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Originally Posted by Some Nut
Age and Parity of Broodmares at Parturition, and Racing Performance of Offspring
Several reports have been published on epidemiological studies carried out on this topic in the USA and U.K. In these countries, the age and parity of dams are regarded to be important factors because it is thought that the dam contributes 55 to 60 percent to the performance of her foal, while the sire donates 40 to 45 percent. Although it is difficult to fairly compare the results of these reports because they use different investigative methods and subjects, we would like to briefly present some of these studies in order to describe a general trend.
The results of one study show that the age of dams at the time of parturition of 1,420 racehorses that had won graded races in Western countries between 1992 and 1994 was 6 to 9 years old. Another study in the United States investigated the parity of the dams at the time of parturition of 680 racehorses that had won graded races. The results of this study showed that many of these horses were born as early parities. The ages of the dams at the time of parturition of racehorses that had won graded races in the U.K. were 7 to 11 years old. Another U.K. study investigated 100 broodmares and 1,196 of their offspring. For this study each broodmare had to fulfill the following criteria: she had to have produced eight or more live foals between 1947 and 1986, and at least two of these offspring must have obtained a Timeform rating of 110 or more. It was found that the best racehorses were born when the dam was 9 years old, and it was her fourth parturition.
Based on the above data, it is said that the age of 16 is the optimum retirement age for broodmares, with respect to the quality of their progeny. This is consistent with the suggestion by Ricketts, who based his analysis on changes in the uterus. All reports suggest that foals born as early parities and from younger mares may be more successful on the racetrack than those born as later parities from older mares.
In Japan, we investigated horses with respect to the age of broodmares at parturition and the racing performance of the offspring, although we did not set the selection criteria as restrictively as was set in the U.K. study. We used the Standard Starts Index (SSI) set by the Japan Bloodstock Breeder's Association as our racing performance index. SSI is a numerical expression in which the earnings of each racehorse per flat race (start) are compared with the average earnings of active racehorses of the same breed, gender and age. SSI allows us to compare the racing performance of racehorses over different generations. The results showed that the SSI of racehorses born when the dam was between the ages of 10 to 12 was high. The graphs on page 7 show the parity and age of dams at parturition for 72 racehorses that won six prestigious Japanese races between 1990 and 2001 (Japanese 2000 Guineas, Japanese Derby, Japanese St. Leger, Tenno Sho: Spring, Tenno Sho: Autumn and Grand Prix). Race performance tends to be better in foals produced at parities 1 to 5, and in racehorses born when the dam was 6 to 12 years old. As such, it is appears that the data from the studies conducted in Japan show a pattern similar to those conducted in Western countries. This suggests that the intrauterine environment, which fosters the embryo and fetus, greatly influences the racing performance of the offspring, and that aging and repeated parturition are detrimental to the uterus."
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