KILLING FROST AND MANAGEMENT OF ALFALFA AND SUDANGRASS
areas may receive the first frost of the season tonight. Most corn and soybeans
in SE Iowa are mature or close enough to
maturity that affects on yields should be minimal. Most questions that come
this time of year regard managing forages.
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 would be entirely at the
expense of root reserves. To optimize plant development and its
over-wintering ability, allow the plant to grow until a killing frost or
mid-October; which ever comes first. If no killing frost occurs by
mid-October and a harvest is desired, harvest the forage. The short daylengths and cold autumn temperatures will minimize the
use of root reserves prior to the "soon-to-come" killing frost.
However, it is often difficult to make hay in mid-late October because of the
There is an old, persistent false-hood around that alfalfa becomes toxic
following a frost. Alfalfa does not contain any "toxic" compounds
that arise from exposure to frost. Where this rumor may have started from
is the fact that alfalfa can cause bloat, and immediately after a frost
alfalfa's bloat potential is higher than normal. Once the frosted parts
of the plant dry, alfalfa's bloat potential is back to normal.
Sudangrass and sorghum-sudan hybrids
require 28 degrees 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. A "light"
frost may damage just the tops of plants. If this occurs, delay grazing
or harvest 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. Although
haying these forages this late in the season is nearly impossible because of
poor dry-down conditions. 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 a proper moisture for your storage structure to ensure
proper 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!