University of Nebraska-Lincoln research has repeatedly shown the positive yield results from planting soybean early. But, according to Jim Specht, UNL agronomist, producers must also recognize that there are three increased risks: A late spring killing frost, germination failure and the migration of overwintering bean leaf beetles to early planted soybean fields.

If weather issues pop up again before planting, you will still be better off staying with your chosen seed varieties, plus populations, until Memorial Day for corn and mid-June for soybeans.

(Original publish date: Sept. 28, 2009) Just what is a killing frost? The temperature has to get down to 28 degrees F for a complete kill on corn and soybean plants. Temperatures above 28 degrees F don't kill the entire plant, but will damage the leaves and the upper stem. That's the answer given by Brian Lang, Iowa State University Extension field agronomist at Decorah in northeast Iowa.

Frost-killed plants can no longer accumulate carbohydrates in the grain so the maximum yield potential is reached when a killing frost hits, he explains.  While temperatures above 28 degrees F do not kill the entire plant, the damage to the leaves and upper stem tissue reduces the photosynthetic area of the plant and its ability to transport carbohydrates from these areas to the grain.

What is the thumbrule on yield loss for frosted crops?
Corn that is within one week of physiological maturity ("black layer") killed by frost would be at about three-fourths milk line and would suffer a yield loss of only about 3%.

Soybeans that are within one week of physiological maturity and are killed by frost would suffer a yield loss of about 5% to 10%.  Soybeans that are within one week of physiological maturity are two-thirds through the R6 stage of growth.  Soybeans that still have green beans in the pods should be left in the field for normal dry down, and then left in storage long enough for the green beans to turn to the normal yellow-brown color, advises Lang.

He says alfalfa usually requires 24 degrees F to completely kill its topgrowth.  Temperatures above 24 degrees F will cause visible damage, but the plant will continue to grow using the remaining leaf area. The main reason not to harvest alfalfa after a light frost is that the harvest would remove all of the leaf area, and the plant's continued development until a true killing frost and plant dormancy would be entirely at the expense of root reserves. 

When should you time the final cutting of alfalfa?
"To optimize plant development and its over-wintering ability, you should allow the plant to grow until a killing frost hits or until mid-October; whichever comes first," says Lang. "If no killing frost occurs by mid-October and a harvest is desired, harvest the forage.  The short daylengths and cold autumn temperatures of mid- to late October will minimize the use of root reserves prior to the soon-to-come killing frost."

There is an old, persistent false-hood that you often hear, that alfalfa becomes toxic following a frost. "Alfalfa does not contain any 'toxic' compounds that arise from exposure to frost," says Lang. "However, alfalfa can cause bloat, and immediately after a frost alfalfa's bloat potential is higher than normal."

 An over-simplified and very brief explanation for this is: Bloat is largely caused by a rapid release of soluble proteins into the rumen.  Alfalfa has considerable amounts of soluble proteins. As cattle eat alfalfa, their chewing action breaks up plant cells and slowly releases the soluble proteins into the rumen. If the cattle eat recently frosted alfalfa, they eat already-broken cells from the frost which rapidly releases soluble proteins (not a slow release), which increases the chance of bloat. Once the frosted parts of the plant dry, alfalfa's bloat potential is back to normal.

Beware with sudangrass, sorghum-sudan to avoid prussic acid
Sudangrass and sorghum-sudan hybrids require 28 degrees F for a killing frost, says Lang. However, he points out that even a "light" frost requires special management. He offers the following explanation and recommendations.

Prussic acid accumulates in the frosted tissue of these plants within a few hours after thawing and wilting. A "light" frost may damage just the tops of plants. If this occurs, you should delay grazing or harvest—delay it by a few days after frost to allow the prussic acid to dissipate from the plant tops.  Livestock can be returned to frost-injured sudangrass (18 inches or taller) and sorghum-sudan (28 inches or taller) after 5 to 7 days.

Sometimes a "light" frost enhances development of young shoots from the base of the plants. If this occurs, delay sending livestock to graze this forage since these new shoots would be high in prussic acid. Ideally, wait for the new shoots to get to a proper grazing height, but more than likely a complete killing frost will occur before that would happen. Once a complete killing frost occurs, wait at least 10 days (wait until the frosted tissue is drying out) before grazing or harvest.

If haying the forage, the curing process decreases the prussic acid content as much as 75%, which removes the feeding concern. If green-chopping the forage, chop only as much forage as the cattle will consume in 4 to 5 hours. Never green-chop the forage and let it sit on the wagon overnight. If ensiling, harvest at proper moisture for your storage structure to ensure good fermentation. This takes a minimum of 4 weeks. The fermentation process will reduce the prussic acid content. Since immature plants can contain higher prussic acid levels, leave this forage ferment for at least 8 weeks before feeding. Never allow horses to graze sorghums or sudangrass at any time.