Potassium and harvest time interaction effect on alfalfa production and profitability.

Researchers have extensively studied and documented the effects of potassium (K) fertility on alfalfa (Medicago sativa L.). Yet, additional research is needed to determine how interactions of K, cultivar, and harvest management influence the K needs of alfalfa. To explore these interactions, we conducted 5 years of field research at the University of Wyoming James C. Hageman Sustainable Agriculture Research and Extension Center in Lingle, WY. Treatments were (a) four K rates (0, 56, 112, and 168 kg K2O ha−1 year−1) applied before planting in the fall of 2016 and after the final harvest in the fall of 2017–2020, (b) two cultivars (Hi‐Gest 360 and AFX 457), and (c) two harvest times (early harvest, late bud to early [10%] bloom, and late harvest, 7 days after early harvest), arranged in a 4 × 2 × 2 factorial under random complete blocks with four replications. At 168 kg K2O ha−1 year−1 and early harvest, a consistently significant (p < 0.05) higher yield response was observed. The same response was seen at 112 kg K2O ha−1 year−1 and late harvest. This occurred at a site with moderate‐to‐high soil K levels throughout the study period. There was a linear (p < 0.001, R2 = 0.66) and quadratic (p = 0.006, R2 = 0.61) response of forage accumulation to K rate at early and late harvest, respectively. Similar trends were also seen for stem count, relative water content, root uptake of K, and tissue K. Time of harvest showed immense potential for optimizing K's effect for a consistent high‐yield response. However, fertilizing alfalfa with 112 kg K2O ha−1 year−1 gave the most profitable production under both harvest times. If K fertilizer prices drop over time, high profits could be attained with higher K fertilization rates. After 3 years of production, average forage accumulation increased under an early harvest system and decreased under a late harvest system. Growers in Wyoming and similar regions are encouraged to consider fertilizing alfalfa with moderate K rates (∼112 kg K2O ha−1 year−1) on soils testing moderate‐to‐high in soil test K, implement a late harvest system for the first 3 years after planting, and transition to an early harvest system after the initial 3 years to maximize alfalfa profits. Core Ideas: There have been conflicting reports on the effect of potassium (K) fertilizer on alfalfa (Medicago sativa L.) production.Research is needed to quantify the K needs of alfalfa as influenced by the interactions of K rates, cultivar, and harvest time.Alfalfa responded significantly to K fertilizer even in a moderate‐to‐high exchangeable K soil environment.Alfalfa cultivar did not influence K requirements, and late harvest reduced K requirements by 56 kg K2O ha−1 year−1 for optimal yield response.The 112 kg K2O ha−1 year−1 rate was most economically optimal for both early and late harvests, suggesting that K rates can be similar for both harvest times. [ABSTRACT FROM AUTHOR]