Maternal effect on first-year growth of Hokkaido native foals kept outdoors all year round

In this study, we investigated maternal effects on the growth of Hokkaido native horses kept outdoors throughout the year. We used the data on body weight (BW), height at withers (HW), heart girth (HG), and cannon circumference (CC) of 517 foals during the first year of life to (1) examine the growth patterns by comparing six linear mixed models and (2) analyze the maternal effect on BW, HW, HG, and CC for each month by estimating variance components. The segmented polynomial third-order regression equation was selected as the best model for all the traits. The estimated proportion of variance components for the effects of the dam were 0.25-0.38 for BW, 0.19-0.28 for HW, 0.19-0.34 for HG, and 0.10-0.21 for CC. A higher effect of the dam compared with that of the sire was observed on BW and HG at all months of age and HW at 0-2 months of age. Therefore, the dam contributed to foal growth not only through half of its heredity but also through the maternal effect derived mainly from its milk. These results provide valuable information for the breeding of Hokkaido native foals.