Research articleThe reproductive performance of Thoroughbred mares treated with intravaginal progesterone at the start of the breeding season
Introduction
The mare is seasonally polyestrous with regular ovulatory cycles occurring in response to increasing day length [1]. The Thoroughbred breeding season officially commences on September 1 in the southern hemisphere; since there is a desire to breed mares as early as possible in the season, most non-pregnant mares are still in the transition phase from winter anestrus to normal ovulatory estrous cycles [2]. The transition period lasts between 60 and 80 d and is characterized by erratic estrous behavior and the growth and regression of ovarian follicles which fail to ovulate [1], [3].
Several methods have been utilized in an attempt to advance the onset of normal ovulatory estrous cycles in transitional mares. These methods include the use of artificial light exposure commencing several weeks before the start of the breeding season [3], [4], [5] and pharmacologic methods, such as: a) GnRH or its analogues; [6], [7], [8] b) oral progestagens; [9], [10] c) progesterone administered parenterally; [11] d) eFSH administration; [12], [13] and f) dopamine antagonists, such as domperidone or sulpiride [14], [15], [16]. The effectiveness of these methods varies considerably, and in addition, all involve at least once daily administration.
The intravaginal route offers a relatively cheap and convenient method of administering progesterone to mares that avoids the need for daily treatment. Intravaginal devices designed for cattle (CIDR, PRID) have been used off-label in mares and are effective in stimulating follicle growth in transitional mares [17], [18], [19], [20]. Generally, cattle devices have not gained widespread acceptance because clinically they are associated with discomfort and marked vaginitis [18], [21]. We recently reported that treatment of transitional mares with an intravaginal progesterone-containing device specifically designed for mares (Cue-Mare, Bioniche Animal Health Australasia, Victoria, Australia) for 10 d, resulted in follicle growth and ovulation within 4 d of device removal [22], and that the devices were associated with minimal discomfort and vaginitis [23]. Such a treatment therefore offers a potential method of managing the transition period on commercial stud farms.
The aim of this study was to investigate the effects of intravaginal progesterone on the reproductive performance of transitional Thoroughbred mares on commercial stud farms.
Section snippets
Animals
Two hundred twenty-seven (227) non-lactating Thoroughbred mares between 4 and 18 y of age (mean 9.4 ± 3.2 y) located on three stud farms were used. The study was performed during four consecutive breeding seasons (2007–10). All mares remained outside on pasture (consisting of perennial ryegrass) under natural light for the duration of the study and had ad libitum access to water and hay. The study location was the Waikato region of New Zealand (latitude 37°49′ S). Mares were enrolled for
Results
During the treatment period none of the intravaginal devices was expelled. Thirteen mares (10.3%) had the device removed on Day 7 when a follicle ≥ 35 mm in diameter was detected. The remaining mares (89.7%) had the device removed on Day 10. Reproductive outcomes for mare age, farm and year of the study are shown (Table 1).
Discussion
In both the Northern and Southern hemispheres, the Thoroughbred breeding season commences at a time when most non-pregnant mares are in the transition phase from winter anestrus to normal, regular estrus cycles. Early foals are desired because they are generally larger than their later-born peers by the time they are sold at yearling sales at approximately 14 to 17 mo of age [24]. In addition, earlier born foals are more physically advanced than their counterparts when they start racing as
Acknowledgments
Cue-Mare devices were kindly provided by Bioniche Animal Health Australasia Pty, Ltd. The participation of the mare owners and stud managers and their staff is gratefully acknowledged. Funding support for this project was provided by the New Zealand Racing Board through the equine Partnership for Excellence.
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