Soybeans still green at harvest

Many growers this year had soybeans still green at harvest. We experience some degree of this problem every year, but this year we saw more than we have ever seen before. Stinkbugs, aerial blight, three-cornered alfalfa hoppers, the use of foliar fungicides, late-season cercospora and anything else that might alter seed set have the potential to cause this problem.

Fields that remained green were not unlike a condition observed frequently in Louisiana known as green bean syndrome. What we observed in Mississippi were plants that stayed green, but pods matured. Ironically even with this problem, all of the fields we have looked at had an excellent yield potential, even though harvest may have been slowed.

The problem did not occur in every field, nor did it occur in 100 percent of any one variety. In our 65 fields in our SMART program the last two years we have seen this problem only in one field (on Oct. 10, 2002), and it was a late-planted field behind wheat. Still, it has an excellent yield potential.

Several varieties have exhibited the problem — some more than others. Some of the varieties many have expressed concern about have cut beans from the mid 70s to the low 80s.

I believe stinkbugs contributed significantly to the problem. We have seen stinkbugs cause fields to stay green around field borders in years past.

Stinkbug numbers have continued to build and over the last few years they have been found farther north than ever before. Several mild winters, lack of control in bean fields, and the reduction of spraying due to the boll weevil eradication program have contributed to an increase in problems due to stinkbugs.

Twenty years ago it was a common occurrence for producers in south Mississippi to spray stinkbugs every year, often twice. This was a widespread practice and stinkbugs were more concentrated south of I-20.

This is the third year we have dealt with high numbers and it will probably be no different next year. Please get prepared and scout fields at least once a week from bloom through the end of pod-fill.

Even with high numbers, stinkbugs have been very variable. They occurred in some early-planted fields early and not others. Later in the season they became more widespread as corn, rice, and grain sorghum began to dry down. As plants mature, you can tolerate higher numbers, but you have to scout for them to accurately determine what and when to spray.

Our current thresholds do not appear to be adequate. For example, from mid-pod fill through when pods have become golden in color, the threshold is nine per 25 sweeps. In my opinion, if I have watched a sub-threshold population of stinkbugs of four per 25 sweeps for two weeks, I am most likely going to spray.

Current guidelines do not address resident sub-threshold populations; this is where you have to apply some sound reasoning. Experience has shown us what they are doing, and I think we need to get them under control before they begin reproducing in the field. Allowing sub-threshold numbers to feed for an extended time has proven to be a mistake.

I know many will disagree with this comment. But if you spend as much time in the same fields week after week as we do, I think you can understand the confidence we place behind this strategy. The number one objective is to scout your fields. Many growers do not want to look because they do not want to find stinkbugs, worms, or whatever else is in a field.

However, we have reached a new plateau in Mississippi regarding soybean yields. We are encountering new but manageable problems.

As I mentioned earlier, numerous things can cause soybean plants to stay green but I believe stinkbugs are a major contributing factor based on what we know today. We see them more consistently and more widespread and we see the results in our SMART fields where we have controlled them.

Alan Blaine is the Mississippi Extension soybean specialist. e-mail: [email protected]

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