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'Weather nightmare may continue for South African corn'

The historic drought afflicting South Africa has already cut its corn crop in half.

FILE: The drought itself might spell even more trouble for corn, especially for those fields that were planted late. Picture: Christa Eybers/EWN.

JOHANNESBURG - To add insult to a growing season that has already been injured by drought, South African corn farmers could well be grappling with frost damage in the coming months.

The historic drought afflicting South Africa has already cut its corn crop in half. Although the country is typically a net corn exporter, the crop losses already incurred this year will have it seeking to import several million tonnes of the grain.

But the drought itself might spell even more trouble for corn, especially for those fields that were planted late. Overly dry conditions encourage a large range in daily temperature, especially in autumn, South Africa's current season.

If temperatures get low enough, both yield and quality could be affected. As such, South African corn production could take a devastating blow since the current estimate of 6.5 million to 7 million tonnes has been driven downward mostly by losses to area and not as much yield. So any considerable hits to yield or quality could have South Africa importing even more corn than originally thought.

The reason drought and temperatures are closely linked brings us back to high school science and the concept of specific heat capacity. Because water's heat capacity is very high, it takes much more energy to change the air temperature when moisture is present, resulting in lower daily temperature ranges.

But dry conditions lack the temperature "moderation" from moisture, meaning that high temperatures can soar and low ones can plummet. And on a cloud-free, windless night, the earth rapidly radiates heat back to space, further dropping the temperatures. This is very commonly observed in desert climates.

The coldest months in South Africa are June and July, and in its interior, where the corn belt lies, it is not uncommon for temperatures to approach freezing during this time. But given the current drought, nights could become even colder more often.

The corn harvest has already begun for farmers who were able to plant last October, on the early side of the calendar. But many farmers planted late or even past the recommended early-January cutoff, so some corn fields could still be in critical development stages by June.

COLD-SCENARIO RESULTS

If South Africa faces a frigid end to its growing season, significant damage to crop yield and quality could arise. The risk to corn from frost events is highly variable and depends on multiple factors including stage of development, variety, weather conditions and topography.

A light frost can occur if temperatures are near 0 degrees C or slightly below for one or two hours. A killing frost occurs if these freezing temperatures sustain for four to five hours, or if temperatures drop to -2 C for even 10 minutes.

The less mature the corn is the more damaging a frost can be. If frost shows up on the tail end of the growing season when corn is at full maturity, there would be no penalties to yield or quality. But there can be large losses from a frost just a few weeks prior to maturity.

When the corn ear enters the dough stage, up to six weeks prior to maturity, a killing frost can chop yields in half and have a severe impact on the quality. A light frost during the dough stage would reduce yields by a quarter, but the damage to quality could still be severe.

About three or four weeks prior to maturity, corn will enter the dent stage. Frost has only a moderate impact on grain quality at this point, and the yield can shrink anywhere from 15 percent to 25 percent depending on the severity of the frost. Later in the dent stage, the heaviest frost can only dock yields around 10 percent, while quality is usually unaffected.

Warmer temperatures can speed up the time it takes for corn to move from pollination to maturity, but the general rule of thumb is that it lasts around nine weeks. Therefore, the timing on the current corn crop's growth will be very sensitive as South Africa moves toward its colder time of year.

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