Published in the Chat 'n Chew Café (7 July 1999). Also published in the Purdue Pest Management & Crop Production Newsletter (9 July 1999).

Effects (or Lack Thereof) of Recent Heat Wave on Corn

R.L. (Bob) Nielsen
Agronomy Department
, Purdue University
West Lafayette, IN 47907-1150
Internet address:

Three to four days of temperatures in the 90’s plus excessive humidity levels wilts most humans, but what effect has it had on Indiana’s corn crop? Given that soil moisture throughout most of the state is reasonably adequate, the answer is that the above normal temperatures have likely NOT been detrimental to grain yield potential. In fact, with adequate soil moisture, the above normal temperatures (day and night) have helped the crop develop faster (move through leaf stages faster) than normal.

Usually folks worry most about the effects of excessive heat on corn during the critical pollination process (described elsewhere in the P&C Newsletter). There are two reasons why this has not been a concern yet in 1999. First of all, little of the state’s crop has reached the pollination stage (5% as of July 4 according to the Indiana Ag. Stats. Service). Secondly, excessive heat by itself usually has little effect on pollination in Indiana. Rather, drought stress accentuated by excessive heat causes greater problems for the pollination process. Fortunately, most areas of Indiana are not faced with severe drought conditions at the moment.

A caveat is warranted relative to fields with significant soil compaction. One of the nasty effects of soil compaction is the reduction in plant-available soil moisture. Soil compaction plus above normal heat at times in June was a common contributing cause of the uneven stands of corn that developed throughout the state. Such fields will also suffer more readily from excessive heat during pollination, even though technically there has been adequate rainfall.

Later on in July and August, as the majority of the crop enters the grain filling stage, excessively warm temperatures could then reduce yield potential. The effect of warm temperatures is a two-edged sword. On one hand, warmer temperatures mean faster crop development (generally a favorable effect). On the other hand, warmer temperatures (especially at night) during grain filling results in less photosynthate converted to plant carbohydrates (dry matter). During excessively warm nights (mid-70’s or greater), the corn plant "burns up" excessive amounts of photosynthate through dark respiration and evolves carbon dioxide back to the atmosphere. This loss of potential plant carbohydrate means there is less available to fill the ear.

So, for the time being, let’s enjoy the fact that the 1999 Indiana corn crop is speeding on its way to pollination and hope that reasonable temperatures and timely rains continue throughout the state.

Corn Growers GuidebookFor other information about corn, take a look at the Corn Growers Guidebook on the World Wide Web at

End of Document