Chad Lee, Conner Raymond, and Carrie Knott; University of Kentucky

Freezing temperatures were recorded across Kentucky Monday morning, April 24, 2023. The coldest temperatures were mostly in central and eastern Kentucky, but freezing temperatures were as far west as Trigg and Webster counties. Temperatures fell to or slightly below freezing in the following counties from Sunday to Monday: Butler, Caldwell, Carroll, Christian, Crittenden, Graves, Grayson, Hardin, Logan, Meade, Ohio, Taylor, and Webster counties (Table 1, at the end of this article). Webster and McLean County were the coldest at 30°F. Frosts likely occurred west of these counties. The good news is that soil surface temperatures likely stayed in the low 50’s to mid-40’s. This is based on soil surface temperatures measured at UKREC in Princeton, KY.

About 36% of corn acres and 20% of soybean acres were planted as of April 23, 2023, according to the USDA-NASS.

Corn and Soybeans at Risk

Corn and soybeans are at more risk to death from the freeze events at specific growth stages and in certain conditions. The following scenarios go from greatest risk to least risk of plant death from the freeze events.

Soybeans at the “crook” stage where the stem is emerged and bent over like a shepherd’s crook were the most susceptible to the freeze (Figure 1). These plants were most likely to be killed by the freeze or frost. At crook stage, typical damage is along the stem with some yellowing of the cotyledon. This will be followed by plants snapping off where damage was observed (Figures 2 and 3).

Corn and soybean seeds and seedlings in furrows that were not fully closed are at risk of being killed by the freeze.

Corn or soybean seeds that were planted shallow had a slight risk of freeze damage, although plant death from the freeze is unlikely.

Corn plants emerged may have tissue above the soil surface die off from the freeze, but the growing points should have been insulated beneath the soil surface.  Those corn plants should recover well. No yield loss is expected.

Soybean plants that have FULLY emerged and are at the VE growth stage (emergence) should survive the freeze event, based on observations during freeze events in late April 2021 and early May 2020. If the soybean cotyledons survive, the soybean plants will survive, and no yield loss will occur. If the cotyledons do not survive, the plant will not survive, either. 

Corn and soybean seeds at proper planting depths are at very little risk from the freeze. Corn and soybean radicles (the shoots emerging from the seeds) that are still below the soil surface likely were insulated and will survive.

We need about 5 days of warm weather before symptoms are easy to see. Based on current forecasts, it may take six or seven actual days to get the 5 days of good growing conditions. Plants or plant parts that have turned black or brown and have lost turgor pressure are easy to identify.

Corn plants need to be examined from the seed upward. We are assuming that the roots are deep enough to not be a concern. Dig up some corn plants and look for any signs of brown/black areas from the seeds upward. If plants are white to yellow beneath the soil and turgor pressure is good, then the seedlings are likely to survive.  

Maybe Just a Chill

Corn and soybean seeds that are in the process of germinating during the freeze are at risk of taking in cold water (imbibitional chilling) within the first 24 to 48 hours after planting. If the soil temperatures were below 50F for an extended period during those 24 to 48 hours, then the seeds are more likely to be damaged. There is some debate about how long the soils need to stay below 50F before severe damage is done from the imbibitional chilling. We can say those seeds are at risk. At this point, either the seeds were damaged, or they were not from imbibitional chilling. Emergence will be slower in these fields. The fields can be scouted in about five days or so to determine the health of germinating seeds and/or emerged plants.

Table 1: Low temperatures recorded across the state from 4/21/23 through 4/24/23. Freezing temperatures are highlighted in light blue. Weather data from the Kentucky Mesonet.

Resources

Coulter, Jeff. 2021. Spring Freeze Injury in Corn. University of Minnesota Extension. https://extension.umn.edu/growing-corn/spring-freeze-injury-corn

KY Climate Center. 2023. Kentucky Mesonet. https://www.kymesonet.org/ accessed April 24, 2023.

Lee, C.D. Evaluating early season frost damage to corn. AGR-192. Univ. of Kentucky Cooperative Extension Service. http://www2.ca.uky.edu/agcomm/pubs/agr/agr192/agr192.pdf

Nielsen, R.L. 2020. Assessing Frost/Cold Temperature Injury to Young Corn. Corny News Network. http://www.kingcorn.org/news/timeless/FrostedCorn.html

Nielsen, R.L. 2020. Cold Soils & Risk of Imbibitional Chilling Injury in Corn. Corny News Network. Purdue Univ. https://www.agry.purdue.edu/ext/corn/news/timeless/ImbibitionalChilling.html

Staton, Michael. 2021. Assessing frost/freeze damage to emerged soybeans. Michigan State University Extension. https://www.canr.msu.edu/news/assessing-low-temperature-injury-to-soybeans

Taylor, M., A. Nygren, J. Rees., J. Specht and A. Timmerman. 2020. Evaluating freeze and chilling injury in corn and soybeans. Nebraska Cropwatch. https://cropwatch.unl.edu/2020/evaluating-freeze-and-chilling-injury-corn-and-soybeans

USDA-NASS. 2023. USDA-NASS. Crop Progress and Condition. https://www.nass.usda.gov/Statistics_by_State/Kentucky/Publications/Crop_Progress_&_Condition/cw23/CW042423.pdf

Ric Bessin, UK Extension Entomologist

The UK Ag Weather Center’s degree day model for alfalfa weevil indicates that many counties in Kentucky have exceed 190 DD used as a starting point to begin scouting by the third week of March. In fact alfalfa weevil damage has appeared in some fields. Once temperature accumulations reach 190 DD, growers are advised to look at their alfalfa fields and conduct weekly alfalfa weevil larval counts and compare those to the economic thresholds listed below.

Figure 1. When degree-day totals reach 190 Degree Days, it is time to begin scouting for alfalfa weevil larvae. Scouting continues on at least on at least a weekly schedule until regrowth after the first cutting. Treat the DD totals for the counties as estimates.

Fall laid alfalfa weevil eggs are the first to hatch in the spring. These eggs hatch earlier than those laid in the spring and 190 DD approximate when first leaf feeding damage becomes noticeable. Temperatures extremes during the winter help to limit the survival of alfalfa weevil eggs that were laid in stems in the fall. Damage by the young larvae will first appear as tiny pin holes in the leaves.

To scout for alfalfa weevil, we use the stem sampling method. While walking in a “U” or “Z” pattern through a field, collect 30 alfalfa stems, carefully cup the top of each stem in one hand and break it off the crown with your other hand and place it bud end downward in a plastic bucket. Be sure your samples are at least 20 feet from the edge of a field so that they are representative of the entire interior of a field. Knock the stems groups of 4 or 5 stems at a time against the inside of the bucket to dislodge the larvae. Count the number of larvae. Measure the length of 10 random alfalfa stems.  If the field is close to harvest, harvest can be an alternative to spraying, but producers need to watch for damage to the regrowth, there are similar scouting tables for regrowth after the first cutting.

Alfalfa Weevil Larvae Thresholds for Spraying 190 to 225 Degree Days (Check your degree days)

• Apply a long residual insecticide if the number of larvae is greater than the number in the table for the average height of alfalfa sampled.
• Sample again in 2 days if the number is above 15 but less than the number in the table.
• Sample in 7 days if the number is less than 15 in your sample of 30 stems.

Alfalfa Weevil Larvae Thresholds for Spraying 226 to 275 Degree Days

• Apply a long residual insecticide if the larval number per 30 stems is greater than the number in the table above for the height of alfalfa sampled.
• Sample again in 7 days if you find less than the number of larvae for the appropriate alfalfa height.

For degree day accumulations above 275, use the economic threshold tables in ENTFACT 127 or ENT-17 to determine the need to spray the field for alfalfa weevil.

If you de need to treat for alfalfa weevil larvae, keep in mind that insecticide resistance has been an issue in some areas.  The best strategy to manage resistance is to use an insecticide only when necessary and to rotate modes of action each year. For many other pests we would rotate more often, but alfalfa weevil has only one generation per years.  To rotate modes of action, select insecticides that have a different IRAC group number on the label.

UK College of Agriculture, Food & Environment Corn & Soybean News (August 2022)

SOIL TESTING for the next crop is important this fall. The summer season’s drought, after spring wetness (with compaction issues), is causing lower, more variable, corn and soybean yields. Lower grain yield means lower nutrient removal, but this is not perfectly predictable from a yield monitor. Drought affected grain is usually nutrient rich compared to rainy season grain. More corn acres will be harvested for silage rather than grain and nutrient removal is greater with silage. Soil test ‘problem’ fields/areas identified earlier this season. If you don’t do your own soil sampling, you might want to book sampling services early – this year there are more questions that need samples to inform deci-sion-making.

SOIL ACIDITY hurts root activity – a bigger problem in droughty seasons. Once soil test results are in, take a close look at soil pH. If needed, and if weather permits, lime should always be applied in the fall. Good quality lime takes time to dissolve and cause the carbonates to neutralize soil acidity.

DECIDING WHETHER TO APPLY fall nutrients, especially for corn and soybean, is more difficult this year. The decision generally depends on the target crop (wheat/forages vs. corn/soy); economics/value of fertilizer, time, and equipment; and the soil test value (low values mean higher recommended rates – better nutrient use efficiency when needy soils are fertilized to better match crop demand = spring for summer crops like corn and soybean). Fertilizer prices are lower (except for potash) now, but still high relative to prior years.

WHEAT follows corn in many areas. This year, most wheat will not need fall nitrogen (N). Lower corn yield causes less N removal. Tissue N will be higher in corn residues, giving greater N availability as residues decompose. Many grain producers have fields in forage production. Likely under fertilized this year, these crops/fields may really need some fall fertility to improve stand health, winterhardi-ness, and both forage quality and stand competitiveness with weeds next spring.

A WINTER COVER CROP can contribute. In addition to protecting against soil erosion (especially with less full-season soy residues this year), cover crops cause greater nutrient retention against fall-winter losses. One ton of rye dry matter (good stand, 12 to 18 inches tall) contains about 35 lb N, 45 lb K2O, and 10 lb P2O5. These nutrients won’t all be immediately available with rye termination next spring, but $32(N) + $33(K2O) + $7(P2O5) = $72 worth of nutrients, considering the most recent aver-age retail fertilizer price levels (https://www.dtnpf.com/agriculture/web/ag/crops/article/2022/08/02/summer-slump-retail-fertilizer), are retained.

FALL NUTRIENT SOURCE DECISIONS might also be difficult. This fall, the need for fertilizer N will be significantly lower. Fall application of N, regardless the nutrient source, will be less economical and losses are more likely, given likely greater fall background soil N levels. Nutrient sources containing N and other important nutrients (DAP, 18-46-0; MAP, 11-52-0; poultry litter) are usually priced consid-ering their N content, making them less desirable for fall application to wheat, corn, and soy acres this fall. DAP, 18-46-0, is a popular fertilizer P source and the most recent DTN survey average retail price (the URL just above) was $1005/ton. Urea, 46-0-0, was $836/ton ($0.909/lb N). This means that the 360 lb N in one ton of DAP was worth about $327, and the phosphate value was $678/ton DAP ($0.737/lb P2O5). About a third of the price of DAP is in the value of N it contains – N that is less likely to be needed this fall. You might ask your fertilizer retailer to bring in triple super phosphate (0-46-0) so that you can meet your fall phosphate needs without losing money on unnecessary N.

FERTILIZER PLACEMENT (banding) improves fertilizer P and K use efficiency, relative to broadcast fertilizer. AGR 1 (http://www2.ca.uky.edu/agcomm/pubs/agr/agr1/agr1.pdf) indicates that in spring, if soil test P and/or K are very low or low, one-third to one-half the recommended rates of P2O5 and/or K2O for corn can be used if it is banded 2 to 4 inches from the row. Relevant research for Kentucky soils is not available, but I’d estimate that precision (GPS guided) banding fall applied P and K would similarly improve their use efficiency relative to fall broadcast P and K. Precision fall banding would likely be superior to spring broadcasting, though not as good as spring banding, as long as corn is planted 2 to 4 inches from the banded P and K. Precision fall placement anticipates precision spring planting.