Winter hazards to forages: Heaving

Craig Sheaffer, Extension forage agronomist, Nathan Drewitz, and Troy Salzer, Extension educators

Heaving of alfalfa. Alfalfa can be killed if roots and crown are
exposed to freezing temperatures or if plants are detached
from the soil and cannot take-up water.

In early January we discussed the beneficial effects of protective snow cover as a protective insulation layer for alfalfa plants over the winter. Surface, soil temperatures have remained near 32 F throughout the winter, a level that is not stressful for perennial plants. However, with melting snow in late winter and spring and saturation of the soil, forages are subject to the peril of heaving.

Heaving mechanisms

Heaving is a volumetric expansion of the soil caused by the segregation and expansion of frozen water (ice) in the soils. There are two forms:

Primary heaving: This is caused by ice needles jacking up plants out of the soil during short duration, alternative freeze-thaw cycles in the spring.
Secondary heaving: This is caused by deep frost penetration into frozen soil during winter with laminar ice lenses produced parallel with the soil surface.

The maximum heaving pressure developed by minor freeze thaw cycles is small and only capable of partial lifting of adult tap-rooted plants. It has a major impact only on seedling plants or poorly rooted plants. Secondary heaving is capable of significant load lifting like pavement heaving and can significantly uproot large tap rooted plants such as mature alfalfa and red clover.

Impact of heaving on plants

Heaving injury includes breaking of the plants' tap roots and shearing of lateral roots. When the soil subsides, the roots may also settle through elastic recoil; however, crowns of tap rooted plants like alfalfa and red clover can remain above ground. Even moderate levels of heaving of an inch or less disrupts the finer roots and nodules of legumes and the exposed roots and crown are subject to temperature extremes and drying. More fibrous rooted plants like grasses have some resistance to heaving because of their numerous fibrous roots which have a greater soil surface contact than tap root legumes.

To minimize heaving risks:

Utilize mixtures of perennial grasses with legumes. Grasses cover the soil around legumes and reduce adsorption of radiant energy and heating of the soil. The fibrous root system of grasses is less prone to heaving and can provide more soil stability.
Avoid fall harvest. The stubble can prolong snow cover, and shield the soil from the sun, reducing the energy input and loss from the soil; thereby reducing thawing and freezing cycles. Fall cutting is especially harmful when used in combination with frequent summer harvests that can stress plants (Table 1).
Follow late-summer and fall planting guidelines for legumes and grasses. Late planted grasses and legumes with small root systems are very susceptible to heaving. For central and southern Minnesota, optimum seeding date is from August 1st-15th When combined with adequate moisture, seedlings have time to develop adequate root systems and legumes can undergo contractile growth pulling the crown below the soil surface.
Select fields that are well drained with good drainage. The freezing of soil water is the primary mechanism in heaving. Fall irrigation can also add moisture to the soil profile and should be avoided where possible.
Plant disease resistant and adapted varieties of alfalfa. Diseases such as Aphanomyces have been shown to weaken root systems and reduce prevalence of lateral roots in alfalfa and increase the risk of heaving.
Plant alfalfa varieties with creeping roots and traffic tolerance with low set crowns. These types of plants have greater resistance to heaving than standard varieties.

Table 1. Cutting management effects on alfalfa heaving and root weight. Note: Cutting management has a small effect on root weight and TNC (total nonstructural carbohydrate).

Cutting*	Heaving	Root weight
	%
3 @ 45 d	4	3.2
3@45 d + fall	28	3.0
4@ 35 d	7	3.0
4@ 35 d + fall	42	3.0
5@ 30 d	54	2.4
5@30 d + fall	44	2.7
LSD 0.05	8.0	NS

*3 or 4 cuttings were taken with a fall cut to remove stubble in late-October. The 3-cut system was initially taken at first flower, while the 5-cut treatment was at early bud.

Responding to heaving

There are no management strategies to reverse heaving once it has occurred. Rolling or harrowing heaved forage stands with the goal of pressing plants into the ground or covering crowns can do more harm than good.

Evaluate the extent of heaving. Stands with heaving of 1 inch or less, have the potential for recovery and development of new roots. Again, some settling can also occur, but because of root damage and crown exposure these stands will likely be less productive. The harvest date following a major heaving event should be delayed until the flowering stage with cutting height raised above the crown.
Assess the overall health of the stand. Estimate plant numbers and stems per foot square from at least 10 areas within a field to assess yield potential. Combined with heaving, if the whole field on average is below the threshold considering the stand age, consider terminating or rotating to a new crop. A healthy, potentially full yielding stands should have a alfalfa plant and stem counts that are about 5 plants/fts² and 40 stems /ft². See Alfalfa winter injury assessment and management for more information.
If there are patches of dead plants, consider selectively interseeding with red clover (10 lb/acre) or a red clover -Italian ryegrass mixture (5lb/acre) to potentially extend the stand for another two years. This should be completed as early in the spring as possible as these forages take time to develop.

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