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.
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.
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.
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.