Effect Of Dietary Cation-anion Balance On The Performance Of Heat-stressed Dairy Cows

The objective of this study was to determine the effect of dietary cation-anion balance (DCAB) on milking performance, physiological status and blood biochemical indices of heat-stressed dairy cows, to investigate an appropriate DCAB value for dairy cows under heat-stressed condition. This experiment was conducted with single factor experiment in Meijiadun dairy farm of academy of agricultural science in Huanggang, Hubei province from July to September. Thirty multiparous Holstein cows in mid-lactation (with average body weight:550～600kg,2～3 parity, milk yield:19.61±2.27 kg/d) were allocated to 6 treatment groups (5 in each group) differed with DCAB value, which were:group A:501.69, group B:401.58, group C:309.67, group D:211.56, group E:120.99 and group F:31.32 meq/kg of dry matter.The experiment lasted for 42 days, day 1 to 14 was preliminary period and day 15 to 42 was experimental period. Dairy cows were fed and milked twice daily and drank with freedom.Daily average temperature humidity index (THI) ranged from 78.16 to 82.96 during experimental period with average THI was 79.78. Respiration rate ranged from54-64 times per minute during the experimental period. Respiration rate was affected positively when DCAB increased from 31.32 meq/kg DM to 501.69 meq/kg DM, and the lowest respiration rate was in group C and the highest was in group A. Rectal temperature ranged from 38℃～40℃ with no influenced by DCAB during ·’ experimental period (P> 0.05).There were significant differences on dry matter intake (P<0.05), milk yield (P<0.05), protein content (P<0.05) and lactose content (P=0.05) when DCAB increased from 31.32 meq/kg DM to 501.69 meq/kg DM. The milk fat content (P> 0.05) and FCM yield (P> 0.05) of heat stressed Holstein dairy cows were not affected by DCAB. DCAB had a positive effect on serum cortisol(P< 0.05). Serum cortisol content increased after decreased with the decreasing DCAB level and the lowest cortisol content was in group D. Serum cortisol content decreased 16.31% and 15.68% respectively, for the first and second sampling time. DCAB had a significant effect on serum lipopolysaccharide content (P<0.05). Serum lipopolysaccharide content reached the lowest when DCAB was 211.56 meq/kg DM during the experimental period and the whole period. The lipopolysaccharide content in group D significantly lower than that in other treatment groups, and lipopolysaccharide content increased after decreased with the decreasing DCAB level.In conclusion, the dry matter, milk yield, milk protein and lactose content of Holstein cows in mid-lactation could be improved significantly, serum cortisol and LPS could be decreased significantly and the loss of heat stress could be alleviated when DCAB increased from 31.32 meq/kg DM to 501.69 meq/kg DM.