Oxidative stress markers, ascorbic acid, and sulfhydryl (SH) residue concentration in primiparous cows' plasma and their relationship to milking performance during hot seasons were investigated. The rectal temperature of cows correlated negatively with SH residue (r = -0.38, n = 38, P < 0.05) and ascorbic acid (r = -0.34, P < 0.05) concentrations in the cows' plasma. The group with a higher concentration of ascorbic acid over the mean value produced significantly more milk (P < 0.05) than did the group with a lower ascorbic acid concentration. Although the cows' milk production showed a positive correlation with ascorbic acid concentration in plasma (r = 0.47, P < 0.05), the relation of SH residue concentration to milk yield was not constant. The plasma SH residue concentration during the hot season correlated positively with milk protein % (r = 0.38, P < 0.05), lactose % (r = 0.35, P < 0.05), and solid-non-fat (SNF) % (r = 0.47, P < 0.05), respectively, but not with fat %. On the other hand, ascorbic acid concentration in plasma showed negative correlations with milk fat % (r = -0.34, P < 0.05) and protein % (r = -0.49, P < 0.05), but correlated positively with lactose % (r = 0.52, P < 0.05). The produced amount (g/day) of milk protein (r = 0.42, P < 0.05), lactose (r = 0.61, P < 0.05), and SNF (r = 0.56, P < 0.05) showed positive correlations with ascorbic acid concentration in plasma.
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