Germination: Cold soil temperatures (sub 50o F) can trigger imbibitional chilling injury or cold temperature injury. These phenomena occur when either seed kernels imbibe water that is colder than what is considered ideal resulting in ruptured tissue during cell expansion or from damage to the mesocotyl during cell elongation. Plant death is possible but more commonly observed are corkscrewed mesocotyls that are responsible for arrested development or delayed emergence.
Emergence to V5: Very little effect, even if crops are completely defoliated, as the growing point is still about one-half inch below the soil surface. New growth should be visible the day after frost as long as the temperature rises above 65 degrees F. Stands can be evaluated after a few days to determine whether certain plants will survive.
V6 or later: Growing point is now above ground. Lodged plants can recover and continue to grow, but plants snapped off at the base are dead. Plants that were borderline 5-6 leaf should be monitored for three or four days to see if regrowth occurs. Stem bruising can be a problem that manifests later in the season. Young stalks will be able to stand with bruising, but, if significant, plants in later stages may not be able to support their own weight and will be more prone to lodging.
V6 to V8: Leaf loss at this stage has a little impact on final yield provided there is no damage to the growing point. Corn can sustain up to 50% leaf loss with almost no yield reduction and 75% leaf loss relates to only about 5% yield reduction. In fact, even when all leaves have been stripped, one can expect only about 10% yield reduction to occur as long as the growing point is undamaged.
9-11 leaf stage: A 50% leaf loss will result in an approximately 5% yield reduction, and 75% leaf loss will result in an approximately 10% yield reduction. Complete (100%) leaf loss will result in an approximately 15% yield reduction.
12-14 leaf stage: A 50% leaf loss will result in an approximately 5% yield reduction, and 75% leaf loss will result in an approximately 20% yield reduction. Complete leaf loss will result in an approximately 35% yield reduction.
15+ leaf stage: A 50% leaf loss will result in an approximately 10% yield reduction, and 75% leaf loss will result in an approximately 40% yield reduction. Complete leaf loss will result in an approximately 60% yield reduction.
Tasseling: A 50% leaf loss will result in an approximately 20% yield reduction, and 75% leaf loss will result in an approximately 65% yield reduction. Complete leaf loss will result in a total yield loss.
Late Milk: A 50% leaf loss will result in an approximately 10% yield reduction, and 75% leaf loss will result in an approximately 35% yield reduction. Complete leaf loss results in an approximately 50% yield reduction.
Buggywhipping: Corn that has been subjected to hail at any stage is susceptible to a phenomenon known as “buggywhipping” where leaves become tightly wound in the whorl and the tassel has difficulty emerging. There is no surefire way to determine when or if these leaves will unfurl, and agronomists should not count these as viable plants when taking stand counts.
Stand Loss at any stage: University research suggests that a 25% stand loss will result in a 10% total yield loss, a 50% stand loss will result in a 25% yield loss, and a 75% stand loss will result in a 45% yield loss.
SOYBEANS AND DRY BEANS
Seed Germination: Soybean will readily germinate at soil temperatures near 50o F. However, the concern relating to soybean that germinate in cold soils is that the germination processes are dramatically repressed thus maintaining the seedling at a susceptible stage for a longer period of time for injury from seedling rot pathogens to occur. Dry bean are not as tolerant to cold or frost as soybean and generally require planting at or near the last frost free date for a given geography.
Cotyledon: Soybeans can survive temperatures down to around 27 degrees F for several hours.
Unifoliate to Trifoliate Stages: Soybean at these stages are more sensitive to frost than when in the cotyledon stage. However, plants can compensate readily for frost or hail damage even when the terminal bud is destroyed because soybean are capable of growing from axillary buds. However, if the plant is cut off below the cotyledons of the plant, the plant will not regrow and, like corn, soybean stem bruising from hail can cause plants to be more prone to lodging at later stages.
Always wait a few days after injury before assessing bean stands for replant decisions. Soybean plants may take up to 4 to 7 days after injury before regrowth becomes evident. The general rule of thumb is that if 4 plants per foot of row remain, the crop can still produce appreciable yields.
Prior to or at 5 leaf stage: Very little effect, even if crops are completely defoliated, as the growing point is still below the soil surface. Small grains may exhibit leaf injury symptomology, but the growing point is well protected and plants can often withstand temperatures in low 20’s without experiencing yield losses.
Jointing: Damage to the growing point will cause that tiller or main stem to be nonproductive. However, if the growing point is injured prior to flag leaf, additional tillering may occur. Defoliation after jointing will reduce yield even if the growing point is not damaged due to the loss of photosynthetic activity.
Boot: Hail damage during boot and later stages will have a significant impact on yield due to both loss of leaves and damage to the growing point.
Heading: Stems bent from hail will result in a dramatic yield reduction. Bent wheat stems will lose about 66% of their production, and bent barley stems about 33%. For example, if 50% of wheat plants in a field have bent stems, the grower can expect a 33% loss of yield (50% x 0.66 = 33%).
Hard Dough Stage: Yield loss is solely due to shattering and lodging.
Frost or hail damage to sunflower has the largest impact when in seedling or budding stage. Sunflower in the bud stage can withstand temperatures down to approximately 26 degrees F. Sunflowers in the 2-6 leaf stage are not as tolerant to cold temperatures and significant losses can occur if the growing point is injured. Stand reduction and growing point loss will result in significant yield reduction. Defoliation and injured tissue can also lead to yield reductions due to loss of photosynthetic activity and disease infestation of stems. 50% stand reduction will result in an approximately 12% yield reduction during vegetative stages and a 20% (R1) – 45% (R6) yield reduction during reproductive stages. 75% stand reduction will result in yield reductions of approximately 25% when in vegetative stages and 35% (R1) – 60% (R6) when in reproductive stages. Leaf defoliation of 75% or greater during the bud stage can result in yield reduction of approximately 50%.
Sugarbeet are relatively hardy to early planting and cold temperatures. However, extended temperatures below freezing can kill the growing point. Plants that survive frost or hail damage in the early season have minimal yield loss as long as they are able to recover and regrow within a few days. Sugarbeet replant decisions are both population and timing dependent. Consult your local cooperative Ag-Staff for specific geographic and calendar date recommendations. Mature sugarbeet can tolerate cold temperatures down to approximately 25 degrees F. However, tonnage losses are exacerbated when defoliation due to hail occurs during the midseason months (June or July) than if defoliation occurs later in season (August or September). Weather conditions prior to a frost can impact the effect of the cold temperatures on the plant. As with many crops, cool or cold conditions prior to frost can “harden off” the mature crop and it is able to better withstand cold temperatures. It is best to wait a few days before evaluating the impact of frost or hail on the crop. Pesticide applications after a frost or hail event should be delayed until new growth is observed.
Canola can compensate very well for hail damage as long as the growing point survives. Plants in early growth stages that lose the growing point from being cut off below the cotyledons will die. However, even when stands are reduced from an average of 12 plants per square foot to 4 plants per square foot, yield is only reduced about 10%. Plants that are farther along in their vegetative growth typically experience yield loss relative to the amount of leaf loss. Generally, yield loss is equal to about 25% of the loss of leaf area due to the reduction in photosynthetic potential. For example, a 50% loss of leaf area from a hail event will likely result in a 12.5% loss in seed yield. Plants losing flowers or buds due to hail will rapidly compensate by developing flowers that would have otherwise been aborted since the plants do not typically support all of the flowers they have the potential to produce. Additionally, crops can also form branches lower on the plant which are capable of producing additional flowers and buds. Final yield loss is dependent on the crops’ ability to compensate for lost branches, flowers, or buds, and this can be variety dependent. Finally, hail that occurs during pod fill will result in direct seed yield losses based on the amount of branches, pods, and seeds that are removed from the plant. Any flowers that are generated after a portion of the plant is lost due to a hail event during pod fill will likely not contribute to yield because it will be too late in the season for those flowers to produce pods and seeds.
FIELD PEA, LENTIL, and CHICKPEA
These legumes are not real tolerant to hail damage. Losses depend on extent of damage, growth stage, varietal variability, and environmental conditions after a hail event. Damage during vegetative stages prior to flowering can be compensated for depending on the ability of the plant to regrow from lower nodes. Disease problems can be more prevalent due to the bruising on the plant, so fungicide programs should become more aggressive than originally planned. If the top node is cut off, plants will likely stop growing vertically and have increased growth from lower nodes, resulting in a shorter, bushier plant. Yield losses from hail damage that occurs during flowering, pod development, or pod fill are hard to estimate until plants reach full maturity. Losses can only be compared to healthy plants as pod fill is dependent on environmental conditions during vegetative stages. If hail damage results in pods being set lower than normal, pods may not fill as large as if they had been set higher. Growers and insurance agents usually need to wait until full maturity in order to get a true assessment of yield loss. Hail damage during pod fill can lead to actual seed loss as well as quality loss, making some seed be unacceptable for commercial grade.
Based on work done in the Pacific Northwest, stems that are broken or cutoff during vegetative stages will generally result in yield losses that are approximately 50% of the percentage of the cutoff stems. Additionally, stems that are cutoff during flowering stages will generally result in yield losses of about 75% of percentage of cutoffs. However, these numbers are for use in making estimates only as they are only arbitrary numbers.