Assessing Frost Damage in Crops

REGINA - Aug 23/04 - SNS -- The impact of last week's frost on crops pulse crops in Saskatchewan and Manitoba depends on the stage development, stresses Sherrilyn Phelps, Crop Development Specialist with Saskatchewan Agriculture and Food.

"During flowering (pulses) are affected by -2 to -3 degrees C while those in the podding stage are a bit more tolerant but will be damaged by -3 to -4 degrees C," Phelps wrote in an assessment of the frost which struck large areas in Saskatchewan and nearly all crop districts in Manitoba.

"During early pod fill a frost can cause discoloration 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 often 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," Phelps reminded growers.

Mustard and Canola

In the case of mustard seed and canola, flowers and pods freeze before the leaves. The leaves can tolerate -3.5 to -4.5 degrees C while the flowers and developing pods can be affected by -2 to -3 degrees C. Immature seed that contains 50 to 60% moisture can be severely damaged by -3 degrees C while those that are close to swathing stage (35% moisture) may escape damage.

Phelps said to escape most frost damage the moisture content should be at least 20% or lower. Frost at flowering will delay maturity but only causes minor reductions in yield by causing flower abortion. Flowers that are open are the most affected. Buds and already formed pods will continue to develop normally if not frozen. However, frost after flowering and during pod fill will cause significant yield reductions and possible down grading.

A fall frost may not injure the leaves but could have a significant effect on the yield and quality of the oils. 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 0 to 5 % color change (about two weeks from normal swathing stage).

However, at the swathing stage (30-40% seed color turn) temperatures of -3 degrees C for 60 minutes will have no effect on chlorophyll content. As temperatures reach -7 degrees C the chlorophyll content will be much greater. Swathing the crop 48 hours prior to -7 degrees C will reduce the chlorophyll content compared to leaving the crop stand.

Once a frost has been received on canola or mustard it is important to assess the damage and extent of the damage. Swathing immediately can save the frozen seed (at later development stages) but may be too early for the undamaged seed.

"Delaying swathing can result in shattering so it is critical to make a proper assessment of the field and swath accordingly. Assessing the field (damaged vs. undamaged seed) is best done at 2 to 3 days after the frost or later. If the majority of the seed is damaged, then swath the crop immediately<" Phelps said.

"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 to shrink and no longer be firm. They will become watery masses that look like pepper once they dry. The more mature kernels will remain hard but will retain the green color which causes quality reductions.

Immature seeds of flax can be killed by temperatures from 0 to -4 degrees C depending on the length of exposure, Phelps said.

Research trials conducted by Agriculture and AgriFood Canada looked at frost tolerance of crops. Flax was found to show considerable damage to immature flax bolls when temperatures reached -3 to -5 degrees C (for one hour) while stems required -6 to -7 degrees C. After flax seed reaches the dough stage it is more resistant to frost.

Typically wheat is more tolerant than barley, and barley is more tolerant than oat to fall frost, Phelps said.

Cereal plants exposed to frost at the late dough stage sustain less injury than plants exposed at the early dough stage. In the milk stage temperatures below 0 degrees C can result in shriveled kernels. After mid-dough stage, temperatures down to -4 degrees C can result in bran frost, kernel shrinkage and possibly a reduction in germination.

Frost not only affected yields but can reduce the germination of those seeds that are exposed. The earlier the frost, the greater the impact on the germination and seedling vigor for the subsequent year's crop.

Larry Gusta and research associates have found that in lab experiments wheat, barley, and oat at the late dough stage show little yield reduction when exposed to -8 degrees C for one hour. However, germination was reduced in the barley and oat.

At the early dough stage the crop yields were affected by one hour at -6 degrees C and all had reduced germination Research with AAFC has shown that hard red spring wheat swathed at up to 45% moisture (mid-dough stage where it can be easily crushed between finger and thumb) can have good bushel weight and make the ##035;1 CWRS grade.

Drying conditions will influence the quality obtained as wet weather could cause mildew and heating damage. With cereals frost damage first appears as soft watery kernels that ooze water upon squeezing. This is evident at the milk stages and early dough and can be seen within a couple days of the frost. With frost damage at these stages the kernels will shrink and become very light.

At earlier stages, such as kernel development, the damage is harder to identify and may take longer to access. The developing kernels contain a lot of moisture normally so the method does not work as well. It may take up to 7 to 10 days to truly evaluate the damage. The developing kernels are halted in development and will become soft and start to shrink up. At later stages of development such as mid to late dough stages frost damage occurs in the form of bran frost and possibly some shrinkage. This is not evident until at least 7 to 10 days.