Study On Dietary Optimum Calcium And Phosphorus Level In Later Lactation Cows Under Heat Stress

The objective of this article was to study the effects of dietary calcium and phosphorus level on production performance, blood biochemical parameters and hormone concentration of late lactating dairy cows under heat stress, to investigate an optimum dietary calcium and phosphorus level for dairy cows under this condition, and provide scientific basis for cows’fine breeding under heat stress condition.In experiment 1, twenty lactating Chinese Holstein cows were randomly allocated respectively into 4 treatments with 5 duplicates each. The dietary calcium contents of 4 treatments were 0.62%,0.72%,0.85% and0.97%(DM basis), respectively. The trial lasted 30 days with 10-d adaption period and 20-d experimental period. Samples were collected in the last 3 days. The results showed as follows:1) Dry matter intake of 0.85% group was significantly higher than other three treatments (P<0.05), and there were no significant differences among them (P>0.05).2) The milk yield of 0.62%,0.72% and 0.85% treatments were significantly higher than 0.97% treatment (P<0.05), and the milk yield of 0.85% treatment was 24% higher than 0.62% treatment (P<0.05).3) Dietary calcium level showed no effects on the milk composition and feed efficiency (P>0.05).4) With dietary calcium level increasing, apparent calcium digestibility was significantly decreased(P<0.01).5) The contents of hydroxyproline in the serum of 0.62% treatment was the highest(P<0.05), and there were no significant difference among other three treatments (P>0.05); with dietary calcium level increasing, serum ALP activity was increased, and 0.96% treatment was significantly higher than the other three treatments (P<0.01). This indicated high dietary calcium level may promote the deposition of bone calcium in cows, but serum hydroxyproline content and alkaline phosphatase activity in each treatment group cows are still within the normal physiological range, indicating that cow’s bone metabolism in each group were in a normal state.6) With dietary calcium level increasing, serum PTH concentrations was decreased, but serum BGP and CT were increased; compared with 0.62% treatment, the serum PTH concentration of 0.72%,0.85% and 0.97% treatments was decreased 18%(P<0.01),36%(P<0.01) and 40%(P<0.01).respectively, and the serum BGP concentration was increased 0.36ug/L (P<0.01),0.85 ug/L (P<0.01) and 0.95 ug/L (P<0.01) respectively; serum CT concentration of 0.97% group increased 39%(P<0.01),23%(P<0.01) and 3% (P>0.05) compared with 0.62%,0.72% and 0.85% respectively. The change of serum hormone content in each group reflected:with dietary calcium level’s increasing, the bone calcium mobilization reduced, the calcium deposition enhanced and bone turnover accelerate. The bone turnover rate in 0.85% group was significantly higher than the 0.62%(P<0.05) and 0.72% group (P<0.05), but there was no difference significant with the 0.97% group (P>0.05). It is concluded that the optimum dietary calcium level for late lactating dairy cows under heat stress is 0.85% (DM basis)In experiment 2, fifteen lactating Chinese Holstein cows were randomly allocated respectively into 3 treatments with 5 duplicates each. The dietary phosphorus contents of 3 treatments were 0.29%,0.35% and 0.42% (DM basis), respectively. The trial lasted 30 days with 10-d adaption period and 20-d experimental period. Samples were collected in the last 3 days. The results showed as follows:1) There were no significant differences on dry matter intake (P>0.05).2) The milk yield of 0.35% and 0.42% treatments were significantly higher than 0.29% treatment (P<0.05), and there were no significant differences between the 0.35% and 0.42% treatment (P>0.05).3) Dietary phosphorus level showed no effects on the milk composition and feed efficiency (P>0.05).4) With dietary phosphorus level increasing, apparent phosphorus digestibility was significantly decreased (P<0.01).5) With dietary phosphorus level increasing, phosphorus in serum was significantly increased (P<0.01); the contents of hydroxyproline in the serum of 0.29% treatment was the highest (P<0.05), and there were no significant difference between the other two treatments (P>0.05); serum ALP activity in 0.29% treatment was significantly higher than the other treatments (P<0.05).This showed that 0.29% of dietary phosphorus level can’t still fully meet phosphorus requirement of late lactation cows under heat stress, resulting in bone degradation of bone phosphorus released into the bloodstream.6) With dietary phosphorus level increasing, serum PTH and BGP concentrations were increased, but serum CT was increased; compared with 0.29% treatment, the serum PTH concentration of 0.35% and 0.42% treatments was increased 5.00%(P<0.01) and 5.69%(P<0.01) respectively, and the serum BGP concentration was increased 0.11 ug/L (P>0.05) and 0.16 ug/L (P<0.01) respectively; serum CT concentration of 0.42% group decreased 3.86%(P<0.01) and 1.21% (P<0.01) compared with 0.29%and 0.35% respectively. The change of serum hormone content in each group reflected:under heat stress, the 0.29% dietary phosphorus level could not still fully meet the phosphorus requirement of cows in late lactation, but 0.35% dietary phosphorus level was completely satisfy. It is concluded that the optimum dietary phosphorus level for late lactating dairy cows under heat stress is 0.35%(DM basis).