Breeding the ‘difficult mare’

Mares can be classed as ‘difficult’ breeders if suboptimal pregnancy rates are experienced despite adequate reproductive management. This includes breeding with semen of acceptable quality, in an appropriate time frame with relation to ovulation, with any resulting fluid being treated appropriately. Reported per cycle pregnancy rates for natural mating are 63–65% (Allen et al, 2007), with pregnancy rates for chilled semen reported to be between 44–65% per cycle (Squires et al, 2006; Douglas-Hamilton et al, 1984). First cycle pregnancy rates for frozen semen of 51.3% have been published (Loomis, 2001), and around this mark is widely accepted internationally. If a mare fails to get in foal, or if her pregnancy rate per cycle is dramatically below these averages, she may be classed as a difficult breeder.

There are multiple reasons why a mare might be a difficult breeder, and more than one factor may exist in the same mare. For example, an aged mare with uterine degeneration may retain fluid, and this may be compounded by poor vulval conformation (Figure 1). It may be obvious what is causing a mare to be difficult to get in foal, or diagnostics may be required to ascertain the cause. Sometimes a presumptive diagnosis can be made retrospectively, after a mare is bred, with information gained from a post-breeding examination. Before a mare is classified as difficult, her breeding history should be thoroughly reviewed, both to ensure adequate management has been employed and to obtain an indication as to what the problems may be. As much information as possible should be sought, including the number of times she has been bred; the type and quality of semen used; timing of breeding with relation to ovulation; timing and type of ovulation agent used; as well as post-insemination monitoring and management.