As July turns to August, bringing the possibility of even hotter temperatures and less rainfall, farmers who planted Group 3 and Group 4 soybeans are looking forward to turning off the irrigation water and beginning harvest.

But with Chicago soybean futures still trading at better than $14 a bushel at this writing, growers may want to reconsider when they would normally cut off the flow of water to those early-planted soybeans, Extension specialists say.

“Traditionally, when the beans are touching in the pod, many quit irrigating,” says Trey Koger, Extension soybean specialist with Mississippi State University. “But some of our better waterers irrigate their soybeans one more time. They believe that last irrigation pays.”

Koger, a speaker at the Terral Seed, Inc., Corn, Soybean and Sorghum Research Field Day in mid-July, says he knows many growers are anxious to turn off the pumps after spending more for diesel fuel and electricity than they anticipated this year.

“Due to extremely high fuel prices, timely and efficient irrigation practices are of the utmost importance this year,” he said. “Growers have to make good decisions on when to begin and when to stop irrigation.”

In contrast to recent years, Mid-South farmers have two soybean crops in 2008: The one they planted with Group 3 and 4 soybeans in April and May, and the one they planted after harvesting wheat in late May and June. Some wheat fields weren’t seeded until July.

Many farmers are reluctant to irrigate small soybeans, such as those that have emerged after wheat in recent weeks, but Koger says growers shouldn’t hesitate to irrigate those plants if they need water.

“Small soybeans can be irrigated without significant injury to the plants if the water is moved across and leaves the field in a timely manner,” he said. “The key is to not let the beans get stressed for moisture no matter the stage of development.”

Another consideration is to not get behind on irrigation. In Mississippi, the average water use for a soybean crop will be 0.2 to 0.25 inch of water per acre per day during peak water demand. Peak water demand is affected by plant size, growth stage and weather conditions.

“Small soybean plants that have not begun to flower have water use requirements,” he said. “Even though these requirements are not as great as for a plant in the reproductive growth stage, water is still critical to the health of small plants.”

The type of irrigation system can make a difference when farmers are trying to decide when to irrigate, notes Koger, who came to Mississippi State via the USDA Agricultural Research Service Soybean Production Research Unit at Stoneville, Miss.

“When comparing furrow irrigation with center pivots,” he said, “a single furrow irrigation can provide up to 2.5 inches of water per acre while a center pivot will put out 0.75 to 0.8 inch of water per acre. Typically, three circles with a pivot will provide the same amount of water as a single irrigation with furrow, border or flood irrigation.”

But farmers need to use caution when applying irrigation water down the furrow to make sure they get the water on and off the field and avoid injuring the soybeans.

“Allowing flood, furrow or border irrigation sets to run for much over 12 hours does open the door for injuring small and large soybean plants,” says Koger. “I like 12-hour sets. That seems to work better for many farmers and on my family’s farm.”

In most cases, farmers should turn off or move to another irrigation set after 12 hours and come back if the water didn’t get all the way to the end of the row on that set. For long runs, farmers should use large water volume and large holes in plastic irrigation tubing to push the water through the field in a timely manner.

“Utilizing small holes in low volume streams in a single long line and allowing the water to run for more than 24 hours can result in significant injury to soybean due to waterlogging,” he says. “This injury typically increases as the plants become more stressed prior to irrigation.”

Koger suggests growers use a soil probe to help them determine how much moisture is available in the soil. If a probe is not available, use a shovel, auger “or any tool which allows you to get down 24 inches deep in the soil profile, depending on the root depth.”

Samples should be taken from the edge of the crop canopy and from a depth of at least 8 inches. Select areas that are representative of conditions in the field for sampling.

“Generally, soil moisture should be monitored at a depth of at least 8 inches below the soil surface,” says Koger. “However, the entire root zone should be checked regularly to evaluate soil moisture through the entire plant root zone.

“Soil moisture conditions depend on a lot of factors including soil types, traffic pans, soil depth, plant size, plant vigor and the environment. The root zone of a healthy plant should never go below 50 percent moisture.”

In clay soils, soil taken from the probe or shovel that forms a ribbon when you squeeze the soil between your thumb and index finger should be above 50 percent water holding capacity and not require irrigation. If the soil will not form a ribbon and is dry, hard or crumbles, the moisture level is likely below 50 percent, and irrigation should be initiated.

Sandy soils will not form a ribbon and may require a different method for determining moisture-holding conditions. “If you can leave your finger or palm print in the soil when balling it up in your hand, there likely is sufficient moisture at that point,” says Koger.

“You can also take the soil and try to compress it into a clump by making a fist. If the soil doesn’t have enough moisture to make a clump or the clump breaks into small pieces when you toss it from hand to hand, limited moisture is present and irrigation is needed.”

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