-based total mixed rations (TMR) to finish yaks. However, corn is markedly deficient in lysine and, therefore, we hypothesized that feedlot yaks supplemented with rumen-protected lysine (RPLys) would improve performance. To test this hypothesis, twelve 2.5-year-old male yaks (122 ± 5.3 kg) were selected, and divided into a control (CON) and RPLys-supplemented (RPL) group. All yaks were provided with a pelleted diet that consisted of 25.0% corn stalk, 31.6% corn grain, and 24.0% corn by-products; while RPL yaks were supplemented with 37.0 g/d RPLys. Dry matter intake was not affected (<i>p</i> = 0.671) by RPLys supplementation, but the average daily gain was greater (<i>p</i> < 0.05; 1.46 vs. 1.25 kg/d) and the feed-to-gain ratio was lesser (<i>p</i> < 0.01; 3.39 vs. 3.90) in RPL than CON yaks. Serum urea nitrogen concentration and aspartate aminotransferase were greater (<i>p</i> < 0.05) in the CON than the RPL group. However, plasma lysine concentration was greater (<i>p</i> < 0.05), while threonine tended to be greater (<i>p</i> = 0.065) in RPL than CON yaks. Rumen ammonia-N concentration was lesser (<i>p</i> < 0.05) in RPL than CON yaks, but pH and volatile fatty acids concentration did not differ (<i>p</i> > 0.10) between groups. The relative abundances of the ruminal bacterial phyla of Firmicutes and Elusimicrobiota were greater (<i>p</i> < 0.05), whereas of the phylum Bacteroidota and genus <i>Butyrivibrio</i> were lesser (<i>p</i> < 0.05) in RPL than CON yaks. In general, the rumen microbiota was altered toward more abundant N utilization taxa in RPLys-supplemented yaks. RPLys-supplemented yaks had elevated plasma lysine and improved feed conversion ratio, providing the first evidence that bypass lysine improves the growth performance of yaks on corn-based diets in feedlots.