There's no doubt that irrigation boosts soybean yields, especially during dry years such as 1995. The problem often is getting water on the field when it's needed most.
Thanks largely to grower checkoff funding, the Cooperative Extension Service, University of Arkansas, has a reliable, research-backed computer program that tells you when to irrigate.
Work on the computerized Irrigation Scheduling Program was begun about 15 years ago by UA agricultural engineering professor Dr. Jim Ferguson and agronomy professor Dr. Don Scott. Much of their research was funded by the Arkansas Soybean Promotion Board.
"Originally, a lot of irrigation timing work was done with tensiometers, which are devices that measure relative soil moisture," says Extension ag engineer Phil Tacker. "But it became evident that many farmers wouldn't use tensiometers.
"You have to check tensiometers every other day. You need two or three of them per field. And there are maintenance problems. They just aren't practical."
The computerized scheduling program, on the other hand, requires very little input on the part of the farmer. "Initially," says Tacker, "you enter the name of the crop and the number of acres.
"If the crop is soybeans, you specify the maturity group. And you enter the emergence date."
Once the one-time entries are made, the grower regularly enters the maximum daily temperatures and the rainfall amounts. This information doesn't have to be entered every day - it could be entered on a weekly basis but the grower has to keep up with the daily readings.
"Using your updated information for the past few days, the computer program calculates how much water the crop has used," notes Tacker. "And based on what you've told the program, it calculates the allowable limit how much water you can allow the crop to use before you need to irrigate.
"The Cooperative Extension Service gives the farmer guidelines on when to irrigate based on the soil type, the crop being grown and whether the fleld is flood, furrow or center pivot irrigated."
Tacker explained that the "irrigation deficit" determines when irrigation is called for. If, for example, you're growing soybeans on silt loam with a pan, the recommended deficit is 1.75 inches. If the soil doesn't have a pan, you can allow the crop to use more water.
"Ongoing research is looking at a range of deficits and the effects they have on yields. The grower guidelines for selecting the proper irrigation deficit are constantly being updated."
Roger Eason, director of the University of Arkansas Experiment Station at Pine Tree, is sold on the computerized Irrigation Schedul ing Program.
'We irrigate about 350 acres of soybeans on the station, and we use the program on all of them," says Eason.
'We've used tensiometers in the past, but they can be aggravating to fool with. We've put our trust in the computer program."
Eason doesn't consider himself a computer person. "As a matter of fact, this is one of the few programs that I run. I'm comfortable with it. My wife teaches computer courses, and she says it's user-friendly."
Eason says the irrigation scheduling program has been especially helpful to him in the production of early maturing, Group III and IV soybeans.
"The program says water such-and-such field of Group IV soybeans on May 25. I scratch may head and say that can't be right, but when I go look at the soybeans, sure enough they're blooming and need irrigation.
"Knowing two or three days ahead of time that you'll need to irrigate is a big help in terms of moving equipment, getting levees prepared if you're flood irrigating, and getting ready to divert water if you're irrigating rice from the same well.
"Invariably, your rice and soybeans will need to be watered at the same time. The water is going to go to the rice first. If you know a week ahead of time that you're going to have to water the soybeans, you can try to get the rice taken care of and then move on to the beans."
Forrest City farmer Tony Wilkie used the Extension computer program to help him schedule flood and center pivot irrigation on 1,200 acres of soybeans in 1995. 'We tried to run our pivots when the program said we had a 1.5 inch moisture deficit,'' says Wilkie. "With our flood irrigated fields, we got ready to water when the deficit reached 2 inches."
He considers the program a good guide to irrigation timing. "In the past, with flood irri gation, we almost waited until plants started to wilt. Like everybody else, we always waited too late. I'm doing things a lot quicker, now.
"By modifying what we do, we're going to be able to irrigate more successfully."
Wilkie used the computer program at his office to schedule irrigation with six center pivots and for five flood irrigated fields during the '95 season.
The irrigation scheduling program for soybeans, as well as cotton, corn and grain sorghum, is available at your county Cooperative Extension Service office. The program costs $15.