Alfalfa response to low potassium under different harvest regimes in Coastal Plains

Potassium fertilization and harvest timing both influence the stand life and performance of alfalfa ( Medicago sativa  L.), but many producers do not apply sufficient K fertilizer as recommended by soil test recommendations. The objective of this trial was to determine the impact of reduced rates of K fertilization on cultivar Bulldog 805 alfalfa managed under different harvest regimes on forage yield, stand persistence, and nutritive value when grown in the southern Coastal Plains. Plots were harvested at bud, 10, 30, and 50% bloom growth stages. Potassium was applied three times across the season at total rates of 0, 67, 101, 134, and168  kg K 2 O ha −1 . Aboveground and belowground plant and soil responses were evaluated. Evaluated levels of K were not sufficient to elicit a positive response in aboveground variables, as all treatments exhibited K deficiency as confirmed by tissue analysis at the end of the study. Harvest timing affected many of the aboveground yield components, however no clear trends existed. As expected, material harvested at later growth stages tended to have higher fiber and lower digestibility than alfalfa harvested at less mature stages. Reduced rates of K fertilization helped maintain soil K levels above the final level in the untreated control. Harvesting alfalfa at later (50% bloom) growth stages decreased starch and nonstructural carbohydrate (NSC) content in the roots. To optimize both alfalfa yield and nutritive value, current University recommendations for alfalfa harvest timing and K fertilization should be maintained in the southern Coastal Plains.