Assessing the Frost and Flood Damage in Your Alfalfa

Hay Fields

Just as in annual crops, the concern with forages is protecting the growing point. Growing points in grasses are all at the soil surface or below ground, and protected from frost injury. On the other hand, legumes elevate their growing points as the growth stage advances, leaving them susceptible to frost injury. The crucial advantage perennial crops have over annuals are the crown and root reserves. Although growing tip damage may occur, the plant will always have the capability of sending out new growing points, as long as the crown is not damaged. Temperatures needed to affect alfalfa crowns vary depending on slope position, amount and type (“wet or dry”) of snow cover, soil temperature, soil moisture, companion crop, and wind. Most studies estimate that alfalfa crown damage may become an issue when air temperatures reach -5C for more than 4 hours. Stand assessment should take place 3 to 5 days after the frost.

Producers can expect to see some minor leaf burn due to frost damage; however, there is much less concern for crown injury.

Established fields

Established alfalfa seeded last spring or earlier will recover to take advantage of the additional moisture quite nicely. Plant tops that remain green and upright will keep growing as normal.
However, plants in younger stands (1-3 years) generally have fewer crown buds developed and available for rapid re-growth. Yield potential may be affected. Plan on monitoring the stand for plant stand reductions into the early summer.
If the top 4" or more on a plant is wilted, if plants are drying out, and/or if they're becoming discolored, they will stop growing for a short period of time. Eventually, new shoots will develop -- some from branches on the main stem and some from the crown. Growth will be set-back slightly, and yield may be affected.

Below is a guideline to use when assessing stand damage:

If less than 30% of stem tops show wilting/browning from frost, do nothing. Enough stems remain to provide good growth and yield of first cutting. Stand will have some yield reduction of first cutting but will recover completely on second cutting.
If most or all stem tops are damaged and stand is less than 10” tall, do nothing. The growing points have been killed but the alfalfa will form new buds at lower leaf junctures (ancillary buds) and continue growing (first cutting might be delayed). Alfalfa may demonstrate some horizontal growth. Mowing existing top growth will not enhance recovery. If stand is over 12” inches tall, harvest and allow to regrow.
If all stems on a plant are frozen back to the ground, the plant is dead. This extent of frost damage has not occurred in Manitoba to our knowledge. However, if observed and fewer than 5 plants per square foot remain, consider rotating to another crop and replanting alfalfa in another field to avoid autotoxicity.

Seedling fields (Seeded in spring or fall)

At emergence, alfalfa and most winter hardy forage grass and legume seedlings are extremely tolerant to cold. But, frost tolerance changes with age. Four or more hours of temperatures around -5C may kill new seedlings, especially once the seedling has reached the 3rd – 4th trifoliate stage. Heat from the soil often protects them from brief, cold temperatures. Seedlings frozen so that all trifoliate leaves are discolored and dying will not re-grow. Seedlings with 1 trifoliate remaining should regrow. Alfalfa seeded with a companion crop survives lower temperatures and longer exposure times before showing frost damage. Reseeding your stand should not be required if more than 20 plants per square foot are remaining; however, less than 15 plants per square foot should trigger a consideration to reseed or broadcast additional seed on the field. Since the plants are less than two years old, autotoxicity is not an issue.
However, as most spring seeding of forages takes place in late May to mid-June, few if any fields are in the germinating to emerging stage at this time of year. Therefore, field conditions should be excellent when the time comes to seed a new forage field. In fact, this extra moisture received might also give producers a chance to clean up that last flush of weeds.

Pastures

Pastures can be thought of in the same manner as established hay fields. Since most pastures are mainly grass, frost injury is not a significant concern. Much like the hay fields, the added moisture will improve stand health and may increase productivity, depending on the amount of overgrazing that has taken place and the fertility program.

Spring Flooding

Excessive moisture in the fall, significant snow fall and spring rains can cause the moisture level of forage stands to be high. Flooding a forage stand limits the amount of oxygen in the soil profile and since plant roots require oxygen to remain healthy, plant productivity and survival is reduced when soil moisture levels are too high. Below is a list of the flooding tolerance of a few common forage crops.

Spring flooding tolerance levels of a few common forage crops.

Alfalfa: 2 weeks
Alsike Clover: 2 – 3 weeks
Smooth brome grass: 2 – 3 weeks
Timothy: 5 – 7 weeks
Meadow Foxtail: 5 – 6 weeks