MINNEAPOLIS — The drought in many regions of Minnesota caused significant problems with alfalfa establishment this spring (Figures 1 & 2). Although periodic rainfall has occurred, most of the state continues under moderate to severe drought (https://www.drought.gov/states/minnesota).
On some farms, alfalfa seeds germinated this spring but failed to thrive due to limited moisture. This has resulted in thin or uneven stands in many fields.
In some cases, areas of fields with heavier soils produced partial alfalfa stands while lighter, sandy soils have no alfalfa. Drought-tolerant weeds have become prevalent in fields and are competing with alfalfa seedlings.
Historically, late summer seeding in August often results in successful establishment of alfalfa. Although this is a risky practice in a drought year, late summer seeding avoids many of the annual weed problems associated with spring seeding of alfalfa.
Late summer is also an excellent time to establish alfalfa-grass mixtures because herbicides are often not necessary. Decreasing air temperatures in August and September also help promote development of cool-season forages.
Autotoxicity, or the killing of new seedlings by older plants or their residues, will not be a problem when re-seeding spring stands in the late summer. If the spring seeded stands were thin, autotoxicity will not occur.
Appaise your alfalfa stands
A goal is to have a uniform stand of about 25 plants per square foot in the fall of the first year. Under normal winter conditions, about half of these plants will die. A stand of about 12 plants per square foot is adequate for high yields the year following establishment.
However, a common challenge is variable stands within a field. Portions of the field will have greater alfalfa populations then others. We recommend walking and mapping fields by taking at least three random plant counts per acre or from areas within the field with extremes in alfalfa populations. 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. For profitable long-term production, an alfalfa field should have about 80% of the field with a population of over 25 plants per square foot in the fall of the seeding year.
Keys for success
While general guidelines for successful establishment are provided here (https://extension.umn.edu/forages/planting-forages#alfalfa-701910), specific recommendations related to seeding during late summer and/or reseeding include:
- Crop insurance program. Consult with insurance providers and government agencies to determine your coverage and how reseeding will affect your coverage.
- Seeding date: Seeding alfalfa and grasses should occur about 6 to 8 weeks before the average date of a killing frost. This translates to an ideal seeding date between August 1 and 15 in the central and southern regions of the state.
- Perennial grasses should have a minimum of 4 leaves before frost, while alfalfa should have 6 true leaves.
- For both legumes and grasses, the crown and root system should be well-developed to survive the winter and the potential for frost heaving. This requires alfalfa to undergo contractile growth, a process by which a crown is formed below grown. The crown is the most winterhardy part of the plant and is the site for bud development for spring regrowth.
- Time of seeding also affects yield the following year. Researchers have found that for each day seeding is delayed after August 1st, alfalfa forage yields the following year decline an average of 135 pounds per day (Figure 3).
- Soil fertility: Soil test and apply recommended fertilizers following University of Minnesota guidelines for alfalfa establishment to ensure recommended pH, phosphorous, potassium, sulfur and boron levels. If fertilizers were applied for spring planting, there is no need to retest and reapply as there was likely little uptake or loss of nutrients without crop growth or rainfall.
- Seeding rate. Seeding rates should be about 12 pounds per acre of pure live seed to provide 60 seeds per square foot. The rational for this recommendation is that alfalfa has about 220,000 seeds per pound and a seeding rate of 1 pound per acre will provide 5 seeds per square foot. Even if half of the seedlings die, fall populations should be greater than 25 plants per square foot, an adequate level for good production the following year. Higher seeding rates in the 15 to 20 pound per acre range are sometimes used for insurance purposes; however, these have not been found to increase yields with good seeding equipment and conditions. When calibrating your seeding machinery, be aware of the effect of seed coatings on seed weight. For example, a seed coating that is designed to supply rhizobium for nitrogen fixation or fungicides can add 20% to the weight of a seed and necessitate increasing the actual seeding rate by 20% to achieve the same pure live seed rate.
- Control weeds. Many drought-stricken fields now have populations of weeds that will compete with new seedlings. It is remarkable how drought tolerant and resourceful weeds like marestail, common lambsquarters, and pigweed species are. These weeds will have established root systems and will need to be killed by tillage or herbicides prior to planting. With late summer seeding there is seldom a need for additional weed control from weeds emerging from the seedbank.
- Seeding method. Alfalfa can be seeded by broadcasting (cultipacker seeders or air seeders) or with drills. The critical factors for successful establishment are shallow seeding (¼ to ½ inch; deeper for sandy soils) and achieving soil-seed contact by using press wheels, rollers, or packers.
No-till seeding in combination with a pre-establishment herbicide for weed control (e.g. glyphosate) is a good practice for many fields that have a well prepared seedbed. No-till seeding will eliminate the need for tillage and potential soil and water loss. 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 (Figure 4). Used in combination with Roundup-ready alfalfa, glyphosate can be used to control weeds in newly established alfalfa.
- Forget the companion crops. Companion crops, like oats or barley, provide benefits for weed control and reduced soil erosion in the spring. With late summer seedings, companion crops will provide a high level of competition for water and light. Companion crops should be used only if there is high wind or water erosion potential in the field. Competition with grass companion crops can be reduced by spraying appropriate post emergent herbicides after companion crops reach the 3-leaf stage.
But what about the drought?
Even if you follow the best agronomic practices, seeding in August this year appears to be a very risky proposition. It is difficult to predict future weather, but we are not optimistic about having adequate soil moisture in August and September to establish new stands of forages and cover crops.
The following insight was provided by Minnesota DNR climatologists Kenny Blumenfield and Pete Boulay, who have reviewed forecasts by the NOAA’s Climate Prediction Center (https://www.cpc.ncep.noaa.gov/). They have noted that outlooks by NOAA’s Climate Prediction Center largely reflect persistent warm and dry conditions, which have resulted in drought escalating across the state in recent weeks:
“The outlooks indicate that the forecasters are betting on a continuation of the dry, generally warm weather we’ve been experiencing, right into the late summer or early fall,” said climatologist Boulay, adding that uncertainty always grows and confidence shrinks the farther into the future these forecasts venture.
“We don’t know exactly where we’ll be in 2-3 weeks, let alone a month or two from now,” said Blumenfeld. “But right now, there is no clear sign of the pattern breaking, and that’s what the forecasters are getting at in their various outlooks.”
Both climatologists noted that Minnesota’s hydroclimate conditions can “turn on a dime,” and cautioned that no two droughts are identical. “People want to compare this to other years,” Boulay remarked, “But the fact is, we don’t know exactly which way it will go.”
— Craig Sheaffer, Professor, Department of Agronomy and Plant Genetics, Krishona Martinson, Extension equine specialist, and Jared Goplen, Extension educator-crops, University of Minnesota Extension
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