Assessing and preventing forage winter kill

by Tamara Scully

Whether you’re grazing animals on perennial pastures or baling your alfalfa for hay, keeping your stands healthy and high-yielding takes management. Some factors can be controlled, while others, such as damage from winter kill, are contributed to Mother Nature. While farmers can’t control the weather, there are some manageable factors which impact the winter hardiness of forage stands.

Early spring assessment

If alfalfa forage stands are low-yielding, it may actually cost money to harvest them. Harvest is a fixed cost, and if you don’t have an acceptable yield, the actual value of the crop may be less than the money spent harvesting it, Christine O’Reilly, forage and grazing specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs, said during a Beef Cattle Research Council webinar.

There are ways to mitigate low yields due to winter kill, beginning with assessing stands for damage early in the season. Early assessment can predict stand yield and allow decisions on whether to replant or renovate existing forage stands.

“Because we do that [assessment] right at green-up, you’ve got a chance, if you notice a problem…[to] figure out how you are going to deal with it,” O’Reilly said. “Depending on the severity, you may decide to patch that field, to put a grass or some red clover in it just to get it through this year. If it’s very severe, you might decide to terminate that stand, and rotate it into something else, and plant alfalfa in a different field somewhere else.”

Conduct early spring plant counts in the field at spring green-up. Depending on the age of the stand, the number of green crowns present can provide an initial estimate of potential yields. In pure alfalfa fields, a new seeding should have a minimum of 20 plants per square foot. One-year plantings should have at least 12 – 20, while two-year old stands should have eight – 12 plants per square foot. For third-year alfalfa, five plants per square foot is normal. Below these counts, yield is being sacrificed.

When assessing stands of mixed grass and alfalfa, use the lower range in each category. These numbers tell you how many plants actually overwintered. The next step is to determine the health of those plants, by pulling up some root samples across the field.

The taproot of alfalfa should be creamy white, like a potato. Roots should not be stringy or discolored, which indicate disease.

A more accurate determination of expected yield is a stem count a bit later in the spring. Healthy alfalfa stands, with maximized yield, will have at least 55 stems per square foot. Forty – 50 stems indicate a stand will yield 75 – 92% of its genetic capability. If there are less than 40 stems per square foot, the alfalfa stand should be renovated or terminated.

Winter hardiness

When selecting varieties of alfalfa, full dormancy ratings and true winter hardiness are not fully correlated. Full dormancy ratings are reported on a 1-6 scale, with “1” being the most able to grow in cold temperatures. But if the crop grows too long in autumn or starts too early in spring, it is more susceptible to winter kill.

Fertility factors into winter hardiness. Purdue University studies show that alfalfa plots with phosphorous added, but no potassium, did worse than controls where no fertility was added. Adding fertility with various ranges of P:K ratios worked better than not adding any potassium at all. Alfalfa also has high sulfur requirements – about five pounds per ton of hay produced.

Not allowing a forage crop to fully rest can also weaken the crop and make it more susceptible to winter kill. Leaves grow before roots do, so allowing animals to graze too low or too soon or cutting too soon before energy stores can be generated by the plant will damage the crop. While grasses store some energy in their lower stems, alfalfa uses its taproot. Allowing forages to store energy decreases plant stress.

“The biggest winter killer…is overgrazing,” Graeme Finn of Southern Cross Livestock said. “[Some farmers] think that if there’s some foliage left on, they’ve wasted feed.”

If the plants go into winter with little foliage, the root system is exposed and damage occurs during dormancy. Foliage adds moisture-holding capacity too, and leaving foliage also allows solar energy to be captured and stored in the root system.

Winter kill

Cold temperatures and their duration, fluctuating temperatures, ice sheeting, lack of snow cover and excess soil moisture are contributing factors to winter kill, said Bill Thomas, forage agronomist at BT Agronomy. Different forages have different tolerance to these factors.

Soil conditions can contribute to the amount of winter kill experienced. Excessive soil moisture allows ice heaving to occur, as layers of ice in the soil go through cycles of freezing and thawing. Wet autumns increase this risk. This causes roots to be lifted out of the soil and exposed to the elements. Fibrous roots, such as those found in timothy or smooth bromegrass, aren’t as susceptible to frost heaving, and actually add protection to alfalfa’s more susceptible taproots.

Snow provides insulation to plant roots, with as little as four inches of snow able to regulate temperature fluctuations and prevent winter kill.

Prolonged exposure to ice, which depletes oxygen in the soil and traps carbon dioxide under the ice layer, leads to plant death. If this ice sheeting occurs for more than seven days, alfalfa will die, Thomas said.

Forages are rated by the Lethal Ice (LI) number, which indicates how many days of ice sheeting are needed before 50% of the plants will die. Timothy grass is very tolerant of ice with an LI of 37. Reed canary grass, which grows via rhizomes, has a LI rating of 26. Orchard grass is rated at LI 9, while meadow fescue is LI 13.

The primary cause of winter-killed forages is subfreezing temperatures, Thomas said. Perennials can withstand low temperatures, but at some point the water in their cells will freeze, causing the cell membranes to burst. The Lethal Temperature (LT) is used to denote the temperature at which 50% of plants are likely to suffer winter kill, and varies by species. It is a measure of winter hardiness. Alfalfa is considered to have good winter hardiness.

Temperature fluctuations can also cause plants to break dormancy. When temperatures warm and decrease again, it can cause winter injury or kill. Some plants, such as timothy, are also responsive to day length, and their winter hardiness decreases as days grow longer.

Wet soils, soil pH and fertility balance are also contributing factors to winter kill. The age of the forage stand plays a role too, with older stands less hardy. The growing degree days (GDD) between second and third cuttings of alfalfa hay fields can also play a role in winter hardiness, with alfalfa requiring at least 500 GDD to achieve good winter hardiness.

“With climate change, the amount of forage stands being injured or killed by environmental stressors is predicted to increase,” Thomas concluded.

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