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What if this growing season is dry?

Talk to any farmer for a minute or two this winter and the lack of subsoil moisture is sure to come up. Depending on what part of the state you’re located, you haven’t seen a good rain for some time. It’s no secret that soil moisture reserves are nearly gone.

Tiles stopped running last summer. The 2011 crop was saved by the plentiful subsoil moisture reserves. Many farmers worry that our continuing lack of rain and snow, plus low subsoil moisture reserves, equate to looming disaster for the 2012 growing season.

Most soils in Iowa hold 10 inches of plant-available moisture in the upper 5 feet of the soil profile. That’s roughly 2 inches of plant-available moisture per foot. A growing soybean crop requires about 20 to 22 inches of moisture throughout the season. A typical growing season receives roughly 25 inches of rain. If rainfall is less than 15 inches during the growing season, significant crop stress results. In soybeans it’s critical that adequate moisture be available in July and August.

With these facts in mind I wondered what a dry growing season would mean to Iowa farmers. These are my thoughts.

Management considerations

Use your normal soybean seeding rate; however, use the lower end of the recommended seeding rate. Work fields as little as possible. Every pass takes away roughly ¼ inch of moisture. No-tillers and strip tillers will have a definite advantage in a dry spring.

You might want to plant beans a little deeper than normal, but don’t go deeper than 2 inches. Expect uneven stands if surface soils are dry. A nice rain right after planting will get the beans up evenly.

The good news is that many seedling diseases are favored by cold, wet soils. You won’t have those in a dry spring. Rhizoctonia root rot is more serious in dry years. Check varieties for expected tolerance and seed treatments for control of Rhizoctonia root rot. The other bit of good news is that sudden death syndrome requires cold, wet soils to infect plants. A dry spring will definitely reduce the SDS risk in 2012.

Dry weather brings a whole new set of insects. Green cloverworm and grasshoppers are two traditional dry weather pests. These insects defoliate the crop — often very quickly. Soybeans can tolerate up to 35% defoliation in a wet year, and 20% is pretty much the top limit in a very dry year. Japanese beetles are a new pest for Iowa, and they are voracious feeders. They work on the top canopy and are present from mid-June through late July. ISU Extension recommends treating when 20% defoliation has occurred.

Will aphids return again?

If it’s dry, expect soybean aphids to be a problem again. Remember, soybean aphid reproduction really falls off when we have very hot weather. That may work in our favor this year, but it seems that there are always enough aphids — hot or not — to warrant careful monitoring.

Damage from soybean cyst nematode is much more serious in dry years. If SCN has ever been a problem in the fields you’ll plant to soybeans this year, you should select a variety to plant that has excellent SCN resistance.

This cannot be overstated. SCN literally explodes in dry years. There’s no yield penalty if you plant an SCN variety and SCN pressure is low. I strongly recommend planting a soybean variety that has excellent resistance to SCN in 2012.

Hopefully, our weather patterns will shift back to more normal. If not, this is what we can expect if the growing season turns out dry. For additional soybean management information, contact your local ISU Extension field agronomist or Andy Lenssen, our new Extension state soybean specialist at Ames.

His email is alenssen@iastate.edu and phone is 515-294-1360.

Holmes is ISU Extension field agronomist at Clarion in north-central Iowa.

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

All rights reserved. Copyright Farm Progress Cos. 2012.