Alfalfa regrowth after the last summer cutting. Image Credit: Hanna Wells.
One of the main factors that affects winter survival of alfalfa stands is having a good supply of carbohydrates stored in the roots going into winter. Potassium (K) plays an important role in helping the plant to create and transport carbohydrates to the root. Lack of sufficient potassium during the critical rest period for alfalfa in fall can negatively affect carbohydrate stores and winter survivability.
Overall, potassium management in alfalfa should be informed by soil testing and records of crop yields. Soil testing will identify how much potassium is needed to bring the soil into the optimal range and crop yields will guide how much potassium fertilizer needs to be added to replace what the crop removed, thus maintaining soil potassium levels in the optimum range. Because plants can take up excessive potassium when soil levels are high, and because excess potassium in forages can cause milk fever in dairy cows, it’s important to only add potassium fertilizer when it is called for by a recent soil test (taken within the last 3 years).
Depending on the existing soil potassium level, the soil test will call for a certain level of potassium fertilizer to be applied to established alfalfa stands. This rate is the recommended requirement for the whole year, but it can be split into multiple applications over the season. A common management recommendation is to apply half the annual requirement after the first cutting in spring and the other half after the last cutting in late summer. Splitting the applications in this way prevents excessive potassium levels in the first cutting, replenishes soil potassium levels for the second and third cuttings, and ensures sufficient potassium is available to alfalfa during the fall rest period. It is important that the second potassium application be made shortly after the last summer cutting so that the plants have access to it when it is needed for helping to make and transport carbohydrates to the roots. Potassium applied late in the fall (October) will provide limited benefits for winter survival.
Different types of potassium fertilizer can be used to meet the needs of the crop. Manure is also a good source of potassium and can be applied to alfalfa if needed. Whatever source of potassium you use, be sure to know the potassium analysis (%K2O by weight) to verify that your application rate will be sufficient to meet the needs. Pay careful attention to liquid fertilizer products that claim to provide a benefit with low application rates, as the application rate may not actually apply as much potassium as needed.
Alfalfa can be an expensive crop to establish and is an important component of feed rations. Paying attention to fertility management to help winter survival of the stand can help make this investment pay off for as long as possible.