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Frequently
Asked Questions: Early Frost and Nitrates
Updated:
July 2006
Ruminant
Nutritionist, Agriculture Knowledge Centre, SAF
What happens to the plant after a frost?
Essentially, the cold temperature freezes the water in the plant cells
creating crystals that enlarge and expand, creating a rupture and leakage.
When the plant cells are warmed by the morning sunlight, the water crystals
melt leaving damaged cells behind. The longer the duration of the frost
on the plant, the more severe the damage will be. The damage to the plant
will depend on the duration and temperature affecting the plant. Some
reports indicate that prolonged exposure of several hours with an air
temperature of -2oC should be considered a killing frost. Other
reports indicate that damage can occur at an air temperature of 0oC
as the plants will feel the cold four to five degrees cooler.
What parts of the plant are most critical to look
at in the event of a frost?
The first part of the plant to look at is the seed head or pod, if it's
in the flowering to seed filling stages. Most commonly, killing frosts
will stall any further development of the seed head or pod. If the plant
is in the flowering stage, it is unlikely that conditions would allow
for further seed development. Should the plant be in the seed filling
stage, the plant is more mature and less susceptible to frost. A factsheet
is available at www.agr.gov.sk.ca/docs/crops/frostdamage.pdf
discussing the effects of frost on oilseeds, pulses and cereals.
How can I tell how severe the frost was?
Usually it takes 24 to 48 hours to determine the extent of the damage.
This can be prolonged in cool, damp weather. Damaged leaves, pods or seeds
will appear watery and lose rigidity. Remember that different plant species
have different frost tolerances. As well, damage will be variable between
cereals, pulses, oilseeds and forage crops, and will vary depending on
what stage of maturity the crop is at. Older crops are more capable of
handling frost than younger crops.
Where do nitrates come from?
Under normal growing conditions, roots are continually absorbing nitrogen
in the form of nitrate for storage in the stalks and stems. The leaves
are the metabolic engines of the plant that utilize the nitrate to build
protein and other nitrogen-based compounds in the plant and seeds. In
the event of a killing frost, the plant development is arrested and the
leaf tissue cells are potentially damaged beyond repair, but the roots
will still continue to transport nutrients from the soil to the stalks
and stems. This is how the potential nitrate build up could occur. In
the event of a frost, one should expect that nitrates will exist in the
plant tissues.
When should I cut my crop as a forage crop?
Several points must be addressed before deciding on when the crop should
be cut for forage, and what method should be used to cut it. To decide
when to cut a forage crop to preserve quality, evaluate and determine
the extent of the damage to the plant during the:
Flowering Stage
– Severe Frost
Under severe frost conditions, growth and maturation of the seed head
or pod will be stopped. The crop will not be harvestable as a grain crop.
If leaf damage is also severe, consider cutting the crop as a forage crop
as soon as possible to capture forage quality, and plan to deal with nitrates
in a feeding management program. Test the forage for nitrates to get an
accurate account of nitrate levels in the forage.
Flowering Stage
– Mild Frost
If the frost is not severe, consider waiting and watching the crop for
two to four days to see if the seed heads or pods continue to mature and
develop. The plant may recover and continue to develop. However, if the
frost was severe, growth will be stopped and one should consider cutting
it immediately to capture forage quality. Test the forage for nitrates
to get an accurate account of nitrate levels in the forage.
Seed Maturation/Milk/Soft
Dough Stage – Severe Frost
Under severe frost conditions, growth and maturation of the seed head
or pod, and filling of the seed head or pod, will be stopped. It is unlikely
that the crop can be harvested for grain. However, this is a good stage
to be cutting the crop for optimal forage quality. If leaf damage is severe,
consider cutting the crop as a forage crop as soon as possible to capture
forage quality. If leaf damage isn’t too severe-the grain heads
or pods are just starting to fill and the environmental conditions aren't
conducive to cutting and baling-there is more flexibility in waiting several
days versus cutting the day after the frost. Test the forage for nitrates
to get an accurate account of nitrate level in the forage and plan to
deal with nitrates in a feed management program.
Seed Maturation/Milk/Soft
Dough Stage – Mild Frost
Under mild frost conditions, examine the crop for damage to the seed head
or pods. It may take a couple of days to ensure that the crop will continue
to develop as normal. If head, pod or leaf damage is noticeable, a decision
can be made to consider cutting the crop as a forage crop as soon as possible
to capture forage quality. If leaf damage isn't too severe-the grain heads
or pods are just starting to fill and the environmental conditions aren’t
conducive to cutting and baling-there is more flexibility in waiting several
days versus cutting the day after the frost. Test the forage for nitrates
to get an accurate account of nitrate levels in the forage and plan to
deal with nitrates in a feed management program.
What
about millet and frost?
Millet is a relatively new, high yielding crop available to Saskatchewan
producers. However, it is severely affected by an early fall frost. Temperatures
of 0oC to -5oC causing frost may result in some millet
varities turning dark green to black the morning of the frost. Crown or
proso millet, pearl millet and sorghum sudangrass are easily killed by a
frost. The foxtail millets, such as Golden German millet and Siberian millet,
may recover from a light frost.
After a frost, producers
should examine the crop to determine whether the whole plant was affected
or just the top leaves. If the whole plant has been killed by the frost,
producers can cut immediately to preserve forage quality and bulk tonnage,
while planning to do a forage nitrate analysis (note: millets tend to
accumulate nitrates quite quickly, so a nitrate analysis should be considered
regardless of frost). If weather does not permit cutting, the crop can
be left standing until more favourable weather develops, as millets do
not lose their leaf matter as quickly as cereals when affected by frost.
With a light frost, the top leaves may only be damaged and will turn a
tan colour two to three days after the frost. With a light frost, the
growing point of the plant will remain viable and will produce a new leaf
in five to seven days. With damage from a light frost, cutting can be
delayed depending on the amount of growing weather remaining in the season.
Grazing the millet may also be an option should the field be grazing capable
(see below).
There
are rules of thumb that suggest a) cutting immediately the day after frost,
b) waiting two to four days or c) waiting seven to 10 days after a frost.
Which rule of thumb do I follow?
a) The rule of thumb
for cutting the crop either the day before or immediately the day after
a killing frost: cutting before the frost would prevent the stockpiling
of nitrates; cutting the crop immediately the morning after the killing
frost would reduce the time allowed for the roots to store nitrates in
the plant, minimizing the final level in the plant; and cutting immediately
may also maximize the potential forage feed quality, again depending on
the stage of maturity of the crop.
b) As the survival
rate of the leaves goes up, the wait period to cut the forage would go
down. Therefore, consider waiting from two to four days, as there are
more leaves available to utilize the nitrates. Keep in mind, though, that
waiting to cut may result in reduced forage quality and a poorer feed.
c) A waiting period of several days after a frost is based on the following
theory: even after a killing frost, the roots of the plant will continue
to absorb nitrates from the soil and store them in the stem and stalk.
If a large majority of the leaves are killed by the frost, they will be
unavailable to utilize the nitrates, resulting in an accumulation of nitrates.
The rule of thumb would be to wait seven to 10 days to allow the remaining
leaves to metabolize and utilize the stockpiled nitrates.
Which
is the higher priority - concern with nitrates or forage quality?
The decision on when to cut a crop for forage after a frost should be
based on forage quality first, and nitrate content second. Essentially,
a severely frozen crop will have a damaged seed head and will not mature
as it would under normal growing conditions. Further waiting may also
result in the plant losing leaf matter to leaves drying up or rotting,
potentially further reducing forage quality.
Also, one cannot determine
nitrate content of a forage immediately after a frost while it is still
standing in the field. The producer should first cut the forage to capture
forage quality, assuming that there is an opportunity for nitrates to
be present, then plan to test the forages for nitrate content at a later
date. The decision to wait on a crop to eliminate nitrates involves more
guesswork than control, as there is no indication if nitrates are even
present in the first place.
Waiting may occur
by default as haying conditions may not be present at the time one intends
to cut. Producers can deal with the nitrate issue with a feed management
program.
Can
I graze a frost damaged field with my cattle or sheep?
Turning cattle or sheep out onto a field affected by a severe frost requires
extra management and attention. The cattle should only be allowed into
the crop for 30 minutes to one hour the day following the frost, AFTER
being fed in the morning, and then they should be removed from the area.
Repeat the next day and slowly introduce the cattle/sheep to more and
more of the crop over a five to seven day period to allow the animals
to adjust to the nitrates and the higher quality forage. There will be
an adjustment period by the stomach bacteria over that time period. Caution
should be exercised, as sudden introduction to the crop will result in
nitrate overload and nitrate toxicity.
What
about cutting the forage crop for silage? Will that reduce the nitrate
content?
In order for good silage to be made, the crop has to be cut at the most
opportune stage of growth. For cereals, this will normally range from
the late milk to mid dough stage of maturity. Making silage may or may
not reduce nitrate content. If inadequate packing of the silage occurs,
nitrate content may be reduced at the cost of making poor silage. If the
crop is packed into a ground pit silo, leaching may occur, allowing for
some nitrate loss. Take samples of the silage as it is dumped into the
silo and when the ensiling process is complete to fully determine what
the nitrate content is.
What
nitrate levels should I be concerned about?
Pay attention to a nitrate level of 0.5 per cent (dry matter basis). Any
levels above 0.5 per cent will require extra management decisions to be
made. A fact sheet on nitrate toxicity and management can be found under
Production | Livestock | Health.
Practical Conclusions
and Final Decisions
Theory suggests that undamaged plant leaves can use stockpiled nitrates.
However, when cutting immediately after a frost or waiting several days
post frost, the presence of nitrates will depend on fertilization levels,
weather conditions during the growing season, and severity of the frost.
The most simplified approach is to first assess the damage, then decide
on a course of action based on the severity of the frost. If plant development
is affected, and a decision to harvest the crop for forage is made, consider
cutting immediately to prevent any further deterioration and capture the
best forage quality possible. If there is a concern about overall nitrate
level, one should consider waiting several days before cutting, but only
if there are adequate numbers of surviving leaves to utilize the nitrates.
Finally, plan to have
nitrates in the forage after a severe frost regardless of when the crop
is harvested. It is also important to prepare a feed management plan when
utilizing a forage containing nitrates.
Recommendations on
how to utilize high nitrate forages/feeds are available on this site or
contact: Agriculture Knowledge Centre at 1-866-457-2377
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