Common usage for Thoroughbreds
Sheerin notes that reinforcement breeding in Kentucky is more common than it was five years ago. Part of the reason can be
found in data from a study by Blanchard and colleagues that looked at reinforcement breeding, which was done while he was
resident veterinarian at Hill 'n' Dale Farm, Lexington, Ky.1 That study was presented at the American Association of Equine Practitioners' (AAEP's) 2006 conference.
Records from 2005 were analyzed of mating results for 2,171 estrous cycles of 1,406 mares bred to one of 13 stallions. An
attempt was made to reinforce-breed all of the mares returning for mating on second or later estrous cycles.
"Mares receiving reinforcement breeding had a greater chance of getting pregnant than mares not reinforced, which resulted
in an average increase of 12 percent per cycle pregnancy rate in eight of 13 stallions in the study," says Blanchard. "The
effect of reinforcement breeding was influenced by the number of sperm recovered, such that mares receiving greater than an
estimated 200 million total sperm had a higher per-cycle pregnancy rate." Study results demonstrated that considerable improvement
in the pregnancy rate could be achieved in some Thoroughbred stallions using the reinforcement-breeding technique.
"We did a multiple logistic regression analysis, adjusting for all the other factors that significantly affected pregnancy
rate," says Blanchard. "That's important for reasons other than just looking at numbers. Using that rigorous statistical model,
we found a significant increase in pregnancy rates in Thoroughbred stallions when reinforcement breeding was used. The dose
affected the outcome. If we had more than an estimated 200 million total sperm that we got from drippings from the penis and
put that in the mare, it was almost a 12 percent advantage; if we got less than that, it was about a 6 percent advantage.
"We've published a couple of other papers looking at factors that affect Thoroughbred stallion fertility, and we've always
found an advantage for some (not all) stallions," Blanchard says.2
In a typical example, one expects good first-cycle pregnancy rates, Blanchard says. "But maybe those mares that are coming
back for the second cycle need a little extra help. Perhaps the failure to get pregnant is a mare problem, or perhaps the
stallion-mare interaction failed to result in a good cover. When only second-cycle pregnancy rates were evaluated (reinforcement
vs. nonreinforcement breedings), reinforcement breeding improved pregnancy rates in 11 out of 13 stallions during 2005."
So, if there's some resistance to using it, Blanchard says, perhaps farm and stallion managers could see if pregnancy rates
in mares that are returning for second or later cycles are improved by reinforcement breeding. "In those instances in which
pretty dramatic improvement in pregnancy rates occurs, you might want to institute it as a standard practice for the rest
of the season for that stallion."
In a 2010 AAEP paper, Varner and colleagues wrote that, in the five stallions examined, three had demonstrable improvements
in per-cycle pregnancy rates when reinforcement breeding was used.3 "One of those stallions had a specific type of ailment—a particular type of breeding pattern—such that we thought it would
help," Varner says. "The horse came off mares prematurely upon ejaculation. In the other two stallions, we didn't have anything
we'd suspect would improve by using the technique. But reinforcement breeding did indeed improve those horses, even more so
than the horse in which we suspected it would show an improvement."