Bill Halfman, Monroe County Ag Agent, Carrie Laboski, Soil Fertility/Nutrient Management Specialist
Management decisions made now can impact how alfalfa stands make it through next winter.
This year there were a lot of reports of alfalfa stands coming through winter but were weak and not yielding up to normal expectations. Certainly, some of that is due to last winter’s weather, but management practices and overall health of the crowns can influence how well the plants make it through the winter.
Crown health is a strong indicator of an alfalfa plant’s likelihood of making it through the winter in good shape. This should be assessed by digging up crowns in several places in each field to determine if they are healthy or not. Crowns that are in poor health will have lower yield anyway, and those fields should be considered good candidates to rotate out of alfalfa. The UW Extension publication titled: “Alfalfa stand assessment: Is this stand good enough to keep?” located at: http://learningstore.uwex.edu/assets/pdfs/a3620.pdf has pictures of alfalfa roots and also indicates their probability of winter survival. This guide can help assess stand health when planning crop management and rotations. Overall crown health declines with age and older stands are typically less tolerant of stress compared to younger stands (18-36 months). Younger stands commonly have lower levels of disease incidence and less physical damage from wheel traffic.
Potassium is vital for carbohydrate movement to the tap root. Therefore, stands with optimum or higher levels of potassium (K) in the soil are at lower risk to experience winter injury than stands growing on low fertility sites. Sufficient potassium levels must be present before the fall rest period. Applying potassium in August/ September is ideal for the alfalfa plant to prepare for winter dormancy. Topdressing in October/November is too late for the plant use the potassium for winter preparation.
Soil pH is also important to alfalfa crown health. Alfalfa grows best at soil pH near 6.8. Because ag lime needs 3 years to be fully effective, lime applications prior to stand establishment are most effective. Top hay producers maintain soil pH and fertility levels throughout the crop rotation.
Another important factor affecting winter survival of alfalfa is a fall resting period. Alfalfa must build a good store of protein and carbohydrates in the crown and taproot to survive the winter. Alfalfa uses root reserves for early growth in the spring and after each cutting. As regrowth occurs, protein and carbohydrate reserves are replaced in the root. If alfalfa is cut, and regrows to 6 or 8 inches and freezes, it will suffer more winter injury and death than if more or less growth occurs before dormancy. This is because root reserves will be used, but not enough regrowth will have occurred to replenish the root reserves.
Late fall cutting can also stress alfalfa. It removes top growth, which provides insulation for the crown, primarily by catching and holding snow. Alfalfa will generally be killed when the crown reaches minus 15 degrees F.
Overall crown health, soil nutrient management and late summer/ fall cutting management can all influence an alfalfa stand’s ability to survive through the winter.