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 seas on, energy balance, temperature humidity index (THI), and postpartum o vulation 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 week ly. plasma progesterone, plasma metabolites related to energy and mine ral balance, and haematological measurements were performed. Milk prod uction was measured, body condition score was estimated, and trans-rec tal ultrasound examinations of the ovaries were each undertaken once a week. The interval between calving and first ovulation was significan tly longer in cows calving in summer (22.8 vs. 17.6 days, P < 0.05). I nterval from calving to the first postpartum ovulation (FOVL) was inve rsely related to the mean plasma glucose concentration for the first 9 weeks after carving (GLU): FOVL=80.0-17.9GLU, (R-2=0.25, P < 0.001). Plasma progesterone concentration during the lift of the second corpus luteum after carving was negatively correlated with THI during the fi rst 2 weeks after calving (r=0.55, P < 0.001). Plasma glucose concentr ation (GLU) was negatively correlated with milk yield (MYD) and octal temperature (RT), and positively correlated with plasma calcium concen tration (Ca) according to the following regression equation. GLU=33.1- 0.02MYD+0.91Ca-0.48RT, (R-2=0.58, P=0.0001). (C) 1997 Elsevier Science B.V.