Relationships among calving season, heat load, energy balance and postpartum ovulation of dairy cows in a subtropical environment.

The study was designed to examine the relationships among calving season, energy balance, temperature humidity index (THI), and postpartum ovulation in high producing cows in a subtropical environment. Holstein cows calving in a feedlot dairy in southeast Queensland during winter (n = 23) and summer (n = 21) were monitored during the first 9 weeks of lactation. Cows were weighed and blood samples collected twice weekly: plasma progesterone, plasma metabolites related to energy and mineral balance, and haematological measurements were performed. Milk production was measured, body condition score was estimated, and trans-rectal ultrasound examinations of the ovaries were each undertaken once a week. The interval between calving and first ovulation was significantly longer in cows calving in summer (22.8 vs. 17.6 days, P < 0.05). Interval from calving to the first postpartum ovulation (FOVL) was inversely related to the mean plasma glucose concentration for the first 9 weeks after calving (GLU): FOVL = 80.0-17.9GLU, (R2 = 0.25, P < 0.001). Plasma progesterone concentration during the life of the second corpus luteum after calving was negatively correlated with THI during the first 2 weeks after calving (r = 0.55, P < 0.001). Plasma glucose concentration (GLU) was negatively correlated with milk yield (MYD) and rectal temperature (RT), and positively correlated with plasma calcium concentration (Ca) according to the following regression equation. GLU = 33.1 - 0.02MYD + 0.91Ca - 0.48RT, (R2 = 0.58, P = 0.0001).