CROP MANAGEMENT
Heat, High Yields and Hybrid Selection
By Greg Stewart, OMAFRA Corn Specialist

Corn and Heat
Heat: it’s what almost anyone involved in the corn industry says it takes to grow a good crop of corn. Corn physiologist Prof. Thys Tollenaar of the Department of Plant Agriculture at the University of Guelph believes the ideal growing season for corn in Ontario requires above-average temperatures in the beginning and end of the season but with normal- to below-normal temperatures during the middle of the growing season. The principles at work here are fairly straight forward. First, our short season demands that for maximum productivity we get our corn off to a fast start, to create a plant canopy that will maximize sunlight interception. Second, excessive heat during the middle of the season can often create yield-limiting stresses, especially if soil moisture is low. Finally, a warm end-of-season allows grain filling to occur more efficiently and allows for more acceptable grain harvest moistures. Considering these factors, 1998 may have been one of the finest corn growing seasons that we have yet experienced in Ontario...even though we had some frost and drought.

Early Season Advantage
Last year provided an abnormally high heat unit accumulation in May and June. Table 1 outlines Ontario Corn Heat Unit (OCHU) accumulation at three sites across the province and points to the pattern that generally across the province heat unit accumulation was 20- to 30-per cent above normal by June 30. Early heat drives soil temperatures up which in turn drives germination, root growth and leaf development. Early planted corn in 1998 generally had a significant canopy developed by mid-June and was intercepting that intensive late June sunlight that often eludes us in seasons when early growth is delayed by cool temperatures in May.

Table 1 - Early season heat unit accumulation
Location	Ontario Corn Heat Units accumulated by June 30. (1998)	Ontario Corn Heat Units accumulated by June 30. (30-year normal)
Ottawa	1282	885
Guelph	1019	77
Ridgetown	1281	1008

Hot Air in Ottawa
The yields from many sites across the province were impressive this year. But the high yield results from the Ontario Hybrid Corn Performance trials from the Ottawa valley were particularly interesting and sparked a detailed examination of temperatures and heat accumulation in some more detail. As illustrated in the previous table, Ottawa got off to an early start and continued on to register a total OCHU accumulation of 3322 for the season. Pretty impressive! The intriguing part of the heat story is that although Ottawa had record heat accumulation, farmers there escaped much of the intense July/August heat normally associated with these warm seasons. Table 2 compares three of the highest heat unit accumulation seasons in recent years and illustrates that 1998 had significantly fewer days in July and August where temperatures reached over 30 degrees C than in two other relatively warm seasons. In fact, the area had fewer hot days in 1998 than even the average for the past 11 years.

Table 2 - Heat unit accumulation and high temperature days in Ottawa.
Year	Total Season OCHU	Days over 30 degrees C in July and August
1988	3095	17
1991	3111	15
1998	3322	8
Normal	2833	--

Undoubtedly this lack of mid-season high heat stress contributed to the high corn yields recorded in this area. It was probably a significant factor in producing above-average corn yields in other areas of the province, especially those which received considerably less rainfall than the Ottawa area. For example, in 1998, Ridgetown recorded only six days in July and August where temperatures reached over 30 degrees C, spread fairly evenly over the two months. A warm spring and a moderate summer was generally followed in most areas by a fall which was free of any prolonged cold snaps which tend to uncouple grain filling, lower test weights and raise harvest moistures.

1999 Recommendations
Talking about the weather is fine. Most of us get a fair bit of practice. However, there are some points for corn producers to consider for 1999 that stem from weather issues.

1. Odds are significantly against having another year like 1998 in 1999. Long-range forecasts are calling for a delayed, cooler than normal spring in 1999. This may or may not be correct, but don’t let the fact that full season hybrids for your area were harvested at 18 per cent moisture this year cause you to select too many hybrids that are full season or beyond for your farm.
2. Bt hybrids seemed to be particularly suited (and perhaps need) a longer growing season like 1998. Be particularly careful of planting a high percentage of full-season Bt hybrids if the season gets off to a slow start.
3. If 1999 comes in warm and early like 1998 and soil temperatures get above 10- to 12-degrees C in late April, plant at least some of your corn and take advantage of the potential early growth. Being overly preoccupied by the potential of a killing June frost is pointless.
4. We certainly can’t count on Mother Nature to buffer our corn crops against heat stress the way she seemed to in 1998. The greatest buffer to corn plant stress that producers have – and that we can do anything about – is the soil. Although economic realities make it difficult we must continue to look for ways to keep cereals in our rotations, underseed red clover or other cover crops and use tillage systems which conserve top soil and soil moisture.

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