Effects of Supplementation with Rumen-Protected Methionine on Milk Performance, Plasma Biochemical Indices and Amino Acid Concentration in Dairy Goats Subject to Heat Stress

This study examined the effects of four levels of rumen-protected methionine (RPM) on the milk performance of dairy goats subjected to severe heat stress (temperature humidity index >90). Seventy-five Guanzhong dairy goats (52.6 +/- 4.9 kg) with the same farrowing period and lactation day (120 +/- 10 d) were randomly divided into five groups of 15 head and fed a 13.55% crude protein diet supplemented with 0, 1.5, 3, 4.5 or 6 g/d RPM in the concentrate. The trial included nine days adaptation time and 30 days of sampling and analysis. The addition of RPM did not change feed intake but 1.5 g/d RPM significantly increased milk protein content, the ratio of milk to feed, and the economic returns. No significant changes in milk fat or urea-nitrogen concentrations or somatic cell count were observed. Plasma urea -nitrogen was significantly lowered in the 1.5 g/d RPM group. RPM supplementation increased plasma methionine concentration and total amino acid concentration in a dose-dependent manner. The highest dose of RPM (6 g/d) enhanced plasma immunoglobulin G concentration. It is demonstrated that supplementation with an appropriate dose of RPM to dairy goats fed a relatively low protein diet and subject to heat stress can increase milk protein production and improve economic returns.