Green Soybean Stems, Light Frost On Forages Raise Questions

Green Soybean Stems, Light Frost On Forages Raise Questions

Soybean plants and stems are remaining green in some fields that are otherwise ready to harvest. Pods and seeds are mature, but stems are still green and tough and that slows down the combine.

A lot of harvesting progress was made across Iowa this week. It's hard to believe Iowa could go that long without getting rain. Both corn and beans are showing a lot of variability in yields - some very good yields and some very bad yields, sometimes both in the same field. The very wet 2010 growing season has caused a lot of variability.

Farmers in many Iowa areas are disappointed with their corn yields compared to a year ago, but are happy about what they are seeing in soybeans. "Bean yields this year have been fairly decent, except where sudden death syndrome disease has cut down on the yield," says Jim Fawcett, Iowa State University Extension field agronomist in east central Iowa. "Corn yields are down from last year certainly and we wouldn't expect corn to yield what it did last year, considering how wet it was in this year's growing season. We lost a lot of nitrogen and had some corn plants dying an early death due to leaf diseases."

Light frost in some areas of Iowa is raising the grazing issue

There was a light frost in some areas of Iowa several days ago - any issues with that? "Yes, there are some areas that had a light frost this week," says Fawcett. "And this time of year the main concern is where you have sorghum-sudan grass pastures for forage."

"When there is a frost, you can get prussic acid building up in the frosted tissue of sorghum-sudan and sudangrass," says Fawcett. "The prussic acid is toxic to cattle. And sometimes, in the days following the frost you will see young shoots starting to regrow, and that's where you have the highest concentration of the prussic acid--in those small, short shoots. So you need to take some extra precaution this time of year. A good rule of thumb is when in doubt, don't turn the cattle out. Don't let them in there to graze sudangrass or sorghum-sudan."

Green soybean plants and stems pose problems for some farmers

Fawcett mentioned the sudden death syndrome or SDS disease killing soybeans in areas of fields in much of Iowa this year. Is SDS related to the green stem problem that some farmers are reporting this fall? "Some farmers are seeing stems staying green on the soybean plants, stems not maturing and drying down like they normally do," he says. "This is especially seen with severe SDS, where there weren't any pods on the plant. The soybean plants are staying green."

Those green plants in those areas of fields are staying tough and it's hard to get the combine through. "I'm not sure we've seen much of this in Iowa before, but it is something that is showing up in fields this year," he observes. "Perhaps there is no place for the plant to put its photosynthate, so the plant just kind of stays green and is probably going to stay green until we have a hard, killing frost."

By and large, how does Fawcett summarize the early start to this year's harvest? "Things are going well. It's certainly helped the dry down of the crop. Farmers don't have to spend all their money on LP gas for grain drying this fall, like they did last fall, when the harvest was so wet and the grain was coming out of the field at high moisture content," notes Fawcett.

What is soybean "green stem syndrome" - what are the causes?

Vince Davis, a University of Illinois soybean agronomist, says the biggest issue concerning growers in his state continues to be green stems and green plants that remain in fields that are otherwise ready to harvest.

"Green stems, sometimes referred to as green stem syndrome or green stem disorder, occur when stems remain green even though pods and seeds yield well and mature just fine," he says. "The condition can range from a nearly normal number of pods on a plant with green stems, to entire plants that remain green with few pods and no seeds developed."

Entire plants that remain green can easily persist until a killing frost occurs. These cases can also range from entirely genetic to entirely environmental causes. "Genetic causes in nature are due to male sterility, causing plants to set about 85% fewer pods resulting in 4.5 times greater carbohydrate concentrations in roots, stem and leaves. In 2006 Curtis Hill and fellow researchers evaluated 1,187 different maturity group 1 and maturity group 2 cultivars in Illinois from 2001 to 2004 and found some relationships between percentages of green stem to certain cultivars suggesting better variety selection may be possible."

Are your soybean plants and stems staying green this fall?

Unfortunately, the syndrome is elusive under different environments, and there is likely little information for growers to access to aid in their seed selection, says Davis. "While genetics may play a role, symptoms can also be environmental. It is commonly associated with viral infections. It can also be caused by insects feeding on the soybean flowers. In addition, other stress factors that increase flower abortion and cause pod loss can play a role."

 With the number of potential causes for this syndrome, Davis says it's difficult to pinpoint the culprit when scouting beans at the end of the growing season. "The good news is these green plant issues tend to appear in fields with average to high yields," he notes. "Green stems are a sign of favorable growing conditions throughout the maturity of the other plants. The only real concern for most growers is how much these green plants and stems reduce harvest speed."

In most cases the percent of green plants is 1% or less of the field. Harvest speed is not affected greatly at those levels when harvest conditions are dry. In severe cases where green plants can be 10% or greater, harvest speed can certainly be reduced. "No clear answers exist for why these symptoms appear and little can be done about it," says Davis. "In cases with high percentages of green plants, delaying harvest until after a killing frost may be an option, but you should monitor the weather and the other plants in the field so you don't lose yield due to lodging or shattering."

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