Swathing when hot increases yield loss

With day time highs in the 30s or high 20s, growers should wait for cooler days before swathing the crop, if possible. Cutting canola in hot conditions will lead to rapid dry down and desiccation, which increases seed shrinkage — and leads to yield loss.

Swathing on hot days is especially risky for canola crops with low levels of seed colour change. Swathing canola prematurely in hot, dry weather is a bad combination, causing the highest yield and quality losses.

If growers feel they must swath, consider swathing at night to take advantage of cooler temperatures and any moisture from dew. Night swathing gives the plant 4-5 hours to shut down slowly, and nights are usually more humid, cooler and less windy, providing more moisture for chlorophyll clearing enzymes to function. The next day, plants in the interior and bottom of swaths will continue to be sheltered from warmest temperatures and from wind. With today’s larger swaths, the sheltering benefit of the windrow is that much greater.

Swathing early does not mean combining early. Canola swathed green can take a lot longer to cure than canola swathed at 50-60% seed color change (SCC). Whether growers swath a green crop now or wait a week, combining could occur around the same time. And by waiting to swath, the crop has a better chance of meeting its yield and quality potential.

How fast can SCC occur? With cool wet weather, moisture loss in the seed will be less than one percentage point per day, and seed may even gain moisture with rain. On an average early fall day, moisture loss may be about 1-2 percentage points per day. In areas with cool nights (e.g. foothills, Parkland transition zones) and later in September, more typical dry down would be 1-1.5 percentage points per day. Under warm to hot and windy conditions, moisture loss can be as high as 2-3 percentage points per day.

A rough guide is that a 10% increase in seed color change occurs with about 5 percentage points of moisture loss. So the period from 30% seed color change to 50% seed color change would need a 10 percentage point drop in moisture. This may take 10 days in cool weather to only 3 days in hot windy weather.

Click here to download the CCC’s free Canola Swathing Guide.