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A total of 19,833 records on 1,992 Thoroughbred foals out of 813 dams and by 365 sires was used to estimate heritability of weight, height at withers, and front cannon bone circumference. Measurement ages ranged from 0 to 174 days. Records were adjusted for the fixed effects of age of dam, sex of foal, and month and year of birth of the foal. Body weight, height at withers and front cannon bone circumference tended to be rather lowly heritable during the first few months but were moderately to strongly heritable by 1 year of age.

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.