Farm Horizons, August 2009

What is a normal growing season?

By Dave Schwartz

Many years ago, a veteran at one of the Minnesota Ag Experiment Stations told me, “A normal growing season is an abnormal growing season.”

He explained, rarely do we experience a growing season in which air temperatures and rainfall are normal each month. Temperatures and rainfall normally run higher or lower than normal, so it’s the abnormal years that, over time, create average temperatures and rainfall.

The past three growing seasons have been abnormally dry. Rainfall has come in tenths, rather than inches. This year, we started out dry and cool, which encouraged excellent root development.

In my mind, we have a deeper root system this year than normal because the growing season began dry, requiring roots to go down for moisture. This deep root system is responsible for our crop hanging on better than some might have expected.

The other factor that has been in our favor is cooler air temperatures. Plant transpiration rates have been lower than normal, with air temperatures in the 70s and low 80s throughout much of June and July.

Weather conditions are critical for corn in July, and August is the important month for soybeans. Corn plants normally tassel in July. The combination of high temperatures and low soil moisture can reduce yields up to 10 percent each day at this critical stage of growth. Silk emergence can be delayed, setting up the potential for poor pollination. For soybeans, pods abort under moisture stress and plants produce more two-bean pods, as well as smaller seeds, if moisture stress continues into late August and early September.

In general, whatever growers can do to reduce stress will help plants get through a drought with less yield-loss. Weeds take away more from a crop in drought years, so fields with good weed control will stand out. Fields that were mudded-in and have compaction will be harder-hit by the drought. I have noticed fields that have livestock manure appear to get through droughts much better.

Growing degree units have been running behind normal for much of the growing season. From May 1 through July 19, growing degree units are running approximately 10 percent behind normal for this part of the state.

I feel we still have the potential for a good crop in this area, if we pick up timely rainfall in late July and August and avoid an early frost.

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