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Alfalfa (Medicago sativa) stands need a break from harvest during the 6-8 weeks prior to the first killing frost. In northern areas, this timeframe is roughly the 1st of September through mid-October and later in more southern regions.1 Having this rest period allows plants to build up adequate reserves of carbohydrates (CHO) in the roots before winter begins. This process can help plants survive winter temperatures and use the stored CHO to regrow in the spring. If plants are cut during this rest period, the speed of spring regrowth the next season may be reduced, plant stands may thin, and yield potential may be lower. Research has shown that a late fall cutting can be made as long as there will be less than 200 growing degree days (41°F is the standard temperature base) accumulation after the cutting. This GDD benchmark generally prevents significant regrowth and depletion of stored CHO’s in the roots going into the winter, which improves winter survivability.2 Farmers may be tempted to harvest alfalfa during the rest period if significant growth has occurred, but doing this would initiate regrowth and reduce CHO root reserves during a critical time. The risk of stand damage is somewhat reduced under the following conditions: in areas with less severe winters;1 if the stand is in bloom at the time of the fall cutting, if it has been at least 45 days since the last harvest, and if the stand is old it will be rotated to another crop in the spring.3 Fields that are at high risk if harvested without a rest period included those that less than a year old in production, stressed with inadequate fertility, and where the alfalfa product does not carry multi-pest resistance.3 Waiting and making a final cutting after the first hard freeze (24 °F or lower) may not hurt alfalfa and may help reduce pest problems.3 For example, Minnesota researchers found that the highest yields came from 3 cuttings during the growing season followed by a late-fall cutting.1
Figure 1. Alfalfa stem count method.
Fall is the best time to evaluate established alfalfa stands for future productivity and decide whether it will be better to keep or rotate out of a stand. In established stands, when stems are 4-6 inches tall stem counts provide a more accurated estimate of yield potential then counting plants (Figure 1). Older stands tend to have fewer plants but more stems/plant. To estimate the yield potential of the stand, begin by counting the stems within an area (either 17x17 inches or a 2 square feet frame) at 4-5 random field locations. Take the average of the counts and divide this number by two to determine the average stems/square foot. If the count is more than 55, stem density has 100% yield potential; whereas 40 stems have about a 72% yield potential and replacement should be considered.
Each ton of alfalfa dry matter removes about 14 pounds of phosphate (P2O5), 58 pounds of potash (K2O), 30 pounds of calcium, 6 pounds of magnesium, and 6 pounds of sulfur.1 A current soil test should be used to determine existing soil nutrient levels, especially pH, phosphorus (P), and potassium (K), to avoid over or under fertilizing in the future. Soil tests are the most reliable method to prevent future nutrient deficiencies, which can decrease alfalfa yield potential and quality. However, for sulfur a tissue test is the most accurate method to assess the need. The recommended soil pH level is between 6.8 and 7.0 and alfalfa responds well to fertilization with P and K. Split applications of P and K, based on yield goals, are recommended after the first cutting and again in late summer. If all fertilizer is applied as one annual application, the task should be completed in early fall which may help minimize the potential for winter injury.
Figure 2. Regrowth of Roundup Ready® Alfalfa after a postemergence application of a Roundup® brand glyphosate only agricultural herbicide (left) compared to conventional alfalfa after a postemergence application of a conventional herbicide (right).
Limited seedling establishment can occur when seeding into existing stands or into a field where alfalfa was recently removed. Autotoxicity is a problem in alfalfa stands that are two or more years old. However, reseeding into an existing stand is generally successful if the stand is less than one year old and soil pH and fertility in those areas is optimum. In situations where alfalfa stands are thin but it is not practical to destroy the stand, consider interseeding grasses or clover to meet forage needs.
Conventional alfalfa products have limited weed control options, which can hinder stand establishment and persistence resulting in lower yield potential (Figure 2, Page 1). A Roundup Ready® Alfalfa system gives alfalfa producers the advantage of broad-spectrum weed control and application flexibility with Roundup® brand glyphosate-only agricultural herbicides and may reduce potential crop injury or future rotational concerns. Based on the weeds present, one or more herbicides with different effective sites of action should be used at least once during the middle years of the stand to help reduce the potential for selecting herbicide resistant weed populations. However, if using a Roundup ready product, Roundup will not be an option for weed control.
Additional information on best management practices for alfalfa can be found in the Alfalfa Management Guide.3 This guide was developed by Extension specialists from several universities to give an overview on production practices and can be found online at: https://www.agronomy.org/files/publications/alfalfa-management-guide.pdf