By Craig Sheaffer and Dave Nicolai
Because of deep and extended snow cover that extended from December to March throughout most of Minnesota, soil temperatures at alfalfa crown depth were near 30 F, well above lethal temperatures for alfalfa (15 F). Overall, there was little reported winterinjury.
However, alfalfa producers should still scout alfalfa fields to determine if areas of these fields have lower-than optimal stands due to cumulative effects of disease, traffic damage or winter injury. Ice sheeting may also have occurred in some portions of poorly drained fields. Conducting a stand assessment this spring will help make decisions related to stand management, cutting schedules, fertilizer applications, or stand replacement.
Optimum stand density varies with stand age
Alfalfa stand density naturally declines with stand age. Stands that are 1 year old or less should have a high stand density. As alfalfa stands age, competition for resources, diseases, and crown injury due to winter and traffic reduce plant numbers. Decreases from 25 plants/ft2 during the first year to 5-10 plants/ft2 by the end of the second production year are commonly reported. This reduction in plant density is compensated for by an increase in plant root and crown size and stems per plant. However, injury to older crowns by traffic or disease can reduce the numbers of stems and productivity of individual stems. A population of 4-5 healthy plants can result in productive stands but lower populations will likely not be as productive.
Assessing alfalfa stands
Assessing the health of the stand will help to estimate future yield potential. Plants from 4 to 6 representative areas of a field should be dug to a 6-inch depth. The plants should be examined for regrowth symmetry from the crown and the number of shoots present. The crown and root should also be examined externally for incidence of decay and then the root cut lengthwise and examined for disease. The best plants have symmetrical crowns and white pithy roots. With disease and injury plants typically have more asymmetrical crowns with root and crown disease (see photos 1-4).
More details and photos for rating alfalfa on a scale of 0-5 are provided in an excellent publication from the University of Wisconsin, Is this stand good enough to keep?
To further evaluate the potential productivity of an alfalfa stand, stem density should be determined by counting stems from 4-6 representative square foot areas within your field. Only stems that are at least 6 inches tall should be counted. Research in Wisconsin and Minnesota has shown that for maximum productivity stem density of at least 55-70 stems/ft2 is desirable. A stem density of 40/ft2 indicates that yield potential is likely to be low and that rotation or renovation is warranted.
Alfalfa stands that are thin or patchy will lack yield potential. There are several options that are influenced by how many additional years are desired out of the stand and by seed and equipment availability, as well as. Supplemental forages are ideally seeded with a no-till drill in the affected areas, but often conventional drills will work if the ground is somewhat soft. Several recommended options are listed in the following publication: Evaluate alfalfa stands this spring
Commonly recommended options for damaged stands
Source : umn.edu
- Stand termination and rotation into a corn crop will allow utilization of alfalfa nitrogen to replace fertilizer nitrogen. This option should be considered if stands are thin throughout the field and contain many weeds. For more information about managing the rotation from alfalfa to corn, terminating alfalfa, and managing the two subsequent corn crops please refer to the University of Minnesota Extension webpage on “Rotating from alfalfa to corn.”
- Interseeding rapidly growing grasses like annual ryegrass or Italian ryegrass at 5-10 lb/acre into portions of the field that have significant thinning.
- True annual ryegrass goes to seed in early summer and will regrow after harvest, but will not survive winter. There are diploid types that are small leafed and more prostrate; as well as tetraploid types that are larger leafed with fewer tillers. Seed of the two types are often mixed togethers and sold together in blends.
- Italian ryegrass, is a biennial and won’t form seed during the year of planting, but instead, goes to seed the next spring. It has more even yield distribution during the seeding year. Its winter survival is reliant on snow cover and is not dependable.
- Seeding red clover at 6-10 lb/acre together with annual ryegrass. Red clover has very good seedling vigor and can be harvested multiple times during the growing season. Mammoth or annual types behave as true annuals, while medium types that predominant in the marketplace are true perennials that persist for two or three years. Red clover forage is wetter than alfalfa’s and requires a longer drying time.