Abstract

In recent years, high temperatures during rice ripening have caused the impairment of rice grain filling, resulting in cracked grains and chalky grains in rice for Japanese sake brewing (brewer's rice) and reduced suitability for brewing. In particular, cracked grains break during rice polishing, making it difficult to use the rice for brewing. Because brewer's rice with many cracks has a lower quality grade, for farmers it is crucial to suppress grain cracking. A previous study reported the effectiveness of silicate fertilizer as a technique to suppress grain cracking and chalky grains in 'Toyonishiki,' which was a rice cultivar used by brewers . In the present study, we examined whether the application of steel slag-based silicate fertilizer to 'Yamada-Nishiki,' the most commonly grown brewer's rice cultivar in Japan, could effectively suppress grain cracking under high-temperature stress during the grain-filling period. In 2021–2022, brewer's rice 'Yamada-Nishiki' was cultivated in a paddy field in Tainai City, Niigata Prefecture, Japan, by applying a standard amount (1000 kg/ha) of steel slag-based silicate fertilizer (23% soluble silicate, 12% total iron, 40% alkali constituents). For high-temperature treatment, small plastic greenhouses were set up in the paddy field during the heading period for 20 days. The two-year field cultivation test results demonstrated that applying silicate increased its concentration in hulls. Moreover, the high-temperature treatment led to a significantly higher percentage of cracked grains, whereas silicate application resulted in a notably lower percentage. This revealed a strong negative correlation between silicate concentration in rice hulls and the incidence of cracked grains. In addition, when the harvested rice was air-dried at 50°C in a ventilated dryer, the rate of decrease in the moisture content of unhulled rice was slower in the silicate treatment and the ratio of cracked grains of silicate-applied rice was also lower. These findings suggest that the use of silicate fertilizers led to significant silicate deposition in rice hulls during ripening, and this deposition slowed down the rate of moisture content reduction in unhulled rice during drying, consequently suppressing cracking in Yamada-Nishiki.