Abstract:

16 Chinese Holstein lactating dairy cows were selected randomly to study the influence of heat stress (from mild to medium) on physiological index and milk yield. The result show that: (1) The physiological index were no significant difference (P>0.05) between the mild heat stress period (THI 74.5±1.0) and no heat stress period (THI 65.2±4.1). The physiological index were significant difference (P<0.05) between the medium heat stress period (THI 84.0±2.4) and no heat stress period (THI 65.2±4.1). From mild to medium heat stress period, while THI rise 1, the respiratory rate of dairy cows increase 3.4n/min, rectal temperature increase 0.04℃;（2）During mild and medium heat stress period, the milk yield of dairy cow decrease 13.8% and 26.9% respectively (P<0.01), the relationship between milk yield and THI were negative (P<0.01) (R=-0.82), whileTHI rise 1, the milk yield of dairy cow decrease by 1.9%.