Abstract:
The investigations for the effects of misty-fan cooling and supplemental of rbST on physiological responses in relation to digestive process of crossbred 87.5% Holstein gene were performed. Eighteen primiparous cows were used for four experiments. Cows in each experiment were divided into two groups and assigned under the normal shaded barn (NS) as non-cooled cows and shaded barn with misty-fan cooling (MF) as cooled cows. The NS barn was separated from MF barn by longed metal sheet wall from floor to roof. Each cow was injected subcutaneously with 500 mg of rbST in every 14 days for 3 consecutive doses in each stage of lactation. Cows were fed the same total mix ration ad libitum and water was freely offered. The experimental results demonstrated that an application of MF cooling could reduce ambient temperature (AT) and temperature humidity index (THI). A low respiratory rate (RR) and rectal temperature (RT) were occurred in cooled cows. The marked effects of MF cooling could reduce the negative effect of high temperatures on digestive function via an increase in the digesta passage rate resulting in an increase in feed intake. An increase in dry matter intake (DMI) in response to both cooling system and rbST supplementation would be partly attributed to an increase in rumen fermentation with increases in VFA, NH3N and microbial protein. It was also found that an increase in water intake accompanying with an increase in DMI was apparent in rbST-supplemented cows under misty-fan cooling. An increase in gut water and liquid outflow rate from the rumen were apparent in rbST supplemented cows. The effect of MF cooling influenced to an increase in net water transfer through the ruminal wall. The rbST-supplemented cows under MF barn also showed a high level of water absorption through ruminal wall. These changes would be in part accounted for an increase in total body water (TBW). The low level of plasma leptin concentration accompanying with an increase in DMI were observed in rbST-supplemented cows under MF barn. The present results indicate that the rbST exerts its galactopoietic action, in part, through changes in body fluids associated with increased in gut water regulation and rumen function, which would be the consequence in distribution of nutrients to the mammary gland and for thermoregulatory mechanisms. The effect of exogenous rbST on the regulation of feed intake would play a role via a reduction of leptin secretion.
