Effects Of Rumen-protected Gamma-aminobutyric Acid On Heat Stressed Dairy Cows And The Underlying Mechanisms Of Alleviating Heat Stress

This experiment was conducted to investigate the effects of rumen-protected gamma-aminobutyric acid (GABA) on performance, rumen fermentation, nutrient digestibility and blood metabolisms in heat-stressed dairy cows. The disappearance rates of RPG coated by acrylic resin were under 10% and 24% respectively following 6 hours of incubation in artificial saliva and 12 hours of incubation in rumen. At a ruminal outflow rate of 0.06/h, effective ruminal degradability was calculated as 24.89% for GABA. The above dates demonstrated that this RPG product had good stability and digestibility.Sixty Holstein dairy cows (141±15 DIM,35.9± 4.3 kg of milk/d, and 2.0±1.1 parity) were randomly assigned to 1 of 4 treatments according to a completely randomized block design. Treatments consisted of0 (control),40,80, or 120 mg true GAB A/kg of DM. The trial lasted 10 weeks. The average temperature-humidity indices at 0700, 1400 and 2200 h were 78.4,80.2 and 78.7, respectively. Rectal temperatures decreased linearly at 0700,1400, and 2200 h with increasing GABA. Supplementation of GABAhad no effect on respiration rates at any time point. Dry matter intake, energy-corrected milk,4% fatcorrectedmilk, and milk fat yield tended to increase linearly with increasing GABA. Supplementation of GABA affected, in a quadratic manner, milk protein and lactose concentrations, and milk protein yield, and the peak values were reached at dose 40 mg/kg GABA. Milk urea nitrogen concentration increased linearly and quadratically. Total solid content increased linearly with increasing GABA. Supplementation of GABA had no significantly linear or non-linear influences on milk yield, lactose production, total solid, milk fat concentration, somatic cell scoreor feed efficieney.Cows supplemented with GABA had higher concentrations of ammonia nitrogen, total volatile fatty acid, acetate, propionate,and microbial crude protein, but ruminal pH, concentrations of butyrate and valerate, ratio of acetate to propionate, and enzyme activities of carboxymethylcellulase and xylanase were not differentamong the treatments. Concentrations of iso-butyrate and iso-valerate in ruminal fluid were increased by 80 or120 mg/kg GABA supplementation compared with control.Apparent total tract digestibilities of dry matter, organic matter, crude protein, neutral detergent fiber, and acid detergent fiber were similar among treatments. As the GABA increased, the immunoglobulin (Ig) Gcontentand the proportions of CD4+andCD8+T lymphocytes increased linearly(P< 0.05),and the IgA contentincreased linearly and quadratically (P< 0.05),whereas concentrations ofinterleukin (IL)-2 and IL-6 decreasedlinearly (P< 0.01), concentrations of IL-4and tumor necrosis factor-a (TNF-a) decreased linearly and quadratically (P< 0.05).The activities of superoxide dismutase (SOD), glutathione-peroxidase (GSH-PX) and total antioxidant capacity (T-AOC) increased linearly (P< 0.05), whereasmalondialdehyde (MDA) content decreased linearly and quadratically (P< 0.05) with increasing GABA.These results indicate that rumen-protected GABA supplementation to dairy cows can alleviate heat stress by reducing rectal temperature, increase dry matter intake and milk production, and improve milk composition.Moreover, rumen-protected GABA supplementation to heat-stressed dairycows can improve their immune functionand antioxidant activity.The appropriate supplemental GABA level for heat-stressed dairy cows is 40 mg/kg of DM.