Association among calving season and measures of energy status, resumption of ovulation and subclinical endometritis in early lactating dairy cows

A total of 65 multiparous cows that calved during the hot season (n = 21), temperate season (n = 23) or cold season (n = 21) were used to investigate the relationships between heat stress, seasonal changes, metabolic traits, body condition score (BCS), resumption of ovulation and occurrence of subclinical endometritis (SE). Cows were monitored for the first 7 weeks of lactation by ultrasonographic examination. Blood sampling and BCS evaluations were performed on a weekly basis. Plasma progesterone (P4) and blood metabolites related to energy status including -hydroxybutyrate, nonesterified fatty acids, total cholesterol, blood glucose and blood urea nitrogen were analyzed. Resumption of ovulation postpartum was confirmed by the first detection of a corpus luteum or a rise in P4 ≥1 ng/ml. Moreover, subclinical endometritis was diagnosed by brush cytology on day 40 ± 2 of lactation and cows having polymorphonuclear cell percentages (PMN%) ≥5 were considered SE positive. As expected, mean temperature humidity index differed (P < 0.001) among hot (75-85), temperate (50-65) and cold seasons (35-45). The BCS in cows that calved during the hot season was lower (P < 0.001) when compared with those that calved during the cold and temperate seasons. The percentage of cows that resumed ovulation in the first 45 days of lactation was greater (P < 0.05) for those that calved during the hot season (90%) than the temperate season (60%). The concentration of NEFA was greater (P < 0.05) for cows that calved during the cold season than the hot season during different weeks postpartum. The concentration of glucose in weeks 4 to 7 postpartum was greater (P < 0.01) for cows that calved during the temperate season when compared to cows that calved during the hot season. There was no difference in the prevalence of SE according to calving season. The percentage of PMN tended (P < 0.1) to be greater in cows with SE during the temperate season than during the hot season. In conclusion, calving season may influence BCS, blood metabolites and resumption of ovulation whereas no association with prevalence of subclinical endometritis was observed.