As a rule, a soybean crop produces about 2 bu./acre for every inch of water it uses through the growing season.

But when moisture comes can be as important as how much. If soybeans get thirsty during the bloom-to-pod-fill stage of production, yields can suffer dramatically.

“Year in and year out, the biggest yield reducer is the August drought,” says Tommy Carter, soybean breeder at North Carolina State University (NCSU). Carter is working on soybean lines that will withstand those mid-summer rainless periods with little or no yield loss.

“Under dry conditions, several lines of our newer material beat commercial varieties in yield,” he says. “But they lag those commercial varieties in seasons with better rainfall. While we have developed slow-wilting lines that have good drought tolerance and strong yields, we can do better.”

However, breeding drought tolerance into soybeans is a slow, painstaking process. The ability to thrive through dry weather involves several traits. Carter has worked at solving the problem for more than 20 years, and he's joined by plant breeders at other land grant universities, all of whom compare notes on developments and swap genetic materials.

Drought tolerance requires soybeans with more massive, unusual root systems; better tolerance of soil salts, such as aluminum; better nitrogen fixation as soils dry out; and other traits not normally bred into commercial varieties.

“It takes at least two genes — usually more than that — to get true drought tolerance,” says Carter. “We're using material from Japan, Nepal, Egypt and other areas of the world. We now have identified genes that impart the slow-wilting trait and have stacked those traits in several experimental lines.”

Pengyin Chen, plant breeder at the University of Arkansas, is working closely with NCSU. “We're incorporating some of their slow-wilting traits into Arkansas-adapted soybeans. We're especially looking at better nitrogen fixation. Some lines have the ability to continue to fix nitrogen in drought conditions because the rhizobial activity persists in water-deficit situations. Soybean plants with these characteristics can withstand longer periods of drought.

“We're incorporating drought tolerance into early maturing varieties, too,” Chen adds. “Many Midsouth growers are going to early maturing soybeans not just for double-cropping, but as main crop soybeans. We have some pure slow-wilting lines out now, but the yield potential is not quite as good as we'd like. We're working to increase the yields of these varieties.”

Early on, the United Soybean Board (USB) has provided much of the funding for drought-tolerance work, says Carter. “USB members understand the importance of soybeans that better withstand drought periods,” he adds. “Few if any commercial companies are working on drought-tolerant varieties.”

What's the potential for developing transgenic drought tolerance?

“It's not that great,” says Carter. “For example, glyphosate resistance (Roundup Ready) didn't exist in any known soybean line; it had to be incorporated from outside sources. We have many soybean lines with different traits needed to provide drought tolerance. Between 20,000 and 40,000 genotypes go back as far as 1900. And we have barely scratched the surface of all that diversity. We've used less than 2% of those in our breeding programs.”

Now, much of the detective work has been done to screen and identify traits that impart drought tolerance. The big job is pyramiding these traits into true slow-wilting soybean varieties — varieties that are also competitive in yield during moist or dry conditions.

“It may still take a few years before we have a true slow-wilting variety that measures up in yield,” says Carter. “But we're getting closer all the time.”