Extreme weather makes winterkill tough to stop

By Nathan Gregory
MSU Extension Service

In less than half a year, Mississippi pastures have endured drought conditions and subfreezing temperatures, but landowners can soften the blow winterkill deals to their winter grazing systems.

Rocky Lemus, forage specialist with the Mississippi State University Extension Service, said moderated grazing, cover crop planting and adequate soil nutrition can keep winterkill in cool-season annual grasses from being widespread. Some forage loss is still to be expected.

Lemus said the extremes in weather have created challenging growing conditions for even the most cold-tolerant varieties of winter grasses, including annual ryegrass, cereal rye, triticale and wheat. Oats are also widely used in Mississippi, though not as resilient against extreme cold.

“Predicting winterkill is very difficult because it can happen due to different factors such as low temperature kill, ice cover, crown hydration, overgrazing and lack of canopy cover, poor fertilization and stand age,” Lemus said. “The rapid decline in soil temperature during freezing and thawing cycles favors low-temperature kill.

“In a cool season annual forage system, most of the winterkill will be related to grazing management, heavy traffic areas, lack of fertilizer, spots with poor drainage, shaded areas along fence lines, and areas covered by ice for 10 days or longer,” he said.

Crown hydration may be the most destructive type of winterkill in Mississippi, Lemus said, because the crown is where grasses develop their root system and new shoots. This is often observed in pastures growing in lowland areas or poorly drained soils where the soil remains frozen below the surface.

“When temperatures start to increase and ice melts, these forage crops begin to de-acclimate and the crowns become hydrated,” he said. “If a drastic freeze occurs during the thawing phase, ice forms inside the crown and ruptures the cell membranes causing loss of moisture and the formation of ice crystals between cells.”

Low-temperature kill can occur when cold temperatures set in and cool-season grasses undergo a dehydration process in response to the lower temperatures, shorter photoperiods and cloudy days.

“This dehydration process increases the potassium and sugars in plant cells that allow the plant to acclimate to freezing temperatures,” Lemus said. “Low soil potassium, low pH and overgrazing can delay this process and make forages more susceptible to the formation of ice crystals in leaves, shoots and crown tissues.

“Avoid grazing or applying nitrogen until the plants have recovered for at least three to four weeks after a drastic winter event depending on temperature changes,” he said. “These limitations restrict their ability to fuel rapid growth and conserve energy for survival until more favorable conditions return.”

Another form of winterkill is ice encasement, also known as anoxia. This occurs when forages are covered by ice for several days, causing a reduction in gas exchange between the leaves and the atmosphere.

“As these active cool-season annual forages continue to respire and oxygen is depleted, they can build up toxic gasses such as carbon dioxide, butanol and ethyl butyrate that can kill the plant,” Lemus said. “The degree of damage may depend on the thickness of the ice, the duration of the ice on the surface and the forage species.”

Producers use a variety of alternatives to keep their livestock fed in addition to rationing forage and storing extra hay. One of these is stockpiling pastures — blocking off some fields before the season’s first frost exclusively for winter grazing.

“Having access to stockpiled pastures is one other way to have some roughage source other than hay or commodity supplementation,” said Brett Rushing, MSU Extension forage agronomist. “Unfortunately, by this time of year, utilization of stockpiled pastures has already been conducted, and producers are forced to dive into hay stocks during these weather events.”

Pastures with widespread winterkill will require more time, heat and care to re-establish.

“With increasing temperatures, we have seen quick response to stands that were knocked back, and we expect full recovery for late winter and early spring grazing,” Rushing said. “Just give it some time, limit grazing and follow nitrogen recommendations as daytime temperatures increase.”

Each month, MSU forage specialists provide guidance and industry updates in the email newsletter Forage News. To subscribe to the newsletter, email Lemus at rocky.lemus@msstate.edu .

More issues of Forage News are available at http://extension.msstate.edu/n ewsletters/forage-news .

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