Post-partum reproductive performance of Zebu (Bos indicus) cows in the humid tropics.

The productivity of cattle largely depends on their reproductive efficiency. The interval from parturition to resumption of ovarian activity in Zebu cattle, raised under tropical conditions, is usually more than 100 days. The duration of this period is affected by factors such as nutrition, suckling, management and environment; but the effects of each factor has not been clearly established. The general aims of the present studies were to obtain information on factors influencing the resumption of ovarian activity in Zebu cows, during the postpartum period, raised in a humid tropical climate. In the first study we investigated how resumption of ovarian activity was related to body condition/body weight and changes in blood chemistry profiles. The animals that resumed cyclicity were in significantly better body condition and had a significantly higher mean body weight than anestrous cows. Albumin was significantly higher and urea significantly lower in the cows resuming ovarian activity, and calcium levels decreased significantly during the three weeks immediately preceding the first rise of progesterone in the first normal estrous cycle. The results suggest that cows that maintain their body condition from calving through the early postpartum period will have shorter intervals to first postpartum estrus compared to cows which loose body weight during the postpartum period. A second study was carried out to determine the effect of body condition and restricted suckling on the interval from parturition to resumption of ovarian activity. There were significant differences in body weight and body condition between cyclic and non-cyclic cows. In addition, there was a significant difference in the interval from parturition to resumption of ovarian activity between animals continuously nursing their calves and cows subjected to two periods of restricted suckling. A transient increase in progesterone concentration was observed in half of the animals which resumed cyclicity. The results show that the response to restricted suckling in postpartum Zebu cows can be affected by body condition. The third study was carried out to investigate the effects of stress factors on progesterone and cortisol patterns in bilaterally ovariectomized Zebu cows. The study showed that the stress of blood sampling can produce an increase in cortisol levels. ACTH administration resulted in significant increases in cortisol and progesterone secretion. The increase in progesteron after ACTH injection suggests that stress is a factor that must be considered when reproductive performance of the Zebu cow is studied. The adrenal glands may be an extra-ovarian source of progesterone, which could confound the interpretation of results of studies of ovarian function in Bos indicus cattle. The forth study investigated the effect of progesterone (Syncro-Mate B) treatment, with or without suckling manipulation, and temporary weaning on ovarian activity in anestrous Zebu cows. Progestogen treatment was capable of enhancing the percentage of postpartum cows displaying estrus whether or not the calf was separated for 48 h or not, but was unable to promote the establishment of cyclicity. Non-treated animals started to cycle concomitantly with the treated cows, underlining a possible carry-over effect from treated to untreated.