Farm News, IA
07-22-07
Lack of moisture affecting pollination
Experts: crop stress could affect yields
By KRISTIN DANLEY-GREINER, Messenger staff writer
With the critical corn-growing period upon the state, the hot, dry weather many parts of Iowa has b een experiencing won’t help yields.
Pollination requires a significant amount of moisture reserves in the soil and when rain is at a minimum, plants will begin to show leaf rolling or curling, which is a sign of moisture stress, said Roger Elmore, corn specialist at Iowa State
University Extension.
“A sizable portion of the state is dry,” he said. “At pollination, yield potential is at its maximum given the conditions the corn has experienced. So, stress from now on will all reduce that potential, largely through decreased kernel number and size.î”
Still, Elmore said that producers shouldn’t give up on their crops yet, because they can be very resilient.î
Joel DeJong, ISU Extension crops field specialist based out of LeMars
, said that there’s quite a large area of corn in his territory that’s been showing stress for awhile. But, it recovers by morning.
“So far pollination has been okay, but we seem to be on the edge of a real problem, I think. The weather forecast isn’t good, and we have a lot of acres to pollinate,î” he said. “So, I think this could be a week that reduces our potential for this year at least in some neighborhoods. Others are hanging in okay. Some areas are under an inch of rain since early May — obviously they are the worst.î”
DeJong added that the soybeans are suffering, too.
“Beans seem short — 30-inch rows are not closing, showing that they are experiencing some stress, too,î he said. ’’Some aphids in beans very few treated.
Also, we can find some fields with a lot of rootworm beetles in silks. They need to be watched to ensure good pollination occurs.î”
Pioneer agronomist Dan Thompson said other factors also will impact this year’s corn crop, including crop stage, variety type and its drought tolerance, soil type, soil conditions at planting, root mass development or lack of it, type of root mass
— fibrous or penetrating, humidity, wind, day and nighttime temperatures.

“One of my driest areas is southeast of Fort Dodge where there has been little to no measurable rain since the first week of June,î” he said. “The weather was pretty good earlier in the season, so a lot of the corn that is pollinating is pollinating ears 14-18 rows around and 40-plus kernels long. The early planted corn that tasseled around the Fourth of July is 50 to 80 percent pollinated. The silks are continuing to elongate to catch more pollen to complete this part of the plant’s lifecycle. Those fields overall look pretty good.î”
Thompson said that even with leaf firing on the lower leaves and the plants rolling up during the day, they are respirating at night in the cooler temperatures.
“We just need some timely rains to aid in grain fill. Some of the later planted corn has not shot a tassel yet. Part of that could be due to the dry weather and the plant’s natural defenses to drought and some of that could stem back to a late planting date,î” he said. “One thing that’s pretty common I am seeing is that any field that was planted into a seed bed that was still a little damp are the fields that are showing the greatest moisture and nutrient stress. In some cases, the field cultivator created a smear layer three to four inches under the soil surface and the corn roots are hitting that zone and pancaking out.
“In other instances, the planter created sidewall compaction and the roots are struggling to branch out through the seed furrow wall, causing the roots to run down the seed furrow trench giving them a tomahawk type of look,î” Thompson continued. “In both cases, the root masses are restricted making it more difficult to find water and nutrients. Because of a weekened root system, these plants seem to be showing a lot more nutrient deficiencies and drought stress in general.”î
In general, Thompson said that during pollination, the corn plant uses approximately 0.33 inches of water per day. In the time leading up to silking, corn under moisture stress can lose two to three percent yield per day. So, four consecutive days of severe moisture stress can cause 40 to 50 percent yield reduction under silk emergence.