Abstract
ABSTRACTHead rice recovery is known to decrease with the delay in harvesting during ripening, but this effect may differ between wet season (WS) and dry season (DS) and among different paddy drying methods. In five experiments, rice was harvested by hand between 25 and 45 days after flowering in DS and WS when temperature around harvest was slightly above and below 30°C, respectively. The highest head rice recovery was generally obtained when rice was harvested at 25 days after 75% flowering. Delaying harvest to 30 days resulted in significant reduction in head rice recovery in 3 out of 7 cases and further delay to 35 days resulted in significant reduction in 8 out of 10 cases. For the crops harvested 25 days after flowering and dried with a flatbed dryer head rice recovery improved compared to sun drying in two experiments. When harvesting was delayed, sun drying tended to reduce head rice recovery compared to artificial drying. Sun drying only in the morning also improved head rice recovery compared to drying for the whole day. For a given drying method, head rice recovery decreased with increase in heat sum from flowering to harvest. It is concluded that rice crops should be harvested in both DS and WS around 25 days after 75% flowering when heat sum with base temperature of 10°C was around 450–500 degree-days. If an artificial dryer is not available, rough rice should be sun dried only in the morning with frequent stirring and mixing to promote more even drying.Abbreviations: WS: wet season; DS: dry season
Highlights
Important physical quality measures of rice grain include total milled rice and head rice recovery, both of which are important characteristics for marketing purposes
Recent work conducted in wet season (WS) crops in Cambodia under similar rice growing conditions to Laos showed a sharp decline in head rice recovery when harvest was conducted at 35 compared to 25 and further decline when harvest was delayed to 45 days after 50% flowering in both hand and combine harvested crops (Bunna, Sereyvuth, et al, 2018)
At 35 days after flowering, grain moisture content was still high (24–25%), yet grain fissuring was observed at harvest, and this appeared to have caused kernel to break during milling which resulted in reduced head rice recovery
Summary
Important physical quality measures of rice grain include total milled rice and head rice recovery, both of which are important characteristics for marketing purposes. Siebenmorgen et al (2013) reviewed preharvesting factors affecting head rice recovery including harvesting time and moisture content at the time of harvest. Delay in harvesting time generally increases broken rice and reduces head rice recovery. This is related to reduced grain moisture content at the time of harvesting (Thompson & Mutters, 2006). At 35 days after flowering, grain moisture content was still high (24–25%), yet grain fissuring was observed at harvest, and this appeared to have caused kernel to break during milling which resulted in reduced head rice recovery. In Laos, Proper drying technique to reduce moisture content in rough rice is a key factor in improving milling quality. Different sun drying methods were compared with the artificial drying using a flatbed dryer for grains harvested at different times during ripening
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