Canola in the heat: What to do?

Canola is a cool season crop, which makes it well-suited to “normal” growing conditions on the Canadian Prairies – warm-hot summer days and cool nights. Heatwaves are hard on canola, especially flowering canola. Canola Encyclopedia describes the physiological effects of heat at flowering. With baking hot weather, growers and agronomists will be looking for solutions. Unfortunately, the only in-season solutions that show consistent results are cooler temperatures and moisture (because moisture loss a common co-conspirator with Western Canadian heatwaves). The only application with any science behind it is boron, and results are not consistent.

Boron application on flowers. 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.

For a clue to explain why boron might not work consistently, especially in dry conditions, consider the fundamentals of plant physiology and the movement of nutrients. In the soil, boron moves with mass flow (with water) so dry soils reduce or stop the movement of boron to the root. Dry soil also reduces the overall plant-available supply. John Heard, crop nutrition specialist with Manitoba Agriculture, says boron deficiency is often called a “dry weather disease” because there is reduced mineralization of soil organic matter and crop residue when dry. He says prairie soils would normally have good cycling of boron from decomposing organic matter, but this cycling does not during really dry conditions. In the plant, boron translocates through the phloem – a process that also requires water. In dry or drought conditions, the physiological mechanisms taking up boron and transporting it within the plant are not working efficiently due to the lack of water. The variable results seen in the field may be a result of some boron moving through the plant when there is adequate soil moisture. However, when the plant is under drought stress, boron is likely not taken up when applied as a foliar application and does not mitigate heat stress. 

Researchers around the world have tried boron on other crops, and rice seems to be most responsive to foliar boron application for mitigating heat stress at flowering. Rice typically has adequate moisture at flowering, which may explain the more consistent response.

Results from a canola boron trial carried out through the Ultimate Canola Challenge are available here. Canola Encyclopedia also has a chapter on micronutrients, which leads with boron.

Other options. No other treatments have been given rigorous scientific study on canola in Canada, so it comes down to buyer beware. Some companies claim their products can reduce heat stress in canola, but these may not necessarily be a fit for your farm. Ideally, you want to evaluate products using good data, independently verified across several years and environments. This article provides some key tips on how to critically assess the potential for product efficacy before investing in an unfamiliar product. This data often isn’t available because the work wasn’t done or won’t be shared.

On-farm research trials. One way to test a product, especially one that hasn’t been studied on canola in Canada, is to leave check strips to determine if there was return on investment for your farm. How to set up a proper, robust trial. It is important to leave several check strips (and mark them) for evaluation at harvest.

Pre-planning

Growers have two other options that might reduce the amount of yield loss during a heatwave.

Seed earlier. 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. Seeding any time in the first half of May usually means canola starts to flower around the first week of July. But early seeding is not fail-safe. For one thing, seeding early presents challenges to stand establishment: frost risk is higher, and slow growth in cool soils increases the risk from flea beetle damage and seedling diseases. Seeding early is also no guarantee that fields will avoid the heat. That was the case in 2020 and certainly in 2021.

Choose more heat-tolerant hybrids. AAFC research scientist 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. This is not an advertised trait, but growers could look at cultivar comparison data and plot trials from areas that had heatwaves to check for differences that may indicate some added level of heat tolerance. 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.

More…

• Hot temperatures and pest management (weeds and flea beetles)
• Effect of heat on flowering – Canola Encyclopedia