Soil, Water & Climate

Agriculture Climate of the Eastern Canadian Prairies

HEAT

Corn Heat Units

Corn is an economically important crop in Manitoba. As a result, the heat requirements for this crop have been studied in some detail. In central Canada, where the majority of corn is grown, studies have led to the development of the "corn heat unit."

The basic concept of the corn heat unit calculation is similar to the degree-day system; that is, the rate of growth is assumed to increase with increasing temperatures. However, day and night temperatures are treated separately. It is assumed that no growth occurs with night temperatures below 4.4°C or day temperatures below 10°C. In addition, maximum growth occurs at 30°C and decreases with higher temperatures, accounting for the detrimental effects of very high temperatures.

Corn heat units (CHU) for each day are calculated by the formula:

CHU= (1.8(Tmin - 4.4)+3.3(Tmax - 10) - °.084(Tmax - 10)2)

2.0

Where Tmax = Daily maximum temperature (°C) and Tmin = Daily minimum temperature (°C)

In southern Manitoba and Saskatchewan, most corn hybrids have been specifically rated for the region. A listing of these hybrids can be found in publications such as the current issue of Seed Manitoba. On the eastern Prairies, corn hybrids grown for silage require 2,000 to 2,100 CHU annually, while the earliest grain corn hybrids require 2,200 to 2,400 CHU to reach physiological maturity.

CHU were accumulated over the growing season (May 15 to the date of occurrence of the first fall frost of -2.2°C).The map of the average CHU accumulation (Figure 20) shows that the best area for corn production is south of Lake Manitoba and Lake Winnipeg, between the Manitoba escarpment and west of a line from Steinbach to Pinawa. Within this area, Pilot Mound, Morden, Altona, Emerson, Portage la Prairie and Great Falls all receive 2,600 to 2,700 CHU on average. In an average year, the Interlake regions, as far north as Eriksdale to Vogar, and southwestern Manitoba, are viable locations for successful grain corn production with a hybrid rating of 2,400 CHU. Average CHU is lower in the Manitoba upland areas above the escarpment and in Saskatchewan, the best areas accumulate only 2,300 CHU, while the Riding Mountain and Kamsack to Porcupine Plain areas receive less than 2,000. In an average year, these areas are not suited for corn production, not even for silage.

At a 25 percent risk level, or one-in-four year occurrence (Figure 21), a hybrid with a 2,400 CHU rating will mature in only five areas: Portage la Prairie, Morden, Altona, Emerson, Winnipeg, Great Falls and Vogar. In other areas, a 2,400 CHU hybrid will not mature in one year out of four. For Saskatchewan, only the earliest hybrids, that is, those with 2,200 CHU ratings, can be expected to mature three years out of four, and only in southern areas.

Assessment of minimum CHU accumulation at a 10 percent risk level (Figure 22) shows that most of the area east of the Manitoba escarpment, south of Lake Manitoba and Lake Winnipeg, and west of Steinbach to Pinawa, will mature a 2,200 CHU rating hybrid to the grain stage nine out of 10 years. There are no areas in Saskatchewan where maturity of even the earliest grain hybrids can be assured nine years out of 10. As for silage, production is restricted to southern Manitoba. There are no areas in Saskatchewan where high quality silage corn can be reliably produced nine years out of 10.