Alfalfa Winterkill Weather Factors

Table of Contents

1. Introduction
2. Weather Risks
3. Early Spring Signs
4. Plants per Square Foot
5. Check Root Health
6. Heaving
7. Stem Counts
8. Strategies

Introduction

Alfalfa winterkill has a way of catching us by surprise in the spring! Monitor alfalfa fields, particularly in areas that have a history of winterkill. If winterkill is evident on your farm, have a strategy ready to ensure adequate supplies of quality forage.

Weather Risks

Winterkill is determined by numerous cumulative factors, and can be very unpredictable. Management factors, including stand age, variety, soil pH, soil potash level, drainage, and harvest frequency, add to weather risk factors that include:

• Dry summer weather can force farmers to cut during the Critical Fall Harvest Period, to cover feed shortages. This reduces the root energy reserves needed for overwintering.
• Mild fall with rapid fall growth, late freeze-up, and more frequent rain, may result in reduced winter "hardening" of the alfalfa plants. Hardening prepares the plants to withstand freezing injury by decreasing water content, increasing root carbohydrate and protein, and converting starch to sugar.
• Warm spells can cause a loss of cold hardening. Also, a few days of 10°C weather in late winter begins to break alfalfa dormancy. The alfalfa develops new shoots that can be frozen back. Newer shoots can be regenerated, but this uses more stored energy and further stresses the plants.
• Lack of snow and bare fields reduces the insulation effect, exposing crowns to cold and fluctuating temperatures.
• Rapid thaws and winter rains result in "ponding" on level fields and subsequent ice sheeting.
• Cycles of spring freezing and thawing, as well as the winter-long heave cycle, can result in alfalfa heaving, particularly on heavier clay soils. Tap-roots are pushed out of the ground, similar to a heaved fence post.

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Early Spring Signs

Check for bud and new shoot vigor. Healthy crowns are large, symmetrical and have many shoots. Watch for delayed green-up, lopsided crowns, and uneven growth of shoots. If you notice any of these characteristics, investigate further by checking for root rots and broken roots.

Plants Per Square Foot

Assuming good crown and root health, future yield potential can be estimated by looking at plants or stems per square foot. Stem counts are more accurate than plant counts, but in early spring it is only possible to count the crowns. Be prepared to replace an older stand if it has less than 5 plants per square foot.

Check Root Health

Dig several plants and cut the roots open lengthwise. Healthy roots have a white or creamy colour inside, are firm, and resist pealing when scratched with your thumb nail. Dying plants will have a discoloured crown and root, with a spongy texture.

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Heaving

If heaving is evident, dig some plants to determine if the tap-root is broken. Plants with broken tap-roots may green-up, but perform poorly and eventually die. Slightly heaved plants can survive, but their longevity and productivity will be reduced. Crowns that are heaved 1 inch or less are not as likely to have a broken tap-root. With time these plants can reseat themselves. Raised crowns are susceptible to weather and mechanical damage. Raise cutter bars to avoid damaging exposed crowns. Using a cultipacker or roller to push the crowns back in the ground can do more harm than good by damaging crowns and breaking taproots.

Stem Counts

When alfalfa growth is 4 to 6 inches in height use stems per square foot as your density measure. Stem density of 55 per square foot has good yield potential. Expect some yield loss with stem counts between 40 and 50. Consider replacing the stand if there are less than 40 stems per square foot, and the crown and root health is poor.|

Strategies

The action needed will depend on how extensive the alfalfa winterkill is, forage inventories and requirements, alternate forage options, how much grass is left in the stand, rotational requirements, and weed pressure.

• It is not recommended that interseeding alfalfa be done to thicken an established (older than one year) alfalfa stand, due to autotoxicity. In an emergency situation, interseed thin spots with red clover instead.
• Ensure adequate fertility. If alfalfa winterkill is extensive, nitrogen applications to promote grass growth in the stand will increase yield and quality.
• For emergency forage alternatives, consider using spring cereals, cereal-pea mixtures, annual ryegrass, forage sorghum or pearl millet.
• Delayed harvesting of stressed alfalfa will compromise quality, but allows plants to restore carbohydrate levels and will increase long-term stand survival.

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