By Shannon Chant, Regional Crops Specialist, Swift Current
Hopefully frost won’t be a problem for producers this fall but some crops were seeded late and the risk of frost increases when we get into September.
The extent of damage caused by frost depends on the temperature, length of exposure time, humidity levels, and how long it takes to reach freezing temperature. Due to the many factors involved, it is very hard to give a definite temperature to which crops can tolerate frost. Even if the air temperature reaches zero Celsius the crop itself can be four or five degrees cooler, because plants can lose heat faster than the surrounding air.
Evaluating the damage is difficult and should be done approximately 24 to 48 hours after the frost for initial symptoms and up to a week to ten days for full extent of damage. A white appearance to the crop is a good early indicator of some frost damage. Heavily damaged crops will quickly show signs of frost injury including discolouration, darkening, water soaked appearance of fleshy tissue and pods. Slightly damaged pods or heads may show very little symptoms but the seeds within the heads may be damaged. Seed harvested from crops exposed to frost must be vigour tested prior to using the seed for next year’s crops.
Typically, wheat is more tolerant than barley, and barley is more tolerant than oat to fall frost. In the milk stage temperatures below zero Celsius can result in shriveled kernels. After mid-dough stage, temperatures down to minus four Celsius can result in bran frost, kernel shrinkage and possibly a reduction in germination. This is not evident until at least seven to 10 days.
Canola and Mustard
With canola and mustard, pods freeze before the leaves. The leaves can tolerate minus three Celsius to minus four Celsius while the developing pods can be affected by minus two Celsius to minus three Celsius. Immature seed that contains 50 to 60 per cent moisture can be severely damaged by minus three Celsius, while those that are close to swathing stage may escape damage. To escape most frost damage the moisture content should be at least 20 per cent or lower.
Most of the damage occurs as a result of green seed at later stages of development which results in downgrading. Swathing at least 24 hours prior to a frost, and preferably 48 to 72 hours prior, can reduce the green seed count even at early stages such as zero to five per cent colour change (about two weeks from normal swathing stage). However, at the swathing stage (30 to 40 per cent seed colour turn) temperatures of minus three Celsius for one hour will have no effect on chlorophyll content. As temperatures reach minus seven Celsius the chlorophyll content will be much greater.
Once a frost has been received on canola or mustard, it is important to assess the extent of the damage. Assessing the field (damaged versus undamaged seed) is best done at two to three days after the frost or later. If the majority of the seed is damaged, then swath the crop immediately. If not, then leave to proper swathing stage. Note that if the crop has frozen and the pods begin to turn white, then the crop should be swathed as quickly as possible as the pods will start to shatter.
To evaluate whether there is frost damage in canola, look for the immature seeds. They will become watery masses that look like pepper once they dry. The more mature kernels will remain hard but will retain the green colour which causes quality reductions.
Plants in the podding stage will be damaged at minus three Celsius to minus four Celsius. During early pod fill, a frost can cause discolouration and deformation of seeds. Frost damaged seeds will be water soaked and no longer firm as they start to ‘leak’. Heavily damaged pods will have a rubbery wilted appearance.
As pulses mature from the bottom of the plant toward the top, frost injury may be much greater on plant tops. Seeds near the ground may have no frost damage and care should be taken to focus harvest efforts on these seeds.
Immature seeds of flax can be killed by temperatures from zero Celsius to minus four Celsius depending on the length of exposure. Flax has been known to recover from frost damage so care must be taken when evaluating the extent of damage. One should wait at least seven to 10 days to really see the effects of the frost. Frost will cause the bolls to become soft and the developing kernels start to leak moisture. The immature kernels will shrivel up and the bolls turn brown prematurely.
For more information on seed quality, see the Effect of Fall Frost on Seed Quality fact sheet on the Saskatchewan Agriculture website or available at your local Saskatchewan Agriculture office.
For more information on fall frost and assessing damage, contact any Regional Crops Specialist or your local agronomist.