dejavu - according to this article, your information is incorrect. What the information does not tell me is whether it is O.K. to cut and feed the frosted alfalfa/grass mixture. The cell damage is not reversed by cutting, but the plant's physiology is changed when it it cut. But I don't know if the risk is reduced or eliminated by cutting.
What Is a Killing Frost? How Cold Does It Have to Get to Stop Corn and Soybeans?
Published: Sep 28, 2009
. . . That's the answer given by Brian Lang, Iowa State University Extension field agronomist at Decorah in northeast Iowa . . .
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."
There is an old, persistent false-hood that you often hear, that alfalfa becomes toxic following a frost. "Alfalfa does not contain any 'toxic' compounds that arise from exposure to frost," says Lang. "However, alfalfa can cause bloat, and immediately after a frost alfalfa's bloat potential is higher than normal."
An over-simplified and very brief explanation for this is: Bloat is largely caused by a rapid release of soluble proteins into the rumen. Alfalfa has considerable amounts of soluble proteins. As cattle eat alfalfa, their chewing action breaks up plant cells and slowly releases the soluble proteins into the rumen. If the cattle eat recently frosted alfalfa, they eat already-broken cells from the frost which rapidly releases soluble proteins (not a slow release), which increases the chance of bloat. Once the frosted parts of the plant dry, alfalfa's bloat potential is back to normal.
Beware with sudangrass, sorghum-sudan to avoid prussic acid
Sudangrass and sorghum-sudan hybrids require 28 degrees F for a killing frost, says Lang. However, he points out that even a "light" frost requires special management. He offers the following explanation and recommendations.
Prussic acid accumulates in the frosted tissue of these plants within a few hours after thawing and wilting. A "light" frost may damage just the tops of plants. If this occurs, you should delay grazing or harvest—delay it by 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, 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, but more than likely a complete killing frost will occur before that would happen. Once a complete killing frost occurs, wait at least 10 days (wait until the frosted tissue is drying out) before grazing or harvest.
If haying the forage, the curing process decreases the prussic acid content as much as 75%, which removes the feeding concern. If green-chopping the forage, 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. This 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.
For entire article go to:
http://www.wallacesfarmer.com/story.aspx?s=31862
