When the first frost hits in the fall each year, the question regarding possible prussic acid poisoning of grazing livestock—or livestock that are being fed green-chopped forage— always pops up. Most concerns about the first fall frost deal with managing sorghum species including sudangrass.

When these forage species are injured with the frost, the plant tissue releases prussic acid for a few days, which can be toxic to livestock. If plants are only partially killed, which is likely with a light frost, new shoots may start to grow which have more concentrated levels of prussic acid.

BEWARE OF THESE TWO: Sorghum and sudangrass are the two forage species you have to be wary about when first frost hits in the fall. The first few frosts of fall bring out the potential for prussic acid poisoning of cattle and other livestock that are fed forages, to the surface. Historically, in Iowa there are very few documented cases of prussic acid poisoning. But the risk is there, so you need to manage the risks.

* Sorghum and sudangrass. Livestock should be removed immediately if a frost occurs and kept out for 5 to 6 days, says Steve Barnhart, Iowa State University Extension forage agronomist. If new shoots develop, livestock should be removed until the shoots are the appropriate height to graze (18 to 20 inches for sudangrass and 24 to 30 inches for sorghum-sudan), or until 5 to 6 days after a hard freeze which kills all shoots completely.

For more information on managing sorghum species in the fall, read Steve Barnhart's article here by clicking on this link. His article is titled "Prussic Acid Poisoning Potential In Frosted Forage."

Is it safe to feed alfalfa to livestock after field is hit by first fall freeze?

* How about alfalfa? Because of the concerns with sorghum species, there is a false belief that alfalfa also becomes toxic with the first fall freeze. Barnhart says that is not correct. Alfalfa does not become toxic with the freezing temperatures, but there is a slightly greater risk of bloat, so the standard bloat prevention practices should be used (For example, don't turn hungry cattle out into an alfalfa field where they might gorge themselves on the lush alfalfa).

* Alfalfa needs a fall rest period. Because of the shortage of hay and high price of hay this year there will be the temptation to try to get a last cutting of alfalfa this fall. Alfalfa has been under a lot of stress with the drought, which makes the fall rest period even more important this year, says Virgil Schmitt, ISU Extension field agronomist in eastern Iowa. Temperatures in the upper 20 degree range will not stop alfalfa growth, and more grow of the plants is likely in the next week to 10 days.

It's best to wait until after a killing freeze (mid-20s) to try to take another cutting of alfalfa to minimize the stress on the plants going into the winter. "However, the later into October we get, the less will be any new top growth during the remainder of the year," says Schmitt.