When a rice field can be safely drained without sacrificing yield is a topic that has generated a lot of discussion. The University of Arkansas recommends draining rice when it has passed physiological maturity to prevent water stress during grain-filling. According to predictions by the rice DD-50 computer program, that is normally 25 days after 50 percent heading for long grain varieties, 35 days after 50 percent heading for medium grain varieties, and 40 days after 50 percent heading for short grain varieties.
However, 10 to 14 days after rice has headed, farmers may consider turning off well pumps to prepare for harvest if there is an adequate flood on the field to prevent drought stress during grain fill.
This only means turning off the well, not draining the field. If temperatures are hot with little chance of rainfall, continue pumping for five to 10 more days, particularly on sandy fields that do not hold water well. Removal of the flood prior to this stage may result in grain yield and milling yield reductions.
Research conducted in Arkansas by Paul Counce, a university researcher, suggests that draining earlier than 25 days after 50 percent heading may be done safely under the right circumstances. He reported that rice yield and quality were reduced by draining at 50 percent heading in small-plot studies conducted at Stuttgart, Keiser and Pine Tree.
However, draining two weeks after 50 percent heading did not reduce yield or quality. However, the practice of early termination of irrigation (draining before 25 days after 50 percent heading) has had little testing in commercial rice fields.
A major advantage to early termination of irrigation lies in maintaining good soil conditions for the next crop. Fields that do not dry quickly, primarily heavy clay fields, are less likely to be rutted during harvest if irrigation is terminated early. This reduces the number of tillage passes across the field, which in turn reduces costs associated with producing the next crop.
Many producers have invested significantly in precision land forming to improve water management on their farms. However, if fields don't dry sufficiently for harvest, farmers must make a decision to either wait past the optimum harvest time for the rice or rut the fields to get the crop out.
Both scenarios cost producers money.
Farmers tend to know their fields better than anyone else. If you create tire ruts during harvest because the soil is not dry enough, these fields are probably good candidates for early irrigation termination. While the process may result in some monetary savings in the right years, be sure to weigh the risks.
The decision for early irrigation termination should be based on individual field conditions. This should be done on fields that have an adequate flood established, slow drying potential because of weather, soils such as clay that hold water or fields that do not dry quickly after draining.
If you're interested in early flood termination, try it on a single field before committing the whole farm.
Charles E. Wilson Jr. is the Extension rice agronomist at the University of Arkansas Rice Research and Extension Center in Stuttgart. e-mail [email protected].