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Solar radiation and wheat yield

A New Zealand farmer in 2002-03 achieved a world record wheat yield of 15.048 tonnes per hectare, or 225 bushels per acre. Phil Needham directed me to a report from the New Zealand Institute for Crop and Food Research, which highlighted this achievement and explained how it happened.

The winter wheat was irrigated heavily and got lots of fertilizer. Specifically, the two high-yielding crops got around 200 pounds of nitrogen per acre. And they were irrigated six times with 1.5 inches of water each time, plus had 20 inches of natural rainfall from seeding to maturity. This is not a realistic combination for most growers in Western Canada.

The most significant message for Canadian wheat growers is the link between solar radiation and yield. The report’s author Tabitha Armour wrote that the recipe for a world record begins with the right variety seeded at the right time so grain fill occurs during the solar radiation peak.

The sun’s energy drives photosynthesis, and photosynthesis contributes around 75 per cent of grain yield in a high-yielding crop, Armour writes. So the more solar radiation the plant absorbs from head emergence to maturity, the higher its yield.

Interestingly, the New Zealand study also noted that sunny days and cooler temperatures are the ideal combination. “The duration of grain fill is determined by temperature, with cooler temperatures extending the grain fill period,” Armour writes. This supports Needham’s point that early seeding is best.

When does solar radiation peak?

Andy Nadler, agricultural meteorologist with Manitoba Agriculture in Carman, dug up solar radiation data for me. As you can expect, June and July are the peak months. Environment Canada’s 30-year mean values for the month of April in Winnipeg are 17.74 megajoules per square metre. This rises to 20.90 in May, 22.74 in June, 22.99 in July, then drops to 19.00 in August and 13.32 in September. This data should be similar for farms across the west on the same latitude, Nadler says.

Based on solar radiation averages, you want your wheat heading out in June and filling through June and July if your goal is to hit the peak radation periods.

Of course not every year is the same. When you look at year-by-year comparisons, you’ll notice wide variability. Nadler put together a graph comparing solar radiation levels at Forrest, north of Brandon, Man., for 2007 and 2008. The highest levels for both years were in the May to July period. This is not surprising, Nadler notes, because June has more daylight hours than any other month, with July and May right behind. But you can see how each year had distinct differences. In 2008, early May had more solar radiation than early July.

To further the point about variability, Armour from New Zealand noted that the record-breaking harvest benefited from higher-than-normal solar radiation. Mean daily solar radiation during the grain-fill period was 26.2 megajoules per square metre. The 10-year average leading up to that year was 23.

As noted earlier, temperature is also a factor. Mean daily temperature was 16.5C during the record-breaking grain fill. The 10-year average was 15.7C for the period. “Interestingly, if our coolest mean temperature of 14.5C coincided with the high solar radiation, the combination could theoretically lead to a grain yield of 18.9 tonnes per hectare at 14 per cent moisture content,” Armour wrote.

— Jay Whetter

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