Optimizing potassium applications and harvest strategies to boost productivity of depleting alfalfa stands

Abstract

Rejuvenating thinning alfalfa (Medicago sativa L.) stands with additional forage species have been ineffective in boosting productivity. Research is needed to identify practical solutions to extend the life of declining alfalfa and increase forage production. A 2-year field study at the University of Wyoming James C. Hageman Sustainable Agriculture Research and Extension Center in Lingle, WY, investigated the interaction effect of K and harvest time on thin alfalfa stands. Treatments were (a) six K rates (0, 56, 112, 168, 224, and 280 kg K₂O ha⁻¹ year⁻¹) applied to a ∼10-year-old alfalfa in the fall of 2019 and after the final harvest in the fall of 2020, and (b) two harvest times (early harvest, late bud to early [10%] bloom; late harvest, 7–10 days after early harvest), arranged in a 6 × 2 factorial under random complete blocks with four replications. Results showed a higher yield response to K at 224 kg K₂O ha⁻¹ year⁻¹ and early harvest, and at 168 kg K₂O ha⁻¹ year⁻¹ and late harvest particularly in soils with high soil test K levels (>300 mg kg⁻¹). Harvest timing influenced alfalfa's K needs, with quadratic responses of forage accumulation to K rate observed at early (p < 0.001, R² = 0.96) and late (p < 0.001, R² = 0.84) harvests. The 168 kg K₂O ha⁻¹ year⁻¹ increased stem density, increasing forage accumulation and profitability under both harvest systems. Stem density significantly impacts weed competition and stand productivity, with stem counts above 590 m⁻² maintaining optimal yields. Alfalfa producers worldwide, particularly in Wyoming and similar environments, can rejuvenate depleting stands by applying moderate K rates (∼168 kg K₂O ha⁻¹ year⁻¹) and adopting early or late harvest strategies when stem densities fall below ∼430 m⁻². However, when stem counts remain above this threshold and weeds are well managed, continued harvesting may be an efficient strategy.