Availability of moisture always ranks near the top when growth and yield factors are rated according to importance. Yields, up to a point, are determined by the availability and use of moisture. Irrigation is a means by which an adequate moisture supply to the crop can be better assured.

This provides a potential for increased yields over dryland production and the opportunity to stabilize year-to-year fluctuations in yield and seed quality. This yield stabilization can allow a more aggressive marketing program. In addition, loaning agencies in some areas are evaluating the percentage of a grower’s soybean acreage that can be irrigated before they issue the producer a crop production loan.

By the soybean’s reproductive growth period, when irrigation is often first needed, approximately 50 to 60 percent of the production costs are already invested in the crop. Irrigation serves as insurance against a drought that can result in yields that do not even cover production costs, especially for double-crop soybean production.

Water requirements
A soybean crop will produce about two bu/A for every inch of water it uses through the season. Yields in the 40 to 50 bu/A range require 20 to 25 inches of available soil moisture during the growing season. The irrigation water needed will vary depending on the beginning soil moisture and the rainfall received during the growing season. An irrigation system needs to be capable of providing 10 to 15 inches of water during the season to assure an acceptable yield.

Six Soybean Irrigation Tips

1. An irrigated crop will not achieve its full potential if adequate surface drainage is not provided. 2. A realistic irrigated yield goal for most acreage is 45 to 50 bu/A. 3. Adequate moisture is essential throughout the growing season for maximum yields. 4. Commitment to irrigation scheduling helps a producer better achieve the benefits of irrigation. 5. It is recommended that water not be allowed to stand on any area for longer than two days. 6. Furrow irrigation requires a water supply of at least 10 gpm per irrigated acre.

Daily water use varies as the crop develops. Germination and seedling stages use 0.05-0.10 inch per day, and the rapid vegetative growth stage uses 0.10-0.20 inch per day. Once the plant reaches flowering to pod fill (full canopy), it uses 0.20-0.30 inch per day. And from maturity to harvest, the plant uses 0.05-0.20 inch per day.

Early and late-season irrigation
Moisture stress anytime after planting can reduce growth and yield. The goal of early irrigation (prior to bloom) is to promote adequate vegetative growth and node development. Pre-bloom irrigation is almost always needed on late-planted and double-crop soybeans. The crop should be irrigated as needed to avoid moisture stress and to provide good soil moisture at seed fill (R5-R6 growth stage), ensuring that the seeds achieve their maximum size.

Most growers realize the need to irrigate when the crop is blooming and setting pods. However, experience indicates that many growers tend to be late with the first irrigation and then quit irrigating before the crop can reach its full potential. The lack of early and late-season irrigation is often responsible for a soybean crop not reaching its irrigated yield potential.

Although irrigation is an important factor in consistent and profitable soybean production, it is not a cure-all. Maximum yield and profit will be achieved only when irrigation is coupled with other production practices that establish profitable yield potentials.

University of Arkansas Division of Agricul-ture provided information for this article.