Inconsistent Responses of Corn to Seeding Rates in Field‐Scale Studies

Published in Agron. J. 105:693–704 (2013) doi:10.2134/agronj2013.0008 Copyright © 2013 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. M hybrids require greater plant densities than older hybrids to optimize yield (Hammer et al., 2009). The introduction of Bacillus thuringiensis (Bt) hybrid corn to resist European corn borer (ECB) (Ostrinia nubialis Hubner) damage has reduced the risk of stalk lodging at these higher plant densities (Stanger and Lauer, 2006). Furthermore, modern hybrids have greater stress tolerance, which reduces the risk of barren plants at high plant densities (Campos et al., 2004). Consequently, growers in the northern U.S Corn Belt have increased seeding rates, as indicated by final corn densities in Iowa and Minnesota that averaged 75,350 plants ha–1 in 2011 (National Agricultural Statistics Service, 2011) compared with 63,500 plants ha–1 in 1996 (National Agricultural Statistics Service, 1996), the year that Bt corn was introduced. Another factor contributing to greater corn plant densities is the narrowing of row spacing in northern latitudes, which may allow greater optimum plant densities (Butzen and Paszkiewicz, 2008). Seed prices, however, have increased significantly in the last 10 yr (US$1.00–$3.40 per 1000 kernels in Iowa; Duffy, 2002, 2012), which is a deterrent to higher seeding rates. On the other hand, market prices for corn have increased significantly in the last 5 yr, which may provide incentive for higher seeding rates. As row spacing narrows, seed costs increase, grain market prices vary, and uncertain weather conditions prevail, the response of new corn hybrids to seeding rates should be monitored periodically to determine the economically optimum seeding rates and final plant densities across a range of locations and growing conditions (Cox, 1997; Widdicombe and Thelen, 2002; Stanger and Lauer, 2006). Optimal plant density for corn can vary by location, primarily latitude, with greater plant densities in northern latitudes in part because shorter season hybrids adapted to these latitudes require higher plant densities for maximum yield (Sarlangue et al., 2007). Furthermore, heat and drought stress are less common in northern latitudes, which reduces the risk of yield losses at high seeding rates. Optimum plant densities have been reported to exceed 80,000 plants ha–1 in the northern Corn Belt states of Wisconsin (83,300 plants ha–1; Stanger and Lauer, 2006), Iowa (~90,000 plants ha–1; Coulter et al., 2010), and Minnesota (82,000–84,500 plants ha–1; Van Roekel and Coulter, 2011, 2012). Optimum plant densities, however, rarely exceeded 74,000 plants ha–1 in Ohio (Thomison et al., 2011), 69,000 plants ha–1 in Indiana (Boomsma et al., 2009; Robles et al., 2012), and 61,800 plants ha–1 under rainfed conditions in Nebraska (Shapiro and Wortmann, 2006), probably because of increased heat and drought stress in the more southern states of the U.S. Corn Belt. Corn hybrids with the Bt trait represented 67% of corn in the United States in 2012 (Economic Research Service, 2012). Plant densities for maximum yield of Bt hybrids (104,500 plants ha–1) exceeded that of non-Bt hybrids (98,800 plants ha–1) in a Wisconsin study in part because of 22% less lodging (Stanger and Lauer, 2006). In studies where lodging was not significant, however, Singer et al. (2003) and Coulter et al. (2010) reported no differences in optimum plant densities between Bt and non-Bt hybrids. Stanger and Lauer (2007) did report that lodging of Bt ABSTRACT Corn (Zea mays L.) seeding rates have increased because new hybrids lodge less, have improved drought tolerance, and may respond to higher rates in narrow rows. Farmer–researcher partnerships were formed to evaluate two hybrids at two 0.76-m-row sites and a twin-row site at four seeding rates (61,750, 74,100, 86,450, and 98,800 kernels ha–1) and a narrow-row site (0.51m rows at ~5000 more kernels ha–1 across seeding rates) in field-scale studies in 2011 and 2012 in New York. Partial budget analyses were conducted to aid in future seeding rate decisions based on 2011–2012 market grain prices (US$265.76 Mg–1) in New York and grower seed (US$225 per 80,000 kernels) costs, in addition to drying and hauling costs. Yields averaged 11.8 Mg ha–1 in 2011 and 10.5 Mg ha–1 in 2012, but yield and relative profit responded inconsistently to seeding rates across sites, between hybrids, and between years at the narrow-row site. Maximum relative profit exceeded the recommended seeding rate of 74,100 kernels ha–1 at the twin-row site (76,000 kernels ha–1), at the narrow-row site in 1 yr (85,000–95,000 kernels ha–1) but not at 0.76-m-row sites. Lack of consistent responses to seeding rates is probably related to dry July conditions in both years, contributing to decreases of 4 to 5 kernels plant–1 and 0.5 to 1 mg in kernel weight for each 1000 kernel ha–1 seeding rate increase at most sites. Based on these field-scale studies, recommended seeding rates in New York will not change.