Early Frost And Nitrates - FAQ
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 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 varieties 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) 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/sheep 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.
Fall Management of Perennial Forage Stands
Charlotte Ward, MSc, PAg – Regional Forage Specialist - Yorkton
The typical production schedule for a perennial forage crop usually consists of 3 to 5 years of high yield (honeymoon period) followed by a rapid decline in productivity. Once this 3 to 5 year honeymoon period is over, factors such as declining soil fertility and decreasing plant numbers typically result in lower production. Another key factor that will affect long-term stand productivity is timing and frequency of harvest – whether or not producers take a 2nd cut of hay or grazing and the timing of that harvest.
Proper management of these stands from now until freeze up can help to mitigate the natural decline in productivity.
How can soil fertility affect stand productivity and why should be thinking about it this fall?
As we harvest perennial forages and remove the plant material from the field, we are also removing a tremendous amount of nutrients from the soil as well. High yielding forages will remove more nutrients than low yielding forages, which will in turn likely remove more nutrients than if the stand were grazed. Over time we need to replenish these nutrients. Manure, commercial fertilizer and even feeding livestock on the field can all be used as tools to import nutrients.
If alfalfa makes up 50% or more of the production in the stand and was properly inoculated at seeding, nitrogen is generally not a concern in the forage stand as the alfalfa will fix all the nitrogen that the stand needs. Phosphorus, potassium and sulphur are the three nutrients that we tend to focus more on in stands that contain a significant portion of legumes as legumes are high users of these nutrients. Phosphorus and potassium are of particular importance as they contribute to root and nodule health and the over-wintering capability of the plants. Both of these nutrients can be fall applied as they are relatively immobile in the soil and they will not leach or volatilize to the atmosphere like nitrogen will. Fall application of these nutrients can also help to decrease the work-load next spring. The most cost-effective way to know where your stand is at in terms of soil fertility is to do a fall soil test.
How does the timing of harvest and frequency of harvest affect forage yield?
Improper cutting of alfalfa stands can lead to winter kill. Understanding alfalfa physiology can help to avoid this problem. In the plant, energy is produced in the leaves through the process of photosynthesis and is used to fuel plant growth. As the plant produces excess energy, it is stored in various plant parts such as the roots and crown. The stored energy is used for regrowth following cutting, plant maintenance over winter and growth during early spring.
When an alfalfa plant is cut, few leaves remain and the plant may draw on stored energy reserves for regrowth. Generally the plant will need 6 weeks to replace leaves and replenish stored reserves to pre-cutting levels. Thus there is a 6-week critical period after cutting that the plant needs to ensure that it has good energy reserves going into winter. This is of particular importance to producers looking to take a second harvest off their fields, either through mechanical harvest or grazing livestock.
Plants harvested after August 15th may not obtain six weeks of good weather of replenish reserves before a hard frost and can go into winter with low energy levels. Plants with low energy levels are more susceptible to winter kill. If it is necessary to take a second harvest, producers should wait until after a killing frost as the plants will shut down and will not try to mobilize energy reserves.
Fall harvests should be approached with caution as harvesting plants after August 15th reduces the stubble height of the field which is important for trapping snow and spring moisture next year. Standing alfalfa will not only trap snow this winter, but will also cover the soil next spring and summer to minimize moisture lost through evaporation.
What can producers do this fall to optimize forage yield next year?
· Avoid cutting between August 15th and the first killing frost
· Ensure there is adequate soil fertility, including phosphorus and potassium which is particularly important for root health and development as well as nodule health
· Manage stubble for maximum snow cover
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