Effect of oestradiol benzoate given after prostaglandin at two stages of follicle wave development on oestrus synchronisation, the LH surge and ovulation in heifers
Introduction
The interval to oestrus following prostaglandin-induced luteolysis during the luteal phase of the bovine oestrous cycle is variable (Macmillan and Henderson, 1984) and is dependent on follicle wave status at the time of treatment (Kastelic et al., 1990b, Savio et al., 1990, Adams, 1994). This variation in interval to oestrus is an important factor limiting the uptake of advanced reproductive technologies in cattle (Adams, 1994). It is also the main factor responsible for the reduced pregnancy rates reported following the use of fixed-time inseminations after both prostaglandin-induced luteolysis and withdrawal of progestagen devices (Hafs and Manns, 1975, Macmillan and Peterson, 1993). Oestradiol, progesterone and GnRH all have possible uses for the exogenous regulation of follicle waves and oestrus and ovulation (Roche et al., 1998).
A number of authors have reported the successful use of oestradiol benzoate (ODB) during the pro-oestrous period subsequent to progestagen (Hansel et al., 1975, Hanlon et al., 1997) and prostaglandin treatment (Nancarrow and Radford, 1975, Ryan et al., 1995) to reduce the interval to onset of oestrus (Nancarrow and Radford, 1975, Peters et al., 1977) and ovulation (Ryan et al., 1995). It has also been suggested that the use of ODB during pro-oestrus, subsequent to a progestagen-based synchronization regimen can facilitate the use of fixed-time artificial insemination without decreasing pregnancy rates (Hanlon et al., 1996). Previous reports indicate that treatment 24 h after induced luteolysis may be the optimum timing (Nancarrow and Radford, 1975, Ryan et al., 1995) and that the injection of 0.5 mg ODB (Hansel et al., 1975, O’Rourke et al., 2000) will induce a peak concentration of serum oestradiol similar in magnitude to that occurring at a natural oestrus (Glencross and Pope, 1981).
Follicle wave status at initiation of treatment affects the response to methods currently used to synchronize oestrous cycles in cattle (Roche et al., 1998). This study was designed, to evaluate the effect of treatment with ODB at follicle wave emergence or the first day of dominance of the second follicular wave on the synchronization of oestrus and ovulation in heifers.
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Animals and treatments
Thirty cycling Charolais crossbred or Simmental crossbred heifers, 23–26 months of age, weighing 553±5.8 kg at the start of the experiment were housed on slatted flooring with ad libitum access to grass silage and water and were supplemented with 2 kg per head per day of a 16% crude protein concentrate. To presynchronize oestrus, each heifer received a controlled internal drug releasing (containing 1.9 g of progesterone, CIDR™, InterAg, Hamilton, New Zealand) device with an attached gelatin
Animals excluded from analyses
In three heifers (one emergence-control and two emergence-ODB) the dominant follicle from the first wave resumed growth after prostaglandin treatment and became the ovulatory follicle, despite the detection of a new wave of follicles. In one heifer from both the emergence- and dominance-ODB groups luteolysis failed to occur after prostaglandin treatment based on ultrasound examinations. However, the emergence-ODB animal was observed in standing oestrus between 44 and 56 h after prostaglandin
Discussion
The administration of 0.5 mg ODB 24 h after prostaglandin induced a predictable onset of oestrus, LH surge and ovulation, regardless of whether the treatment was given at follicle wave emergence or at dominance. Oestrus was detected close to the time of the pre-ovulatory LH surge in all animals, but it was variable with times ranging from 4 h before to 12 h after. Similar wide ranges have been reported previously (Cavalieri et al., 1997). Our results confirm that the administration of 0.5 mg of ODB
Acknowledgements
We thank N. Hynes and G. Claffey for assistance with hormone assays, T. Harte for overall care and provision of the animals, A.F. Parlow (NIDDKs National Hormone and Pituitary Programme) and J. Roser for the provision of reagents for the LH assay.
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Present Address: Department of Veterinary Preclinical Studies, University of Glasgow Veterinary School, Bearsden Road, Glasgow G61 1QH, UK.