A gronomy J our n al • Volume 10 0 , I s sue 3 • 2 0 0 8 643 ABSTRACT Crop rotation and N are management methods that can increase corn (Zea mays L.) grain yields. Our objective was to determine the corn grain yield response to six crop rotation sequences and four N rates in a long-term (35-yr) study. Th e rotations were continuous corn (CC), corn-alfalfa (Medicago sativa L.) (CA), corn-soybean [Glycine max (L.) Merr.] (CS), corn-corn-corn-alfalfaalfalfa (CCCAA), corn-corn-oat (Avena sativa L.) with alfalfa seeding-alfalfa-alfalfa (CCOaAA), and corn-soybean-corn-oat with alfalfa seeding-alfalfa (CSCOaA). From 1970 to 2004, fi rst-yr corn grain yields (CCCAA, CCOaAA, and CSCOaA) increased from 79 to 100 kg ha -1 yr -1 . Increasing N rates did not infl uence grain yield trends, indicating that an alfalfa crop produced the N required by fi rst-yr corn. However, 224 kg N ha -1 was needed to improve second and third-yr grain yield trends 69 and 58 kg ha -1 yr -1 , respectively. Grain yield trends for CC did not improve despite increasing N treatments, although grain yield tended to increase over time at 224 kg N ha -1 (P < 0.10). From 1989 to 2004, corn grain yield trends of CA and CS decreased by 161 kg ha -1 yr -1 if no N was added. Th e 2-yr rotation was not suffi cient to improve grain yield trends, whereas the 5-yr rotation was able to enhance corn grain yield and decrease the need for fertilizer N. Eff ects on pathogens and insects were not evaluated but warrant further investigations. Overall, this data shows that extended rotations involving forage crops reduce N inputs, increase corn grain yields, and are more agronomically sustainable than current short-term rotations.

Bt (Bacillus thuringiensis) corn (Zea mays L.) hybrids resist European corn borer [Ostrinia nubilalis (Hü bner)] damage and lodge less, creating interest among growers, agronomists, and seed companies in their yield response to increasing plant population. Corn hybrids with Bt and non-Bt traits were evaluated from 2002 to 2004 across 10 locations in Wisconsin in 76-cm rows at target populations from 61 750 to 123 500 plants ha 21 to (i) determine the agronomic and economic optimum plant population for corn and (ii) identify agronomic and economic optimum plant populations for Bt and non-Bt hybrids. The quadratic model for both grain yield and grower return response to plant population was significant. The maximum yield plant population (MYPP) for Bt and non-Bt corn was 104 500 and 98 800 plants ha 21 , respectively. The overall MYPP for corn was 102 400 plants ha 21 , which is 28 300 plants ha 21 more than the current Wisconsin recommendation of 74 100 plants ha 21. Planting corn to the MYPP increased grain yield by 4.2% over the current population recommendation. However, the economically optimum plant population (EOPP) for both Bt and non-Bt corn was 83 800 plants ha 21. It was concluded that Bt corn hybrids require higher plant populations for maximizing yield potential; however the higher harvest costs related to those greater yields and the higher seed costs associated with attaining those populations resulted in no difference in the EOPP between Bt and non-Bt corn. Plant population recommendations for corn should be near 83 800 plants ha 21 , the point where the EOPP was achieved. Since this recommendation is affected by rising seed and management costs and variable market prices, a periodic evaluation of plant population response for newly released hybrids should be done.

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. N itrogen has been considered as one of the best crop-input investments that a farmer can make in terms of return on dollars spent (Pikul et al., 2005); however, N is the most expensive nutrient for growing grain crops. Bundy et al. (1999) estimated that in the 12 states of the North Central United States, at least 3.3 million tonnes of N fertilizer was applied annually to corn at a cost of $800 million. Nitrogen fertilizer is universally accepted as a key component to high corn grain yield and optimum economic return. Overapplication is more frequent since producers have an economic incentive to err more frequently in that direction. Th e cost of unneeded N fertilizer in areas of overapplication is less than the cost of lost yield potential in areas of underapplication (Scharf et al., 2005). Crop rotation has been shown to increase corn yield 5 to 30% and soybean yield from 8 to 16% compared to continuous production of either crop (

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Current research indicates that plant populations should increase above the current Wisconsin recommendation of 74 100 plants ha 21 for higher corn (Zea mays L.) grain yields. However, stalk lodging is a major constraint to increasing plant populations. One option growers have is using Bt (Bacillus thuringiensis) hybrids, which are known to lodge less. A new method for assessing stalk lodging risk is needed. The objectives of this study were to (i) assess rind strength of corn hybrids with or without the Bt trait to increasing plant populations at natural levels of European Corn Borer [ECB, Ostrinia nubilalis (Hü bner)], and (ii) determine the potential of rind strength measurements for predicting stalk lodging. In 2003 and 2004, rind penetrometer resistance (RPR) was used to measure rind strength for four Bt and five non-Bt hybrids with plant populations ranging from 64 220 to 123 500 plants ha 21 at 10 locations in Wisconsin. Stalk lodging increased from 6 to 18% as plant population increased, and only at one location, Arlington, did Bt hybrids lodge significantly less (20 vs. 42%) than non-Bt hybrids. The RPR of both Bt and non-Bt hybrids decreased from 3.9 to 3.7 load-kg plant 21 with increasing plant population. Plants with rind strength measurements , 3.9 load-kg plant 21 taken at physiological maturity showed more lodging. Thus, such measurements appear to be a good indicator of stalk lodging potential in a high plant population situation. This new method has potential to help growers prepare for grain harvest by identifying fields with plants having average RPR below a 3.9 load-kg plant 21 threshold, and adjusting harvest schedules of suspect fields earlier to avoid yield losses caused by stalk lodging. Further research using more hybrids across a range of environmental conditions and plant populations will be necessary to fully understand the importance of this tool and its potential importance in understanding and identifying high lodging potential environments.