Effects of Dietary Phosphorus Level on Production Performance and Blood Indicators of Dairy Cows under Heat Stress

This experiment was conducted to investigate the effects of dietary phosphorus level on production performance and blood indicators of heat-stressed dairy cows. Fifteen Chinese Holstein cows with similar body weight,days in lactation,parity and milk yield were randomly assigned to 3 groups with 5 cows in each group.The dietary phosphorus level in 3 groups were 0.29%,0.35% and 0.42%( dry matter basis),respectively.The results showed as follows: 1) there was no significant difference on dry matter intake among groups( P0.05); milk yield in 0.35% and 0.42% groups were significantly higher than that in 0.29% group( P0.05).2) Milk fat percentage in 0.29% group was significant lower than that in the other groups( P0.05),however,there were no significant differences among groups on milk protein percentage,lactose percentage,milk total solid percentage and feed efficiency( P 0.05). 3) Serum phosphorus content was significantly increased with the elevated dietary phosphorus level( P0.01); the content of hydroxyproline and the activity of alkaline phosphatase in serum in 0.35% and 0.42% groups were significantly lower than those in 0.29% group( P0.05). 4) With the increasing of dietary phosphorus level,serum parathormone and bone gla protein contents were increased,and 0.29% group was significantly lower than 0.35 % and 0.42% groups( P0.01); while serum calcitonin content was decreased,and 0. 42% group was significantly lower than 0. 29% and 0. 35%groups( P0.01). It is concluded that the optimal dietary phosphorus level is 0.35% for dairy cows under heat stress considering growth performance and blood indicators.