Transgenic Corn Rootworm Protection Enhances Uptake and Post‐Flowering Mineral Nutrient Accumulation

Published in Agron. J. 105:1626–1634 (2013) doi:10.2134/agronj2013.0230 Available freely online through the author-supported open access option. 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. C rootworm is among the most damaging insect pests of maize production in the United States costing farmers an estimated U.S.$1 billion annually as a result of yield loss and chemical control measures (Metcalf, 1986; Agricultural Research Service, 2001). While CRW larvae primarily feed on maize roots (Gray et al., 2009), adult beetles consume silk, pollen, and kernel tissue (Moeser and Vidal, 2005). This damage to aboveand belowground tissues reduces grain yield, total dry weight production, and accumulation of key nutrients (Kahler et al., 1985; Godfrey et al., 1993; Rice, 2004). Control of CRW larvae and adult insect populations to mitigate root feeding and yield loss traditionally involved the use of crop rotation and insecticides. The reduced effectiveness of these strategies occurred with the development of insect resistance to seed and foliar-applied insecticides (Meinke et al., 1998), and CRW behavioral changes including extended egg diapause and loss of ovipositional fidelity to maize for Northern CRW (D. barberi Smith and Lawrence) and Western CRW (D. virgifera virgifera LeConte), respectively (Gray et al., 2009). Transgenic hybrids expressing the Bacillus thuringiensis (Bt) toxin, developed for control of CRW, have been rapidly adopted during the past 10 yr in the United States. While the benefits of Bt hybrids include improved consistency of insect control, healthier root systems, greater yields, improved N use, and improved grain quality (Rice, 2004; Folcher et al., 2010; Haegele and Below, 2013), important agronomic questions remain unanswered. Specifically, it is unknown if mineral nutrient uptake patterns, especially in more stable, higher yielding Bt hybrids (Edgerton et al., 2012), differ in comparison to their non-Bt (refuge) counterparts. While 17 widely accepted plant essential nutrients exist, recent work has documented that N, P, K, S, and Zn are required for maize production in greater quantities, and with the exception of K, these nutrients have relatively high nutrient harvest index (HI) values (Bender et al., 2013). To maximize nutrient use, maize production will require fertilizer availability during key growth stages, especially for these nutrients. Physical transport of soil nutrients is achieved by mass flow and diffusion through soil solutions (Barber, 1962). According to Barber (1994), as much as 93% of P uptake and 80% of K uptake in maize is acquired through diffusion compared to mass flow, which accounts for up to 79 and 100% of N and S accumulation, respectively. The majority of N and K accumulation occurs during vegetative growth, in contrast to P, S, and Zn, which are primarily acquired during grain fill (Sayre, 1948; Hanway, 1962; Karlen et al., 1988; Bender et al., 2013). Total dry weight production after the initiation of reproductive growth (i.e., silk emergence) is partitioned directly into developing grain as opposed to other plant tissues. As a result, it is not surprising that nutrient accumulation during grain fill, especially in nutrients with high harvest index values (e.g., N, P, S, and Zn), is partitioned into maize grain (Bender et al., 2013). Although N is the only nutrient with a relatively high harvest index and relatively low post-flowering uptake, N is rapidly translocated to grain tissues during grain fill. ABSTRACT Although modern maize (Zea mays L.) hybrids with transgenic insect protection from corn rootworm (CRW) (Diabrotica spp.) demonstrate improved yield and insect control compared to their non-protected (refuge) counterparts, no comprehensive studies have documented the impact of transgenic insect protection on nutrient uptake and partitioning. The objective of this study was to investigate the effect of transgenic protection from CRW on the timing and quantity of uptake for key nutrients such as N, P, K, S, and Zn. Results from two similar experiments across 5 site-years were analyzed and summarized. In the first experiment, transgenic hybrids averaged greater grain yield (10%; 0.9 Mg ha–1), total biomass (7%; 1.2 Mg ha–1), and grain nutrient accumulation of N (8%), P (12%), K (9%), S (9%), and Zn (12%) compared to non-protected hybrids (P £ 0.05). In the second experiment, the yield response associated with transgenic insect protection varied among hybrids. Those hybrids which exhibited a yield response compared to their non-protected counterparts resulted in greater post-flowering acquisition of N (31%), P (24%), and K (38%) (P £ 0.05). The results indicate that in favorable environments, transgenic CRW protected hybrids not only produce more total biomass and yield, but also maintain greater rates of nutrient acquisition during grain-filling.