Abstract: 45 Holstein lactaing cows with similar lactaion period, calving number and milk yield were randomly divided into 3 groups(n=15), in order to research the effects of diet adding potassium chloride on production performance and blood indexes of lactating cows during July to September. The diets of three experimental groups were basal diet(comprising corn silage, green hay and concentrates) (control group), basal diet＋50 g potassium chloride (groupⅠ) and basal diet＋100 g potassium chloride (groupⅡ) respectively. The results showed that there were no significant differences between groups in content of free fatty acids(NEFA), glucose(GLU) and urea nitrogen(BUN) in serum (P>0.05). β-hydroxybutyrate (BHBA) content was significantly higher in control group than in group Ⅰand Ⅱ(P<0.05),whereas no significant difference was found between group Ⅰand Ⅱ. Compared with the control group, decreasing degree of daily milk yield was less 1.6 kg (P<0.05) and 0.7 kg (P<0.05) for group Ⅰand Ⅱrespectively. There were no significant difference between groups in milk composition (P>0.05).The results indicated that potassium chloride supplementation can relieve the decreasing of milk production in summer.

Key words: lactating cows; potassium chloride; heat-street; serum indicators; milk production