No-tillers should prepare to monitor the effects of high temperatures hitting the Corn Belt on their corn crops, university experts warned this week.

In Iowa, the short-term forecast includes daily high temperatures above 90 through Thursday, say Roger Elmore and Elwynn Taylor from Iowa State University's department of agronomy.

The long range forecast, both the 6 to 10, and the 8 to 14 day forecasts call for above-average temperatures, and average or below average precipitation. Low daily temperatures will reach only into the mid-seventies.

Corn will rapidly accumulate heat units, as many as 30 per day. How will corn fare?

Corn development stages

Late planting this spring, cool soils and close to average heat-unit accumulation since is responsible for slow crop development. This report summarizes crop status through Sunday, July 10. Both silking and tasseling have since progressed rapidly. Most fields are either very close to tasseling and silk, or those landmark processes have already started.

Water use, pollination and silking

Unfortunately, maximum corn water use occurs at tasseling, approaching 0.35 to 0.40 inches per day. Our better soils can hold two inches of crop available water per foot. Availability of pollen is usually not a problem with modern hybrids for a couple of reasons. 

First, at its peak a plant produces 500,000 pollen grains per day! There is usually more than enough pollen to go around. Secondly, most pollen shed occurs during the morning when temperatures are cooler and moisture stress less evident.

Unfortunately, when stress occurs, pollen shed is often not affected while silking is delayed. Breeding efforts over the last few decades though have improved stress-tolerance of hybrids significantly. The time between pollination and silking – also known as the anthesis-silk interval, ASI - is very short with modern hybrids; sometimes silking actually precedes pollen shed. The shorter ASI results in few barren plants. In older hybrids, silking always followed initial pollen shed by at least several days.

Stress during pollination and silking could result in shorter ears, increased tip back and fewer kernels per ear. All of these contribute to less yield potential.

Some positives

We have a few things going for the corn crop as heat approaches. Soil moisture conditions are excellent across Iowa. Near normal – in north central, northeast, and east central Iowa;  ‘Unusual moist’ in west central and central Iowa; “Very moist” in northwest Iowa; “Extremely moist” in south central and southeast Iowa.

Likewise, the crop moisture index shows that all of Iowa sits at the midpoint, “Slightly dry/ Favorably moist.” A good share of our soils have high water holding capacity. As the heat spell continues, the differences in mid-afternoon corn leaf rolling between soils with better moisture holding capacities than others will be evident.

High temperature impacts on corn

The forecast heat wave may have a double impact on the crop. The first is the increase in rolling of corn leaves in response to moisture deficiency. By rule-of-thumb, the yield is diminished by 1 percent for every 12 hours of leaf rolling - except during the week of silking when the yield is cut 1 percent per 4 hours of leaf rolling. Unfortunately, most of our crop will be silking next week.

The second impact is less obvious initially. When soil moisture is sufficient, as it is for the most part this July, the crop does not have a measurable yield response to one day of temperatures between 93 F to 98 F.

However, the fourth consecutive day with a maximum temperature of 93 F or above results in a 1 percent yield loss in addition to that computed from the leaf rolling. The fifth day there is an additional 2 percent loss; the sixth day an additional 4 percent loss.

Data are not sufficient to make generalizations for a heat wave of more than six days, however firing of leaves then becomes likely and very large yield losses are incurred. 

Generally a six-day heat wave at silking time is sufficient to assure a yield not to exceed trend (Iowa trend yield is near 174 bushels per acre). Should warmer than usual nights continue for a six-week period the state is assured a below trend harvest. None of these three factors are assured but the possibility is very real.

The picture in Pennsylvania

Penn State University grain crop management specialist Greg Roth says corn can tolerate pretty severe drought, but when severe drought is combined with high temperatures over 90 degrees F, then the impacts of drought are exacerbated.

One impact of high temperature stress is the effect on pollination and pollen viability. Another impact is increased respiration when night-time temperatures are high and this could lead to more kernel abortion during early grain fill.

Last week I noticed many fields that had already silked in the some south-central counties of Pennsylvania, which is good. Many of these likely have reasonable pollination. Even here in Centre County silking is underway in many of the earlier planted fields but many other fields will be struggling to silk this week.

My observation from field scouting locally is lots of variability in plant development with some plants near normal and shedding pollen, some showing pollen shed with no silking and others under severe stress with no silk or tasseling evident.

Reports from around the state indicate the soils with lower water holding capacity are showing the most severe stress, and the crop on the deeper soils is holding on but these will be impacted soon too. I am growing more concerned about the yield potential of later planted fields. A recent report from Kansas State (Sindelar et al., 2010) showed that the effects of delayed planting were really increased in high stress environments.

If the drought persists for another 10 days or so, we will need to start to think about more management options like those in our drought management factsheet: