Effects of rumen-protected tryptophan on growth performance, fibre characteristics, nutrient utilization and plasma essential amino acids in Cashmere goats during the cashmere slow growth period

Tryptophan (Trp) is a limiting amino acid in growing ruminants and supplementation of Trp in a rumen-protected form may have positive effects on performance in ruminants. Trp may affect cashmere fibre growth by regulating plasma melatonin secretion. From December to February, identified as the cashmere slow growth period, cashmere growth rate slows down and plasma melatonin secretion declines. Thus, Trp supplementation may affect cashmere fibre growth positively during this period.This study was conducted to investigate the effects of rumen-protected tryptophan (RPT) on Cashmere goats during the cashmere slow growth period. Optimal RPT supplementation level was determined. The Thirty-six Liaoning Cashmere goat wethers (41.63 ± 0.37 kg) aged 1.5-year-old were stratified by weight and randomly assigned to the following treatments: without RPT (control), low RPT (LRPT), medium RPT (MRPT) and high RPT (HRPT) at RPT levels of 0, 2.0, 4.0 and 6.0 g per goat per day, respectively. LRPT and MRPT supplementation increased average daily gain (P < 0.05) and N retention (P < 0.05).The length and growth rate of cashmere fibre increased both linearly (P < 0.05) and quadratically (P < 0.05) with increasing RPT supplementation. Plasma Trp concentration increased linearly (P < 0.05) with increasing RPT supplementation. Conversely, RPT supplementation decreased plasma isoleucine, valine and total essential amino acid concentrations (P < 0.05). In conclusion, RPT supplementation potentially enhanced growth performance, cashmere fibre growth and N utilization in Liaoning Cashmere goat wethers. In experimental conditions of the current trial, the optimum RPT supplementation level was 2.0 g per goat per day during the cashmere slow growth period.