Healthy soil and sustainable growing

["TradingPostPaul" <tradingpost@riseup.net>]

>http://acreage.unl.edu/news/News/FreezeForage.htm

Freeze Effects on Forages

In the next couple of weeks, we will most likely see a hard freeze in
Washington County. This frost will signify the end of the 2006 growing
season. A hard freeze will also have ramifications for those who are
grazing forages.

When plants freeze, changes do occur in their metabolism and composition
that has the potential to poison livestock. There are procedures that can
be done to prevent this from happening. Sorghum-related plants, like cane
and sudangrass, can be highly toxic, particularly for ruminant animals such
as beef animals, for a few days after frost. Horses and swine are less
susceptible to prussic acid.

Freezing breaks plant cell membranes. This breakage allows the chemicals
that form prussic acid to mix together and release this poisonous compound
rapidly. Livestock eating frozen sorghum can get a sudden, high dose of
prussic acid and potentially die. Fortunately, prussic acid soon turns into
a gas and disappears into the air. By waiting three to five days after a
freeze before grazing sorghums, the chances of poisoning become much lower.

If you do expect a freeze, move cattle to a non-prussic acid pasture the
night before. Otherwise, animals may start eating frozen and risky plant
materials early in the morning before you can move them.

When sorghum- related plant tops only have been frosted, new shoots may
regrow at the base of the plants. These also can be very dangerous because
of high prussic acid content, and because cattle will electively graze
them. It is recommended that farmers do not graze frosted summer annuals
until the regrowth of shoots is 15 to 18 inches tall, or until several days
after the entire plant and shoots are killed by subsequent frost.

Freezing also slows down metabolism in all plants. This stress sometimes
permits nitrates to accumulate in some plants that are still growing,
especially grasses. Since grazing animals primarily consume the leaves of
the plants, the hazard to grazing animals is not great. Unlike prussic acid
which has a greater effect on the leaves of the plant, it is generally the
stems of the plant that accumulate the nitrates. However, greenchop or hay
cut right after a freeze can be more dangerous because it does include the
stems.

Alfalfa reacts two ways to a hard freeze, down close to twenty degrees
causing alfalfa plants to wilt. Here, nitrate levels can increase, but
rarely to hazardous levels. Freezing also causes alfalfa to be more likely
to cause bloat for a few days after the frost. Then, several days later
after plants begin to wilt or grow again, alfalfa becomes less likely to
cause bloat. Waiting to graze alfalfa until well after a hard freeze is a
good, safe management practice.

Freezing of forages is not something to lose sleep over. However, it does
pay to be cautious following a freeze for a period so that problems do not
occur.