The last week in April, air temperature descended to 23?F for as long as 5 hours in some locations in Northern Nevada. Winnemucca reported over 6 inches of snow on April 30th. So, how do these low temperatures affect the alfalfa crop?
Literature indicates that frost damage in alfalfa will occur when non-dormant alfalfa plants are exposed to temperatures below 25?F for at least two hours. Cold injury risk is reduced where vegetative growth or cover is protecting the new seedlings or forage growth lower in the plant canopy.
Air temperature, a few feet above a bare or grass covered soil surface, is what is measured and reported. Plant tissue temperature is influenced by leaf surface color, density of the plant canopy, air movement within the canopy, the temperature of the soil, and likely more subtle conditions.
The air within the forage canopy is likely ‘layered’, meaning the temperature at the top of the canopy is colder than the temperature at the soil surface, and below the soil surface in the taproot and crown area. Simple statements about the influence of the reported temperature can be misleading. To complicate things a bit more, tolerance of leaves to frost varies somewhat among varieties and individual plants, and is not always related to winter hardiness of the variety.
Alfalfa stems can take much lower temperatures than leaves. A hard frost will cause the alfalfa stems to bend over with a “shepherd’s hook” or act as a lazy stem. If after a few days the stems straighten back up, the stem is uninjured and will resume growth with no yield penalty.
Plants use up some of their root reserves for their initial spring growth. After growing points are frozen, these plants will then have to initiate regrowth from new crown or axillary buds. This will delay the growth and developments of the crop as well as use up more of the remaining root reserves.
Healthy stands will recover more quickly. Depending on weather conditions, some first-cut yield reduction and a delay in maturity can be anticipated. Where damage is uneven across a field, there can be some unevenness in maturity. If possible, delay cutting of severely affected fields to allow rebuilding of root reserves and full recovery.
New Forage Seedings: at emergence, alfalfa and most winter hardy forage grass and legume seedlings have good cold tolerance. However, spring cold snaps can hurt new seedings too. For alfalfa, at second trifoliate leaf stage (and older) seedlings become more susceptible to cold injury and may be killed by four or more hours at 260 F or lower temperatures. Alfalfa seeded with a companion crop survives lower temperatures and longer exposure times before showing frost damage.
Established Stands: well established, developing forage plants have lost their winter cold hardiness. Exposed tissue is susceptible to cold temperature injury. Several hours of 25-270 F temperature, or lower, will damage leaf tissue and may seriously damage buds and growing points. Upper 20s0 F air temperatures will likely damage one to several sets of trifoliate leaves that were exposed at the top of the canopy. The buds and growing stem tips are somewhat more protected and often continue to grow normally.
There will likely be leaf tissue damage where overnight temperatures go 250 F to 270 F or lower for several hours. The only management suggestion at the moment is to wait a week or so to see what the damage is.
If plants do not recover from the initial wilting and die back to the ground, the plant is dead. At least one set of leaves must have escaped damage for the plant to recover. If more than 20 plants per square foot remain, stand will survive in good shape. If stands are lower than 20 plants per square foot consider reseeding.
If new seedings were permanently damaged, consider re-seeding as soon as possible. Keep the good areas and drill into thin or damaged areas. Tillage may not be necessary.
For established stands; a “light” frost/freeze where temperatures do not go below around 270 F or so for very long is likely to freeze several sets of trifoliate leaves on the alfalfa tops and set back growth rates for a while, but plants will grow out of it. No need to cut, although some growers seeking very high quality might do so if standing yield is high enough to justify harvest with the understanding that plants will be weakened by early cutting and should be allowed extra time to recover before the next cutting.
A freeze that penetrates about halfway down into the alfalfa canopy will likely kill the top-most stem tip/growing points. These plants will continuing to grow too, producing more branching below the freeze zone; others may produce new stems from below ground, crown buds. This kind of regrowth will be slow to initiate.
If yield is high enough to justify harvest, and plants have reached 15 to 20 inches of height or bud stage, the stands probably should be cut, knowing that extra time will be needed for recovery before the next cutting.
If yield of standing crop is low, or the plants were not nearly ready to cut, probably best to just wait out the delay in regrowth. It will be hard to justify the time and expense of cutting/shredding with no immediate harvestable crop.
A colder, freeze likely will freeze plants all the way to the ground and kill the above ground stems. April frosts and freezes are not likely to be cold enough to damage underground crowns, so the plant is not dead. Harvest of frozen plants is warranted if yield is sufficient, but must be done immediately. If frozen plants collapse much of the biomass will be lodged.
Frozen leaves will shatter quickly from stems as they dry, so good handling and harvest management is needed to salvage the forage. If you are not going to harvest this frozen plant material, there is no benefit from shredding damaged tissue, the new stems will grow up through the frosted, lodged stems.
After a freeze that causes visible damage to alfalfa tissue, the plants are under some stress, and will be more susceptible to damage from foliar diseases and sometimes, insects, on regrowth, so continue scouting fields.
Replanting severely damaged established fields may be necessary. If there has been widespread, sever cold injury to established stands, consider replanting a new alfalfa stand in an adjacent field due to autotoxicity effects of older stands.
