Frost and Hail Damage in Field Crops
Corn - Soybeans - Cereals - Dry Edible Beans - Canola
Corn plants damaged by hail may experience a reduction in leaf surface area, bruising of the stalk and ear, and in serious incidences, stalk breakage. Hail damage may also provide an entry point for diseases such as smut. Yield loss due to hail is dependent on the stage of the crop at the time of the hail event and the level of defoliation. Yield loss is greatest when the corn is defoliated during tasselling. Younger plants may experience a delay in growth and development due to hail, but yield loss is usually minimal. Yield loss is minimal when defoliation of plants occurs near maturity. See Publication 811, the Agronomy Guide for Field Crops for Estimated percentage corn grain yield loss due to defoliation at various growth stages (page 38).
Hail damage is most harmful if defoliation occurs during tasseling.
Corn - Frost Damage - Early-Season
Frost damage in May or June will generally have little impact on the crop, provided the growing point of the corn plant is still below the soil surface. This is the case until the young plant reaches roughly the sixth-leaf stage (V6). On more advanced plants and/or where damage is more severe, split the stalks to see if the growing point has been damaged. This procedure will require some time to make the correct recommendation. It takes about 3-5 days following a frost to accurately determine the degree of damage to verify the presence of healthy growing points (yellowish-white and firm) or to see new leaf growth.
Frozen leaf tissue bleaches to a straw colour several days after freezing. In some cases, it also develops a "knot," which may restrict expansion of the undamaged tissue lower in the whorl. Producers have attempted to mow frost injured fields to clip these knots and help the plant recover though research has shown plants can recover as quickly and yield just as much if they are left alone.
If the forecast calls for a risk of frost, consider delaying inter-row cultivation, nitrogen side-dressing or herbicide applications until warmer temperatures return. Soil disturbance at the surface introduces more air into the soil and insulates the corn plants from the heat of the soil mass, thus increasing the risk of frost damage.
Similarly, crop residues and weeds act as a barrier for heat transfer from the soil to the corn plant. Dry soils are more prone to frost damage due to their lower capacity to store heat during the day and thus less heat to transfer and protect the corn plant overnight.
Frost injury on corn in mid-June. Smaller plants can recover, but growth in larger plants may be restricted by frost-injured dead tissue.
Corn - Frost Damage - Late-Season
Cold temperatures during the grain-filling period in August and September may cause yield and quality losses. The extent of these losses depends on the developmental stage of the corn and the temperatures recorded. As temperatures drop to 0°C, frost damage first occurs to the leaves of the corn plants. This damage will eliminate any further photosynthesis, reduce grain filling and will often have a negative effect on stalk strength. However, as long as air temperatures do not fall below -2°C, stalk tissues will remain viable and stalk constituents will be mobilized to fill the ear as much as possible. If temperatures fall below -2°C, both leaves and stalks may be damaged and no further photosynthesis or remobilization can occur. This will terminate grain filling, and kernel black layer will develop. Table 1-27, Estimated risks to grain corn yield and quality from late-season frost damage outlines the potential risks to yield and quality for grain corn experiencing different levels of frost damage is available in Publication 811, the Agronomy Guide for Field Crops.
Generally, the early dent stage is the cut-off point where corn can withstand frost damage to the leaves and still produce a reasonable grain yield. This stage is characterized by having kernels showing small indentations in the crown of the kernel, at least in the lower half of the cob.
Plants damaged below the cotyledons by early-season frost or hail will not recover. If frost or hail damages the growing point of the seedling, but not the stem portion below, the plant will send out new shoots from the base of the leaves or cotyledons. Wait 3-4 days and watch for new growth to emerge from the point where leaves attach to the stem (leaf axils). Research trials show that leaf loss at early growth stages has little impact on final yield or maturity.
Soybeans are most vulnerable to hail damage during flowering and pod fill.
Soybeans - Stem Damage
Broken or cut-off stems have greater impact than leaf loss on yield and maturity. If stem loss is under 50% prior to flowering, yield loss will be less than 10%. When evaluating hail damage, check for bruising on the plant stem. Severe damage to the stem will make it more difficult for the plant to recover. It can also make the plant more susceptible to disease. Bruising, which does not cause stem breakage, results in minimal yield loss. In terms of yield reduction, soybeans are most vulnerable during the flowering and seed fill period. This is particularly true if stems are broken, resulting in a reduction in the number of pods and seed size. Delays in maturity also occur.
Soybeans - Late Season Cold Temperature and Frost Injury
Soybeans are regarded as a warm-season crop and are therefore more susceptible to cold temperatures, especially during flowering. It is believed that sustained cold temperatures (less than 10°C) during flowering affect proper formation of pollen in the flower. Sustained cold temperatures result in poorly developed pods called parthencarpic pods (also known as monkey pods). There is some variety difference in tolerance to cold temperatures. Varieties that have tawny pubescence (i.e., yellowishbrown hair) are often more tolerant of cold than those with grey pubescence.
Soybeans are easily injured by frost until they reach physiological maturity, which is attained at the R7 stage (when one pod has changed to brown/grey on main stem). Frost after physiological maturity generally does not damage soybean plants if pods remain intact. Prior to this stage, grain and seed quality will be affected in injured soybeans. A severe frost during flowering or pod fill can reduce yield by up to 80%. Freezing during pod fill will result in severely damaged beans with a greenish, candied appearance. Even moderately frosted beans with a greenish colour and slightly wrinkled seed coat are considered damaged and can be discounted when present in excess of limits. The seed will eventually dry down with a wrinkled seed coat. Frost-injured plants may reach maturity earlier but will have seed moisture equal to non-frosted plants. Germination will also be severely reduced. The Grain Commission classifies frost-damaged soybeans as those whose cotyledons, when cut, are green or greenish-brown in colour with a glassy, wax-like appearance.
Yield reductions from late season frost injury are smaller as the crop matures. Frost during the R5 stage reduces yield by 50%70%. Frost at the R6 stage will cause losses of 20%30%. Once the crop reaches the R7 stage only a 5%10% yield loss is expected. No yield reductions occur once the plants have reached full maturity.
The freeze/thaw cycles of early spring are one of the main reasons for winterkill in Ontario. The risk is highest in heavy-textured soil and/or soils with limited sub-surface drainage. As frost goes into the ground, it works under the crown and "lifts" the plant up. If these freeze/thaw cycles are repeated, the plant is ejected or "jacked" out of the soil. Roots are broken and left exposed above the soil, causing death of the plant due to desiccation. This process is referred to as frost heaving. Deep-seeded wheat is not more resistant to frost-heaving injury. The primary root system does not anchor the plant in the soil. The secondary root system anchors the wheat plant in the soil, protecting against frost-heaving injury. The secondary root system of the wheat plant cannot develop deeper in the soil than the depth of the seed, see Figure 41, Days to emergence at various seeding depths in Publication 811, the Agronomy Guide for Field Crops. When wheat is seeded deep, the plant develops the crown and secondary root system at about 22.5 cm (0.751 in.) deep, as the crown develops in response to light. Regardless of planting depth, the secondary roots will not develop below 2.5 cm (1 in.). To maximize resistance to frost heaving, wheat plants need an extensive secondary root system developed as deep as possible. In frost heaving prone soils, increased seeding rate can also reduce damage. At higher seeding rates, and with some root growth, roots interlock and plants are more resistant to frost heaving action.
Frost heaving of winter wheat is caused by freeze/thaw cycles of early spring, lifting up the crown.
Both frost and hail can be devastating to a bean crop. The extent of early-season frost damage will depend upon where the plants were damaged. If the plant is damaged below the cotyledons, it will not recover. If the growing point is damaged, but the lower stem remains intact, the plant will send out new shoots from the base of the leaves or cotyledons. Wait a few days before replanting to see if these shoots appear. Dry edible beans have a much more limited ability to recover from hail than soybeans. Determinate plant varieties are less likely to recover than Type II indeterminate types. When evaluating hail damage, check for bruising on the plant stem. Stems damaged during the vegetative stage may not be able to support the weight of pods. In addition, wounds from hail damage serve as entry point for bacterial blight pathogens to infect plants. When the pods are damaged by hail, the seeds or entire pods will often rot.If frost occurs close to maturity, pods that are yellow to brown in colour are often sufficiently mature to escape damage. Green beans will shrivel, retain their off-green colour and result in increased "pick." Delaying harvest until the beans dry down sufficiently will help prevent staining and improve separation.
Canola seedlings can recover from a light spring frost if the growing point is not damaged. Prior to taking any action, wait 45 days to assess damage. Check the growing point for green colour at the centre of the leaf rosettes. Although the cotyledons or other leaves may be black, re-growth can occur in 410 days depending on weather conditions if the growing point is alive.
New growth from a seedling canola plant that has been affected by frost, but still has the growing point intact.
Seedlings are more tolerant of frost at the 34 leaf stage than at the cotyledon stage. Ice crystals can form on the surface of plants without necessarily causing serious damage. Water within plant cells contains dissolved compounds that lower the freezing point by several degrees over water outside the cell. The length of time the plant is exposed to freezing temperatures is important, since a sharp frost that lasts only a short period may cause less damage than a mild frost that lasts all night. Rapidly growing plants are less tolerant of frost than plants that have undergone several days of cold weather (hardening).
Frost can be more damaging to canola that is in the flowering stage. Open flowers may be aborted and there may be a delay in maturity. While mature seeds under 20% moisture should remain unaffected by frost, developing seeds that experience a severe frost may not fill and can become shriveled. Check pods for damaged seeds that have lost their green colour and turgidity.
Spring and Winter Canola - Hail
Plants will not usually survive if hail removes both cotyledons or the stem is broken below the cotyledons. However, because canola plants branch significantly in thin stands the yield loss may not be severely affected by the loss of plants. If hail occurs during vegetative growth and causes a loss of leaf area, yield will be reduced. Stem bruising and breakage will result in higher losses. If hail occurs during flowering, plants can compensate by developing secondary clusters and new branches, refer to Publication 811, the Agronomy Guide for Field Crops for Table 6-8, Percentage yield loss due to the destruction of branches during flowering in canola. Yield losses will be highest when hail occurs during late flowering and during pod fill. Canola has poor ability to recover from hail once the pod-fill stage is reached. Hail between flowering and pod fill will result in uneven maturity due to a later flush of growth and flowers.
Download a free copy of Publication 811, the Agronomy Guide for Field Crops to access additional information.
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