Weight, height, and cannon bone measurements were obtained on 1,992 Thoroughbred foals. Colts were heavier than fillies at birth and the differences increased with age. Dams under 7 years of age and older than 11 years had foals of lighter weight at birth than mares 7 to 11 years of age and the differences persisted at 510 days of age. Foals born in April, May, or June were heavier than foals born in January, February, or March. Year of birth also influenced body weight. The trends for height at withers and cannon circumference were similar to those of body weight.
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Hintz et al. (1979) studied this question.