Typically, farmers are cautioned about harvesting alfalfa within 5 to 6 weeks before a killing frost occurs. Agronomists generally advise that you are better off to not harvest alfalfa after mid-September. When the first killing frost for alfalfa occurs - usually late October in northeast Iowa - then you can make the cutting, if you really think you need the feed and must take a fall cutting of the crop.
A killing frost for alfalfa is when the temperature dips to about 24 degrees F for 4 to 6 hours, says Brian Lang, Iowa State University Extension field agronomist at Decorah in northeast Iowa. "Harvesting alfalfa between mid-September and the first killing frost puts the stand at higher risk of winter injury," he explains. "By not harvesting it after mid-September you give the alfalfa stand a 5 to 6 week rest period in the fall. That allows the plants to store some food reserves in the roots so the stand can better survive the rigors of winter."
However, there is another challenge that is often encountered with a late October harvest of alfalfa. You are likely to have poor drying conditions for the hay. That's why most farmers would like to harvest the hay in late September or early October, while the weather is still more favorable. "If you do wait until late October or when the first killing frost occurs, you may decide to not make hay out of the alfalfa. Often, the last harvest is best harvested as haylage," notes Lang.
Leave a high stubble of 4 to 6 inches to help catch snow for winter
Another consideration is economics. "You want to leave a high stubble height of 4 to 6 inches to insulate the stand and help catch snow for the winter," says Lang. "If the fall stand is only a foot tall or so, and you need to leave some stubble, you need to ask yourself - is there enough harvestable forage to justify your harvest expenses? Plus you will put a little more stress on the stand with the harvesting of it versus not harvesting it. Research also shows that next spring's harvest of the crop tends to be a little higher if no fall harvest is taken versus if a fall harvest is taken. It will grow taller next spring if you don't take a fall cutting from the alfalfa stand this fall."
Occasionally, Lang will hear from some farmers who think they need to take a fall harvest of a good growth of alfalfa because if they don't do it, the alfalfa will lodge and smother the stand over winter. "This is NOT true," he says. "Alfalfa will not smother itself. That's only a possibility if you also have a high percentage of grass in the field, with grass providing the risk of smothering the alfalfa stand."
Finally, you need to realize that "fall" harvest of alfalfa is just one stress factor that will contribute to possible winter injury, he says. Other factors include alfalfa variety selection (inherent winterhardiness and disease resistance), soil fertility, pest management, wheel traffic, fall soil moisture conditions (wet falls increase risk of winter survival), and leaving some forage cover (stubble height) for insulation into the winter. "The better you manage these factors, the less stress you will put on the alfalfa, and the better the alfalfa stand can tolerate your less than ideal fall harvest management," says Lang.
How should you manage sorghum-sudan harvest in the fall?
What about sudangrass or sorghum-sudan? These crops require 28 F for a killing frost, however even a "light" frost requires special management. Prussic acid accumulates in the frosted tissue within a few hours after thawing and wilting, he explains. A "light" frost may damage just the tops of plants. If this occurs, you need to delay grazing or delay harvest for 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, Lang says you should 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 (sudangrass 18 inches or taller and sorghum-sudan 28 inches or taller), but more than likely a complete killing frost will occur before that would happen. Once a complete killing frost occurs, wait until the frosted tissue is drying out (usually about 10 days) before grazing or harvest.
You're better off green chopping sorghum sudan this time of year
"If you are haying the sorghum-sudan forage, the curing process decreases the prussic acid content as much as 75%, which removes the feeding concern," he notes. "However, haying these forages this late in the season—in October--is nearly impossible because of poor dry-down conditions." You're better off green chopping sorghum-sudan in the fall instead of trying to bale it.
If green-chopping the forage, Lang recommends you 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. Good fermentation 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," he warns.