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Has your alfalfa been nipped by frost?

Jeff Caldwell 04/26/2012 @ 8:17am Multimedia Editor for Agriculture.com and Successful Farming magazine.

The alfalfa crop in much of the Midwest has gotten a boost from the early start to spring. But, temperatures have dipped in the last month, leaving some fields susceptible to frost and freeze damage. So, how can you tell if your crop's been nipped by a spring chill?

"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," says Iowa State University Extension forage management specialist Steve Barnhart. "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."

If spring temperatures get down to around 27 degrees or lower, damage to both established stands and new seedlings is likely. It typically takes about a week, Barnhart says, to tell if damage has occurred. Alfalfa plants gradually lose their winter hardiness as spring advances, so the later the freezing temps hit, the worse the damage can be.

"Well established, developing forage plants have lost their winter cold hardiness. Exposed tissue is susceptible to cold temperature injury. Several hours of 25-27 F temperature, or lower, will damage leaf tissue and may seriously damage buds and growing points," Barnhart says. "At emergence, alfalfa and most winter hardy forage grass and legume seedlings have good cold tolerance. But, spring cold snaps can hurt new seedings too."

The key thing in gauging potential damage is the amount of time temps sunk to the mid-20s. If it's been for "several hours," you're likely facing more damage. But, if it's that cold for a shorter time, damage could be lighter.

"A 'light' frost/freeze where temps don't go below around 27 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," Barnhart says. "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."

If your crop has been weakened by a frost, the effects may not be immediate. It may instead come later in the season in the form of greater disease susceptibility, Barnhart says. "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," he says.

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