Dietary supplementation of acetate-conjugated tryptophan alters feed intake, milk yield and composition, blood profile, physiological variables, and heat shock protein gene expression in heat-stressed dairy cows.

Abstract

The purpose of this study was to investigate the effects of dietary supplementation of rumen-protected tryptophan (RPT) at four levels on milk yield, milk composition, blood profile, physiological variables, and heat shock protein gene expression in dairy cows under conditions of moderate-severe heat stress (MSHS, THI=80~89). Sixteen early-lactating dairy cows (body weight=719±66.4kg, days in milk=74.3±7.1, milk yield=33.55±3.74kg, means±SEM) were randomly assigned in a factorial arrangement to one of the four treatments: control group (n=4, no RPT supplementation), 15g/d RPT (n=4), 30g/d RPT (n=4), or 60g/d RPT group per cow (n=4) supplemented to the TMR. A higher dry matter intake (DMI) and milk yield were found in the 30g RPT group compared with the other groups, and the 3.5% fat-corrected milk yield, energy-corrected milk yield, milk fat, protein, β-casein, mono-unsaturated fatty acid, and poly-unsaturated fatty acid contents, and serum glucose content were observed in the 30g RPT group (p<0.05). The milk lactose concentration was significantly higher in the 30g RPT group compared with the control and 60g RPT groups (p<0.05). The plasma cortisol level was lower, while the serotonin and melatonin concentrations were higher in the 30g group compared with the other groups (p<0.05). Heat shock protein (HSP) 70 expression was downregulated in the control and 15g RPT groups, whereas the expression of HSP90 and HSPB1 remained unchanged among the groups. In particular, the 30g RPT group was considered to have an improved DMI, milk yield, and lactose concentration, as well as anti-heat stress effects due to the simulation of serotonin and melatonin during MSHS.