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."