Potassium Management during the Rotation from Alfalfa to Corn

Published in Agron. J. 103:1785–1793 (2011) Posted online 26 Sep 2011 doi:10.2134/agronj2011.0183 Copyright © 2011 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. W 3.89 million ha harvested each year in U.S. Corn Belt states (Iowa, Illinois, Indiana, Michigan, Minnesota, North Dakota, Nebraska, Ohio, South Dakata, and Wisconsin), alfalfa is the fourth most widely grown crop after corn, soybean [Glycine max L. (Merr.)], and wheat (Triticum aestivum L.) (USDA-NASS, 2009). Although alfalfa hay yields for the region average between 5 and 7 Mg dry matter (DM) ha–1 (USDA-NASS, 2009), farmers in the upper 20th percentile of profitability from Corn Belt states in the University of Minnesota Farm Financial Database (Minnesota, Nebraska, North Dakota, Ohio, and Wisconsin) produced an average of 13 Mg ha–1 from 2008 to 2010 (Center for Farm Financial Management, 2011). This disparity between average forage yield and attainable yield raises the question: How can producers improve their crop management practices to increase alfalfa yield? One possible answer is that nutrient application rates need to be increased. Alfalfa is stressed by frequent harvests, field traffic, winter conditions, poor drainage, and drought. Potassium is crucial for tolerance to these stresses and maximum yield because it improves carbohydrate storage in alfalfa roots and helps increase the regrowth rate (Reid et al., 1965; Schnappinger et al., 1969). High levels of soil fertility and pest control have been required to maintain stands and maximize yields (Berg et al., 2005, 2007). Current K fertilizer guidelines for established alfalfa vary widely among Corn Belt states. For example, Minnesota recommends 80 kg K ha–1 to established alfalfa for a yield goal of 11 Mg DM ha–1 when STK is in the medium range (81–120 mg K kg–1 ammonium acetate-exchangeable K) (Rehm et al., 2000). However, the University of Nebraska recommends only 30 kg K ha–1 (ammonium acetate-exchangeable K; Tarkalson and Shapiro, 2005) while the University of Wisconsin recommends 250 kg K ha–1 (Bray 1-exchangeable K; Laboski et al., 2006) for medium STK levels. Furthermore, Wisconsin recommends increasing K rates for established alfalfa by 20% if a stand life longer than 3 yr is desired. In addition to the wide range of K fertilizer recommendations for alfalfa, none of the recommendations for the Corn Belt change for the last production year when stand persistence is not a major concern (Iowa, Sawyer et al., 2007; Illinois, Fernandez and Hoeft, 2009; Michigan, Ohio, and Indiana, Vitosh et al., 1995; Minnesota, Rehm et al., 2000; North Dakota, Franzen, 2010; Nebraska, Tarkalson and Shapiro, 2005; South Dakota, Gerwing and Gelderman, 2005; Wisconsin, Laboski et al., 2006). However, it may be important to maintain alfalfa stands ≥43 plants m–2 in the last production year to maximize the N fertilizer replacement value to the following corn crop (e.g., Rehm et al., 2006) and to avoid nitrate leaching losses during the alfalfa growing season (Entz et al., 2001). At least two concerns about K supplementation face growers. The most important is fertilizer price. In the past three decades (1978–2007), commercial fertilizer prices for the most widely used K source averaged about $176 Mg–1 KCl, but have been three to four times higher beginning in 2008 (USDA-ERS, 2011). This raises the risk of lower profits if crop response to the input is lower than expected. The second concern is due to luxury consumption of K by alfalfa, which poses the additional ABSTRACT High K fertilizer prices in recent years have made it imperative for growers to apply optimum K rates to alfalfa (Medicago sativa L.). Current university fertilizer guidelines in the Corn Belt do not change for the last production year, when alfalfa stand persistence is not a major concern. Furthermore, little is known about carryover of K applied to alfalfa on first-year corn (Zea mays L.) grain and silage yields. In 2008 to 2010, on-farm research was conducted on 10 fields with medium soil test potassium (STK) to determine response to K for alfalfa yield and quality in the last production year, and to estimate K carryover to first-year corn. Alfalfa yield and relative feed value (RFV) and quality (RFQ) did not improve with K fertilization. Herbage K concentration and K uptake increased with K fertilization across sites, indicating that applied K was available during the season of application. When corn relied on carryover K alone, each 100 kg ha–1 increase in the index of available K increased corn grain yield by 0.5 Mg ha–1, decreased stover yield by 0.4 Mg ha–1, and did not affect silage yields. Regardless of K rate applied to alfalfa, additional K applied to corn increased corn stover and silage yields by 10 and 8%, respectively. This suggests that carryover K was less available than K applied to corn. On medium STK soils going into the last year of alfalfa, applying fertilizer K to first-year corn rather than alfalfa may enhance economic return.