Draining rice fields for harvest is a critical timing question for rice growers.

Draining too early risks reduction of rice yield and milling quality while draining too late leads to wasted water, harvesting in muddy soil conditions with equipment problems and increased tillage and land forming costs.

Research on this project has the potential to improve timing, provide a scientific basis for draining rice fields and provide the basis for a recommendation for a safe stage to drain rice.

We have developed a program which takes into account water available to the crop after draining the field, timing between reproductive growth stages and water use during reproductive growth stages. (For more, go to http://cses.uark.edu/1815.htm.)

Results from our previous research indicated farmers could drain long-grain rice cultivars on many soils two weeks after 50 percent heading (50 percent of the panicles in a field were judged to have emerged).

The standard practice and Extension Service recommendation at the time was 25 days after 50 percent heading for long-grain cultivars, 35 days after 50 percent heading for medium-grain cultivars and 45 days after 50 percent heading for short-grain cultivars.

Consequently our research, confirmed by other scientists, indicated farmers could drain 11 days or earlier than was normal practice or recommended. This allowed water savings of $23 per acre.

Farmers have often been reluctant to practice earlier draining for fear of injuring rice. They often do not know when rice is at 50 percent heading and prefer a more visible marker.

The rice growth staging system was developed to provide an objective, uniform and adaptive system for determining the stage of rice development and facilitate communication about production practices and research findings.

The system is objective in that discrete plants characteristics form the basis of judgment for the stage of development. It is also adaptive in that the enumeration and determinations are based on visible plant criteria.

We set about to determine when rice could be safely drained and found — as have other scientists — that rice can be injured by water deficits anytime until the grain has completed filling. We then compared these findings to the actual findings in the field in which rice yields and milling quality are not reduced when draining four weeks prior to harvest.

The answer: the soil profile provides a substantial water reservoir for the rice crop during late reproductive development. Also, as the crop proceeds towards maturity, it uses less water.

In addition, the reproductive development of the rice crop, as with vegetative development, follows thermal time — the DD50. So, the rice crop’s reproductive development can be predicted using projections based on reproductive development and DD50 accumulation.

The program works as follows:

First, determine the amount of water held in the soil profile after draining surface water.

Second, determine in a backward direction from maturity the amount of water used from R9 (grain maturity) to R8 (brown grains on one grain on main stem panicle), to R7 (one yellow grain on main stem panicle), R6 (grain elongated to end of hull), R5 (grain elongating to end of hull on one grain on main stem panicle), R4 (anthesis on one grain on main stem panicle) and R3 (exertion of main stem panicle).

When the cumulative water use from these stages through R9 is determined they can be matched to the water available in the soil at draining. The cumulative stage which is equal to or less than the water available in the soil is the stage at which the rice is predicted to be safe to drain. This computer program can become a useful tool for planning and management of the rice crop.

Cost savings are not trivial. The cost of each 3-inch irrigation is $4 to $19 per acre for pumping depths of 50 feet to 300 feet.

Further savings can be realized by the reduction of tillage costs associated with rutting rice fields.

Additional benefits from early rice draining are due to the availability of water for soybeans at yield critical stages of soybean development.

The program — partially funded by a grant from the Arkansas Rice Research and Promotion Board through rice producer check-off funds — has been field-tested at Arkansas Grand Prairie locations for four years (2005-2008). Another test began in 2009.

So far, results have shown no decrease in rice grain yield or quality from draining by the program compared to conventional, later draining.

For more, contact me at (870) 673-2661 or pcounce@uark.edu.