Potassium fertilization affects alfalfa forage yield, nutritive value, root traits, and persistence

Authors: JUNGERS, JACOB - University Of Minnesota; KAISER, DANIEL - University Of Minnesota; Lamb, Joann; LAMB, JOHN - University Of Minnesota; NOLAND, REGAN - University Of Minnesota; Samac, Deborah - Debby; WELLS, M. SCOTT - University Of Minnesota; SHEAFFER, CRAIG - University Of Minnesota

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/2019
Publication Date: 8/30/2019
Citation: Jungers, J.M., Kaiser, D.E., Lamb, J.F., Lamb, J.A., Noland, R., Samac, D.A., Wells, M.S., Sheaffer, C.C. 2019. Potassium fertilization affects alfalfa forage yield, nutritive value, root traits, and persistence. Agronomy Journal. 111(6):2843-2852. https://doi.org/10.2134/agronj2019.01.0011.
DOI: https://doi.org/10.2134/agronj2019.01.0011

Interpretive Summary: Potassium (K) is one of the most important nutrients in alfalfa growth and development. However, reports of the effects of K fertilization on alfalfa yield, quality, disease resistance, and persistence have been inconsistent. Moreover, the effects of K fertilization on have not been tested across cultivars. Therefore, K fertilization rates that maximize yield and persistence while minimizing forage K concentration need to be identified. An experiment was done with five K fertilizer rates using eight alfalfa cultivars at three locations that varied in the baseline amount of soil K. All cultivars responded similarly to K fertilization. At the site with the lowest baseline values, K fertilizer had a positive effect on yield, which plateaued at a K rate of 296 kg K/ hectare. There was no effect of K on yield at the site with the greatest baseline soil K. There was no effect of K fertilization on crown rot. Accumulation of K in plant biomass increased with K fertilizer rates beyond those which maximized yield to levels that could cause milk fever in dairy cows. This research found that K fertilizer rates for alfalfa should be determined based on expected forage yield, baseline soil K concentrations, and whether or not the forage will be fed to lactating cows in order to reduce over-fertilization and production costs.

Technical Abstract: Potassium (K) is a critical macronutrient for alfalfa growth, however reports of the effects of K fertilization on alfalfa yield, quality, and persistence have been inconsistent. Moreover, the effects of K fertilization on forage yield, quality, and persistence have not been tested across cultivars. Five K fertilizer rates ranging from 0 to 403 kg K ha-1 were applied to eight alfalfa cultivars during the establishment year (2011) and three subsequent production years (2012-2014) at three locations varying in baseline soil test potassium (STK) levels. Alfalfa yield, nutritive value, and STK were measured annually. During the final year, stem density and belowground traits were quantified. Forage yield, nutritive value, and root traits varied by cultivar, but there were no interactions between cultivar and K rate. Crown rot varied by cultivar but was not affected by K rate. Averaged across cultivars, the effect of K on yield varied by location and stand age. At the site with the lowest baseline STK values, K fertilizer had a positive quadratic effect on yield, which plateaued at a K rate of 296 kg K ha-1. There was no effect of K on yield at the site with the greatest baseline STK. Potassium fertilizer reduced forage quality and increased forage K concentration. Above- and below-ground biomass K concentration increased with K fertilizer at rates beyond those which maximized yield, indicating luxury consumption of K. However, mineralization of soil K resulted in a net increase in STK after accounting for K removal during harvest. Potassium removal rates that resulted in maintained STK were 167, 227, and 268 kg K ha-1 year-1 across locations.