February 14, 2002 | By: Laura Skillman

Soybean yields in Kentucky have increased during the past 25 years, yet farmers continue to strive to overcome barriers in their fields preventing their yields from rising even higher.

Four University of Kentucky agricultural specialists recently addressed the topic of yield barriers and the misconception that soybean yields haven't kept pace with corn yield increases. They also pointed to planting date as one key area where yield potential can be affected.

"We've increased soybean yields in the past 25 years by about 11 bushels," said James Herbek, UK Extension grains specialist. "With corn, yields have also increased about 40 bushels per acre. But that is like comparing apples and oranges."

A better estimate of whether soybeans are keeping pace with corn is a comparison based on percentage, and that shows a comparable yield increase, he said.

Over the years, improved variety traits have aided in increased yields and likely will continue to help with yield potential. New production management practices have historically improved yield potential but there is not anything on the near horizon, Herbek said.

One factor limiting yield potential is that soybeans are not as efficient as corn in turning energy into the final product, he said. If that could be changed through continued advances in biotechnology, yield potential could improve substantially.

There is no "magical" production input around the corner to substantially increase yields, Herbek said, but improving current management practices could improve yields on many farms. From an agronomic perspective attention should be paid to planting date, variety selection, row spacing, and crop rotation.

"The critical thing is avoiding plant stress," he said. "It reduces growth and usually reduces yield potential.

"I think some of our recent yield limitation concerns are lack of soybean cyst nematode awareness and management, limited control of sudden death syndrome, increased frequency of viruses and insects, herbicide resistence, less focus on development of certain variety traits and pushing the early planting date."

Diseases can play a mayor role in reducing yields. Soybean cyst nematode, for example, can greatly reduce yields without it being apparent in the fields. Proper use of resistant varieties and crop rotation can go a long way in ameliorating the effects of the disease.

"SCN is relatively easy to control, but there is an over reliance on resistant varieties. We assume that if we go ahead and plant these resistant varieties and do the usual crop rotation, everything will be fine. But, that may or may not be the case," said Don Hershman, UK Extension plant pathologist.

"There is little information available about how a specific variety will handle a specific population of nematode. It is very difficult to get a handle on because of the variability of SCN populations across the state."

Sampling farms on a regular basis to determine nematode populations can monitor SCN populations over time to determine if management practices being done on a farm are working or not.

Sudden death syndrome is also a problem but sometimes looks worse than it is, Hershman said. It is very difficult to control but there is no question that early planting predisposes a field to SDS and mid- to late-maturing varieties are also the worst affected. One key to managing the disease is to avoid planting highly susceptible varieties. Cyst nematode and SDS are often associated with one another.

Proper disease identification is essential. For example, stem canker is often mistaken for SDS, and while SDS is difficult to control there are excellent resistant varieties available for stem canker, he said.

Controlling insects will not increase yield potential, but it can keep farmers from seeing yield reductions, said Doug Johnson, UK Extension Entomologist.

Planting dates also play a role in reducing potential insect damage. If planted too early or late, plants are more susceptible to insects. Scouting for insects can aid farmers in determining what insects may be in their fields and if they are in amounts high enough to pose a problem.

Insects generally have caused only sporadic problems in soybean fields. A new pest, the soybean aphid, has been found in the past few years in Kentucky fields in small numbers, but if its numbers increase substantially it has tremendous potential to cause problems, Johnson said.

UK weed scientist Jim Martin noted the majority of soybeans grown in Kentucky are the Round-up Ready varieties. That has helped fields be cleaner of weeds than in the past, but he said weed competition can still be a limiting factor in soybean yields.

Between four to eight weeks after planting is a critical time in controlling weeds to keep them from competing with soybeans. The larger the weeds, the more likely they are to cause yield losses. Waiting to spray until weeds are above a foot tall generally results in a yield reduction one out of five times, Martin said.

For more information on soybean management practices, contact a county office of the University of Kentucky Cooperative Extension Service.


James Herbek, Don Hershman, Doug Johson and Jim Martin, (270) 365-7541