A lot more mares get pregnant on the first cover these days than in the past, due to all of the razzle-dazzle medical stuff that allows the vet to pinpoint when she ovulates. Back then, they had to rely solely on the mare's reaction to the teaser, and some mares just plain wouldn't 'show,' or would show too late or too early, leading to repeat covers.
This is not a new phenonmenon, by the way. Hambletonian, the mostly TB founder of the Standardbred, covered huge numbers of mares in his day, starting at age 2 (only 4 mares that year). At 4 he covered 101 mares, one of 7 years he did triple digit covers BUT his conception rate was only 69%: that wouldn't cut it with Coolmore (ask George Washington). He sired over 1300 foals in 24 years at stud, an average of 55 a year. Hambletonian was ridden frequently by his groom, which helped keep him in shape for his working season. Too many stallions are allowed to get fat and flabby on the off season, so the breeding season does become a real physical strain.
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