Mild Frost Or Killing Frost, Impact On Crops

Mild Frost Or Killing Frost, Impact On Crops

Freezing temperatures September 15 came almost three weeks ahead of when they're normally expected in central Iowa. But that freeze didn't hurt Iowa's 2011 corn and soybeans as bad as some people thought.

As the 2011 crop growing season is drawing to a close, Iowa had some pretty cold temperatures overnight on September 14. It got down to 32 degrees F and even a little below that freezing mark in a few areas of the state. Actually, the cold temperatures occurred early in the morning on Thursday September 15, notes Jim Fawcett, Iowa State University Extension field agronomist in east central Iowa, based at Iowa City. That's almost three weeks ahead of when the first killing frost in the fall is normally expected to occur in central Iowa.

It raised some concern from an agronomic standpoint but not from an economic standpoint. The markets were unimpressed and corn and soybean prices actually went lower instead of higher. That's because the temperatures didn't turn out to be as cold and didn't do as much damage as the forecasts had predicted.

"It was cold everywhere in the state that morning, but the frost occurred more in the area of the state north of highway 30, and in particular north of highway 20," he notes. "If you live along highway 20 or north of that major east-west roadway across Iowa, you experienced some scattered frost."

How low do temperatures have to go to be considered a killing frost?

On corn, 28 degrees F is considered to be a killing frost, says Brian Lang, an ISU Extension agronomist at Decorah in northeast Iowa. Frost-killed plants can no longer accumulate carbohydrates in the grain so maximum yield potential is reached at the time of the killing frost. Temperatures above 28 degrees don't kill the entire plant, but can damage a percentage of leaf tissue and maybe upper stem tissue. This damage reduces the photosynthetic area of the plant and limits its ability to transport carbohydrates from these areas to the grain.

On soybeans, 28 degrees F is also considered to be a killing frost. As with corn, partial frost will reduce some of the leaf area so the plant can't finish filling the grain as it would have. With the exception of very late planted beans, all soybeans were at least in the early to mid-R6 growth stage, if not further along, when the frost hit on the morning of September 15 last week.

At the beginning of the R6 stage of soybean growth, dry matter accumulation of the grain is only at about 50%, says Lang. When soybeans just start to turn color, they are about half way through the R6 stage (R 6.5) and have accumulated about 90% dry matter in the grain. Once one pod on the main stem reaches a mature brown color, the stand is estimated to be at the R7 stage and nearly 100% dry matter accumulation of the grain has occurred.

Iowa corn, beans are far enough along so frost didn't do much damage

There are some concerns regarding this first fall frost of 2011, and the most concern is with crops in approximately the northern third of Iowa. "Mainly it's forages that you have concerns about with a first fall frost," says Fawcett. "I think corn and soybeans this year are far enough along in maturity, particularly since we didn't get a hard freeze, that this frost isn't a big concern for corn and beans."

Fawcett adds, "However, we always get questions from farmers about the sorghum-sudan and sudangrass forages when a first frost occurs in the fall. Farmers want to know how to avoid toxicity problems with cattle that are grazing or are fed these forages."

Alfalfa doesn't cause prussic acid poisoning, but sorghum-sudan can

People sometimes get this situation confused with alfalfa, and think they may have problems with alfalfa. "Don't be confused on this," says Fawcett. "Frost does not cause any toxicity in alfalfa. So there's no problem in having cattle out there eating after a frost occurs on alfalfa. It can make cattle a little more susceptible to bloat, so it's always a good idea to follow recommended bloat reduction management precautions. The bottom line is, don't turn starving cattle out into lush alfalfa, which you should never do anyway."

The real concern is sorghum-sudan or sudangrass, if you've had a first fall frost. Sorghum-sudan can become toxic. "You get prussic acid formation in the injured tissue of the plant," he explains. "With this light frost the main thing is to let the dead tissue dry down before you turn cattle back into the pasture or field to graze the sorghum-sudan. It might take 5 to 7 days or so that you have to wait."

Later in the fall, when a harder freeze occurs, and maybe it kills the sorghum sudan plants down close to the ground, you can sometimes get new shoots growing out from that, says Fawcett, and those shoots will have a higher concentration of prussic acid. "Thus, be sure you use good management practices this time of year with sorghum-sudan pastures. Don't graze the cattle on them unless you've checked things out and know it is safe to graze them."

Some farmers in Iowa began harvesting corn and soybeans last week

Some farmers in east central Iowa and also in other areas of the state started harvesting corn and soybean fields this past week. "I'm amazed at how fast the beans have changed and matured in a week's time," says Fawcett. "I didn't see many bean fields turning last week and this week there are a number of fields that look like they are ready to be combined and actually some harvesting is starting, particularly in the drier areas of southeast Iowa."

Those that are being harvested early are mainly the early-maturing soybean varieties, so these early-harvested fields aren't a real good indicator of what yields we are going to see this year for Iowa as a whole. "I'm hearing of bean yields in the low-40 bushel per acre range for those early maturing varieties of soybeans," says Fawcett. "Hopefully we're going to hear of some higher yields as farmers get into more fields with the combines in the next week or two."

What about corn yields in 2011 in Iowa? "Most of the corn harvest so far has been chopping for silage," says Fawcett. "That's normally not a real good indication of what yields will be, because farmers usually chop the cornfields for silage that they think aren't going to yield well for grain. I haven't heard of a lot of corn grain yields yet. But I think corn yields are going to be all over the board this year. There are some good yielding fields in Iowa and also some poor yielding fields. I'm thinking we'll have decent yields in Iowa in 2011 overall, but we're certainly not going to break any records with the state average yield this year."

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