Now that corn pollination complete, the corn plant is focused on using its resources to filling the kernels. This is important because we sell grain based on weight, says Mark Licht, Iowa State University Extension agronomist. Yield reductions would come as a result of reduced kernel size or sometimes kernels not filling out toward the ear tip.
Once corn reaches the R4 stage of growth (dough stage), kernel number is established on each ear, says Licht. Some farmers, especially in areas of western Iowa that have been drier during the 2016 growing season, are reporting corn ears that aren’t completely filling out to the ear tips. Higher temperatures at night will increase the number of growing degree units, which will cause higher respiration rates at night. This will result in use of sugars produced during the day that were intended for the purpose of contributing to kernel weight.
Why are some corn ears severely tipped-back this year?
“After walking through corn in much of western Iowa the past couple weeks, it is apparent that the corn crop has been through significant weather stress this summer,” says Shane Brockhoff, an agronomist for AgriGold. “The dryness in early summer and in some areas all summer, coupled with high temperatures are lowering prospects of monster corn yields in many fields this year.”
The environmental stress is being expressed as corn ears that are severely tipped-back with many aborted and/or non-fertilized ovules.
How can this be possible with near perfect moisture conditions through the critical month of July in many parts of western Iowa? There are two reasons, says Brockhoff. First, you need to understand excessive heat kills corn yields through the grain fill period. And second, drought stress that hit in June stunted the ability of corn plants to maximize yield potential.
How hot has 2016 been so far? According to WeatherTrends360.com for Des Moines in central Iowa:
* June 2016 was 5.5 degrees F above average. It was the hottest June in in 25 years, and rainfall was 3.47 inches below average. It was the driest June for Des Moines in 24 years.
* July 2016 was 0.5 degrees F above average in temperature. The month of July for Des Moines ended up with a rainfall total that was 2.51 inches above average. It was the fourth wettest July on record for Des Moines.
The weather statistics for July do not overwhelm a cornfield at first glance, notes Brockhoff. However, the key lies in the heat wave that hit July 18 through July 25.
Hot overnight temperatures right after pollination was bad timing
“We endured eight nights in a row in which temperatures did not go below 70 degrees. And for three nights in a row during that span the temperatures were between 75 and 80 degrees,” he points out. “Unfortunately, this occurred directly after pollination for much of the region when corn was changing from R2 blister stage into R3 milk stage. Bad timing.”
The corn plants could not keep up with the respiration demands to remain cool on this string of hot nights, so they aborted kernels at the tip of the ear to ensure solid seed fill for the lower part of the cob. The result is significant yield loss, says Brockhoff.
Timely rains in early July masked the stress that a record hot and nearly record dry June put on many Iowa fields as well. Many fields in Iowa were severely drought stressed for two to three weeks during June. Corn was rolling and the fields were stringing together plenty of ‘bad days’ exposing issues with soil compaction caused by spring and fall field operations.
Many areas have lost at least 8% to 12% of yield potential
“I estimate we lost five to seven days of grain fill due to the heat wave in mid-July and the short burst heat waves we have endured since,” says Brockoff. “If a ‘normal’ grain fill period lasts approximately 60 days (pollination to black layer stage), then we have lost 8% to 12% of our grain fill window. Will this translate to 8% to 12% direct yield loss? If your field is showing severe tip-back and other signs of heat stress, I would argue your yield loss could be even worse.”
2016 corn yield picture for Iowa is not all doom and gloom
Some areas of the state look tremendous for yield potential especially areas that were lucky enough to receive a solid shot of rain in mid-June. This allowed the roots to get through the dry and compacted layers of soil to reach subsoil moisture. Earlier planted corn looks better overall. Fields sprayed with fungicide at the VT growth stage of corn are holding their tip kernel filling much better (lower respiration). Higher fertility and effective nitrogen management is also paying off this year; the plants are showing less signs of stress in fields where more nitrogen was applied, says Brockhoff.
Some of the issues we are seeing in the field were predicted in advance of this growing season. See this previous newsletter at agrigold.com/Universal/Articles/2016-Grain-Fill-Weather-Hot-and-Dry/. “The heat has definitely made an impact on 2016,” says Brockhoff.