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Replant, thicken or keep?

Every year some farmers are faced with making the decision of how to handle soybeans with reduced stands. Whether stands are reduced by hail, frost, insects, soil compaction or disease, farmers need to decide whether to replant or accept the existing stand.

It has been drilled into my head over the years that it is not a good idea to try to “thicken up” soybean stands by interseeding into existing stands. Whenever I visit fields to help farmers decide what to do, they often want to just plant or drill some more soybeans into the poor stand. I usually encourage them to make a decision one way or the other, either keep it or start over. Much of the time the farmer ignores my advice and plants more beans into the stand, and apparently it usually works out fine.

Finally, I decided to try to find out where this recommendation came from — to not “thicken up” soybean stands. I was surprised to find that very little research has been done on the subject. To try to find out for ourselves whether this is a good idea, my colleagues and I started a study in 2011 at the ISU Southeast Research and Demonstration Farm near Crawfordsville to investigate replanting soybeans versus interseeding more soybeans into an existing low stand.

Soybeans were seeded at four seeding rates: 140,000; 110,000; 70,000; and 40,000 seeds per acre. Some plots of the 40,000 population were interseeded with an additional 70,000 seeds per acre when the original population was at VC growth stage (cotyledons fully developed), and other plots were interseeded when the original population was at V2 (first trifoliate leaves fully expanded).

Some of the 70,000 population plots were interseeded with an additional 40,000 seeds per acre, also at VC and at V2. This was compared to soybeans planted at 140,000 seeds per acre on the same dates that the interseeding was done to simulate soybeans being replanted on those dates.

The results of the study were a bit surprising, in that all of the treatments yielded about 50 bushels per acre, including the 40,000-seed-per-acre planting rate, which had a harvest population of only 35,000 plants per acre. The stems on these plants were very large with a great deal of branching. I’m not ready to recommend seeding rates of 40,000 seeds per acre, but it does show the tremendous ability of soybeans to compensate for low stands.

Potential yields were probably reduced somewhat because of the wet spring, which delayed the initial planting until May 12. This was followed by a dry summer, which would have also cut into the yield potential. With higher yield potentials of 60 to 70 bushels per acre, we might have seen some yield penalty with the low population, as well as with the “replanted” beans, which were planted on June 1 and June 7. There was a trend for a lower yield for the later-planted beans, but it was not significant.

One thing noteworthy is that it didn’t hurt to interseed more beans into the existing stand, but there also was no advantage to interseeding either. It is also likely that if the interseeding had been done later into more developed soybeans, there would have been greater problems with the original population shading out the later planting. This might result in some of late-planted beans acting more like weeds than contributing much to the yield.

Keep the existing stand?

The main point to be learned from this initial year of the trial is that often the best decision when faced with a poor soybean stand is to keep the existing stand. One of the main concerns years ago with a poor stand was weed control. We have better weed management tools today, so that is not as much of a concern.

We know from previous research that soybeans planted in June usually yield less than soybeans planted in late April or early May; potential yield will be reduced with the later planting date if a field is replanted. I usually recommend keeping stands of 50,000 to 60,000 plants per acre that are relatively uniform. I would expect some yield loss with a stand of 50,000 to 60,000, but even more of a yield loss with planting soybeans a month or so late.

The problem is that stands are never lost in a uniform manner, which makes the decision more difficult. Even though this trial showed no advantage to thickening up soybean stands, one place where it might make sense is in fields with large areas of no stand. Some stand will certainly yield better than no stand, and even though thickening up the poor stand may not increase the yield, it may also not hurt anything if done early enough after the soybeans have emerged.

We are repeating the trial this year and plan to include a seeding rate of 20,000 seeds per acre. If that doesn’t give us a yield penalty, we are planting way too many soybean seeds out there.

The results of this trial and other trials at ISU Research farms around the state are posted online at www.ag.iastate.edu/farms/progress_report.php.

Fawcett is an Iowa State University Extension field agronomist at Iowa City. You can contact him at fawcett@iastate.edu.

This article published in the May, 2012 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2012.