Utilizing Preemergence HerbicidesWith and Without Fertilizer CarriersShould you spray or should you use a granular carrier?

by FRED YELVERTON, Ph.D.

TIE USE OF preemergence crab-grass/ goosegrass herbicides assprays versus impregnated or

coated on a fertilizer carrier continuesto be a much-debated topic amongturfgrass managers and distributors ofthe products. Research on herbicideperformance and an understanding ofthe behavior of the various preemer-gence herbicides in the environmentindicate that each method of deliveringpreemergence herbicides has potentialadvantages and disadvantages. Thepurpose of the following discussion isto outline various factors that affectherbicide performance with the twotypes of application and to point outwhere one method may be favored overthe other method.

To cover the pros and cons of usingpreemergence herbicides with andwithout fertilizer carriers, it is necessaryto discuss this topic in three sections:1) herbicide performance, 2) timing ofapplication, and 3) environmental im-plications. The herbicides involved inthis discussion are the major productsutilized for crabgrass and goosegrasscontrol: dithiopyr (Dimension), pro-diamine (Barricade), oryzalin (Surflan),pendimethalin (various trade names),benefin + trifluralin (Team), and oxa-diazon (Ronstar). These herbicidesrepresent three herbicide families: 1)dinitroanilines or DNAs (Barricade,Surflan, Team, and pendimethalin), 2)pyridines (Dimension), and 3) oxadia-zole (Ronstar).

Herbicide PerformanceTo compare and contrast herbicide

performance when applying as spraysversus a dry carrier, it is important tohave a clear understanding of howpreemergence crabgrass/ goosegrassherbicides work. The preemergenceherbicides in question do not preventseed germination; they kill the weed asit grows through the herbicide treatedzone (1, 2). The DNAs and pyridinesare absorbed by roots and shoots ofgerminating weeds and inhibit cell

Drop spreaders and sprayers can be usedin banding areas around impermeablesurfaces and water resources to helpreduce off-target movement of products.

division in the growing points, andthe weeds fail to emerge. Ronstar isabsorbed in emerging shoots (notroots) and affects photosynthesis, andthe emerging shoot dies (1). The factthat Ronstar is not absorbed by rootsexplains why it can be safely used whensprigging many warm-season turfspecies.

Once a product is applied andwatered in, a herbicide barrier is estab-lished in the soil/thatch layer. There-fore, uniform coverage with a preemer-gence herbicide, whether applied asa spray or granular, is necessary foroptimum control. Large gaps in the

herbicide-treated zone can result inweed escapes. Small gaps in this treatedzone, such as those caused by aerifica-tion, generally do not adversely affectherbicide performance (5, 10).

When the herbicide application is ona granular carrier, once the treatedturfgrass area is watered, the granuledissolves, and the herbicide barrier isestablished. Because the herbicides inquestion are readily adsorbed by soilorganic matter, they do not move veryfar from the granule. Therefore, cover-age with a granular product (whethercoated on a fertilizer or on an inert drycarrier) must be uniform and denseenough to prevent large gaps in theherbicide barrier. When comparingcoverage with a granular productversus a properly applied spray, cover-age is usually better with a spray appli-cation. This does not necessarily meancontrol will be better with a spray.

From a coverage standpoint, a cer-tain density of granules per unit area isnecessary for adequate herbicide per-formance. On a granular carrier,particle size and uniformity of particlesize are very important components ofherbicide performance. As particle sizedecreases, the density of particles perunit area increases. This has beendemonstrated by Kelly and Coates (9).In their research, southern crabgrasscontrol increased to a point and thenleveled off with decreasing particle sizeof the granular product. Uniformity ofparticle size is important because whenspreading granular products, smalland large particles can travel differ-ent distances from the applicationequipment.

The effects of coverage with variousgranular formulations on smooth crab-grass control is demonstrated in Table1. In this experiment, Barricade appliedas a 0.29G or as a spray provided bettersmooth crabgrass control than whenapplied as a O.5G formulation. Themost likely reason for these differencescan be attributable to better coveragewith the sprayable or 0.29G Barricade.

MARCHI APRIL 1998 13

Bulk fertilizer and herbicide applications need to be carefully managed, especially onimpervious surfaces. Some materials cannot bind to soil particles and are free tomove with the water. Remove products from these areas before irrigation or rainfall.It's also very important to maintain a safe distance from wetland areas.

VaporPressure7.8 x 10-5

1.1 X 10-4

Herbicidebenefin*trifl uralin *

Table 2Vapor Pressure and

Relative Volatility Ratingsfor Preemergence

Crabgrass/Goosegrass Herbicides(Measured as mm of HG @ 250 C)

RelativeVolatility

RatingModerateModerate-

HighBarricade 2.5 x 10-8 Very Lowpendimethalin 9.4 x 10-6 LowSurflan 2.5 x 10-8 Very LowDimension 4 x 10-6 Low~'*Ronstar 7.8 x 10-7 Very Low'~Team is a combination of benefinand trifluralin

**Dimension has low volatility, butone of the carriers in the Eeformulation is volatile

indicator of how easily a chemical (inthis case a herbicide) changes from aliquid to a gas. Herbicide loss can occurdue to volatility losses. In addition,preemergence crabgrass/goosegrassherbicides vary in their volatility. Therelative volatilities of the various pre-emergence herbicides are listed in Table2. Volatility losses also increase withincreasing temperature. This is whypesticides have a stronger smell whentemperatures are warm in the summerthan in the winter.

One way to help control volatilitylosses is by utilizing a herbicide on agranular carrier. When a spray appli-cation is made, the foliage interceptssome of the herbicide. The herbicide issubjected to volatilization losses andherbicide performance can sometimesbe decreased. One way to help mini-mize volatility losses from sprayappli-cations is to water the he'rbitide inimmediately after application. This willhelp remove the herbicide from thefoliage and move it down in the canopywhere the herbicide barrier is estab-lished. Higher wind speeds have alsobeen shown to increase volatility losses(7). The effect of post-treatment irriga-tion was demonstrated by Gasper et al(8). They showed that pendimethalinefficacy on smooth crabgrass was re-duced when irrigation was delayedlater than the day of treatment. Vola-tility losses of pesticides have also beendemonstrated by Cooper et al (7).

An example of how volatility may beimportant is demonstrated in Table 3.

When bulk spreading granularproducts using large equipment, thereare some areas on the golf course thatare hard to reach, and coverage cansometimes be difficult. An example isa severe slope on the back of a tee.Control in these areas is sometimesless than in fairways due to difficultyin obtaining good coverage. In theseareas, it is a good idea to make appli-cations with smaller granular appli-cators, or to utilize sprays.

Coverage is only one aspect ofobtaining adequate control with pre-emergence herbicides. Volatilizationlosses from herbicides also can impactherbicide performance. Herbicide vola-tility (as measured by vapor pressure) isa measure of the intramolecular bond-ing forces of a chemical, and it is an

91

810.75

0.75

TreatmentBarricade65WGBarricade0.29GBarricadeO.5G

LSD oc 0.05 9.3Hidden Valley Golf Club, Angier, NC.Applications made March 4 and finalevaluations made September 26, 1996.

Table 1Preemergence Smooth CrabgrassControl with Various Barricade

FormulationsYelverton and Hinton, 1996 (14)

Rate %(Ibs ai/a) Control

0.75 98

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TreatmentDimension1EC

When comparing coverage between a granular product versus a properly appliedspray, coverage is usually better with a spray application. This does not necessarilymean control will be better with a spray.

While Dimension is not volatile, one ofthe carriers in the emulsifiable concen-trate formulation is suspected of beingvolatile. In this case, the herbicideswere not watered immediately aftertreatment. Rainfall was relied upon forherbicide activation. This delay inactivation resulted in the lower rate ofDimension providing as good or bettercrabgrass control than the sprayable(EC) formulation.

Table 3Preemergence Smooth Crabgrass

Control with DimensionFormulations

Yelverton, Lewis, andHinton, 1995 (13)

Rate 0/0(lbs ai/a) Control

0.5 90

Dimension 0.38 980.25G

LSD oc 0.05 NSThorndale Golf Club, 1995. Herbi-cides applied March 16 and finalevaluations were September 19, 1995.

Photodegradation, or degradationby UV light, and dislodgeable foliarresidues also may playa role in herbi-cide performance. When a herbicideapplication is made, sunlight may startto degrade the herbicide. If a productremains on the folaige for extendedperiods of time, degradation by sunlightmay occur. Again, irrigation soon aftera spray application can help minimizeany of these potential losses. Follow-ing pesticide applications, especiallywhen applying liquid materials, thereremains some residue of that pesticideon the turfgrass foliage. Research hasshown that some pesticides appliedwithout irrigation had foliar residuelevels four times higher than for irri-gated turf (7). These residues of pesti-cides are known as dislodgeableresidues.

To summarize herbicide perfor-mance, maximum control with agranular product is dependent onachieving a minimum density of par-ticles per unit area. Particle size anduniformity of size are important con-siderations when considering a gran-ular product. Coverage is usually betterwith a properly applied spray applica-tion. Most spray applications need tobe watered in soon after application to

prevent volatility and other losses. Byimpregnating or coating a herbicideon a granular carrier, volatility lossespotentially can be minimized.

Timing of Applicationof Herbicides Appliedon Fertilizer Carriers

When applying herbicides on a fer-tilizer carrier, consideration must begiven to the timing of herbicide andfertilizer application. If it is the correcttime to apply a preemergence crab-grass/goosegrass herbicide and the turf-grass needs fertilizing, then obviously

applying a herbicide on a fertilizercarrier can be advantageous. However,many times the two applications arenot compatible. An example is non-overseeded warm-season grasses in thesouthern United States. Large andsmooth crabgrass can germinate whensoil temperatures reach about 55 de-grees near the soil surface (11). Pre-emergence crabgrass herbicides needto be applied in many areas of thesouthern U.S. in late February throughMarch. At this time, warm-seasongrasses are still dormant and will notbe actively growing for another 4 to 8weeks. Depending on the soil texture,soil type, and type of fertilizer, this canlead to nutrient losses because the turfis not actively growing. However, in thecase of warm-season turf overseeded

with perennial ryegrass, the timingmay be compatible because the cool-season turf may be in need of fertili-zation at this time.

Environmental ImplicationsAs previously mentioned, if ~he tim-

ing of a herbicide/fertilizer applicationoccurs when the turf is not activelygrowing, the possibility exists of nutri-ent leaching through the soil profile.The extent of nutrient loss depends onsoil texture, soil type, type and amountof fertilizer applied, and how wellestablished the turf is. Brown et al (6)

showed various levels of nitrogenleaching losses through a "Tifdwarf"bermudagrass putting green whenfertilized with either ammonium nitratein mid-February or ureaformaldehydein late January. These researchers con-cluded that decreasing fertilizer ratesduring periods of slow growth could re-duce nitrogen leaching losses. Bowmanand Cherney (4) showed that nitrogenleaching losses were greatest on warm-season turf species soon after sodding,and leaching losses declined to verylow levels once the turfgass. becameestablished. It is clear from both re-search projects that nitrogen leachinglosses can be minimized by fertilizingwarm-season turf species only whenthey ~re well-established and activelygrowmg.

MARCH/APRIL 1998 15

The preemergence herbicides dis-cussed in this article are stronglyadsorbed to soil colloids (primarilyorganic matter) and also have lowwater solubilities. As a result, they donot leach through the soil profile andare not subject to runoff unless the soilparticle moves (12). However, whenthese products are inadvertentlyapplied to asphalt, concrete, and otherimpermeable surfaces that may befound on the golf course, they are freeto move with water. While certainnutrients such as nitrogen are moremobile than the herbicides in question,most research indicates little runoffoccurs when products are properlyapplied to well-established turf. But aswith herbicides, when they are appliedto impermeable surfaces, they are freeto move with water, and contaminationof water resources may occur.It is important that these materials

(all pesticides and fertilizers) be appliedonly to established turfgrass areas.When applying granular materials,particularly when applied with largeequipment, it is difficult to keep offthese impermeable surfaces. If theyinadvertently land on these surfaces,care should be taken to remove theseproducts before irrigation or rainfall. Toprevent placement on these surfaces,banding with drop spreaders or makinga pass with a sprayer (10-15 foot band)next to. concrete, asphalt, or ponds,lakes, etc., can be helpful. This willallow the larger bulk spreading equip-ment to stay further away from theseareas and lessen the likelihood of off-target movement of pesticides andnutrients. Another helpful tool is toutilize buffer areas around ponds,lakes, and streams. Baird et al (3)demonstrated the buffer areas can re-duce surface runoff losses of certain

pesticides and phosphate-phosphorusfrom bermudagrass turf.

In summary, there are advantages toapplying herbicides either as sprays oron granular carriers. Performance ofthe herbicide is dependent on thequality of the granular carrier andapplication. While better coverage isusually obtained with sprays, properspray application is also important foroptimum performance. Volatility lossesfrom pesticides can be reduced byutilizing a granular carrier or by irri-gating very soon after a spray appli-cation. This is more important forcertain herbicides than others. Someherbicides have such low volatility,immediate irrigation is not necessary.Utilizing drop spreaders or sprayapplications to band around imper-meable surfaces and water resourcescan help reduce off-target movement ofpesticides and fertilizers on the golfcourse. There is increasing evidencethat buffer areas around water can alsohelp reduce potential soil runoff ofpesticides and nutrients.

References1. Ahrens, W. H. 1994. Herbicide Hand-book. Weed Science Society of America.Seventh Edition.2. Appleby, A. P., and B. E. Valverde. 1989.Behavior of dinitroaniline herbicides inplants. Weed Technol. 3:198-206.3. Baird, J. H., N. T. Basta, R. I. Huhnke,M. E. Payton, G. V Johnson, D. E. Storm,and M. D. Smolen. 1997. Influence ofbuffer length and mowing height on sur-face runoff of pesticides and nutrients frombermudagrass turf. 1997 Agronomy Abst.page 130.4. Bowman, D. c., and C. T. Cherney. 1997.Nitrate leaching from six warm-season turfspecies. 1997 Agronomy Abst. page 132.

5. Branham, B. E. 1985. 1984 herbicideevaluations. page 49-54. In Proceedings ofthe Annual Michigan State University Turf-grass Conference. Vol. 14.6. Brown, K. W., R. L. DubIe, and J. C.Thomas. 1977. Influence of managementand season on fate of nitrogen applied togolf greens. Agron. J. 69(4):667-671.7. Cooper, R. J., J. M. Clark, and K. C.Murphy. 1995. Volatilization and dislodge-able residues are important avenues ofpesticide fate. USGA Green SectionRecord. page 19-22.8. Gasper, J. J., J. R. Street, S. K. Harrison,and W E. Pound. 1994. Pendimethalinefficacy and dissipation in turfgrass as in-fluenced by rainfall incorporation. WeedSci. 42:586-592.9. Kelly, S., and Coates. unpublished data.10. Monroe, J. H., W M. Lewis, J. M.Dipaola, and A. H. Bruneau. 1989.Preemergence herbicides and aerification.1989 Agronomy Abst. page 162.11. Watschke, T. L., P. H. Dernoeden, andD. J. Shetlar. 1995. Turfgrass weeds and theirmanagement. page 6. In Managing Turf-grass Pests.12. Weber, J. B. 1990. Behavior of dinitro-aniline herbicides in soils. Weed Technol.4:394-406.13. Yelverton, F. H., W M. Lewis, and J.Hinton. 1995. 1995 Turfgrass ResearchReport: Weed Control and Plant GrowthRegulators. North Carolina State Univer-sity. 112 pages.14. Yelverton, F. H., and J. Hinton. 1996.1996 Turfgrass Research Report: WeedControl and Plant Growth Regulators.North Carolina State University. 314 pages.

FRED YELVERTON, Ph.D., is AssistantProfessor of Crop Science at NorthCarolina State University. He conductsresearch and extension programs in turf-grass weed management and plant growthregulators.

/41i-gap in herbiCide barrier herbicide barrier

Herbicides do not prevent weeds from germinating,they kill weeds as they grow through the herbicidetreated zone.

Large gaps in herbicide barrier results inweed escapes.

Preemergent herbicides create a barrier in the soil to kill weeds as they grow through the barrier zone. Uniform coverage is criticalfor optimum control.

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