Freeze Damage to Alfalfa
Much of Nebraska was 2 to 4 degree above average for the month of March (Figure 1). There was a 3-day heat wave with 70s and 80s on April 6, 7, and 8 in portions of Nebraska. In Lincoln, a record high was set on April 7 (87 degrees) and a record low on April 10 (17 degrees) and tie for a record low on April 14 (16 degrees). In response to these temperature extremes, the National Weather Service in Omaha called it a “weather whiplash”. What effects will these recent freeze events have on the third most dedicated crop, alfalfa, in terms of acres and value of production in Nebraska?
Taking it to the Field
- Numerous factors influence freeze damage, not just weather station air temperatures
- Given multiple freeze events, wait 2-3 frost/freezes days before assessing damage and 10 days of favorable weather to assess regrowth (Figure 2)
- New Seedlings – One set of leaves that escaped is a good sign of survival. Determine the number surviving alfalfa plants per square foot while in the field
- Established Fields – Expect delay of first cutting and slight yield reductions. Slight and moderate damage does not warrant any action. Hard freeze damage and insufficient prior growth also warrants no action.
Keep reading to learn more about the four highlighted “taking it to the field” comments.
Complexity of Freeze Events
The potential damage to alfalfa is correlated with the amount of spring growth (i.e. more growth, more risk). Beside the growth stage of the alfalfa, numerous factors influence the extent of damage from an early morning freeze event:
- Air temperature (4 to 6 ft above the soil surface or ground-level)
- Near surface air temperature (6 to 12 inches above the soil surface or ground-level for alfalfa this time of year)
- Length of freezing conditions
- Soil temperature
- Local topography
- Alfalfa growth stage
Due to the complexity mentioned above, look at the actual response of the alfalfa plant itself to determine freeze damage rather than rely only on temperature. Also, wait several frost/freeze free days before assessing damage.
New Seedings and Seedlings
New seedlings this early in the spring handle low temperatures (low 20s) better than one often thinks due to natural seedling cold tolerance and being close to the warmer soil. Seedlings from emergence to the second trifoliolate leaf stage are more cold tolerant than later growth stages. Companion crops would help new alfalfa seedlings survive under colder or longer periods of exposure. When seedlings lose all trifoliolates (leaves) and look discolored, they will not likely regrow. Having at least one set of leaves escape the frost damage is a good indicator of plant survival. Determining the number of plants per square foot that survived can also help guide management decisions. Stands that are thinner than 15 plants per square foot may need maintenance and probably some reseeding.
Established Alfalfa and Categories of Damage
Established alfalfa will outgrow the damage from a light freeze (upper 20s for several hours). Observable damage from a light freeze is the wilting and sometimes loss of a few upper leaves and some slight curling of the stem (Figure 3 and 4). Buds or growing points will continue to grow as normal. A majority of the terminal buds should still look green and alive 2 to 3 days after the freeze event. In terms of management, there is no need to do anything under these conditions.
Under moderate (mid 20s for several hours) and hard freezes (low 20s and below for several hours) additional damage can occur. Moderate freeze damage will kill the upper part of the stem and terminal buds, but not warrant any action. Regrowth must occur from lower axillary buds on the stem or from new crown buds. As a result, there will be a delay in growth and the first cutting. A hard freeze causes leaf, buds, and stem tissue death and plants will regrow from crown buds. Plants usually quickly fall over flat to the ground. Plants will still regrow from the crown buds. There will be a delay in the start of the first cutting and possible slight yield reduction.
Numerous factors influence how much alfalfa damage will occur from a spring freeze. It is best to wait several freeze-free days before assessing the final damage. There may need to be some management actions taken after a hard freeze (cutting, grazing, shredding, no action), but usually it is best to do nothing except wait for alfalfa to regrow on its own. Established alfalfa was about 6” tall prior to the freeze on April 10 in areas of southeast and portions of northeast Nebraska. Given the limited amount of growth on most alfalfa fields, cutting for hay would not be practical.
To learn more about assessing freeze damage and management options, please read the previous written article: Assessing Alfalfa Post-freeze. There is also a nice follow-up article in 2012 addressing things learned from the 2007 freeze event: How Alfalfa Responds to Frost.