Evaluating new alfalfa stands

Craig Sheaffer, Extension forage agronomist, Krishona Martinson, Extension equine specialist, and Claire LaCanne, Extension educator-crops

Alfalfa seeded with an oat companion crop. Are
 there enough plants to develop a productive
 long-term stand?

This has been an unusual spring with above normal and intensive rainfall throughout Minnesota. Adequate levels of soil moisture should provide for high levels of germination as seeds normally need to absorb 125% of their weight, but excessive rainfall can cause problems with seedling emergence and persistence. Sources of these issues include:

• Soil crusting. Medium and heavy textured soils, can experience crusting that prevents or delays seedling emergence. Crusting problems are compounded when seeding depths exceed ¼ inch, therefore, controlling seeding depth is essential.
• Washing of seed and seedlings. On soils prone to water erosion, heavy erosion can wash shallow planted seed away as well as uproot small seedlings. Using companion crops on erodible soils can reduce erosion.
• Flooding. Flooding damages and kills plants by creating an anaerobic environment which prevents alfalfa seedling respiration, promotes formation of ethanol in the roots, and inhibits development of biological nitrogen fixation. In addition, saturated soils can lead to greater incidence of root disease such as Phytophthora root rot. Alfalfa seedlings, of various stages of development, have been reported be able to tolerate flooding of 10 to 14 days at normal May temperatures, but high temperatures can accelerate the damage of flooding. Flooding is most detrimental to germinating seedlings and those with 1 to 2 trifoliate leaves than those most advanced with multiple stems (4 + weeks old). Plants inhibited by flooding can slowly recover when water recedes. Disease resistant varieties and seed coatings containing fungicides can overcome some disease issues related to wet soils. 
• Herbicide residuals. Some poor stands might be the result of residual effects of herbicides applied to previous crops. The dry conditions over the past couple of years may play a role in herbicide residual activity, because most herbicide residuals rely on moisture and soil microbes to break down. Many herbicides used for weed control in corn, soybean, and wheat contain restrictions on the label regarding rotation to alfalfa. A rotation restriction is the interval between herbicide application and when alfalfa planting can occur with no injury. If restrictions are listed on the label, rotation to another crops, besides alfalfa, is necessary. For more details see: Alfalfa herbicide rotation restrictions

How many plants is enough?

Figure 1. Alfalfa stand density during the first 24 months
 (seeded at 15 pounds per acre). Source: Radkin, M.
https://fyi.extension.wisc.edu/forage/determining-
the-optimum-alfalfa-seeding-rate/

While we often use a threshold of 3 to 5 plants per square foot as an indication of a viable 2 to 5 year old stand. New seedings need to have much higher populations to provide high seedling year yields and to be productive due to natural stand loss that occurs from year to year. By the fall of the seeding year, plant populations should be 30 to 35 plants per square foot. 15 to 25 plants per square foot should be present at the start of the first production year for maximum yields. Although thinner stands can fill in overtime as alfalfa crowns spread, forage yield will initially be less. Figure 1 shows an example where a 15 lb per acre seeding rate was used. The data shown assumes that there are 199,000 seeds per pound, 100 % pure live seed, and an initial survival rate of 60% (to 3 to 4 weeks). Because of natural competitive thinning, a loss of another 40 to 50% during the seeding year and overwinter occurs. This process allows most fields to arrive at a stand density of about 25 plants per square foot the following spring (12 months later).

Assessing your alfalfa stands

It is always a good idea to assess your stands after seeding by counting plant numbers throughout the field. However, a common challenge is variable stands within a field because it’s common for portions of the field to have greater alfalfa populations than others. Taking at least three random plant counts per acre, or from areas within the field with extremes in alfalfa populations, is recommended. You can estimate populations by counting the number of plants in a known area. For example, use a 17 by 17 inch square made from PVC or rebar which is equivalent to 2 square feet.

How to thicken stands

For fields without adequate populations due to erosion or flooding, there are a few options. You can till and then reestablish by broadcasting or drilling seed, or you can thicken by interseeding. If thin stands can be thickened by interseeding with no-tillage equipment, it's cheaper than starting over. Autotoxicity, or the killing of new seedlings by older plants or their residues, will not be a problem when re-seeding spring stands.

No-till seeding into the already firm seed bed will eliminate the need for tillage, and reduce erosion risks. It is important that you confirm the working condition and calibration of the no-till drill to ensure the proper seeding rate, depth of planting, and adjustment of coulters or disc openers to cut residue and create an opening for dropping the seed.

Even though we are beyond the date for optimum spring seeding, it is worth the risk to seed until mid- July this year because of the existing good soil moisture levels. If that timing is not feasible, then plan on harvesting or mowing the existing crop at about 60 days following planting, and seed immediately thereafter.

Additional information of alfalfa management practices:
https://extension.umn.edu/planting-forages/alfalfa-establishment-management-strategies