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Thwarting alfalfa injury in spring begins in the fall

Jon Pretz for Progressive Forage Published on 02 October 2019

Alfalfa fields are at risk for winter damage or kill due to extended cold temperatures and ice accumulation on an annual basis, particularly in the Midwest.

While fall may not be the typical time of year to begin thinking about winter’s impact on alfalfa fields, preparing for any potential seasonally related harm before it happens can help significantly reduce the impact of winter injury on your alfalfa fields next spring.

We cannot control the weather. We can, however, reduce the risk of winter injury through variety selection, maintaining young stands, potassium fertilization, proper pH, soil drainage, snow retention and timely cutting management. In the event of a tough winter, remember to diagnose the potential injury, determine the yield potential and establish a game plan.

Factors that can affect alfalfa plant hardiness

  • Variety selection: Alfalfa varieties with superior winter hardiness ratings and a high disease resistance index are less likely to experience winter injury.

  • Stand age: Maintain young alfalfa stands, which are less susceptible than older stands thanks to their decreased disease potential and higher plant populations. Plant some new stands prior to Aug. 15 each year to allow for plant development before winter.

  • Potassium: Adequate potassium in the soil results in increased stand density through stronger, healthier root systems, which can tolerate more challenges during the winter.

  • Soil pH: Soils with a pH above 6.6 are less likely to experience winter injury.

  • Soil drainage: Always plant alfalfa on well-drained soils to minimize the risk of winter injury and kill due to ice sheeting and root rot that can happen in saturated soils. Tile drainage can be installed to help keep fields from becoming too wet on the surface.

  • Snow retention: Snow provides insulation to the plants and the crown. The crucial temperature region is 2 to 4 inches below the soil surface, where a large part of the root structure is located. Stands that have at least 6 inches of stubble left will be able to retain more snow cover and, as a result, will be less susceptible to winter injury.

  • Cutting management: Harvest frequency and timing of the fall cutting will affect the winter hardiness of the alfalfa. Stands at greatest risks are ones whose last cutting is taken between Sept. 1 and the middle of October, as these plants will not have enough time to accumulate the appropriate carbohydrate levels in their root systems before winter.

How to diagnose winter injury in the spring

  • Stands that are slow to green up: Compare your stand to other fields in the area. If you notice great variation in your field with some greenup and some areas that are still brown, you should investigate the brown stands for injury or death.

  • Winterkilled roots have a gray appearance: Healthy roots should be firm and white in color, as shown in Figure 1. Roots that are soft and dark are exhibiting potential symptoms of winter-related death.
    Root evaluation
  • Asymmetrical or uneven growth: Asymmetrical growth is likely a sign that a portion of the plant was winterkilled, and only the healthy portion will be productive. If uneven growth is observed, a portion of the buds developed during the previous fall could very well be injured, and it will take time for new buds to form, thus resulting in shoots of different heights on the same plant.

  • Heaving: During heaving, alfalfa crowns and roots are forced above the surface of the soil as a result of freezing and thawing. Heaving occurs more commonly in heavy soils that feature a high moisture content. Repeated freezing and thawing causes soil expansion and contraction, which pushes the taprooted plants out of the soil (as shown in Figure 2) in late winter or early spring. Due to their more developed taproot, older alfalfa stands are more prone to heaving compared to grasses or first-year alfalfa stands, which have more fibrous root systems.

Alfalfa heaving

Determine yield potential

The potential yield of an alfalfa field stand can be estimated by determining the number of stems in a square-foot area. After determining the stem number, use the following formula to calculate the yield potential of that stand.

Yield (tons per acre) = (stems per square foot x 0.1) + 0.38

Example: An alfalfa field with 50 stems per square foot would have a yield potential of 5.38 tons per acre. Remember, this is potential yield; soil elements, nutrient deficiency, insects, diseases and many other factors may affect the actual yield. The following guidelines can be useful when determining whether to keep or replace the alfalfa stand.

Be prepared

It is important to establish a plan to ensure an adequate forage inventory for the coming year in case your fields experience winterkill or severe heaving (Table 1).

Action options in damaged stands

If heaving is confined to patches in the field, consider renovating the stand by reseeding the affected patches. If moderate heaving allows you to keep the field for more than one year, consider planting red clover or a mixture of red clover and grass to ensure an adequate forage yield.

If your goal is to maintain the field this growing season, and if additional legume yield is desired, consider berseem clover, crimson clover or even field or forage peas, depending on the ratio of seed cost to potential yield. But remember: When another grass or legume is patched into an existing stand, the newly seeded area will need time – often a minimum of 60 days after planting – to become established prior to harvest.

If severe heaving or winterkill takes place, and the alfalfa stand is terminated but additional forage is required, other options include warm-season sorghum-sudangrass or their BMR variety counterparts, which can be planted and harvested within a 50- to 60-day window followed by subsequent harvests every 30 days into the fall (in optimal growing conditions). Other cool-season grass options for filling forage inventories include spring cereal grains such as oats, barley, triticale and Italian ryegrass.

Quality forage is a critical component of a dairy herd’s nutrition program. To be adequately prepared for a damaged alfalfa crop or, in the worst-case scenario, an alfalfa crop considered a complete loss, it is essential to understand your options and have a backup plan in place.  end mark

Jon Pretz
  • Jon Pretz

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