To increase the possibility of winter survivability, a late-summer alfalfa cutting should occur early enough so plants can regrow, produce, and store carbohydrates in the roots and crowns before entering fall dormancy. An accurate stand evaluation is critical for estimating yield potential for estimating yield potential and if the existing stand should be kept for the following year. Fertility, especially potassium, should be maintained at high levels for maximizing alfalfa tonnage.
Alfalfa needs a harvesting break of about 6 to 8 weeks prior to the first killing frost. In northern areas, this timeframe is roughly the first of September through mid-October and later in more southern areas.1 The resting period allows plants to rebuild carbohydrate (CHO) reserves that are needed for spring regrowth before winter begins. If the crop is harvested during the rest period, the speed of spring regrowth may be reduced, stands may thin, and tonnage potential may be lower. Research has shown that a late-fall cutting can be made if less than 200 growing degree days (GDD, using the base temperature of 41 °F) accumulate after the cutting. The GDD benchmark generally helps prevent measurable regrowth and depletion of root-stored CHOs before the crop enters the winter season.2
Fall is the best time to evaluate established alfalfa stands for future productivity by counting stems. Older stands tend to have fewer plants but more stems/plant. To estimate tonnage potential begin by counting stems that are 4 to 6-inches tall within a framed area of either 17X17 inches or 24X24 inches at 4 to 5 field locations (Figure 1). Take the average of the counts and divide this number by 2 to determine the average stems/sq ft. If the stem count is more than 55, tonnage potential is considered to be 100%. Stand replacement should be considered if the count is 40 or a tonnage potential of about 72%
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 soil test should be made to determine existing soil nutrient levels, especially pH, phosphorus (P), and potassium (K) to help avoid over or under fertilizing in the future. However, for sulfur a tissue test is the most accurate method to assess the need.
The recommended soil pH level for alfalfa production is between 6.8 to 7.0. Split applications of P and K, based on tonnage goals, are recommended after the first cutting and again in late summer. If all fertilizer is applied as one annual application, it should be applied in early fall to help minimize the potential for winter injury.
Autotoxicity (growth inhibition by same species) can occur if an attempt is made to reseed alfalfa into an existing stand of 2 or more years. However, reseeding into an existing stand is generally successful if the stand is less than 1 year old, and soil pH and fertility are at recommended levels. In situations where the alfalfa stands are thin, but it is not practical to destroy the stand, consider interseeding with grasses or clover to help meet forage needs.
Conventional alfalfa products have limited weed control options, which can hinder stand establishment and persistence resulting in lower tonnage potential. 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 (Figure 2). Based on the weeds present in the alfalfa crop, 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.
The Alfalfa Management Guide is a publication developed by Extension specialists from several universities to give an overview on alfalfa best management practices. The guide can be found at: https://www.agronomy.org/files/publications/alfalfa-management-guide.pdf.
1Undersander, D., Cosgrove, D., Cullen, E. 2011. Alfalfa management guide. American Society of Agronomy, Inc. https://agronomy.org/
2Barnhart, S. 1999. Fall harvest management of alfalfa. Integrated Crop Management. Iowa State University. http://www.ipm.iastate.edu/ipm/icm/1999/9-13-1999/alfalfaman.html
3Lacefield, G.D., Henning, J.C., Rasnake, M., and Collins, M. 1997. Alfalfa - the queen of forage crops. AGR-76. University of Kentucky. Additional
Sources: Summers, C.G, Putnam, D.H, et al. 2008. Irrigated alfalfa management for Mediterranean and desert zones. Publication 3512. University of California. http://alfalfa.ucdavis.edu/IrrigatedAlfalfa/ Kaatz, P. 2015. Assessing sulfur fertility levels for alfalfa. http://msue.anr.msu.edu/news/assessing_sulfur_fertility_levels_for_alfalfa Web sources verified 6/29/18. 130930014001