Thoroughbred Stallions (Natural Cover)

As output of normal sperm in ejaculates declines and pregnancy rates decrease, one approach that may benefit pregnancy rates in an aging Thoroughbred stallion is to diminish the mating frequency (i.e., reduce the number of covers allowed per day and thus the size of the mare book). Evaluating breeding records to compare pregnancy rates among days with differing numbers of covers may provide guidelines for how much to limit mating frequency.

Another management strategy sometimes employed with natural service mating programs involving stallions with lowered fertility is the routine use of reinforcement breeding. Reinforcement breeding is a process wherein the dismount semen sample (or aspiration of semen pooled in the vagina, or both) is collected following natural cover and is infused into the uterus of the mare just covered. The dismount semen sample should be filtered to remove as much debris as possible and immediately mixed with a suitable volume (5 to 20 mL) of prewarmed extender before infusion. Alternatively, extender can be mixed with the dismount sample (or semen sample aspirated from the vagina) before filtering and infusion into the uterus. The process results in an increased number of sperm being placed directly in the uterus, and the added extender may provide some protection to sperm and improve the longevity of motility until they access the oviduct. Pregnancy rates can be compared between “reinforced” and “nonreinforced” mares, and if fertility seems to be improved, reinforcement breeding may be justified as a standard protocol for that stallion. In aged Thoroughbred stallions with declining testicular function and reduced pregnancy rates (i.e., pregnancy rates per cycle of 20% to 40%), reinforcement breeding often results in a 1.2-fold to twofold increase in pregnancy rates per cycle.

Older stallions with declining testicular function may also ejaculate a low number of normal motile sperm of short longevity (i.e., sperm motility rapidly declines in vitro). Managing each mare in the stallion’s book with two to three covers in the period 12 to 36 hours after the administration of an ovulation-inducing drug may improve pregnancy rates. This strategy is directed toward maximizing the number of live normal sperm that will have colonized the oviduct as near to the time of expected fertilization as possible. By necessity, the mare book number must be reduced because of the increase in mating frequency required for each mare bred.