By Ashley Knapton
BSc(Agr), CCA-ON

A historically common experience in Eastern Ontario is to await the regrowth of alfalfa in the spring only to find it doesn’t come up. Winterkill is a challenge we often run into, but what is it exactly?

Winterkill is an overarching term that signifies the failure of a crop, including alfalfa, to regrow. The causes of winterkill can be wide ranging and multifaceted. Anything that impacts the health of the roots will greatly increase the risk of winterkill. For example, roots that are encased in ice are sometimes smothered and unable to release carbon dioxide to the soil. Late fall cutting that does not allow time for the plant to replenish its stores of carbohydrates impacts regrowth ability. Frequent cutting results in an increase in traffic and as such, damage to the crowns at the top of the roots. Fertility also impacts the health of the roots. Something that is important to consider is that a combination of these factors is just as harmful, if not more so. For example, if you cut slightly towards the middle of the critical fall harvest season and you operate in a 4 cut system, you are increasing the stress on that field at a faster rate than someone who is operating a 4 cut system that monitors the critical fall harvest period.

Now that we have a better understanding of what winterkill is, what do we do with this information? In the early spring, we can do a preliminary evaluation of the survival of a field of alfalfa, for example by counting the number of surviving plants per square foot. For fields that are in their second year of production, we will ideally see 15-20 alfalfa plants/ft2. For fields that are 3+ years established, we can see as few as 5 alfalfa plants and still maintain acceptable production, though we can likely subscribe to the mentality “the more the merrier”. While this will not give a definitive idea of yield potential, it will be an early indicator of the viability of a field for the upcoming year.

When counting plants, it’s a good idea to dig a few up and evaluate the health of their root systems. A healthy root will be bright white and firm in density, while an unhealthy root will start to show brown discolouration at the top of the root, known as the crown, as well as softening in the centre of the root. This browning can be an indicator of winter injury, which in turn can provide a hint as to the plant’s ability to yield and remain resilient for the upcoming year.

Weather is a major influencer of winterkill. Lack of snow cover and harshly cold winds can dry the roots out, damaging them. Freeze thaw cycles in the spring can accelerate the heaving of roots, further exposing them to damaging conditions. Pests also impact winter survival. Leaf diseases and notorious bugs like the potato leafhopper add significant levels of stress to the plant and impact its ability to refill the root reserves.

Managing for winterkill is an option. Ensure your alfalfa is planted on well drained soils to allow the roots to breathe. Ensuring sufficient potassium levels in the soil will help to reduce the risk of winterkill because it is a crucial nutrient for allowing the uptake of carbohydrates. When harvesting in the fall, carefully consider your timing to try and avoid the middle of the critical fall harvest period. It is important to also remember that alfalfa does have a limited life span. Before even planting the field, variety selection matters as there have been genetic differences found in the ability of a crop to survive.

Alfalfa can be a challenging crop, but when managed carefully it can provide a large amount of forage per acre, while providing a benefit to soil structure.