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Canola flowers flounder in the heat

Yield loss can be more than we realize — highs over 29.5 C disrupt the flower-fertilization process, which can leave blanks and empty pods up the stem

Missing pods are a symptom of heat stress during flowering.

Lindsay Wytinck was camping with her family on the 2020 July long weekend and did something she doesn’t usually do — turned on the camper air conditioner so they could sleep.

“That was a first for us,” says the farmer from Cypress River, Man. “The nights were so uncomfortably hot.”

Meanwhile back at the farm, the Wytincks’ earliest canola fields were just coming into flower. Those hot days and warm nights not only made camping uncomfortable, they were also reducing canola yield potential.

The yield-damaging effect of heat has been shown over and over in Canadian canola studies. In a study published in 1992, Agriculture and Agri-Food Canada (AAFC) research scientist Wes Nuttall found that canola yields for years at the high end of the normal July-August temperature range were 0.4 tonnes per hectare (approximately seven bu./ac.) lower than yields for years with temperatures at the low end.

In another review 18 years later, Randy Kutcher, then with AAFC, looked at long-term weather data and canola yields, and showed a yield loss of approximately seven per cent for each 1 C increase in average growing-season temperature.

Warm nights compound loss

The conclusion is that warm years have lower yields and cool years have higher yields.

It doesn’t take much heat at flowering to cause yield loss. In plot studies from 1989 to 1991, AAFC research scientist Malcolm Morrison found that yield loss for Brassica napus, B. rapa and B. juncea would start when temperatures during flowering exceeded 29.5 C. More degrees and more hot days compound the loss.

As part of the study, Morrison measured the accumulation of “heat stress units” and found that as the daily maximum temperature increased beyond the 29.5 C threshold and as flowering canola experienced multiple hot days, the yield loss became steadily worse. (See Figure 1 below)

AAFC research scientist Malcolm Morrison found that yield loss for canola plants would start to occur when temperatures during flowering exceeded 29.5 C. The more heat stress units, the greater the yield loss. To provide a simple sample calculation, if a farm has two days of 32 C, the heat stress units will be 5. (32 – 29.5 = 2.5; 2.5 x 2 days = 5.) (Source: Morrison and Stewart, 2002.) photo: Supplied

“You give a cool-season crop like canola one day over 30 C at flowering and it’s going to reduce yields,” Morrison says.

Kutcher expanded on this, concluding that hot days during the first 14 days of flowering had the greatest impact on canola yield. He also dug into the combination of hot days and warm nights and found that “high-yield years had lower nocturnal temperatures throughout the growing season, and low-yield years had higher nocturnal temperatures.”

That long weekend in July 2020, while the Wytincks were sweating in their camper, the temperatures could hardly have been worse for their canola. Weather records for Cypress River show highs of 30.5 C on July 3 and 29.7 C on July 4. Nights provided little in the way of relief. Over those two days, the Environment Canada weather station at Cypress River recorded a stretch of 65 hours when the temperature barely dropped below 20 C. (It went to 19.4 C at 8 a.m. on July 4 and was around 19 C for five hours in the early hours of July 5.)

Flowers don’t fertilize

Wytinck says these canola fields “flowered for quite a long time,” which the crop will do to compensate for lost flowers. But research shows that canola has a hard time compensating for heat damage to those first flowers. In a Canadian study published in 2000, Sangu Angadi showed that while extended flowering and podding on side branches could help, it did not come close to making up for lost yield from damage to those first flowers.

Missing pods are a symptom of heat stress during flowering. photo: Courtesy Justine Cornelsen

Morrison, who has done several canola-and-heat studies over the years, says the plant will block its own fertilization if temperatures are too hot. It produces flower after flower, but won’t fertilize them, which results in blank spaces up the stem. In some cases, pollination will occur without fertilization, so a pod forms but doesn’t have any seeds. These “parthenocarpic” pods are chubby, misshapen and empty. And for a final blow, even when cooler temperatures returned, Morrison observed a few days “lag” time before flowers started to work properly again.

For the Wytincks, canola fields that looked so good earlier in the season didn’t reach their yield goal. After being interviewed for this article, Lindsay compared notes on the summer heat and canola yield with her husband, Trevor. While they don’t recall seeing a lot of missing pods, they did notice pods that were yellow and shrunken — possibly parthenocarpic. “Trevor completely agreed that heat was one factor in lower yields,” Wytinck says.

How to beat the heat?

There are no silver bullets — or bullets of any kind. Management options are more like pebbles for a slingshot.

Seeding early has long been the recommended practice to make sure canola isn’t flowering during the hottest days of the year. This can work to some extent, given that weather averages show that early July is cooler than late July. (See Table 1 below) Seeding any time in the first half of May usually means canola starts to flower around the first week of July.

Table 1: These days align with the typical canola flowering period for the Canadian Prairies. Manitoba summer highs and lows tend to be higher than those in black soil zone canola growing areas of Saskatchewan and Alberta, as shown by the averages for Winnipeg, Saskatoon and Edmonton. Notable: Winnipeg had four days over 29.5 C (including three over 32 C) in July 2020. Saskatoon had six days over 29.5 C (including two over 32 C) in July 2020. Edmonton had one day over 29.5 C (July 28, 29.7 C) in July 2020. photo: Source: Government of Canada

“But early seeding isn’t always possible,” says Curtis Rempel, CCC vice-president of crop production and innovation. “Seeding early is often impossible due to soil moisture, and seeding early has other challenges — frost risk is higher, and slow growth in cool soils increases the risk from flea beetle damage and seedling diseases.”

The Wytincks’ experience in 2020 shows that early seeding isn’t always the answer. An early-July heatwave hit their earliest-seeded canola and a late-July heatwave hit their later-seeded fields. (This is demonstrated in Table 1.) Temperatures around Cypress River cooled off a little after the first week of July but then ramped up again later in the month when a lot of later-seeded canola was in flower. In fact, the hottest night of the year at Cypress River was 21.9 C on July 24. That same temperature cycle was repeated across the province and many other parts of the Prairies. Canola couldn’t dodge the heat no matter the seeding date.

Boron application on flowers has been tried as a treatment, but results are more miss than hit. A four-year study in Ontario looked at boron (0.3 lb./ac.) applied to flowering canola, and the only economic response came on the hottest year. This provided some positive evidence for the practice, but when now-retired Alberta Agriculture oilseed specialist Murray Hartman did a meta-analysis of boron studies, his results showed no relationship between boron response and temperature — even when temperatures were hot at flowering.

Genetic solution?

One pebble that could be chipped into an arrowhead is genetically superior heat tolerance in canola cultivars. Malcolm Morrison showed considerable genetic differences in a growth-chamber comparison of 47 cultivars from 2011 to 2013. Chad Koscielny, in his University of Manitoba PhD thesis, also showed differences in heat tolerance among hybrid parent lines. Both studies suggest that seed companies could improve canola heat tolerance by selecting for the trait from among existing lines.

The challenge for seed companies is that this adds yet another selection parameter into what is already a long list of demands. But perhaps for regions like southern Manitoba that tend to have hotter nights, identifying those hybrids with advanced heat tolerance could go a long way to improving yields. Another possible genetics-related option is to choose earlier-maturing hybrids, which also tend to flower earlier, as a way to dodge the hottest days of July.

Curtis Rempel says farmers could use results from Canola Performance Trials ( to see how hybrids performed with weather in their regions. “By looking at weather records and comparing that to hybrid yield performance, we can use CPTs to tease out which hybrids have increased heat tolerance.”

The bottom line is that while canola genetics have improved a lot over the years, Brassica napus is still a cool-season crop that gets hammered by hot temperatures at flowering. Heat will be one reason why yields in 2020 were lower than people expected.

“And while various factors contribute to yield loss in any given year, a long-term warming trend for July nights may also partly explain why canola yields in Western Canada are not increasing as fast as people might expect,” Rempel says. (See Figures 2 and 3 below)

Dauphin, Manitoba’s 50-year temperature trends for July show that highs are fairly flat and lows are increasing. photo: Supplied
Grande Prairie, Alberta’s 50-year temperature trends for July show that highs and lows are increasing. photo: Supplied

When asked if it’s possible we’re not fully accounting for the yield loss from hot days and warm nights, Malcolm Morrison gave a one-word answer: “Certainly.”

For more on the science behind heat effects on canola, including the physiological effect of hot temperatures on canola reproductive vitality, see the Canola Encyclopedia.

About the author


Jay Whetter is communications manager for the Canola Council of Canada.



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