Monitoring combines can instantly measure the yield of rough rice and be used to develop yield maps. Yield maps can then be used to estimate nitrogen uptake in mature stages and the amount of nitrogen fertilizer required. The present study aimed to clarify the relationship between the dry yield of rough rice and nitrogen uptake and to develop a predictive formula for nitrogen uptake based on multiple regression analysis. We used a dataset obtained from long-term fertilizer experiments conducted at Daisen research station from 2008 to 2017, which included the rice varieties Akitakomachi, Kiyonishiki, and Bekoaoba. In particular, for Akitakomachi, we analyzed the effect of cropping rotation, namely a continuous paddy and a paddy-upland rotation field, on the predictive formula. The dry yield and nitrogen uptake were strongly correlated for the entire dataset (R2=0.87). The slope of the regression equation among the varieties and the intercept among cropping rotations were significantly different. Multiple regression analysis was conducted with nitrogen uptake as the response variable, and the dry yield of rough rice and meteorological factors, namely the cumulative daily mean temperature and daily radiation from heading to maturity stage, as explanatory variables. Moreover, the least AIC model contained both the dry yield of rough rice and meteorological factors as explanatory variables. However, the differences in R2 and RMSE between the least AIC model and a model containing dry yield as the only explanatory variable were no more than 0.00–0.02 and 0.01–0.07 g-N m−2, respectively. The results indicate that nitrogen uptake at the mature stage was accurately estimated based on the dry yield of rough rice alone.
Heat stress during ripening increases the formation of chalky or cracked rice grains, and its occurrence is increasing, possibly owing to climate change. Silicon is known to improve water use efficiency, photosynthetic ability, and root vitality of rice plants, resulting in increased yields and grain quality. However, its effect on the formation of chalky or cracked rice grains remains unclear. We aimed to clarify the effect of silicate fertilizer on grain quality of brewers’ rice cultivar ‘Toyonishiki,’ which exhibits a low tolerance to high temperature. High air temperatures during ripening in 2015 and 2016 reduced grain quality of ‘Toyonishiki.’ The application of silicate fertilizer significantly increased silicon concentration in hulls and leaves and increased the number of panicles relative to the control (no silicate fertilizer). Silicate fertilizer significantly reduced the occurrence of chalky and cracked grains, which were significantly negatively correlated with the silicate content of plants. In addition, the water uptake ability of roots and the stomatal conductance of leaves during ripening tended to increase with the application of silicate fertilizer. We consider that silicate fertilizer improved water use efficiency and photosynthetic activity of rice plants, even under high temperatures during ripening. In addition, we conclude that the application of silicate fertilizer mitigated the production of chalky and cracked grains of brewers’ rice cultivar ‘Toyonishiki.’