Evaluating Alfalfa Fields for Winter Injury & Stand Quality

Each year, our Alfalfa fields are at risk of being injured or killed by winter conditions. Cold temperatures, ice sheets, and crown heaving are some of the main challenges our stands will be subjected to that can lead to winter injuries. When you add in nutrient deficiencies, late fall harvests, and crown or root diseases, your alfalfa fields may struggle to make it through the winter. It’s important to understand this injury early in the spring to make crop rotation decisions.

How do plants prepare for winter?

The preparation for winter begins as days become shorter in late summer and early fall. Once nighttime temperatures drop below 40 degrees, the process of hardening will begin to enable the plant to tolerate freezing temperatures: Cell membranes change allowing them to remain more fluid and more functional at colder temperatures. Sugars accumulate within the cells to lower the freezing point. Cells lose water to tolerate freezing conditions. This does not damage the cell, but helps pull more water from the cell to the cell wall. This dehydration coupled with the absorption of free water in the cell means there is very little water left to freeze inside the cell.

What causes winter injury?

Depending on varieties and past management, alfalfa can tolerate temperatures as low as 5 – 15 degrees F. Below this temperature, water left within the cell freezes and forms ice crystals that puncture the cell membrane. When the cells thaw, they die as water and cell contents leak through the punctured cell membrane.

Ice sheets that prevent air exchange to the alfalfa crowns can cause your plants to die. With the limited air exchange, toxic metabolites such as ethanol, methanol, and lactic acid can accumulate which lead to the death of the plants. Alfalfa plants can tolerate roughly three weeks of this before they are killed.

What conditions enhance the likelihood or winter injury?

• Stand age – older stands are more likely to suffer from winter injuries.
• Soil pH – stands growing in a soil pH 6.6 or higher are less likely to suffer from winter injuries.
• Soil fertility – stands with high fertility, particularly potassium, are less likely to experience winter injuries.
• Soil moisture – alfalfa grown in well-drained soils are less prone to winter injuries.
• Cutting management – stands harvested between September 1st and October 15th are at a greater risk, as they cannot replenish root carbohydrate reserves before winter.
• Snow cover is an excellent insulator. Stands with at least 6 inches of stubble will retain more snow cover and be less susceptible to winter injury

How do I diagnose winter injury and the health of my stand?

• Slow to green up – if other fields are green and starting to grow and yours are still brown, it’s time to check the field.
• Asymmetrical growth – if parts of the root are killed, the living parts will still produce stems for harvest where the dead areas will not.
• Uneven growth – uninjured buds will start growth in the spring while the injured buds will need to regrow causing uneven growth from a crown.
• Root damage – the best way to determine damage is by digging up plants.

Healthy roots will be firm and white in color. Winter killed roots or diseased roots will have a grey water-soaked appearance. Once water leaves the roots, they will become brown, dehydrated, stringy and smell rotten. If 50% of more of the root is blackened from rot, the plant will most likely die during spring green up or after first cut.

As fields dry up and soil temperatures start to maintain 50 degrees, alfalfa stands are going to start to actively grow. Use the outlined factors above to help determine stand health. For any questions or in-depth evaluations, stand stem counts, and accurately affected acres, please contact your local CaroVail location to set up field visits.