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Serving: IA
Alfalfa field
BEST BET: If you make the final cutting of alfalfa too early in the fall, plants regrow too much and may not survive winter.

Fall harvest of alfalfa: Now or after killing frost?

Extension agronomist says don’t harvest now, but don’t wait for a killing frost, either.

When is it best to make the final cutting of alfalfa for the year? Should you do it now, in early October, or wait until after the first killing frost occurs this fall?

“I suggest neither,” says Brian Lang, Iowa State University Extension agronomist in northeast Iowa. “Don’t harvest now, but you don’t have to wait for a killing frost either. However, you should wait at least until the last half of October to make the last harvest for this year.”

On average, the alfalfa killing frost (25 degrees F) in northeast Iowa occurs in the third week of October, but current long-term weather forecasts suggest the first killing frost for 2019 will not occur until mid-November, he notes.

“Not that we can trust long-term forecasts. But it really doesn’t matter relative to the goal at hand,” Lang adds. “What does matter is to avoid accumulating more than 200 growing degree days [base 41 degrees] from the cutting date to the date of the first killing frost.”

Best to wait to make final cutting

“Accumulating more than 200 GDDs between cutting date and killing frost would contribute to a significant depletion of carbohydrate reserves that we count on to get the plants through the winter,” Lang explains.

While it’s too wet to cut now, if you did cut now, the estimated GDD accumulation from Oct. 2 to mid-November would be 350 GDDs, which is well over the max 200 GDDs concern. If waiting until Oct. 15 before cutting, the estimated GDD accumulation for the last half of October (130 GDDs) plus the first half of November (50 GDDs) is 180 GDDs total, which is in the “safe” range of fewer than 200 GDDs.

“Anytime we target alfalfa for a cutting in late fall — near the date of the killing frost — we also would like to leave a good stubble height, approximately 6 inches, to help trap snow and insulate the plants,” Lang says. “Most farmers tell me that modern hay-cutting equipment can’t be set that high, but do what you can.

“If we compared risks to an alfalfa stand of cutting now, in early October, and getting some regrowth yet this fall to catch snow but depleting carbohydrate reserves versus cutting the crop in late fall, in late October, at a short stubble height, the late-short cut is the better scenario of the two. But a late cut with 6 inches of stubble height would even be better.”

How about sorghum-sudan forage?

After fall harvest — keep livestock out of the sorghum-sudan forage fields for now. “This is a reminder that without the occurrence of a killing frost [28 degrees], fall-harvested sorghum-sudan forages will still try to regrow,” Lang notes. “This regrowth of new shoots will be high in prussic acid concentration. So, after harvest, keep livestock off of these fields until after a killing frost followed by the plant tissue turning wet-dark green to black, and then in five days or so dry out to a tan-gray color. Once at the tan-gray color, the forage is safe to graze.”

For a refresher on issues with sorghum-sudan and frost, Lang recommends reading this short article from Michigan State University.

Check corn for stalk rot, ear molds

Lang also reminds farmers they need to prioritize cornfields for harvest. Aside from using grain moisture to rank fields for harvest, also consider standability. Check fields for stalk rot. As cornfields mature (reaching black layer, the R6 stage), they should be scouted for stalk rot.

“If a field has about 15% stalk rot or more, the risk of significant lodging is high enough to justify harvesting the field on the early side,” he says. “Test the stalk firmness by pinching the lower internodes with thumb and forefinger. Healthy stalks are firm and cannot be compressed. If a stalk can be compressed or feels soft, it is rotted and is a good candidate for lodging. Randomly check about 100 plants per field for a good assessment of conditions.”

As you check corn for lodging risk, look at the condition of some ears. Another trigger for an earlier-than-intended harvest is evidence of ear molds, Lang says. If a field has 10% or more of the ears with more than 10% to 20% mold, a field should be harvested as soon as corn moisture content reaches a level that can be harvested. This increases drying costs but is less expensive than loss of crop due to potential for mycotoxin development, he says.

Moldy corn should be dried to 15% moisture or less for short-term storage, and 13% to 14% for long-term storage. Overall, very moldy corn is a poor candidate for storage. If a mycotoxin analyses indicates safe levels, selling the grain or feeding to a less-sensitive livestock species may be a better alternative than storage. To help with identification and management of ear molds, Lang provides this list of free PDF printable resources:

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TAGS: Corn Harvest
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