The sun came up with vapors from frosty plants filling the air as it warmed. I know there’s nothing we can do about the frost, but I appreciate it’s still hard. What we can do is know what to expect next from these replant crops. Lots of information below. A blessing is the warmer predicted temperatures for this week.

Conditions vary due to location, hills/valleys, soil moisture, etc. Temperatures at 31F for a few hours can kill leaves while temperatures at 28F for a few minutes can kill the plant, including the stalk of corn and soybean. Four hours at 24F or lower will freeze the growing point at the top of alfalfa and 26F will kill milo (sorghum). Frost symptoms of water-soaked leaves and/or stems will begin 1-2 days post-frost but it can take as much as 5 days before we see the extent of the damage. One thing I’ve thought about is how we’re blessed with irrigation. I know everyone was tired of irrigating, but many pivots ran before this frost. A higher soil moisture content can allow radiant heat from the soil to potentially keep the canopy warmer than the air temperature outside of the canopy. Hopefully that helps reduce frost impacts.

Corn: If only leaves are killed, the ear will continue to fill because the stalk can continue to move carbohydrates to the ear. If only partial leaves are killed, photosynthesis will continue at a slowed rate. If the stalk dies, yield loss increases. In all these cases, stalk quality will be compromised and with these large ears, be aware of plants going down. Check for stalk rot and harvest timely. Estimated yield losses based on research conducted in 1984 where they used 80 and 105 day hybrids:

• Beginning Dent: Leaves & Stalk killed = 40% yield loss; Leaves only killed: 27% yield loss (23% via National Crop Insurance Services 100% leaf loss at this stage)
• ½ milk: Leaves & Stalk Killed = 12% yield loss; Leaves only killed: 6% yield loss (8% yield loss based on National Crop Insurance Services 100% leaf loss at this stage)

The kernels will eventually form a black layer when there’s no longer nutrients from the placental cells where the kernel tips are attached to the cob. The pre-mature death results in lower test-weight grain (low 50’s to high 40’s) that can be chaffy. Test weights below 53 lb/bu rapidly reduce storage life. Plan on moving low test-weight corn to market before summer and aerating it frequently before then. Kernels can be smaller than normal, misshapen, and may break easily when handled. Grain moisture content will be greater than 35%. Grain drying rates can range from 0.83 to 1.16% moisture less/day when whole plants die prematurely. Research found it took 4-9 more days of field drying to the 22-30% moisture range when corn was frozen before black layer compared to ‘normal’ maturity.

The outer kernel portion will dry slower than the inside part, thus grain moisture will be deceiving. Plan on grain being 1-2% higher than hand-held grain moisture testers. Also plan to dry frost-killed corn 1-2% lower than 14-15% and cool the grain as quickly as possible.

For those planning to dry corn, Peter Thomison from Ohio State shares, “Dry frost-damaged corn at reduced air temperatures (below 160 °F) and store at 14 percent (or lower) moisture. Dry corn as gently as possible, even if it is tempting to crank it up for higher dryer capacity. Also, use slow cooling methods after gas-fired drying to minimize quality problems. If possible, aerate stored grain to cool it to 20 to 30F for winter storage (in the upper Midwest).”

It’s not recommended to make silage from frost-damaged corn because of the excessive moisture in the plant and lower quality. Low test-weight corn and sorghum can be fed to livestock; more information on that here: https://go.unl.edu/0q8i.

Soybean: A killing freeze for soybean is considered 28F. Temperatures below 32F can injure leaves and stems/pods/seeds are injured at 30F. More leaves present can maintain more radiant heat keeping the canopy warmer. If at least one pod was mature on a soybean stem, the remaining seeds on the stem should mature fairly normally with normal colored beans. If just the leaves were froze, but stems remain intact, the beans should continue to fill. I have seen soybean pods that were green in color during a frost result in both round, green-colored beans, or remain green, lima-shaped beans. The beans will be similar to corn in that hand-held moisture testers will read 1-2% low compared to what they actually are. Depending on the percent of green beans at harvest, one may choose to bin the beans and aerate them to allow a reduction in the greenness and moisture in them over time before marketing.

Milo: If the grain hadn’t reached physiological maturity (usually around 40-45 days after flowering), there can be reduced grain yield and reduced test weight when temperatures reach 28F for around 2 hours. Sorghum plants aren’t normally completely killed by frost until temperatures reach 26F. K-State research found yield reduction to be around 20% when the grain is at hard dough stage. They also found a 6.5-27% reduction in grain weight at hard dough and 27-52% reduction at soft dough.

If grazing sorghum forages, it’s best to keep livestock out of them from the first light frost to full hard frost and allow the tissue to completely dry for 5-7 days post hard freeze to avoid prussic acid poisoning.

Alfalfa: Prior to the frost, several had asked about taking another cutting of alfalfa. We don’t recommend that within 6 weeks of a hard frost (24F for 4 hours). One can take a cutting after a hard frost. One can also take a cutting in mid-October even if a hard frost hasn’t yet occurred. The goal with this final cutting is to limit any new regrowth so the plant spends its energy preparing for winter dormancy. I haven’t checked where we’re at, but there’s also a way to consider this final cutting using GDD. Please let me know if you have questions on that and I can share more.

Resources:

Ohio State: https://agcrops.osu.edu/newsletter/corn-newsletter/2019-33/late-season-frost-effects-corn-grain-production-adapted-dr-j

Purdue: https://www.agry.purdue.edu/ext/corn/news/timeless/FrostFreezeImmatureCorn.html

Iowa State: https://crops.extension.iastate.edu/blog/charles-r-hurburgh-mark-licht/harvest-consideration-frost-killed-corn