Abstract
The present paper reports a part of records regarding the crossbred horse population. The horse population concerned in this study was produced following the grading system based on the Hokkaido pony mare, which have been bred for over 80 years since 1878. They are interesting both from ecological and economical standpoints, they have been well accli matized to grazing at all seasons of the year. Their staple feed consist mainly of Sasa apoiensis NAKAI growing in the forest. The inter-sire difference of grazing horse in the forest is foimd to be significant being at 1% letel as seen in the analysis of variance of the gestation length( Tabe I). This indicates that the gestation length is partly influenced by genotype of foetus but cannot be determined by its environment only. The genetic variance occurring in the; gestation length is estimated as 33% by neglecting epistatic gene effect. On the contrary the inter-sire difference of the ligh.t breed, well fed in stable, is probably insignficant because of the environment difference between grazing and stable horses .
