The influence of month of birth on growth and development of thoroughbred foals and yearlings

Thirteen Thoroughbred farms in Virginia provided 260 horses for the study of growth patterns of horses from birth to July of the yearling year. Measurements included heart girth, wither and hip height, body length, chest width, knee to ground length, cannon bone circumference and body weight, as estimated by heart girth. Quadratic regression equations, adjusted for the effects of farm, sex and month were developed to describe the growth of foals. Foals born in January-February were smallest overall at birth, 30 and 90 d-of-age (P< .01) May-June foals were largest at birth. Differences in wither height, body length and body weight of foals born in May-June vs in January-February at 30 d-of-age, were 1.7, 3.8 cm and 21.6 kg, respectively; At 90 d-of-age, differences were 1.6, 4.9 cm and 13.3 kg, respectively. Differences in size due to birth month were apparent up to 270 d-of-age. May-June foals exhibited the fastest initial growth rate. Colts were intermittently larger than fillies subsequent to 150 d-of-age. In January of the yearling year, birth mouch influenced all measurements (P< .01). Foals born in January to March were larger than foals born in April to June. In July of the yearling year, differences were identified less often and actual differences in size were smaller than in January. Average differences in heart girth and wither height of foals born in January to March compared to foals born in April to June were 8.4 and 5.1 cm in January and 5.2 and 3.6 cm in July of the yearling year. Foals born in April to June displayed larger gains during the first 7 mo of their yearling year than did early-born foals. Correlation coefficients between measures of foals up to 400 d-of-age decreased or remained the same. Thus, growth in various proportions was asynchronous. Regression coefficients that described the resemblance between wither heights of parents and offspring increased with age and were highest in July of the yearling year. Offspring-midparent regression coefficients of 1982 foals increased from .50 ±.18 at 30 d-of-age to .78 ±.14 at 400 d-of-age. The regression coefficient in July was .84 ±.15.