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Corn+Soybean Digest

Ask The Experts

Q: What can we gain from longer rotations?

HARMS: Since most growers are interested in the bottom line, the price of crops is a major factor in determining rotations. Soybeans yield more if they're not in close rotation with beans, due primarily to diseases.

Corn also tends to yield more when it follows a legume. It can take advantage of some organic nitrogen, the incidence of disease is reduced, and there's usually less insect pressure. I'm not sure there is any advantage for corn to follow wheat or oats. Continuous corn seems to do better than second-year corn, while bean yields go down each year that beans follow beans.

NAREM: I live on the western fringe of the Corn Belt, where we grew up planting many different crops. Now, most farmers concentrate on corn and soybeans and marvel at the increasing pest problems they face.

Soilborne diseases, herbicide resistance issues, extended diapause in rootworm beetles and other problems we may not recognize prompt us to reconsider lengthening crop rotations. If we repeat our farming practices annually for long, nature's pests will find a way around our best defenses.

Short-term economics tell us to plant corn and soybeans, but rotational crops can have longer-term financial benefits that are hard to quantify.

Q: How high can or should corn populations go?

HARMS: Plant populations should vary with the following:

  1. The hybrid and its ability to flex the ear, withstand drought and utilize nutrients.

  2. The soil and its ability to provide nutrients, including water and oxygen.

  3. Row spacing and configuration.

  4. Environmental conditions, such as sunlight (day length), heat and dry winds.

  5. Susceptibility to disease.

  6. CO2 movement in the field.

We have growers with very good land in central Illinois (rolling and high fertility) who get responses on populations of 36,000. We also have fields where 26,000 is plenty high. My research on split rows indicates that 36,000 is not high enough on some fields in certain situations. My best advice: Experiment to find the best rate in each field.

NAREM: Corn populations and yields have increased dramatically in South Dakota over the past 20 years. Test plots, even this far west, often show yields peaking at 32,000-36,000 plants/acre.

With current genetics, however, I think we're reaching maximum practical populations. The incremental yield increases, predicted as populations exceed 30,000, are rarely worth the standability risk. Late-season moisture stress and increased shading from dense stands increase risk of stalk rot.

Q: What row spacing should I use, and how does spacing affect white mold?

HARMS: We've had questions about row spacing every year for the last 20, but now with white mold we're looking at some different considerations. Normally, expect a 10% increase in yield by changing from 30 in. rows to 7 ½ or 10 in. drilled beans.

This rule should still hold, but my concern is the increase in soybean diseases, especially white mold. White mold is favored by high humidity and newly decaying tissue at blooming time. Going from 7 ½ in. rows to 30 in. rows may not help if you already have a heavy infestation. But it will help to prevent an infestation, or help in a low to moderate infestation.

The best protection against white mold is variety selection. White mold may not live forever, but it lives long enough that it will re-infest itself.

NAREM: Soybean row spacing has come full circle in my area, from corn planter to drill to a resurgence of interest in rowed beans, often in 15-22 in. rows.

Planting narrow-row beans with corn planter units seems like a wonderful compromise. It captures almost all the yield potential of solid seeding while limiting white mold and keeping seed costs down. White mold is only a sporadic problem in my area, and 30 in. rows are a reasonable defensive choice where past infestation dictates caution.

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