Average July temperatures across the state were 2 to 4 degrees below normal. Based on our "comfort level," this perhaps has been one of the most pleasant Julys on record.
Here in Iowa, late July and early August typically is the warmest time of the year. This period also coincides with pollination, the most critical stage of development for corn production. Pick up any text on corn production, turn to the section pertaining to "temperature," and you'll find plenty of information on "heat stress" and the effects on corn development. But how does cooler weather affect corn growth and development?
Corn grows rapidly and yields best when temperatures are moderate and there's an ample supply of water. The ideal temperature for corn growth is cooler than many may think: 75 to 86 degrees. With a plentiful supply of moisture, corn will tolerate temperatures in the mid 90s.
Here in Iowa, above normal temperatures are more advantageous from planting to about mid-June. From mid-June through September, slightly below normal temperatures are preferred. This primarily has to do with the pattern of rainfall during these periods. Generally, there is ample moisture in the first half of the growing season. Consequently, the corn plant can tolerate warmer than normal temperatures. In the latter half of the growing season, there tends to be limited rainfall and therefore, cooler temperatures are preferred.
Historically, we can look back to the 1992 growing season and try to draw some comparisons to 1996. The big difference between these two years was the supply of moisture in the first half of the growing season. Most of the state was experiencing a moisture deficit during May and June of 1992, whereas in 1996, there was an abundance of moisture. With regards to 1992 temperatures, May was warmer than normal but the June-through-August period was cooler than normal. With the exception of the last couple of weeks in June and the first week in July, temperatures in 1996 have been cooler than normal.
Cool temperatures are good for silking and pollination.
These cooler temperatures have been excellent for silking and pollination. The silking pace for 1992 was only slightly ahead of where we are now in 1996. In 1992, many were surprised that a cooler than normal July and August produced a record crop. But remember, the 1992 crop was planted on time and in many areas it was not fully mature and was very wet at harvest. Through the end of July 1996, growing degree days (GDDs) across the state averaged about 150 to 200 behind normal. If you assume an average accumulation of 20 GDDs per day through the growing season, this translates into an approximately 7- to 10-day delay in crop progress.
Many fields across the state will silk after August 1. Iowa State University research has shown a yield advantage to corn that silks before July 25. Corn that silks earlier has more "quality" days for grain fill and drydown. Given the current conditions, I would say the prospects for a late maturing, wet corn crop are good. Let's hope there's a return to warmer weather in August and September.
This article originally appeared on page 151 of the IC-476(22) -- August 19, 1996 issue.