Draining rice is a critical management decision for rice growers. Draining too early can lead to reductions in yield and quality. Draining too late can lead to rutted soil conditions and difficulties in harvesting. Furthermore, pumping water is costly and there are frequently alternative uses for the water used in irrigation.
Over four years, research I did with colleagues Earl Vories and Terry Siebenmorgen on silty clay loam and silt loam soils found no reduction in rough rice yield or milling quality when rice was drained at two weeks after 50 percent heading compared to four weeks after 50 percent heading. Draining at 50 percent heading did result in both rough rice and head rice yield reduction.
Of the four research years, two had above average rainfall for six weeks after heading. Rainfall for the other two was below average for six weeks after heading.
Others have found similar results. Recently, however, some scientists have reported reductions in yields by draining two weeks after 50 percent heading. Some scientists have even advocated leaving the flood on until harvest.
About 20 years ago, the practice of flooding until harvest was tried by some farmers. It provided the farmers with a pretty good challenge in effectively harvesting their rice crop.
Many rice growers have adopted earlier draining of their rice fields with considerable advantages and no apparent loss of either yield or quality. Many soils, including silt loam soils, are good candidates for early draining.
Terry Gray of Delaplaine, Ark., produces zero-grade, no-till rice. Terry has found early draining to be a critical part of his management program.
But many growers have been reluctant to practice earlier draining. Certainly, fields that drain easily - such as hillside rice - may not be good candidates for early draining of rice fields. Obviously, our first goal is providing adequate water to produce the rice crop.
But there is no advantage to wasting water and rutting fields. Work led by Mike Popp indicates water savings of $25 per acre (in terms of 2004 fuel prices) may be realized by draining early. Tillage cost savings amounting to more than $25 per acre may be realized by reduced rutting, according to some rice producers.
Water use by the rice crop is determined primarily by crop leaf area and amount of sunlight. Consequently, evaporative demand may be quite high while temperatures are very mild. Conversely, under cloudy but hot conditions, water use may be quite low.
Water use by the rice crop is greatest at heading. Afterwards, leaf area generally declines, slowly at first and then more rapidly. By harvest, the grain is ripe and the crop is using a fraction of the water used at heading. As with other grain crops, any water deficit during grain development is likely to decrease grain yield, quality or both.
Several considerations should be made in draining rice, including the type of soil, depth of root zone, stage of development, and evaporative demand. Another consideration is labor and difficulty: farmers need a definite drain time so that gates can be removed easily.
We developed a growth staging system for rice which allows the actual “age” of the rice plant or crop to be determined objectively (for more information, see http://www.uark.edu/depts/agronomy/facpage/counce/ricestage.html). Since developing the rice growth staging system, we have recorded the time interval between developmental stages and can predict how long a given rice variety takes to go through a growth stage. We can, therefore, use the DD50 to predict reproductive growth stages.
Previously, the DD50 stops accumulating at 50 percent heading and reverts to days. By the introduction of the growth staging system, we can improve the accuracy of reproductive predictions of rice development and associated management decisions during reproductive development.
By using a combination of DD50 reproductive growth stage predictions and maximum transpiration by the rice crop per day, we can predict how much water will be used between growth stages (R6 to R7, R7 to R8, and R8 to R9). If we sum water use, growth stage by growth stage beginning with the final stages, we can know how much water it takes to get from a particular growth stage to a mature crop.
For instance, a Wells rice crop may use 1.1 inches of water between R8 and R9, 0.7 inch between R7 and R8 and 3 inches between R6 and R7. Consequently, between R6 and R9 the crop would use 4.8 inches of water. Between R7 and R9, the crop would use 1.8 inches of water.
By determining water holding capacities and rooting depths for a particular soil, we can calculate the water held by the soil and available to the rice crop at the time of draining. A DeWitt silt loam soil with an 8-inch root depth would hold 3.52 inches of water at draining.
When the water available is equal to, or greater than, the amount of water used from a particular case, then it is safe to drain the rice field. Consequently, in this particular soil and crop situation, R6 would be too early to drain but R7 would be safe to drain.
The program is being tested and we are seeking input from rice growers about making the program the most useable for rice producers. The program is a comprehensive approach for determining the time to drain rice. This year we are conducting field tests of the system.
Paul Counce is a rice physiologist with the University of Arkansas Rice Research and Extension Center.