Oats are mainly used as forage in Japan. A new cultivation type, in which oats are sown in late summer and harvested as silage or hay in December or later, has been adopted in the warm region of Japan. This type which is referred to as “summer-sown cultivation” enables to make good use of the field from late summer to early winter. However, this system is not satisfactory for oats, because high temperatures persist until mid-September, the day-length shifts from long to short, and the temperature from high to low. We have been engaged in the breeding of oats for summer-sown cultivation to overcome such adverse conditions. Therefore we focused on early heading habit in fall. Based on the evaluation of introduced varieties and lines for heading habit, we observed that some Mexican varieties headed early in summer-sown cultivation. Then we released ‘Haeibuki’ which is a progeny from a cross between a Mexican cultivar, ‘Guelatao’ and an old Japanese one, ‘Hayate’. Haeibuki is characterized by very early heading in summer-sown cultivation and a high dry matter percentage at harvest. Through field trials in the breeding process, we observed differences among cultivars in germination under summer-sown conditions, presumably associated with high temperature.