Winter freeze-thaw swings can threaten forages

Ice sheeting is another threat to grass and forage stands because it provides little protection from low air temperatures

Low snow cover in parts of the Prairies could result in higher winterkill in tame hay and forage fields.

Even mighty timothy, the most cold-tolerant among them, can suffer under prolonged sub-zero cold with no snow as an insulating blanket.

Bill Thomas, an agronomist from Truro, N.S., said that although exposure to subfreezing temperatures is the most important factor in winterkill, a yo-yo effect of freeze-thaw during winter can also be deadly.

Most forages can handle temperatures of -15 to -20 C but below that, water freezes in the plant cells, forming ice crystals that damage tissue. Alfalfa can handle -20 C to -26 C for short periods but a prolonged period even at -10 C without snow cover can bring problems.

Speaking at a webinar organized by the Beef Cattle Research Council, Thomas said timothy and brome grass are typically the most winter-hardy grasses, followed by reed canary grass. Tall fescue and meadow fescue rate fair to good, while orchard grass has only fair tolerance. Perennial ryegrass rates poorly.

As for forages, birdsfoot trefoil, alfalfa and red clover are deemed to have good hardiness, while white clover is poor. Thomas said breeders have made strides in alfalfa winter hardiness over the years so variety selection is important.

Ice sheeting is another threat to grass and forage stands and it isn’t uncommon on the Prairies.

“Ice provides little to no protection to low air temperatures,” said Thomas.

“What happens if you have prolonged ice is that you get a depletion of soil oxygen under the ice. What happens is the soil microbes continue to respire and use up all the oxygen and fill the area under the ice… with carbon dioxide, which is toxic to plants like alfalfa.”

Winter hardy grasses can stand long periods under ice, but alfalfa and other legumes are just the opposite.

“Ice sheeting longer than seven days can cause significant injury, even death to alfalfa plants, and that’s really because of the CO2 levels.”

Wet fall conditions can also be conducive to frost heaves, which thrust plant crowns and roots upward, exposing them to the elements. Again, the fibrous root systems of grasses are more tolerant of this than is alfalfa, Thomas said.

Warmth at the wrong time of the year can also be a threat, and that is definitely a factor during Chinooks. Temperatures of 0 to 5 C can cause some forages to break dormancy and if that is followed by extreme cold, it can be lethal.

Thomas said in general, older stands are more susceptible to winterkill, as are those with poor fertility.

Stands that received plentiful moisture in fall might not have fully hardened off. Saturated soil also has poor aeration and is more prone to frost heaves and ice sheeting during winter.

Come spring, a quick assessment can pay off if winterkill is suspected. Christine O’Reilly, forage and grazing specialist with the Ontario ministry of agriculture, food and rural affairs, said producers should do a plant count as soon as things start greening up.

This provides an “early warning system” in case reseeding is required.

“Ideally take a jackknife with you. Dig up some roots and then you want to cut open that big taproot and take a look at the colour on the inside. So a healthy root should be the creamy off-white colour of most potatoes,” said O’Neill.

If the root is yellow or brown, or if it is ropey or stringy inside, the root is not healthy and should not be included in a plant count.

“The reason I call it the early warning system (is) because we do that right at green up. You’ve got a chance, if you notice a problem, to get on the phone with your seed dealer and figure out how you’re going to deal with it. Depending on the severity, you may decide to patch that field” or terminate it.

O’Neill said a plant count of 20 plants or more per sq. foot, in newly seeded or young pasture, is a good sign. Twelve to 20 plants in a year-old pasture and eight to 12 plants in a two-year-old pasture is also sufficient. Ideal plant counts vary depending on whether the field is straight alfalfa or an alfalfa-grass mix.

Later in spring, a stem count can be useful, O’Neill said.

“What research has found, out of Wisconsin, is that yield is more closely correlated to the number of stems per sq. foot than the number of crowns.”

O’Neill said 55 or more stems per sq. foot indicates full yield potential. Forty to 50 stems indicate 75 to 92 percent of yield potential. If the stem count is less than 40 per sq. foot, it is likely not worth keeping.

“The main reason for that is you… have lost at least 25 percent of your yield right out of the gate but also because of the economics behind it. So essentially when we’re making hay, harvest is a fixed cost… so if you have a very low-yielding hay field, your cost per ton is much higher.”

Producers can reduce their risk of a poor hay crop to some degree, O’Neill added.

It starts with selection of a disease-resistant variety, in the case of alfalfa. Disease symptoms can look similar to those of winterkill — wilting, yellow or reddish leaves and browning of the roots.

Adequate fertility in the field provides further protection, with potassium being especially important for winter hardiness.

O’Neill said alfalfa is a nutrient-hungry crop and managers can’t assume that manure provides a balanced profile. A soil test can tell the real story.

Graeme Finn of Southern Cross Livestock ranches near Madden, Alta. He said winterkill is one of the biggest problems for graziers in Western Canada and overgrazing is the biggest culprit.

“We really believe in rest periods on our place,” he said. Those who think leaving foliage in fall is akin to wasting feed are mistaken, he added. Instead, it provides protection for plants and soil and allows quicker regrowth in spring.

“Leaving forage is not a waste. It’s an asset to the whole farm, the whole grazing operation and to the soil.”

Finn advised ranchers to be generous with seed when planting new pasture and then leave it without grazing for one year.

To guard against winterkill, his advice is:

  • don’t overgraze in the growing season
  • use rotational grazing
  • leave plenty of foliage in the pasture
  • select a forage species suited to the region
  • fertilize any new stand of pasture
  • choose to graze alfalfa rather than hay it.

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