In this study, silage corn cultivars were selected to establish a double cropping system in the southern area of the Kanto Region. Several early-maturity cultivars (relative maturity [RM] 100-115days) and several late-maturity cultivars (RM 120-135) were examined for the first and second cropping, respectively. Dry-matter yield, ripening period, and effective cumulative temperature (ECT) over 10°C required for ripening were compared. The yellow-ripe stage when the dry-matter ratio of the whole plants parts increased to 28% was assumed to be the growth stage appropriate for harvesting. In the first cropping from early April, the very early-maturity cultivars with RM of 100reached the yellow-ripe stage in late July, and required approximately 1,188±33°C of ECT for ripening. On the other hand, in the second cropping from early August, the late-maturity cultivars with RM of 125-135reached the yellow-ripe stage in late November or early December, and required approximately 1,200°C of ECT for ripening. We concluded that the combination of the very early-maturity cultivars (RM100) and the late-maturity cultivars (RM125-135) was suitable for the double cropping system in the area examined.

A field scale trial was conducted over a period of 8 years on heavy clay soil in poor physical condition in order to assess land improvement with heavy rate incorporation of farmyard manure (FYM) at grassland renovation on grass production. The land improvement included pipe drainage, subsoiling, deep tillage to 0.6m and incorporation of FYM at heavy rate, up to 800t/ha. The land improvement without the FYM incorporation did not increase nitorogen (N) uptake by grass and grass dry matter (DM) production of the renovated sward. The land improvement with heavy rate FYM incorporation significantly increased N uptake by grass and grass DM production, with incorporation rate. The N uptake and grass DM production, however, could be explained by only mineral fertilizer, i.e. in terms of response to nutrients applied. From the results, it was suggested that the observed benefits of land improvement with heavy rate FYM incorporation were not as a result of an improvement in soil physical properties. We concluded, therefore, that the land improvement and heavy rate FYM incorporation could be explained in terms of the impact of nutrient supply, similar to the effects of mineral fertilizer application to renovated grassland, even though the land improvement were undertaken at grassland renovation on the heavy clay soil in poor physical condition.