Grain Fissuring and Milling Yields of Rice Cultivars as Influenced by Environmental Conditions

Abstract

Milling quality of rice (Oryza sativa L.) is a primary factor determining its economic value in the U.S. market. It is largely influenced by genotype and environmental conditions prior to harvest. To determine the effect of environment on milling quality of rice genotypes, seven cultivars were evaluated for grain fissuring and milling yield at decreasing grain moisture during 1991 to 1993. Environmental factors, including air temperature, relative humidity, and rainfall, were monitored during grain maturity and drying period. Harvesting was carried out at 2‐ to 4‐d intervals as grain moisture decreased from 270 to 130 g kg−1. High daily minimum relative humidity of 58% and mild temperatures during 1992 caused no significant fissuring until average harvest moisture decreased to 169 g kg−1. Warmer and drier weather during 1993 test period, with 43% daily minimum relative humidity~ caused fissuring to be initiated earlier at a higher grain moisture of 184 g kg−1. With decreasing harvest moistures, head rice yield among all cultivars continued to increase until grain moisture was low enough for fissuring to occur. The optimum harvest moisture with respect to maximum head rice yield among long‐grain cultivars, except for ‘Lemont’, ranged between 179 and 151 g kg−1 during 1991–1993. Within this range, the optimum harvest moisture was highly dependent upon cultivars and environmental conditions. Cultivars with susceptibility to fissuring require higher harvest moistures for maximum milling performance. Optimum harvest moisture among years in grams per kilogram ranged between 200 to 176 for Lemont and 238 to 179 for medium‐grain ‘Bengal’. Despite the strong influence of weather conditions on rice milling yield, certain cultivars, such as ‘Cypress’ and ‘L‐202’, have shown significant resistance to severe environmental fluctuations. Slow drying and fissuring resistance characteristics of Cypress grains provide flexibility in harvesting, transporting, and processing and preserve milling quality.