April 2001
(newsletter issue date)

Alfalfa Heavily Damaged by "Heaving" from Freezing and Thawing this Spring

Alfalfa fields have been hurt by freezing and thawing this spring, especially in North Missouri, report agronomists at the University of Missouri-Columbia.

"I've never seen such carnage," said Rob Kallenbach, extension forage specialist. He has seen 70 percent kill rate in research plots seeded last fall.

The culprit is "heaving," caused when clay soils expand and contract, said MU veteran alfalfa researcher Jerry Nelson.

"Soil lifts as it freezes (water expands when freezing) and drops when it thaws," Nelson said. "This causes the plants to be jacked out of the ground, slowly at first as the root is anchored and elastic, but with repeated freezing and thawing, the root breaks and the plant lifts rapidly."

Late March is the typical time for heaving, Nelson added. Early warm springs the last few years have prevented the problem.

Usually, the damage is worst on older stands, where the thick taproot of the alfalfa plant reaches deep into the subsoil. The expanding and contracting occurs in the top few inches of soil.

However, this is the first time that Kallenbach has seen such heavy damage on young plants. "The seedlings often float with the soil movement, but this year, the small taproots were jacked entirely out of the ground. The plants are lying there, dead."

David Davis, superintendent at the MU Forage Systems Research Center, Linneus, Mo., has found severe damage on parts of a 20-acre field seeded last fall. On some south facing slopes nearly half of the plants have been completely heaved out of the ground. "The last few day-night cycles have added to the heaving situation," he said. "We'll have to evaluate if the stand is worth saving."

On older plants Kallenbach has seen the top six inches of big taproots protruding above the soil surface. "Those plants will be weak and may not recover. They are likely to be cut off below the growing point when the mower comes along."

Alfalfa growers should examine their stands and make some management decisions by mid-April. "Thinned stands will require intensive care this year," Kallenbach said.

For stands less than two-years old there should be 10 plants per square foot surviving. Older stands can remain economically productive with four plants, or even two plants, per square foot. The critical factor for maximum production is 35 stems or more per square foot. Older plants, with large crowns, can have many more stems than younger plants.

For first aid, damaged fields should have adequate fertility. Soil test phosphorus should be above 40 pounds per acre and potassium above 300 pounds.

Next will be weed control to reduce competition. The weeds come in to cover the bare soil and alfalfa does not compete well with weeds.

"There are lots of herbicides available for use on pure stands of alfalfa, but alfalfa-grass mixes will be more of a problem," Kallenbach said.

Farmers should delay hay harvest by a week or more, until past mid-bloom, to allow plants to build root carbohydrate reserves. "With delay, you'll give up some hay quality on the first cutting to gain some longevity in stand life," Kallenbach said.

The delay before cutting also allows some lifted plants to contract back into the ground, Nelson said. This will work where the heaving was an inch or less.

Nelson also said that young stands, seeded last fall, could be thickened or re-seeded with alfalfa this spring. However, older stands will have built up autotoxicity in the soil that will kill new seedlings.

Kallenbach said that on older stands that have declined in vigor, it might be a good year to grow corn. The alfalfa stand has built up a nitrogen reserve in the soil that can be used by the corn crop.

A new stand of alfalfa can be seeded the following year. "The autotoxicity breaks down in about 12 months," Kallenbach said.

The agronomists report that the damage is most severe on south facing slopes. Also, clay soils of northern Missouri, which have high shrink-and-expansion ratios, are most troublesome. Interseeding grass with the legume can help prevent damage.

The damaged plants are more likely to be vulnerable to diseases and insects, Nelson said.

"Many plants with broken taproots will die. Those that survive will need extra time to recover," Kallenbach, said.

  (Photo credit: (MU File Photo by Duane Dailey)
Alfalfa crown, which should be at ground level, has been jacked upward by alternative freezing and thawing. The upward movement breaks the taproot, which reaches deep into the subsoil. Most plants die, although some can recover with intensive management. The problem has been severe in north Missouri, said an agronomist with the University of Missouri.

From: AgEBB

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