Assessment of Weed Control Strategies for Corn in ...

Weed Technology. 2004. Volume 18:203–210

Assessment of Weed Control Strategies for Corn in the North-Central United States1 BRENT E. THARP, JAMES J. KELLS, THOMAS T. BAUMAN, R. GORDON HARVEY, WILLIAM G. JOHNSON, MARK M. LOUX, ALEX R. MARTIN, DOUGLAS J. MAXWELL, MICHEAL D. K. OWEN, DAVID L. REGEHR, JON E. WARNKE, ROBERT G. WILSON, LEON J. WRAGE, BRYAN G. YOUNG, and CALEB D. DALLEY2 Abstract: Field experiments were conducted across the north-central United States to determine the benefits of various weed control strategies in corn. Weed control, corn yield, and economic return increased when a preemergence (PRE) broad-spectrum herbicide was followed by (fb) postemergence (POST) herbicides. Weed control decisions based on field scouting after a PRE broad-spectrum herbicide application increased weed control and economic return. Application of a PRE grass herbicide fb a POST herbicide based on field scouting resulted in less control of velvetleaf and morningglory species, corn yield, and economic return compared with a PRE broad-spectrum herbicide application fb scouting. Cultivation after a PRE broad-spectrum herbicide application increased weed control and corn yield compared with the herbicide applied alone, but economic return was not increased. An early-postemergence herbicide application fb cultivation resulted in the highest level of broadleaf weed control, the highest corn yield, and the greatest economic return compared with all other strategies. Weed control based on scouting proved to be useful in reducing the effect of weed escapes on corn yield and increased economic return compared with PRE herbicide application alone. However, economic return was not greater than the PRE fb planned POST or total POST strategies. Nomenclature: Morningglory species, Ipomoea spp.; velvetleaf, Abutilon theophrasti L. Medicus #3 ABUTH; corn, Zea mays L. Additional index words: Abutilon theophrasti, ABUTH, CHEAL, Chenopodium album, cultivation, economic analysis, field scouting, Ipomoea spp., IPOSS, Setaria spp., SETSS, weed control systems. Abbreviations: EPOST, early postemergence; fb, followed by; POST, postemergence; PRE, preemergence. INTRODUCTION

ual herbicide. This strategy is a proactive approach to weed control and offers simple, cost-effective control of many weed species. However, weeds that are not controlled by this strategy, commonly referred to as ‘‘weed escapes,’’ are important to consider because they may compete with the crop for limiting resources and contribute seed to the weed seedbank. Schmenk and Kells (1998) reported that although sublethal rates of pendimethalin or atrazine reduced velvetleaf growth in corn, velvetleaf was still competitive enough to decrease corn yield. In addition, escaped weeds produce seed that can remain viable in the soil for long periods. Buhler et al. (1997) stated that for most weeds, a small seed reserve could remain viable in the soil for long periods and, upon germination, produce enough seed to replenish the seedbank. Cardina and Norquay (1997) demonstrated that 0.19 velvetleaf plants/m2 produced a cumulative 1,480 seed/m2 and 70 plants/m2 during a 5-yr period in conventional-tillage corn production. Postemergence (POST) herbicide application is one option to control escaped weeds in corn. A POST strat-

A common weed control strategy in corn is a preemergence (PRE) application of a broad-spectrum resid1 Received for publication May 29, 2001, and in revised form October 14, 2003. 2 First, second, and last authors: Former Graduate Research Assistant, Professor, and Graduate Research Assistant, respectively, Department of Crop and Soil Sciences, Michigan State University, East Lansing, MI 48824-1325; Professor, Department of Botany and Plant Pathology, Purdue University, West Lafayette, IN 47607; Professor, Department of Agronomy, University of Wisconsin, Madison, WI 53706; Assistant Professor, Department of Agronomy, University of Missouri, Columbia, MO 65211; Associate Professor, Department of Horticulture and Crop Science, The Ohio State University, Columbus, OH 43210; Professor, Agronomy Department, University of Nebraska, Lincoln, NE 68503; Research Specialist, Department of Crop Sciences, University of Illinois, Urbana, IL 61801; Professor, Department of Agronomy, Iowa State University, Ames, IA 50011; Professor, Department of Agronomy, Kansas State University, Manhattan, KS 66506; Researcher, Warnke Research, Geneva, MN 56035; Professor, Panhandle Research Station, University of Nebraska, Scottsbluff, NE 69361; Professor, Department of Plant Sciences, South Dakota State University, Brookings, SD 57007; Assistant Professor, Department of Plant, Soil, and General Agriculture, Southern Illinois University, Carbondale, IL 62901. Corresponding author’s E-mail: [email protected]. 3 Letters following this symbol are a WSSA-approved computer code from Composite List of Weeds, Revised 1989. Available only on computer disk from WSSA, 810 East 10th Street, Lawrence, KS 66044-8897.

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egy typically includes a decision based on monitoring (scouting) fields for weed infestations. Field scouting is an important component of integrated pest management. A 1993 survey of crop consultants in 12 states indicated that more hectares of corn production were scouted than any other crop grown in the Midwest (Wright et al. 1997). A 1994 survey of 271 farmers in central Illinois indicated that 34% of the respondents based their insect control decisions on economic thresholds, whereas only 9% used economic thresholds as a basis for weed control (Czapar et al. 1995). Instead of economic thresholds, weed control decisions were driven by general field appearance, landlord perception, weed interference with harvesting operations, and the concern of increased weed seed production. In addition to herbicides, weeds can be controlled by mechanical methods, but reliance on mechanical methods alone decreased weed control consistency, crop yield, and economic return (Mount Pleasant et al. 1994). Mechanical weed control can be integrated with herbicides to enhance overall weed control (Buhler et al. 1995; Mohler et al. 1997). Timely cultivation after herbicide application not only enhances weed control but also enables herbicide rates to be reduced from their full recommended rates (Buhler et al. 1992, 1995; Mulder and Doll 1993). Several weed control strategies are available to corn producers. Consistency of weed control, effect on corn yield, and cost of the weed control strategy are major factors that influence which weed control strategy a grower chooses. Research was conducted over a wide geographic region in the north-central United States to evaluate the consistency of several weed control strategies and to assess the value of field scouting and cultivation. MATERIALS AND METHODS

Field studies were conducted at 13 locations across the north-central United States in 1997, 1998, and 1999 (Table 1). Corn hybrids adapted to the environmental conditions for each location were chosen and planted in conventional- or reduced-tillage environments. Soil fertility strategies, tillage operations, and planting methods were conducted in accordance to the customary practices of each region. Scottsbluff, NE, was irrigated using sprinklers and was the only location that was irrigated. Eight weed control strategies were evaluated at each location in each year (Table 2). 204

Planned. Weed control strategies selected before planting were (1) Strategy 1—PRE: A broad-spectrum herbicide premix was applied at planting; (2) Strategy 2— PRE fb POST (planned): A reduced rate of the same herbicide premix as in strategy 1 was applied at planting. This was followed by (fb) a planned broad-spectrum POST herbicide application; and (3) Strategy 3—POST (planned): A single-herbicide application was used to control weeds using the same herbicide premix used POST in strategy 2. Scouting. Weed control strategies that include decisions based on field scouting were (1) Strategy 4—PRE fb POST (scout): The same broad-spectrum herbicide premix used in strategy 1 was applied at planting. Weed escapes were evaluated by principle investigators at each location. From this evaluation, a decision was made as to whether a sequential POST herbicide application was needed. If a POST application was deemed necessary, the principal investigator at each location determined which herbicide would be applied. Need for POST herbicide application was based on field scouting to determine what weeds were present and the personal knowledge and experience of the principal investigator at each location; (2) Strategy 5—PRE (grass) fb POST (scout): A PRE herbicide primarily active on annual grasses (metolachlor) was applied at planting. A sequential POST herbicide application was based on scouting as in the previous strategy; and (3) Strategy 6—POST (scout): Nicosulfuron was applied for control of annual grass weeds. Because nicosulfuron is primarily effective on grasses, a tank mix partner was chosen, if necessary, by the principal investigator based on scouting the plots to determine which weeds were present before the herbicide application. Cultivation. Weed control strategies that combine chemical weed control with cultivation were (1) Strategy 7— PRE fb cultivation: The same broad-spectrum herbicide premix used in strategy 1 was applied at planting. Corn was then cultivated to control escaped weeds between the rows; and (2) Strategy 8—EPOST fb cultivation: A broad-spectrum herbicide premix was applied early postemergence (EPOST). Corn was then cultivated 10 to 14 d after the EPOST application. Statistical Analysis. The trials were arranged in randomized complete block designs with at least three replications. Untreated and weed-free plots were included for comparisons. Weed species and density varied across locations each year. Data for the species most consistent across locations are presented. Visual weed control ratings were collected at various times throughout the seaVolume 18, Issue 2 (April–June) 2004

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Table 1. Soil characteristics, corn hybrid, and corn planting date at each location in 1997, 1998, and 1999. Year

Test site

Soil texture

Organic matter

Soil pH

Corn hybrid

Planting date

% 1997

1998

1999

Ames, IA Arlington, WI Manhattan, KS Belleville, IL Brookings, SD Columbia, MO East Lansing, MI Hollandale, MN Lincoln, NE Scottsbluff, NE South Charleston, OH Urbana, IL West Lafayette, IN Ames, IA Arlington, WI Manhattan, KS Belleville, IL Brookings, SD Columbia, MO East Lansing, MI Geneva, MN Lincoln, NE Scottsbluff, NE South Charleston, OH Urbana, IL West Lafayette, IN Ames, IA Arlington, WI Manhattan, KS Belleville, IL Columbia, MO East Lansing, MI Geneva, MN Lincoln, NE Scottsbluff, NE South Charleston, OH Urbana, IL

Clay loam Silt loam Silt loam Silt loam Clay loam Silt loam Loam Loam Silty clay loam Silt loam Silt loam Silt loam Silty clay loam Loam Silt loam Silt loam Silt loam Clay loam Silt loam Loam Loam Silt loam Silt loam Silt loam Silt loam Silty clay loam Clay loam Silt loam Silt loam Silt loam Silt loam Sandy loam Loam Silty clay loam Silt loam Silty clay loam Silt loam

son. Ratings recorded between 40 and 80 d after planting were determined to provide the best representation of weed control effects. Corn yield was determined at each location and is expressed as a percentage of the mean yield of weed-free corn for each location each year. Boxplot diagrams are used to illustrate the level and consistency of data for each weed control strategy across years and locations (Figures 1–3) (Ott 1993). In each boxplot, the boxes represent 50% of the observations and the lines outside the boxes represent 80% of the observations. The size of the box and the length of the lines represent variability within each treatment. Shorter boxes and lines indicate greater consistency among the observations. Means are listed on the horizontal axis of each figure. Visual weed control ratings and corn yields were subjected to ANOVA procedures, and the means were separated using Fisher’s protected LSD procedure at the 0.05 level of significance. Corn yield from West LafayVolume 18, Issue 2 (April–June) 2004

4.3 3.9 2.5 2.4 3.0 2.7 3.4 4.1 3.0 0.9 3.4 4.8 4.6 4.0 3.9 3.2 2.5 3.3 3.2 3.5 6.6 3.1 1.0 3.1 4.0 3.5 4.3 3.7 2.5 2.2 2.3 2.6 5.6 2.9 0.7 4.9 5.9

6.8 6.3 5.7 6.2 6.4 6.5 6.5 7.8 6.5 7.8 6.0 6.3 6.0 7.1 6.3 5.8 6.3 6.8 6.4 7.0 7.8 6.5 7.8 5.8 6.5 6.3 6.8 6.2 5.7 5.8 6.3 6.4 7.6 6.7 7.7 6.9 6.8

Garst 8640 Dekalb 493SR Dekalb 592SR Pioneer 3335 Pioneer 3730 Pioneer 3395IR Pioneer 3573 Pioneer 3861 Pioneer 3394 Pioneer 3732 Pioneer 3394 Pioneer 3489 Pioneer 3489 Garst 8539 Dekalb 493SR Asgrow RX760IMI Pioneer 3335 Pioneer 3730 Pioneer 3395IR Pioneer 3573 Mycogen 200 Northrup King 4640Bt Pioneer 3732 DK 566RR Pioneer 34A14 Pioneer 34G81 Garst 8550 Dekalb 453SR Dekalb 621Bt Pioneer 33G26 Pioneer 3395IR Pioneer 3573 Dekalb 493 Asgrow RX799Bt Dekalb 493 Pioneer 33G26 Dekalb 589RR

May 12 April 29 April 24 May 10 May 9 April 25 May 13 April 29 May 13 May 7 April 22 May 7 May 17 May 4 April 30 May 5 May 12 April 28 April 25 May 11 April 30 May 11 May 4 April 25 April 28 May 18 May 26 May 4 May 7 May 11 May 3 May 3 April 29 May 20 May 7 April 27 May 5

ette, IN, in 1999 was not included in the analysis because high winds caused severe crop damage. Profitability Analysis. The profitability analysis was based on gross margins over weed control costs for each treatment. Weed control cost assumptions included herbicide treatment, application costs ($9.90/ha PRE, $12.40/ha POST), weed-scouting fee ($3.70/ha), and cultivation ($8.20/ha). Prices for herbicides and custom application fees were approximated from a pesticide distributor. Doanes 1996 Agriculture Report was used to estimate cost for owning and operating a six-row cultivator on 76-cm row spacings. All other production costs were assumed to be fixed across treatments. Gross margins over weed control costs were calculated by multiplying corn yield by corn price ($78.74/metric ton) and subtracting total weed control costs. Gross margins were not calculated for untreated and weed-free treatments because weed control costs could not be fairly estimated. 205


Table 2. Description of the weed management strategies. Weed management strategya

Abbreviation

Herbicide

Application rate Recommended rate for soil type 75% of recommended rate 0.013 1 0.013 1 0.85 kg ai/ha

PRE fb POST (scout)

Metolachlor 1 atrazine Metolachlor 1 atrazine Nicosulfuron 1 rimsulfuron 1 atrazineb Nicosulfuron 1 rimsulfuron 1 atrazineb Metolachlor 1 atrazine

PRE (grass) fb POST (scout)

Metolachlor

Recommended rate for soil type

POST (scout)

Nicosulfuronf

0.035 kg ai/ha

PRE fb cultivation EPOST fb cultivation

Metolachlor 1 atrazine Rimsulfuron 1 thifensulfuron 1 atrazineb

Recommended rate for soil type 0.012 1 0.006 1 1.0 kg ai/ha

PRE broad spectrum Reduced rate of PRE broad spectrum followed by planned POST

PRE PRE fb POST (planned)

Planned POST without scouting

POST (planned)

PRE broad spectrum followed by POST as determined by scoutingc PRE grass followed by POST as determined by scoutingd POST with broadleaf tank mix partner as determined by scoutinge PRE broad spectrum fb cultivation EPOST fb cultivation Untreated Weed free

UNT WFREE

0.013 1 0.013 1 0.85 kg ai/ha Recommended rate for soil type

a Abbreviations: PRE, preemergence; POST, postemergence applications to weeds 3 to 10 cm in height; EPOST, early postemergence applications to weeds 3 to 5 cm in height. b Crop oil concentrate at 1% (v/v) and urea ammonium nitrate at 4.7 L/ha were included. c Plots were scouted to determine if a POST herbicide was needed. For 19 dicamba at 0.14 or 0.28 kg/ha was chosen, for 14 bromoxynil at 0.28 kg/ha, and for three primisulfuron at 0.02 kg/ha plus CGA 152005 at 0.02 kg/ha. Nicosulfuron at 0.035 kg/ha plus a nonionic surfactant at 0.25% (v/v) and urea ammonium nitrate at 4.7 L/ha was tank mixed with the POST broadleaf herbicides in eight instances. d Plots were scouted to determine if a POST herbicide was needed. For 12 dicamba at 0.14 or 0.28 kg/ha was chosen, for 24 dicamba at 0.28 kg/ha plus atrazine at 0.52 kg/ha, and for two bromoxynil at 0.28 kg/ha plus atrazine at 0.56 kg/ha. Nicosulfuron at 0.035 kg/ha plus a nonionic surfactant at 0.25% (v/v) and urea ammonium nitrate at 4.7 L/ha was tank mixed with the POST broadleaf herbicides in 10 instances. e Plots were scouted to determine if a POST broadleaf herbicide was needed. For 16 dicamba at 0.14 or 0.28 kg/ha was chosen, for 20 dicamba at 0.28 kg/ha plus atrazine at 0.52 kg/ha, and for 2 bromoxynil at 0.28 kg/ha plus atrazine at 0.56 kg/ha. f Nonionic surfactant at 0.25% (v/v) and urea ammonium nitrate at 4.7 L/ha were included.

Gross margins over weed control costs for each replication of each treatment were statistically analyzed using ANOVA, and means were compared using Fisher’s protected LSD at the 0.05 level of significance. RESULTS AND DISCUSSION

Planned Weed Management Strategies. Velvetleaf and morningglory control ranged from 0 to 100% and averaged 61 and 63%, respectively, when treated with a PRE broad-spectrum herbicide alone (Figure 1). Control of foxtail species (Setaria spp.) and common lambsquarters (Chenopodium album L.) averaged 84% from this strategy. Weed control increased and was less variable when the PRE broad-spectrum herbicide application was fb a planned POST herbicide application, compared with PRE application alone. The PRE broad-spectrum herbicide fb POST application averaged 90% or greater control of each weed species. Control of foxtail species ranged from 80 to 100% from the PRE broad-spectrum herbicide program fb POST strategy, which was greater than control from any other strategy. Weed control was more consistent when two herbicide applications (PRE fb POST) were used compared with a single-herbicide application (either PRE or POST). 206

Others have shown that two herbicide applications often increase weed control compared with a single application (Johnson et al. 2000; Rabaey and Harvey 1997; Van Wychen et al. 1999). Weed competition reduced corn yield in untreated plots by 15 to 95% of the weed-free plots (Figure 2). Corn yield after the application of PRE broad-spectrum herbicide alone was lower and more variable than all other weed control strategies. This suggests that competition from weeds not controlled by the PRE broadspectrum herbicide likely reduced corn yield. Schmenk and Kells (1998) reported corn yield reductions from velvetleaf treated with sublethal rates of atrazine or pendimethalin. Rabaey and Harvey (1997) showed that corn yield was increased with a PRE fb POST strategy compared with either of these strategies applied alone. Corn yield after application of a reduced rate of the PRE broad-spectrum herbicide with a planned POST herbicide was 96% of the yield of weed-free corn (Figure 2). Corn yield with multiple herbicide applications was greater than yield in treatments with a single-herbicide application. However, the additional herbicide application did not increase economic returns in all instances (Figure 3). The planned POST strategy and the PRE broad-spectrum herbicide fb POST strategy provided Volume 18, Issue 2 (April–June) 2004

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Figure 1. Boxplot figures represent the control of foxtail species, common lambsquarters, morningglory species, and velvetleaf. Means of each treatment are located along the horizontal axis and those that are followed by the same letter are not statistically different according to LSD (P 5 0.05).

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Figure 2. Boxplot figures represent corn yield expressed as percentage of the weed-free control. Means of each treatment are located along the horizontal axis, and those that are followed by the same letter are not statistically different according to LSD (P 5 0.05).

greater economic return than the PRE broad-spectrum herbicide applied alone. Scouting for Weeds. Scouting fields for weeds will likely increase the precision of weed control by allowing growers to assess the level of weed infestation and choose a weed control strategy that will control the weeds present. Investigators scouted plots that were treated with a PRE broad-spectrum herbicide and decided that a POST herbicide was needed for control of annual broadleaf weeds in 36 of 38 instances and that a POST herbicide was needed for control of annual grass weeds in eight instances (Table 2). After scouting plots treated with a PRE application of a grass herbicide, the investigators decided that a POST broadleaf herbicide was needed in every instance and a POST grass herbicide was needed in 10 of 38 instances. Investigators determined by scouting that a broadleaf herbicide was needed in all instances for the POST with scouting weed management strategy. Weed management strategies that included a decision based on scouting increased control of all weeds compared with the PRE broad-spectrum herbicide applied alone (Figure 1). When scouting was included with a PRE broad-spectrum herbicide application, velvetleaf control ranged from 75 to 99% and averaged 92%, and control of morningglory species ranged from 80 to 99% averaging 92%. The PRE application of a broad-spectrum herbicide fb scouting controlled velvetleaf and morningglory greater than a PRE application of a grass herbicide fb scouting. Among all weed control 208

strategies, a PRE broad-spectrum herbicide application fb scouting provided the highest or similar to the highest broadleaf weed control. Control of common lambsquarters and velvetleaf was greater from the POST tank mixture with scouting compared with the planned POST strategy. Weed control decisions based on scouting increased the consistency and level of weed control. Corn yield after only a POST weed control strategy was reduced compared with PRE fb POST applications (Figure 2). Corn yield can be reduced by competition if weeds that emerge with the corn are not removed in a timely manner (Carey and Kells 1995; Johnson et al. 2000; Tapia et al. 1997). PRE herbicides can delay the emergence of weeds after corn emergence and reduce weed competitiveness. However, in instances when weeds are not controlled by a PRE herbicide applied alone, corn yield can be reduced. This research showed that corn yield increased when scouting was used to determine potential POST herbicide application requirements after a PRE herbicide application compared with the PRE–POST planned or PRE alone weed control strategies. Field scouting was most profitable after a PRE broadspectrum herbicide application. Among the weed management strategies that included PRE applications fb scouting, gross margins over weed control costs were greater with a PRE broad-spectrum herbicide than a PRE grass herbicide (Figure 3). Scouting did not always increase gross margins. Gross margins over weed control Volume 18, Issue 2 (April–June) 2004

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Figure 3. Boxplot figures represent gross margins over weed control costs. Means of each treatment are located along the horizontal axis and those that are followed by the same letter are not statistically different according to LSD (P 5 0.05).

costs for the POST with scout weed control strategy were less than the planned POST weed control strategy. Increased costs for herbicide and scouting reduced the economic returns for the POST tank mixture with scouting strategy compared with the planned POST strategy. Cultivation. Cultivating after a PRE broad-spectrum herbicide application increased the control of all weed species compared with the herbicide applied alone (Figure 1). However, cultivation after the PRE broad-spectrum herbicide application did not control broadleaf weeds as well as other weed control strategies. Buhler et al. (1995) showed that cultivation increased control of broadleaf weeds that were not controlled by PRE weed management strategies. An EPOST herbicide application fb cultivation provided the greatest level of broadleaf weed control compared with all other weed control strategies. However, grass control from this strategy was similar to weed control strategies that included only one POST application and was less than PRE fb POST strategies. Culpepper and York (1999) reported increased annual grass control from POST fb cultivation weed management systems. Corn yield when cultivation followed application of a PRE broad-spectrum herbicide program was increased compared with the PRE broad-spectrum herbicide alone (Figure 2). However, corn yield was lower when a PRE broad-spectrum herbicide application was fb cultivation compared with other PRE broad-spectrum herbicide with Volume 18, Issue 2 (April–June) 2004

POST weed control strategies. Using the EPOST fb cultivation strategy resulted in corn yield similar to that of the weed-free control. Corn yield also was similar to those from both PRE–POST weed control strategies (planned and scout). The EPOST fb cultivation strategy had the greatest economic return of all the weed control strategies (Figure 3). Cultivation after a PRE broad-spectrum herbicide application did not affect gross margins over weed control costs compared with the PRE herbicide alone treatment, indicating that the increased yield was equal to the cost of cultivation. Cultivation increased yield and economic return after application of POST herbicides in one of 2 yr of trials conducted by Culpepper and York (1999). Results of this research indicate that a PRE broadspectrum herbicide program applied alone provides inconsistent weed control and diminishes economic returns compared with more intensive weed control strategies. Scouting fields for weeds that escaped a PRE herbicide treatment and applying POST herbicides to control the escaped weeds decreased yield loss and increased net return. After scouting, the investigators of this research determined that a POST herbicide was needed in 36 of 38 instances to control broadleaf weeds that escaped a PRE broad-spectrum herbicide, and POST herbicides were needed in all instances after a PRE grass herbicide. Cultivation after a PRE broad-spectrum herbicide appli209


cation increased weed control and corn yield, but there were no apparent economic benefits compared with a PRE herbicide alone application. Interrow cultivation after an EPOST herbicide application had the highest return of the weed control strategies tested. LITERATURE CITED Buhler, D. D., J. D. Doll, R. T. Proost, and M. R. Visocky. 1995. Integrating mechanical weeding with reduced herbicide use in conservation tillage corn production systems. Agron. J. 87:507–512. Buhler, D. D., J. F. Gunsolus, and D. F. Ralston. 1992. Intergrated weed management techniques to reduce herbicide inputs in soybean. Agron. J. 84: 973–978. Buhler, D. D., R. G. Hartzler, and F. Forcella. 1997. Implications of weed seedbank dynamics to weed management. Weed Sci. 45:329–336. Cardina, J. and H. M. Norquay. 1997. Seed production and seedbank dynamics in subthreshold velvetleaf (Abutilon theophrasti) populations. Weed Sci. 45:85–90. Carey, J. B. and J. J. Kells. 1995. Timing of total postemergence herbicide applications to maximize weed control and corn (Zea mays) yield. Weed Technol. 9:356–361. Culpepper, A. S. and A. C. York. 1999. Weed management in glufosinateresistant corn (Zea mays). Weed Technol. 13:324–333. Czapar, G. F., M. P. Curry, and M. E. Gray. 1995. Survey of integrated pest management practices in central Illinois. J. Prod. Agric. 8:483–486.

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Johnson, W. G., P. R. Bradley, S. E. Hart, M. L. Beusinger, and R. E. Massey. 2000. Efficacy and economics of weed management in glyphosate-resistant corn (Zea mays). Weed Technol. 14:57–65. Mohler, C. L., J. C. Frisch, and J. Mount Pleasant. 1997. Evaluation of mechanical weed management programs for corn (Zea mays). Weed Technol. 11:123–131. Mount Pleasant, J., R. F. Burt, and J. C. Frisch. 1994. Integrating mechanical and chemical weed management in corn (Zea mays). Weed Technol. 8: 217–223. Mulder, T. A. and J. D. Doll. 1993. Integrating reduced herbicide use with mechanical weeding in corn (Zea mays). Weed Technol. 7:382–389. Ott, R. L. 1993. An Introduction to Statistical Methods and Data Analysis. 4th ed. Belmont, CA: Duxbury Press. 100 p. Rabaey, T. L. and R. G. Harvey. 1997. Sequential applications control woolly cupgrass (Eriochloa villosa) and wild-proso millet (Panicum miliaceum) in corn (Zea mays). Weed Technol. 11:537–554. Schmenk, R. and J. J. Kells. 1998. Effect of soil-applied atrazine and pendimethalin on velvetleaf (Abutilon theophrasti) competitiveness in corn. Weed Technol. 12:47–52. Tapia, L. S., T. T. Bauman, R. G. Harvey, et al. 1997. Postemergence herbicide application timing effects on annual grass control and corn (Zea mays) grain yield. Weed Sci. 45:138–143. Van Wychen, L. R., R. G. Harvey, M. J. VanGessel, T. L. Rabaey, and D. J. Bach. 1999. Efficacy and crop response to glufosinate-based weed management in PAT-transformed sweet corn (Zea mays). Weed Technol. 13: 104–111. Wright, R. J., T. A. DeVries, and S. T. Kamble. 1997. Pest management practices of crop consultants in the midwestern USA. J. Prod. Agric. 10:624– 628.

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