WEED MANAGEMENT SYSTEMS for ENVIRONMENTALLY SENSITIVE AREAS

WEED MANAGEMENTSYSTEMS forENVIRONMENTALLYSENSITIVE AREAS

Plant Protection ProgramsCollege of Agriculture, Food

and Natural Resources

IntegratedPestManagement

Published by MU Extension, University of Missouri-Columbia

$3.00 IPM1018

CONTENTS

Effect of management practices on weedcontrol with atrazine . . . . . . . . . . . . . . . . . . .3

Weed control in no-till, herbicide-resistant corn . . . . . . . . . . . . . . . . . . . . . . . . .5

Herbicide activity in corn with cover crop . . .8

Weed interference and nitrogen accumulationby grass weeds in corn . . . . . . . . . . . . . . .10

Acknowledgments . . . . . . . . . . . . . . . . . . . . . .14

Authors

William G. Johnson

Reid J. Smeda

Sarah Hans

Kaleb HellwigDepartment of AgronomyUniversity of Missouri-Columbia

Note: William Johnson’s current affiliation isDepartment of Botany and Plant Pathology,Purdue University.

On the World Wide Web

Updates to this publication will be posted on theWorld Wide Web at:http://muextension.missouri.edu/explore/agguides/pests/ipm1018.htm

Production

MU Extension and Agricultural InformationDale Langford, editorDennis Murphy, designer and illustrator

© 2002 University of Missouri

This publication is part of a series of IPMManuals prepared by the Plant ProtectionPrograms of the University of Missouri. Topicscovered in the series include an introduction toscouting, weed identification and management,plant diseases, and insects of field and horticul-tural crops. These IPM Manuals are availablefrom MU Extension at the following address:

Extension Publications2800 Maguire Blvd.

Columbia, MO 652111-800-292-0969

College ofAgricultureFood andNaturalResources

Atrazine is an important herbicide forbroadleaf weed control in Missouricorn and grain sorghum production.

It is applied to 80–100 percent of corn and grainsorghum acres annually at a rate of about 1.4pounds of active ingredient per acre (lb ai/acre).More than 18 herbicides and tank mixes contain-ing atrazine are available for preemergence andpostemergence application in corn. However,atrazine use has been scrutinized for more than 10years because of its occasional detection in surfaceand groundwater. In 1994 the U.S. Environ-mental Protection Agency (EPA) established amaximum contaminant level of 3 parts per billion

of atrazine in drinking water supplies in Missouri,and 10 drinking water supplies received notices ofviolation (NOV) for exceeding acceptable levelsof contamination. No NOVs were issued in 1995,1996 or 1997; two NOVs were issued in 1998.

These concerns may lead to the restriction ofatrazine use if ways are not found to reduce theamount of atrazine detected in surface andgroundwater. Because atrazine is a key componentof weed management programs throughout theCorn Belt, it is necessary to use it wisely. Researchconducted at the University of Missouri over a 12-year period (1990–2001) explored how atrazinerates, application timing, and tillage practiceshave influenced weed control and grain yield infield corn.

The information presented in this publica-tion was compiled from the results of field exper-iments to assess the performance of atrazine andother herbicides in corn. Visual evaluations ofweed control were collected in July. The speciesevaluated included giant foxtail, cocklebur,common ragweed, common sunflower, commonwaterhemp, ivyleaf morningglory and velvetleaf.Data were sorted by herbicide application timing,atrazine rate and tillage system.

Environmentally sensitive areas arecharacterized by their susceptibilityto the effects of agricultural practices

and require special consideration when selectinga weed management program. These areas areoften associated with watersheds that feed intosources of public drinking water, such as the SaltRiver watershed in northeast Missouri, whichincludes Mark Twain Lake and several otherbodies of water. Water quality is of particularinterest in rural areas with intensive row-cropproduction because of runoff, the potential forincreased erosion of soil particles, or leaching ofinputs such as fertilizers or pesticides into surfacewater. This publication examines four aspects ofcorn production in environmentally sensitiveareas:

1. Influence of management practices onweed control with atrazine.

2. Weed control in no-till, herbicide-resistantcorn.

3. Herbicide activity in corn using a covercrop.

4. Weed interference and nitrogen accumu-lation by grass weeds in corn.

WEED MANAGEMENTSYSTEMS forENVIRONMENTALLYSENSITIVE AREAS

EFFECT OFMANAGEMENTPRACTICES ONWEED CONTROLWITH ATRAZINE

program (Figure 2). This occurred in one of thefirst years that Roundup Ready and Liberty Linktechnology was used in the research program.

Two-pass programs provided the best over-all control in 11 out of 12 years. Two-pass herbi-cide applications also provided commerciallyacceptable control (90% or higher) more oftenthan single-pass programs (Figure 3). Research inno-till corn became more prevalent in theMissouri program in 1997. Figure 3 summarizesinformation gathered between 1997 and 2001 toevaluate the influence of tillage systems on weedcontrol in corn.

How do tillage systems affectweed control with atrazine?

Research shows that attaining 90 percentcontrol of all weeds in no-till systems occurs lessfrequently than in conventional-till systems(Figure 3). Overall weed control greater than 90percent with two-pass programs occurs moreoften in conventional-till than in no-till systems.Two-pass programs provide 90 percent or greatercontrol more often than one-pass programsregardless of tillage system. The addition ofatrazine increases the frequency of attaining 90percent control.

How do atrazine rates affectweed control and corn yield?

No matter what weed management programyou prefer, the addition of atrazine will increasethe level of weed control you can achieve.Including atrazine in a herbicide programincreases the frequency of weed control valuesexceeding 90 percent. Interestingly, Missouriresearch showed no significant weed controlresponse to atrazine rate, but did observe a signif-icant yield response to rate (Figures 4 and 5).

Average overall weed control ranged from79 to 85 percent with atrazine rates of 0.5 to 2.5lb ai/acre. However, corn yields ranged from 101to 160 bu/acre over the same atrazine range. Thisdifference in response could be due to any ofseveral reasons:

1. Corn yield is extremely sensitive to weedinterference. Despite the minimal differ-ences in overall weed control as influencedby atrazine rate, these differences couldhave been significant enough to causeyield differences.

2. Higher rates of atrazine would providebetter late-season weed control, which

How does application timingaffect weed control with atrazine?

In a comparison of one-pass preemergenceapplication of atrazine with postemergence appli-cation and preemergence followed by post-emergence applications (two-pass), the two-passapplication programs provided greater weedcontrol than one-pass programs (Figure 1).

When averaged over 12 years, test resultsfrom postemergence programs only once showedcontrol equal to that provided by the two-pass

Integrated Pest Management4

2-pass(n=1059 observations)

1-pass(n=1131observations)

84*77

Weeds evaluated were giant foxtail, cocklebur, common ragweed,common sunflower, common waterhemp and velvetleaf.

*2-pass weed control programs provided higher control than 1-pass programs at the 0.1 level of significance

14 -12 -10 -8 -6 -4 -2 -0 -N

um

ber

of

year

s

1-passpreemergence(soil applied)

1-passpostemergence

2-pass(soil applied

followed by post-emergence)

0

12

1*

n = 2190 observations

* In 1996 1-pass postemergence programs provided weed control equal to 2-pass programs.

0 5

82

35

53

2413

23

0

30

144

N T C T N T C T N T C T1 - p a s s

p r e e m e r g e n c en = 7 9

1 - p a s sp o s t e m e r g e n c e

n = 8 5

2 - p a s sn = 1 9 4

Till

age

syst

emap

plic

atio

n t

imin

gP

erce

nt

of

trea

tmen

ts

Without atrazine

With atrazine

Weeds evaluated were giant foxtail, cocklebur, commonragweed, common sunflower, common waterhemp andvelvetleaf.

Figure 1. Average weedcontrol (%) in July with1- and 2-pass weedcontrol programs. �

Figure 2. Herbicideprograms that providedbest overall weedcontrol in July. �

Figure 3. Treatmentsthat provided 90% orgreater weed control,NT = no-tillCT = conventional till �

atrazine use to achieve desired weedcontrol levels.

3. Overall weed control is not very responsiveto atrazine rates. However, corn yieldincreases significantly with higher ratesof atrazine application.

The popularity of no-till croppingsystems and herbicide-resistant cornhas grown substantially in recent

years. Between 1990 and 2000, the number ofacres in no-till corn increased by more than9 percent. The acceptance of this practice hasincreased by the availability of improved plantingequipment, more selective herbicides, and herbi-cide-resistant crops. Three of the herbicides forwhich herbicide-resistant corn has been devel-oped include Roundup (glyphosate) in RoundupReady corn, Liberty (glufosinate) in Liberty Linkcorn and Lightning (imazethapyr & imazapyr)in Clearfield corn.

University of Missouri research on the rela-tive efficacy of weed control programs using thesethree herbicide-resistant corn varieties and theirsuitability to Missouri evaluated three herbicidestrategies:

1. Burndown plus full rates of residual herbi-cides applied early preplant (EPP), 1 to 2weeks before planting.

2. Burndown plus a reduced rate of aceto-chlor applied EPP followed by reducedrates of residual herbicides postemergence(SPLIT).

3. Burndown plus full rate of acetochlor EPPfollowed by the appropriate postemer-gence product used in herbicide-resistantcorn plus atrazine (EPP gr/at+) and theappropriate postemergence product.

Table 1 shows the products and applicationrates evaluated in the research.

Weeds evaluated in the Missouri trials weregiant foxtail, common cocklebur, commonragweed and common waterhemp. Weed controlratings taken five weeks after the last herbicide

would result in less weed biomass at harvestand greater harvest efficiency. Additionally,greater late-season weed control wouldresult in less competition for nutrients andwater at a time when these are in highdemand by corn for grain fill.

3. In treatments using lower rates of atrazine,there is a higher frequency of other herbi-cides used to control weeds missed bylower atrazine rates. It is possible that inthe Missouri trials, the use of some ofthese herbicides caused crop injury andyield loss.

Summary1. Two-pass herbicide programs that include

both preemergence and postemergenceherbicide applications provide better over-all weed control and more frequent occur-rence of weed control exceeding 90percent.

2. No-till or reduced-till production systemsprovide unique weed management chal-lenges and are more dependent on

Weed Management Systems for Environmentally Sensitive Areas 5

79 84858377

100

80

60

40

20

0

Wee

d c

on

tro

l (%

)

High

Low

Average

Weeds evaluated were giant foxtail, cocklebur, common ragweed,common sunflower, common waterhemp and velvetleaf.

Atrazine rate (lb/A)0 0.5 1.0 1.5 2.0 2.5

101

160156156

0 0.5 1.0 1.5 2.0 2.5

300

250

200

150

100

50

0

Yie

ld (

bu/A

)

High

Average

Low

Atrazine rate (lb/A)

122

Figure 4. Effects of atrazine rate on high, low and averageJuly weed control in no-till corn.

Figure 5. Effects of atrazine rate on high, low and aver-age yield for no-till corn.

WEED CONTROL IN NO-TILL,HERBICIDE-RESISTANT CORN

application in late July are shown as box-and-whisker plots grouped by herbicide strategies(Figures 6–10).

Note: In Figures 6–10, the thick (red) line isthe mean, the thin (yellow) line is the median, andthe boxes show the 25th and 75th percentiles. Theerror bars show the 10th and 90th percentiles.

Giant foxtail controlThe greatest control of giant foxtail was

attained with SPLIT applications in all cornvarieties (Figure 6). SPLIT applications providegreater control of giant foxtail because of its longemergence period (8 to 10 weeks). Lightning usein Clearfield corn resulted in more consistentcontrol than other herbicide programs thatincluded postemergence treatments. EPP treat-ments resulted in more overall variation in controlthan treatments that included a POST application.

Common cocklebur controlThe most effective control of common

cocklebur occurred with treatments that includedboth preemergence and postemergence applica-tion of residual herbicides (SPLIT or EPP gr/at+)(Figure 7). Treatments in which the residualherbicide was applied at a single time (EPP orMPOST) provided less control and more varia-tion in control. Residual herbicides applied onceare not active long enough to control late-emerging common cocklebur.

Common ragweed controlAs in the control of common cocklebur, treat-

ments that included both preemergence and


Table 1. Treatments used in no-till, herbicide-resistant corn.

Treatment description

EPP burndown treatment(Same for all varieties)

Roundup Ready™ corn POST treatment

Liberty Link™ corn POST treatment

Clearfield™ corn POST treatment

EPPa 1.5 pt Roundup Ultrac

2.5 pt Harnessd

4 pt Aatrexe

none none none

SPLIT 0.75 pt Roundup Ultra1.25 pt Harness2 pt Aatrex

0.75 pt Roundup Ultra1.25 pt Harness1.5 pt Aatrex

28 oz Libertyf

1.25 pt Harness1.5 pt Aatrex

1.44 oz Lightningg

1.5 pt Aatrex

EPP gr/at+ 1.5 pt Roundup Ultra2.5 pt Harness

2.0 pt Roundup Ultra1.5 pt Aatrex

28 oz Liberty1.5 pt Aatrex

1.44 oz Lightning1.5 pt Aatrex

MPOSTb none 2 pt Roundup Ultra1.25 pt Harness1.5 pt Aatrex

28 oz Liberty1.25 pt Harness1.5 pt Aatrex

1.44 oz Lightning1.5 pt Aatrex

EPP/MPOST 2.0 pt Roundup Ultra 2.0 pt Roundup Ultra 28 oz Liberty 1.44 oz Lightning

Notes:a EPP — early preplant c Roundup Ultra 3.0 lb ai/gal e Aatrex 4 lb ai/gal g Lightning 70% ai b MPOST — midpostemergence; 2- to 4-inch weeds d Harness 7.0 lb ai/gal f Liberty 1.67 lb ai/gal

100

80

60

40

20

0

Co

ntr

ol (

%)

CFLLRR CFLLRR CFLLRRCFLLRRCFLLRREPP SPILT EPPgr/at+ MPOST EPP/MPOST

= R o u n d u p R e a dy = L i b e r t y L i n k = C l e a r f i e l dCFLLRR

Figure 6. Giant foxtail control in no-till herbicide-resistant corn.

Co

ntr

ol (

%)


100

80

60

40

20

0


Figure 7. Common cocklebur control in no-till, herbicide-resistant corn.

postemergence application of residual herbicidesprovided the most effective control of commonragweed (Figure 8). However, any treatment thatincluded a POST application of Lightning tendedto produce a lower level of control than the corre-sponding Roundup or Liberty treatment withinthe herbicide strategy. In the strategies that relyon single applications (EPP, MPOST), bettercontrol was achieved with earlier applications(EPP) than with late applications (MPOST)because the common ragweed is smaller andeasier to control and does not emerge later in thegrowing season.

Common waterhemp controlOf the weed-control strategies evaluated in

the Missouri studies, the greatest variability occuredin control of common waterhemp (Figure 9).SPLIT treatment in Roundup Ready and LibertyLink varieties, as well as the use of Roundup in theEPP gr/at+ program, provided 100 percent controlof common waterhemp. Clearfield programstended to provide lower control than the otherprograms. Roundup Ready programs tended toprovide the highest control of this weed. Becausecommon waterhemp germinates throughout thegrowing season, use of residual herbicides beforeand after planting, in addition to POST herbicideswith efficacy on waterhemp, were needed to provideeffective season-long control.

Corn yieldCorn yields tend to follow a trend similar to

that for weed control (Figure 10). Weed controlprograms using SPLIT and EPPgr/at+ applica-tions resulted in the highest yields of the treatmentsin the study, at 94 percent and 90 percent of theweed-free control, respectively. Postemergencetreatments alone without accompanying preemer-gence treatments resulted in yields that were lessthan or equal to 65 percent of the weed-free checks.

SummaryIn no-till, herbicide-resistant corn, herbicide

programs using both pre- and postemergenceapplications of a residual herbicide consistentlyprovided effective weed control with loweramounts of variability. These programs alsoprovided the highest yield. Use of residual herbi-cides in programs with both pre- and post-emergence applications resulted in the best yield.


Co

ntr

ol (

%)


100

80

60

40

20

0


Figure 8. Common ragweed control in no-till, herbicide-resistant corn.

100

80

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40

20

0

Co

ntr

ol

(%)



Figure 9. Common waterhemp control in no-till, herbicide-resistant corn.

140

120

100

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60

40

20

0

Cor

n yi

eld

(% o

f wee

d-fr

ee c

heck

)



Figure 10. Corn grain yield in no-till, herbicide-resistant corn.

What is the cost of implementinga cover crop in corn production?

Many costs of introducing a cover crop intoa conventional corn production system are relatedto establishment of the cover crop. This includesthe cost of the cover crop seed. One to twobushels of seed per acre is the rate necessary toestablish a suitable rye cover crop stand. Anotherinput is the application of nitrogen (30 to 50lb/acre in early spring), which is needed to opti-mize cover crop growth. There are also some in-season costs that are associated with interactionswithin the cropping system. These include,subsoil moisture depletion during the growingseason of the corn crop, potential immobilizationof nutrients required for corn (specifically nitro-gen), and greater pressure of insect pests that mayuse the cover crop as a host.

What are the benefits of using acover crop?

Cover crops provide both short- and long-term benefits. Short-term benefits include contin-ued use of atrazine in areas prone to watershedcontamination, weed suppression by the covercrop residues, and reduced soil erosion and runoffof nutrients and pesticides. Long-term benefitsinclude improvement in soil organic matter anduniform establishment of plant species relativelysensitive to a burndown herbicide in the spring.

What special considerations areinvolved in adopting a cover crop?

Many of the keys to successfully adopting acover crop are related to good management of thefield. These and other considerations requirespecial planning for the cover crop:

• Timely harvesting of the crop in the fall.• Establishment of a cover crop when soil

temperature is suitable for emergence andgrowth.

• Timely application of herbicides in earlyspring to terminate cover crop growth.

• Proper rate and timing of nitrogenapplication.

• Use of an effective no-till planter forestablishing corn.

• Proper use of in-furrow insecticide ortreated seed.

• Minimizing light interference due to thecover crop by rolling down the cover cropshortly before or after corn planting.

Because of the low cost and reliability ofatrazine, alternative corn productionpractices that would ensure continued

use of atrazine are necessary. One such option isincreasing the presence of surface crop residuesin early spring, a time when atrazine is most likelyto move away from target sites (Figure 11). In atypical corn-soybean rotation, the standard levelof soybean residue is 30 percent under no-tillconditions at the time when atrazine in applied.This level of residue may not be enough toprevent atrazine movement in periods of moder-ate to heavy rainfall.

Alternatively, planting a cover crop aftersoybean harvest would increase plant residue tosufficient levels before corn planting. Elevatedplant residue levels may reduce the movement ofatrazine off-site. The use of a cover crop wouldrequire additional management, but the option tocontinue use of an efficacious and cost-effectiveherbicide such as atrazine may offset this cost.

Studies were conducted at the University ofMissouri Agronomy Research Center in 1999,2000 and 2001 to evaluate the use of cover cropsin corn production. All treatments received alabeled rate application of Roundup Ultra(26 oz/acre) 14 days before planting. Treatmentsincluded a preemergence application of Bicep IIMagnum or Dual II Magnum and postemergenceapplication of Marksman or Clarity. Treatmentswere ranked according to the amount of atrazineapplied. All treatments were repeated on bareground and on a rye cover crop. Treatments wereas follows:

High rate — Bicep II Magnum followed byMarksman; 2.5 lb atrazine/acre

Medium rate — Bicep II Magnum followedby Clarity; 1.6 lb atrazine/acre

Low rate — Dual II Magnum followed byMarksman; 0.79 lb atrazine/acre

None — Dual II Magnum followed byClarity; 0 lb atrazine/acre

Giant foxtail and waterhemp control wasrated two weeks after the postemergenceapplications.


HERBICIDE ACTIVITY INCORN WITH A COVER CROP

Figure 11. Cornemerging through theresidue of a rye covercrop.

of common waterhemp (Figure 13). High andmedium rates of application (2.5 and 1.6 lb/acre)resulted in 85 to 92 percent control of commonwaterhemp, a lower level of control than that forgiant foxtail. Low rates of application (0.79 lb/acre)resulted in less than 75 percent control of water-hemp and similar levels in both the rye cover cropand bare ground areas. Without atrazine, water-hemp control dropped to 65 percent in the covercrop areas but was significantly greater than inbare ground areas (46%).

How does a cover crop effectcorn yield?

Significant year-to-year differences in grainyield in the Missouri research (Figures 14, 15 and16) reflect differences in environment and weedpressure. Soil conditions in 1999 were extremelydry, but there was average or above average rain-fall in 2000 and 2001. For each atrazine applica-tion level in the experiment, corn yield in fieldswith rye residues were reduced by an average of6.8 bu/acre in 1999, 2.2 bu/acre in 2000, and 44.2bu/acre in 2001. The loss in grain yield may bedue to differences in corn plant populationsbetween the fields with and without a rye cover

How effective are herbicideapplications in cover crops?

In the Missouri research, weed control wasnot reduced by the presence of a rye cover cropat any rate of atrazine application (Figures 12 and13). However, increased rates of atrazine appli-cation did result in increased control of giantfoxtail and common waterhemp. At the high andmedium application rates (2.5 and 1.6 lb/acre),giant foxtail control was excellent (98 to 100%)for the bare ground and the cover crop treat-ments. Giant foxtail control decreased at the lowatrazine application rate and without atrazine,with a similar level of control in the rye cover cropand on bare ground (Figure 12).

The amount of atrazine in postemergenceapplications clearly influenced the level of control


100

80

60

40

20

0

98

757979

1009899

70

Gia

nt fo

xtai

l con

trol

(%

)

High Medium Low None (2.5 lb/A) (1.6 lb/A) (0.79 lb/A) (0.0 lb/A)

Bare groundRye

Atrazine application (lb/A)

Figure 12. Giant foxtail control 2 weeks after post-emergence application as influenced by cover crop andatrazine rate.

100

80

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0

85

46

7472

858892

65

Wat

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cont

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%)



Bare groundRye

Figure 13. Common waterhemp control 2 weeks afterpostemergence application as affected by cover crop andatrazine rate.

80

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0Cor

n gr

ain

yiel

d (b

u/A

)



1999 Bare ground1999 Rye

Figure 14. 1999 corngrain yield as affectedby cover crop andatrazine use.�

250200150100500G

rain

yie

ld (

bu/A

)




Figure 15. 2000 corngrain yield as affectedby cover crop andatrazine use.�

cations of nitrogen runoff from crop fields.Runoff from overapplication of nitrogen canresult in high nitrate levels in wells and streams.At a larger scale, elevated nitrogen levels arelinked to a hypoxic zone in the Gulf of Mexicowith decreased oxygen available for marine life.

By determining when weed competition fornitrogen is greatest, it may be possible to deter-mine an optimal time for removing grassy weedsto reduce the amount of nitrogen tied up by theseweeds. Recent introduction of herbicide-resistantcorn varieties such as Clearfield, Liberty Link,and Roundup Ready has provided growers withpowerful tools for removing weeds after cropand weed emergence. These tools have thepotential to reduce reliance on soil-applied herbi-cides such as atrazine. However, corn is sensitiveto early-season weed interference, which occursbefore postemergence herbicide applications.

In the first of two separate studies conductedat the University of Missouri with RoundupReady corn, broadleaf weeds were controlled witha soil-applied herbicide, and annual grass weeds(giant foxtail, large crabgrass and barnyardgrass)were allowed to infest the corn. When the weedsreached a specified height, they were sprayed

crop, or possibly immobilization of nitrogen inthe rye, limiting availability to corn. Leaves ofcorn plants in cover crop areas were yellowerthan those in bare ground areas in 2001.

SummaryThe use of cover crops such as rye can

contribute to effective weed control, especiallywhen the cover crop is accompanied by the use ofpreemergence or postemergence herbicides. Thispractice may allow continued use of atrazine insensitive watersheds, because cover crop residueswould minimize soil and herbicide movement ata time of year when little crop residue is typicallypresent. However, other factors, such as nitrogenavailability need to be addressed to optimize cornproduction in a cover crop system.

Nitrogen is a major economic inputfor corn production; with applica-tion rates of 100 to 200 lb/acre are

recommended to optimize corn yield. Nitrogendeficiencies during the growing season result instunted, yellow corn and can lead to yield loss(Figure 17). The effects of nitrogen deficienciesin the soil can be intensified in the crop by thepresence of weeds in the field (Figure 18).

Increased regulation of nitrogen fertilizeruse will require greater efficiency of nitrogen useby producers. These regulations may come fromincreased awareness of the environmental impli-


200

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0Cor

n gr

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d (b

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Figure 16. 2001 corngrain yield as affectedby cover crop andatrazine use. �

WEED INTERFERENCE ANDNITROGEN ACCUMULATIONBY GRASS WEEDS IN CORN

Figure 17. Nitrogen deficiency in corn causes pale greento yellow leaves and yellow or brown midribs.

Figure 18. Season-long interference from shattercane(right) retards corn growth and yield.

The shattercane experiment showed thatnitrogen accumulation by shattercane equals orsurpasses that by corn. Shattercane plants thatwere removed and tested at heights of 15 inchesand 18 inches had entered their rapid growthstage and had accumulated more nitrogen thanthe corn.

The experiments showed that both shatter-cane and annual grass weeds compete successfullywith corn to absorb nitrogen from the soil. Of thetwo types of weeds, shattercane absorbs nitrogenmore aggressively.


Figure 19. Early-seasonnitrogen accumulationin corn, giant foxtail,large crabgrass andbarnyardgrass.�

with Roundup and the corn was kept weed-freeuntil harvest.

In a second experiment, Roundup Readycorn was treated with soil-applied and post-emergence applications of atrazine to control allweeds except shattercane. After reaching a spec-ified height, the shattercane was removed withRoundup. The corn was kept weed-free for theremainder of the growing season after herbicidetreatment.

How much nitrogen do grassweeds accumulate in abovegroundbiomass early in the season?

Early-season accumulation of nitrogen bygrass weeds ranged from 1 to 34 lb/acre (Figure19). On a per acre basis, corn and grass weedsaccumulate similar amounts of nitrogen (Figure20). By the end of the growing season (cropharvest) there is less nitrogen in the abovegroundweed biomass than there was at midseason. Thisdifference is due to the loss of seeds and seednitrogen before the weed plants were removedfor testing. In addition, corn contains at leastthree times more nitrogen (mostly in the grain)at harvest than at midseason.

How do grass species differ inability to accumulate nitrogen?

In experiments with annual grass weeds, cornaccumulates more nitrogen on a per acre basisuntil the grasses reach a height of about 9 inches.When the grasses reach that height, their growthrate increases rapidly, and nitrogen accumulationrates increase as well. Research shows that thecorn and the grass weeds have accumulated equalamounts of nitrogen by the time the grasses are12 inches tall.

Table 2. Grass weed height at each removal timing and corresponding corn growth stage and yield.

Grass height(cm)

1999 2000

Yield (% of weed-free control)

Days after planting Corn growth stage

Days after planting Corn growth stage

Weed free — — — — 100

3 28 V4 42 V5 97

6 34 V5 45 V6 99

9 37 V6 51 V8 87

12 44 V8 63 V11 84

Weedy check — — — — 74

LSD (0.05) — — — — 13

Notes:Grass weeds controlled were giant foxtail, large crabgrass and barnyardgrass.Corn was planted on May 3, 1999, and on April 25, 2000.

CornharvestWeed height (inches) at removal

100

80

60

40

20

0

9 1321

79

343123

5165

Nitr

ogen

(lb

/A)

CornAnnual grass weeds

3 6 9 12

Figure 20. Early-season nitrogen accumulation in corn and shattercane.

3 6 9 12 15 18 CornharvestWeed height (inches) at removal

111 1312129742 4

3218109

100

80

60

40

20

0

CornShattercane

Nitr

ogen

(lb

/A)

Can sidedress nitrogen be usedto overcome the effects of early-season grass interference?

A sidedress surface application of ammoniumnitrate did not overcome the competitive effectsof early-season weed interference in the Missouriresearch (Figure 21). In 1999, a drought year,sidedress nitrogen did not improve corn yields,regardless of the stage at which weeds wereremoved. In 2000, a year with optimal moisture,corn yields improved with sidedress nitrogen,regardless of when weeds were removed. It ishighly likely that the surface-applied nitrogen istied up by soil microbes as they decompose theweed residue. If so, knifing in sidedress nitrogenwould improve the opportunity for yield recov-ery from early-season weed interference.

How does nitrogen accumulationby grass weeds affect corn yield?

Nitrogen accumulation by weeds does corre-spond to a reduction in corn yield (Figures 22 and23). Corn yield was reduced by about 0.55 bu foreach pound-per-acre increase in abovegroundnitrogen (N) accumulated by the annual grasscomplex of giant foxtail, large crabgrass and barn-yardgrass. For shattercane, corn yield was reduced0.26 bu/lb N in 1999 and 1.86 bu/lb N in 2000.Yield determination involves complex interac-tions that extend well beyond nitrogen or mois-ture. Interactions between various yield factorsare often regulated by the most limiting factor ina particular environment. Additionally, eachspecies has a unique response to environmentalstresses. In the Missouri studies, environmentalconditions had a greater effect on corn yield loss

When should grass weeds besprayed to avoid yield losses dueto weed competition?

Yield losses in corn were observed whenweeds in the annual grass complex of giant foxtail,barnyardgrass and large crabgrass were allowed toreach a height of 9 inches. The grasses reachedthis height about 37 days after planting in 1999and 51 days after planting in 2000 (Table 2).

Interference from shattercane caused signif-icant yield losses when the weed was allowed toreach 12 inches in height before removal, about46 days after planting in both years (Table 3).

In each study, corn was in the V6 to V8 stageof growth when yield reductions were firstobserved. Control of grass weeds before this stageof corn development is necessary to minimizeyield loss due to weed interference.


Table 3. Shattercane height at each removal timing and corresponding corn growth stage and yield.

Grass height (cm)

1999 2000

Days after planting

Corn growth stage

Yield (% weed-free control)

Days after planting

Corn growth stage

Yield (% weed-free control)

Weed free — — 100 — — 100

3 24 V3 93 28 V2 91

6 36 V6 91 37 V4 83

9 39 V7 85 42 V5 81

12 46 V8 77 46 V6 76

15 45 V8 59 50 V6 70

180 49 V9 59 52 V7 70

Weedy check — — 15 — — 57

LSD (0.05) — — 20 — — 22

Note: Corn was planted on May 3, 1999, and April 17, 2000.

Grass height in inches

60

40

20

0

-20

0 3 6 9 12 15 18 21 24

20001999 Sidedress at this time in 2000

Sidedress at this time in 1999

Bu

/A

Figure 21. Change in corn yield as a result of sidedress nitrogen application.

due to shattercane interference than on yield lossdue to interference from the other species of grassweeds.

Summary1. Nitrogen content in corn and grass weeds

is influenced by available moisture.2. Corn yield losses due to weed interference

cannot be detected by monitoring cornfor visual signs of nitrogen stress.

3. Annual grasses (giant foxtail, large crab-grass and barnyardgrass) should becontrolled before they reach a height of 9inches and shattercane before it is 12inches tall to avoid yield losses and reduc-tions in the nitrogen content of corn atharvest.

4. Weed size, rather than corn growth stageor days after planting, should be used totarget control tactics to avoid yield loss.


250

200

150

100

50

0Cor

n gr

ain

yiel

d (b

u/A

)

0 20 40 60Nitrogen accumulation (lb/A)

Annual grasses

y = 136 - 0.55xr2 = 0.35

250

200

150

100

50

0Cor

n gr

ain

yiel

d (b

u/A

)

0 10 20 30 40Nitrogen accumulation (lb/A)

y = 180 - 1.86xr2 = 0.13

y = 55 - 0.26xr2 = 0.02

19992000

Shattercane

Figure 22. Influence of nitrogen accumulation by grassweeds on corn grain yield.

Figure 23. Influence of nitrogen accumulation by shatter-cane on corn grain yield.


ACKNOWLEDGMENTS

The activity that led to this publication wassupported in part by University Outreach andExtension Mission Enhancement funds, the

University of Missouri Plant Protection Program,and various crop protection companies.

IPM 1018 New 11/02/5M

■ Issued in furtherance of Cooperative Extension Work Acts of May 8 and June 30, 1914, in cooperation with the United States Department ofAgriculture. Ronald J. Turner, Director, Cooperative Extension, University of Missouri and Lincoln University, Columbia, MO 65211. ■ UniversityOutreach and Extension does not discriminate on the basis of race, color, national origin, sex, religion, age, disability or status as a Vietnamera veteran in employment or programs. ■ If you have special needs as addressed by the Americans with Disabilities Act and need this publi-cation in an alternative format, write ADA Officer, Extension and Agricultural Information, 1-98 Agriculture Building, Columbia, MO 65211, orcall (573) 882-7216. Reasonable efforts will be made to accommodate your special needs.

OUTREACH & EXTENSIONUNIVERSITY OF MISSOURICOLUMBIA

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WEED MANAGEMENT SYSTEMS for ENVIRONMENTALLY SENSITIVE AREAS