Winter Survival of Alfalfa
Every year here in Manitoba alfalfa stands are at risk of being injured or killed during the winter. Winterkill is hard to predict because alfalfa can fail to survive winter for several reasons. The most common reason alfalfa iskilled/injured is when water in the plant freezes, causing the cell walls to rupture. Alfalfa can also be injured or killed by suffocation due to icing or if the alfalfa comes out of dormancy during the winter and uses up its root reserves before spring comes. The last common failure is soil heaving.
Finding alfalfa fields winterkilled in the spring can challenge livestock producers to meet the nutritional requirements of the herd. It is sometimes difficult to balance the competing demands of alfalfa quality, yield and persistence. Having the ability to evaluate this injury early in spring is helpful in making crop rotation decisions. What we are going to look at are some of the factors affecting winter injury and how to evaluate it.
Temperature- Alfalfa begins preparing for winter once shorterdays begin in late summer. As night time temperatures drop below 5 C the plant begins the process of hardening-off. During this process,sugars accumulate within the cells to lower the freezing point. The cell starts to lose water which increases its ability to tolerate freezing. The water in the cell wall freezes drawing more water out of the cell increasing the cells tolerance. This process enables the plant to tolerate temperatures as low as -12 C. Temperatures below -12 C and/ or an inadequate hardening-off period can cause the water in the cell to freeze, rupturing the cell wall.
Adequate snow cover of at least 6 inches insulates the alfalfa crown and root at moderate temperatures. Soils with a lack of snow cover can expose alfalfa crowns to temperatures less than -12 C. This results in freezing damage to cells and plant death. Fluctuating winter temperatures with lows below freezing and highs greater than 5 C, without snow cover, can cause plants to break dormancy and be less resistant to freezing. To follow soil temperatures in your area go to:
Suffocation- Although we tend to think of alfalfa being dormant during the winter, alfalfa does continue to respire using small amounts of its root and crown reserves. Adequate snow cover not only protects the alfalfa crown from freezing but allows air to move freely between the snow particles providing good air exchange. Ice sheeting during the winter prevents this air exchange to the crown. Toxic gases like ethanol, methanol and lactic acid then start to accumulate in the plant. The plant can withstand about 3 weeks of toxin accumulation before damage occurs.Ice sheeting (Snow melt causing ponding water followed by freeze up)can kill localized areas in alfalfa fields. Smothering of alfalfa under ice can cause injury in 1 to 3 weeks and plant death within 2 to 6 weeks (less if soil temperatures are near freezing and longer if the soil is colder).
Dormancy- As the plants break dormancy next spring, root and crown nutrient ‘reserves’ will decrease for a week to ten days while new growth is starting. If a week or two of unseasonably warm weather occurs in January or February, the plants will sometimes ‘break dormancy’ prematurely, and use up some of their stored reserves in their attempt to establish new spring growth. This depletion of reserves can cause injury or slower spring recovery if once the plant starts to grow, we experience cool temperatures, slowingthe plant’s ability to produce its own food through photosynthesis versus drawing from its root reserves.
This break in dormancy in January or February can also contribute to both reducing the plants ability to withstand lower temperatures and increase the possibility of suffocation.
Heaving-High moisture levels or clay soils increase freezing and thawing cycles which “heave” the crowns out of the soil. This heaving tears small roots off, cutting off the uptake of moisture and loss of a secure anchor of the plant. Eventually, the crowns can be lifted out of the soil and exposed to cold, drying winds. Plants that heave can still recover and produce good spring growth.
Factors Influencing Survival
As you are no doubt aware, Alfalfa is a perennial forage that uses sunlight to produce sugar and starches. These carbohydrates are used for producing new growth and when the production exceeds this need, the excess is stored in the roots for future re-growth.Some of the risk factors associated with winterkill we can control, while others we cannot. There are a number of factors that, although they can’t guarantee survival, can help influence the plants ability to withstand winter injury.
Stand age -Younger stands (18-36) months are more stress tolerant when compared to older stands. They tend to have lower levels of disease incidence and show less damage from wheel traffic.
Soil Fertility- Although alfalfa does produce its own nitrogen and is an excellent crop to help build nitrogen and organic levels in soils, it is a high user of potassium (K). A low soil potassium level is a major factor leading to loss of alfalfa in a stand, particularly on loams and sandy loams. Low potassium limits the plant’s capacity to store root carbohydrates, reduces the plants ability to withstand freezing temperatures and resist disease. Potassium requirements of alfalfa are high. A typical yield of alfalfa can remove over 150 lbs of actual potassium per acre annually.However, high soil potassium levels can result in luxury consumption of potassium by the alfalfa and subsequent nutritional problems with dairy pre-calving dry cows. Potassium applications on soils testing over 150 ppm will not significantly increase winter hardiness and are not recommended.Topdressing in October/November is too late for optimum conditions and should be considered as helping next year’s crop.
Cutting Management- Winter survival starts in the fall.Having good root reserves is the first step.The shorter the interval between cuttings during the growing season, the greater the risk of winter injury. This is related to the total amount of carbohydrates that translocate to the roots during the growing season. Similarly, a fall harvest forces the plant to use stored reserves to initiate new growth. If growing conditions following this later harvest do not allow reserves to be replenished, the plant enters the long, high risk, winter period with low root reserve supplies. Harvesting before the "Critical Fall Rest Period” allows the plants to re-grow and build sufficient root energy reserves for over the winter. Adequate root reserves are necessary for winter survival and persistence, as well as vigorous spring growth and good first cut yields. Allowing at least one cutting to reach bloom improves winter survival odds. For more information on cutting management see the article on Critical Harvest Period in the September Newsletter.
Snow Cover-Adequate snow cover of at least 6 inches insulates the alfalfa crown and root at moderate temperatures. Soils with a lack of snow cover can expose alfalfa crowns to temperatures less than -12 degrees C. This results in freezing damage to cells and plant death. Fluctuating winter temperatures with lows below freezing and highs greater than 5 degrees C, without snow cover, can cause plants to break dormancy and be less resistant to freezing. Standing stubble also enable air movement to continue by providing a channel thru the ice that allows for an air exchange.
So how predictable is winterkill as we enter into the winter? Not very predictable at all. In your own area you can only look for the signs of extreme temperatures, lack of snow cover and saturated soils with ice. Hopefully, you will see all your alfalfa make it through to spring.
John McGregor
Extension Support, MFGA