Reducing pesticide drift

There are two kinds of spray drift - vapor drift and particle drift.

Vapor drift is the volatilization of pesticide molecules and their movement off-target. It occurs independently from the application. The potential for vapor drift of any pesticide can be predicted by its vapor pressure, the air temperature, size of treated area and climatic conditions. Vapor drift can travel much further than particle drift. Avoid vapor drift by not using pesticide formulations whose potential volatility is high, given the air temperature and climatic conditions. These include pesticides which will volatilize rapidly from moist soil or higher temperatures. (See Table 1)

Table 1. Relative damage to tomatoes by vapors from 2,4-D. (Baskin and Walker) Temperature and hours of exposure 70-75 F 90 F 120 F 2,4-D Formulation 2 hr 16 hr 2 hr 16 hr 2 hr 16 hr Butyul ester 3.5 6.0 5.8 6.0 6.0 6.0 (High volatile) Butoxyethanol ester 1.0 1.0 2.3 5.7 6.0 6.0 (Low volatile) Dimethylamine 1.0 1.0 1.0 1.1 1.2 1.2 (Non-volatile)

Particle drift is the off-target movement of spray particles formed during application. It is affected by the environment, application equipment, and how the equipment is used. Spray particles less than 200 microns can drift considerable distances. Large spray particles may reduce drift, but may not provide the coverage needed.

Agricultural spray nozzles produce a wide range of spray droplet sizes (10 to >1000 microns). As a comparison, a pencil lead is approximately 2000 microns in diameter. A paper clip wire is 850 microns, a staple wire is 420 microns, a toothbrush bristle is 300 microns, a sewing thread is 150 microns, and a human hair is approximately 100 microns in diameter.

The key is to set up the sprayer to produce the largest droplets that will provide adequate control of the target pest. This is a balancing act between the particle size that is best for drift control and the one that is best for product efficacy. In some situations, efficacy may have to be sacrificed to avoid drift. The drift potential depends not only on the volume medium diameter (VMD), but also the total spectrum of droplet size. A VMD of 300 could mean that half the droplets were 250 micron in diameter and half were 350 microns. Or it could mean half the droplets were 50 microns (very susceptible to drift) and half were 650 microns. The VMD plus the droplet spectrum gives a more accurate estimate of the droplet size relative to drift. If you double the droplet size you have 1/8 as many spray droplets. Spray droplets leave the nozzle at a certain speed. Small droplets (<200 microns), lose momentum faster than large particles and slow to the fall of gravity; when slowed by other forces such as air movement.

Dave Smith, a Mississippi State University ag engineer, analyzed data for more than 100 studies involving drift from ground sprayers. Of the 16 variables he considered, three were the most important: wind speed, boom height, and distance downwind.

1. Wind speed. When the wind speed was doubled, there was almost a 700% increase in drift when the readings were taken 90 feet downwind from the sprayer. Hence the recommendations of spraying in 10 mph wind or less. Be aware that drift potential also may be high at low wind speeds. This is because light winds (0-3 mph) tend to be unpredictable and variable in direction. Calm or low wind conditions may indicate the presence of a temperature inversion. Drift potential is actually lowest at wind speeds of 3 to 10 mph (gentle but steady breeze) blowing away from sensitive areas.

2. Boom height. When the boom height is doubled, for example from 24 to 48 inches, the amount of drift increased 350% at 90 feet downwind.

3. Distance downwind. If the distance downwind is doubled, the amount of drift decreases five-fold. If the distance downwind goes from 100 to 200 feet, you have only 20% as much drift. If the distance goes to 400 feet, you only have 4% of the drift you had at 100 feet. Check wind direction and speed when starting to spray a field. You may want to start spraying one side of the field when the wind is lower or you may choose to only spray part of a field because of wind speed, wind direction or distance to susceptible vegetation. The rest of the field can be sprayed when conditions change.

The most common cause of temperature inversions close to ground level is radiant cooling of the ground -- the ground cools off quicker than the air above it. Clear skies favor radiant cooling and therefore favor the formation of surface inversions. Early morning and late afternoon are the times when surface inversions are most likely to occur. Conditions not favoring inversions include low heavy cloud cover, strong to moderate winds (greater than 5-6 mph), a temperature increase of 5 degrees, and bright sunshine.

It's important to recognize when inversions are present. Bodies of water or well-irrigated fields both favor the formation of inversions. Under clear to partly cloudy skies and light winds, a surface inversion can form as the sun sets. Under these conditions a surface inversion will continue into the morning until the sun begins to heat the ground. Usually if you wait for a 5-degree increase in temperature after sun up the chances for an inversion decrease greatly. Inversions only affect the small pesticide droplets that don't settle quickly. There is a higher potential drift and therefore off-target effects if the application is made during a surface inversion. The small droplets can remain in a concentrated cloud until the inversion dissipates or the cloud of droplets moves out of the area where inversion conditions exist. Minimizing the production of small droplets will minimize the potential or drift under inversion conditions. It may be illegal to start a fire to determine the presence of an inversion or wind direction. Instead smoke bombs or smoke generators are recommended.

Drift reduction nozzles

Many new spray nozzles are designed to reduce drift. Many of these use a pre-orifice which controls the flow rate. The exit orifice controls the pattern formation. The result is larger spray droplets which are less susceptible to drift. Also, some of these nozzles can be used over a wider pressure range, which produces large droplets at low pressure and small droplets at high pressures. The ability of these nozzles to produce good spray patterns over a wide pressure range makes them an excellent choice to use with rate controllers which control the application rate by pressure changes.

While helpful these drift reduction nozzles can still create drift under some conditions, such as when the sprayer speed is increased and the resulting pressure is increased, resulting in smaller spray droplets. At slow speeds the spray droplets may be too large for good coverage. Air induction or venturi style nozzle produces droplets with a VMD of 400 to 600 microns, practically eliminating the more driftable droplets less than 200 micron particles. However, particles larger than 400 microns are problematic because they have a tendency to bounce off vegetation and may provide minimal coverage of weed foliage. The spray pattern produced by venturi style nozzles generally was more variable than the pattern produced the XR flat fan nozzles.

Drift retardants reduce drift by increasing the viscosity or surface tension of a spray solution. The viscosity of spray mixtures can greatly influence the size of spray droplets produced by atomizers. In the 1970s water thickening additives such as swellable polymers that produce a solution of gelatinous particles were developed and used. Modern drift retardants include polyacrylamides, polyethylene oxides and polysaccharides. It has been demonstrated many times that drift retardants significantly increase the VMD of a sprayed solution when added to the water in a spray tank. After several trips through the sprayer pump some of the drift retardants lose their efficiency and the size of the spray particles is the same as before. Research shows that while some drift retardants may help under some conditions, most drift management should include nozzle selection, boom height, pressure, etc.

The environment and human safety are the top priority of any activity. We must handle pesticide safety. With the larger number of people coming into contact with agriculture, we need to be sensitive to their lack of knowledge of agricultural issues. Understanding drift and knowing how to manage it will help most producers avoid problems. Remember, we are responsible for the injury we cause and are accountable for it.

2,4-D

2,4-D is an active ingredient in many compounds. It has been used to successfully control broadleaf weeds for many years. However, each year its misuse affects many non-target species as well.

2,4-D is available in 4 lb and 6 lb formulations. The 4lb is most common, but there is still plenty of 6 lb 2,4-D available. Make sure you understand what rate of what formulation you are going to spray since it only takes 2/3 of a pint of 6 lb 2,4-D to have the same activity as 1 pint of 4 lb 2,4-D.

In addition, 2,4-D is available in ester and amine formulations. Esters vaporize and result in vapor drift which can travel long distances. The low volatility (LV) esters reduce the amount of volatilization, but some will still likely volatilize at temperatures above 85 degrees. The amine salt formulation is non-volatile. During warm weather (temperatures above 85 degrees) only amine formulations of 2,4-D should be used.

The ester formulation tends to penetrate the foliage easier so amine use rates are usually higher than ester rates. The amine formulation typically does not volatilize, however all 2,4-D formulations are subject to particle drift.

Common products containing 2,4-D are: Amine 4, Brushmaster, Champaign, Chaser 2, Chaser Turf Herbicide, Crossbow, Curtail, D-638, DMA4 IVM, Five Star, Formula 40, Grazon P+D, Hi-DEP, Landmaster BW, MEC Amine-D, Oasis, Pathway, Range Star, Saber, Savage, Scorpion III, Shotgun, Starane + Extron, Starane + Saber, Starane + Salvo, Super Trimec, Trimec Classic, Trimec Classic DSC, Trimec Plus Quadmec, Trimec Lawn Weed Killer, Weedone LV4, Weedone 638, and Weedmaster.

Dicamba

Dicamba is the active ingredient in Banvel/Clarity. It is not as prone to vapor drift as 2,4-D, however it is subject to particle drift and each year products containing dicamba damage trees, soybeans, alfalfa and other broadleaf plants in Nebraska. Know your herbicides and the conditions in which you spray. Common products containing dicamba as one of the active ingredients are: Banvel, Banvel SGF, Brushmaster, Celebrity, Celebrity Plus, Clarity, Dicambazine, Dicamba DMA Salt, Distinct, Fallow Master, Fallow Master Broadspectrum, Fallow Star, Fuego, Marksman, Mec Amine-D, NorthStar, Op-Till, Range Star, Super Trimec Classic, Trimec Classic DSC, Trimec Lawn Weed Killer, Trimec Plus Quadmec, Rave, Resolve, Sequence, Sterling, Sterling Plus, and Weedmaster.

Clopyralid

Clopyralid is the active ingredient in Stinger. It is used in crop and brush control situations. While not as subject to vapor drift, it can have particle drift and does have more residual with the potential to carry over into the next year's growing season.

Products that contain clopyralid include: Accent Gold, Broadstrike Plus, Chaser Ultra, Confront, Curtail, Curtail M, Hornet, Hornet WDG, Lontrel, Redeem R&P, Scorpion III, Stinger, and Transline.

Picloram

Picloram is the active ingredient in Tordon. Typically this product is used to control weeds in range and pastures. This product also can have particle drift plus it has long soil residual activity resulting in carryover. Products that contain Picloram include:Grazon P+D, Pathway, Tordon 22K, Tordon RTU, and Tordon K.

Black stem damage of alfalfa

Cause

Spring black stem and leaf spot is caused by the fungus Phoma medicaginis var. medicaginis. This fungus survives the winter as pycnidia or dormant mycelium in overwintered stem lesions or fallen leaves.

Symptoms occur primarily on alfalfa stems and leaves but seedpods, crowns and upper taproots also may become infected. In spring small black spots develop on leaves, petioles and stems of new shoots. The leaf lesions are irregular, and enlarge and merge until much of the leaflet is covered. Infected leaves yellow and drop from the plant. Lesions on stems and petioles turn black. As stem lesions enlarge and merge, most of the stem becomes black. If the stem is girdled by the advancing lesions, it will die. The fungus also can invade the plant base and cause the crown and upper taproot to rot.

Conditions favoring disease

Pycnidia form in abundance in infected stems during late fall and early winter. In spring during wet weather spores are produced inside the pycnidia. As they ooze from the pycnidia, they are splashed by rain onto the leaves, petioles and stems of newly emerged shoots. The new shoots become infected as they grow through the residue from the previous year's crop. Some spores may be spread by wind and insects. The foliage must be wet from dew or rain for infection to occur and spread. The disease continues to move upward in the canopy if cool, wet weather persists through spring.

The fungus also is carried in the seed. A seedling blight can occur when infected seed is planted. The disease will often spread from these seedlings to nearby healthy seedlings. Early seedling loss creates stand establishment problems.

Spring black stem and leaf spot is usually a problem only on the first cutting. Regrowth from this cutting may become infected but disease severity is generally light. If environmental conditions between mid-April and mid-June are favorable (cool and wet) for disease development, plants may become defoliated before cutting. When severe, as much as two-thirds to three-fourths leaf defoliation can occur which results in the first cutting being primarily stems. This not only reduces yield but significantly lowers hay quality.

Spring black stem is present in Wyoming each year and may be particularly severe at higher elevations. In Nebraska it becomes severe after cool, wet weather.

Control

Early cutting is recommended if spring black stem and leaf spot are prevalent on the lower portion of plants. Scout fields weekly in spring to determine the extent of disease development. Decide whether to cut early based on the results of scouting fields and weather forecasts. Don't delay the decision to cut early, otherwise most of the leaves may have been lost by harvest. Ideally, one-tenth bloom is the optimum growth stage for harvesting while maintaining forage quality and limiting premature leaf loss due to disease. If the field is recovering from winter injury, it may be necessary to delay the first cutting at the risk of foliage loss to spring black stem and leaf spot. A few moderately resistant cultivars have been developed. Multi-resistant cultivars with a high level of resistance to spring black stem and leaf spot should soon be available.

Alfalfa stands are sometimes burned in early spring to control alfalfa weevil. This practice also has been shown to reduce the inoculum of spring black stem. Grazing the aftermath after a hard freeze in the fall, which is frequently done in Wyoming and Nebraska, also should reduce pathogen inoculum.

Using certified seed produced in arid areas will insure maximum stand establishment and reduce the chance of seedling blight caused by the spring black stem fungus.

Confusing black stem with insects

There is a tendency to confuse black stem with insect damage. Alfalfa weevils are usually the first suspect; however, a careful examination of the plant damage should help you delineate the cause of the damage so you can select the appropriate option for control.

Alfalfa weevils are small green worms with a light stripe down the back. They feed on the top of newly emerging leaves, causing a shothole appearance. Heavy damage will cause the tops to look frosted as the damaged leaf tissue turns brown. Weevils should be easily found with a sweep net or bucket.

Black stem starts on the lower portion of the plant and works its way up.

Potato leafhoppers also work on the upper leaves as well as the bottom leaves, and will leave a characteristic "v" shape feeding pattern on the leaves.

Regardless of the cause of damage, at this time the best solution is to take a cutting and scout the regrowth. It is NOT necessary to treat with an insecticide immediately after the hay is removed. Give the field time for normal greenup and look for insects. If insects are holding back regrowth, then spray.

Accumulated degree days through May 27, using a 41°F base. Egg hatch usually is completed by 750 degree days (DD). Producers should begin scouting for common stalk borers when 1,300-1,400 DD have accumulated and make insecticide treatments at 1400-1700 DD. (Map courtesy Al Dutcher, NU State Climatologist)

Common stalk borers are rather distinctive in appearance, with three white stripes on a background brownish-purple coloration. The two stripes on the side stop just behind the three pairs of true legs, then continue about half-way down the length of the caterpillar. Stalk borer feeding may kill the growing point if the caterpillar bores into the base of the stalk or it may produce ragged feeding holes in the leaves, if feeding starts in the whorl and then moves down into the stalk.

As of May 19 we have accumulated 750-1300 degree-days (base 41F) since Jan. 1 (see map), depending on location in the state. Based on research at Iowa State University, stalk borer egg hatch begins at about 575 degree days and should be complete by 750 degree days. Scout corn for common stalk borers when about 1,300-1400 degree days have accumulated. Updated degree day maps will be published in future issues of Crop Watch.

Check corn plants bordering grassy areas. Examine several sets of 10 plants. Look for feeding damage and insect damaged plants to see if live larvae are present. If weedy grasses were common throughout the field in the previous year, the whole field may need to be scouted for common stalk borers. Use the information, at left, to determine the economic injury level. To be effective, insecticides must be applied before common stalk borer larvae enter the stalk. In cases where stalk borers begin feeding on grassy weeds or other vegetation in field edges, control is most effective if timed between 1400 and 1700 degree days, which corresponds to the first half of the period when stalk borers are migrating from weedy hosts into corn. If the infestation is restricted to the field margin, use a border treatment.

Table 1. Common stalk borer economic injury levels (% injured plants) (Assumes 80% insecticide efficacy, and $2 bu/acre grain value). Corn leaf stage 125 bu/acre yield potential Corn leaf stage 150 bu/acre yield potential Control costs/acre Control costs/acre $7 $10 $13 $7 $10 $13 1 7 9 12 1 7 9 12 2 8 11 15 2 8 11 15 3 10 14 18 3 10 14 18 4 11 16 20 4 11 16 20 5 11 16 21 5 11 16 21 6 22 32 41 6 22 32 41 7 100 100 100 7 100 100 100

In cases where there is a history of fieldwide stalk borer damage at a site, insecticides applied to corn and timed for egg hatch may reduce damage. The disadvantage of this approach is that there is no effective way to sample for stalk borers at this time, so treatments are made without knowing whether they would be profitable that year.

Insecticides may be mixed with fast-acting herbicides being used to burn down early season weeds, or applied several days after use of slower-acting herbicides. Check the label for compatibility of different insecticide and herbicide mixtures.

Eastern and Central Nebraska

I received a report of wheat streak mosaic severely affecting a field in Gage County. This disease is not common to southeast Nebraska, but under the right conditions will occur. The field was in wheat last year and was hailed, resulting in volunteer wheat. The wheat curl mite and the virus survived the summer on the volunteer wheat and then moved into the current crop last fall. The extended mild fall exacerbated spread of the mite along with virus development in infected plants.

Severe crown and root rot was reported from a continuous wheat field in south central Nebraska. This is not surprising since that disease goes hand-in-hand with drought stress and loose seedbeds in the fall. That portion of the field that had been fallowed was less affected by the disease because it had more soil moisture.

Leaf rust was found on a wheat sample from Saline County this week. The severity was light on the flag and flag-1 leaves. Rust has been slow to develop in Kansas this spring, although Oklahoma is reporting severe leaf rust around Stillwater. Stripe rust has been severe in Texas, Arkansas and southern Missouri but hasn’t moved into the Central Plains to any degree.

• Paul Hay, Extension Educator in Gage County (report added May 28, 2002): Crops in southeast Nebraska are finally feeling the heat and sun. Stands are now looking quite good, despite the slow development. About five days of good planting weather are needed to finish the soybeans and milo. One 35-acre wheat field was destroyed because of severe wheat streak mosaic virus, which is very unusual for eastern Nebraska. High winds and loose soil from tilled fields resulted in a number of corn fields being "sand blasted;" some callers to our office thought the damage might be herbicide injury.

• Ron Seymour, Extension Educator in Adams County: About 85% of the corn crop has been planted, of which about 40% has emerged. Emerged fields are in the one- to two-leaf stages. Many fields have a yellow or purple color indicative of cool weather stress. No insect or disease problems were noticed. About 20% of the soybean crop has been planted but plant emergence has been slow. Recent rains have boosted the wheat crop condition with many fields rated as excellent. Most of the plants are 18 to 24 inches tall, and are in the boot stage with a few heads starting to emerge. Alfalfa fields have budded and the plants range in height from 12 to 18 inches. Alfalfa weevils are common, but infestations are minimal. A few pea aphids and bean leaf beetles also were found but not in a significant number. Winter pasture grasses are growing well and summer grasses are breaking dormancy.

• Karen DeBoer, Extension Educator in Cheyenne County: Our overriding problem is that despite some light rains, our wheat fields are very dry. A freeze last week damaged leaves. It's difficult to determine the extent of damage to the growing point since much of the wheat was just jointing; other tillers may be able to help compensate if there is damage. Crown and root rot from the dry winter conditions can be found in most of the wheat. Conditions are very dry for planting spring and summer crops. Pastures are not greening up in some areas as they should.

• June 15, Saturday, 9 a.m. to 6 p.m.
  
  A bison and grass tour at the Hutchinson Buffalo Ranch will explore how management intensive grazing is practiced in a low stress environment. Visitors will be able to see natural fly control, health care and management that works in harmony with nature. Speakers will include: Allan Nation, editor of Stockman Grass Farmer; Dr. Brian Vonk, an Omaha physician; and UNL Extension Educators Terry Gompert and Bob Scriven. To register, contact Dave at 402-273- 4574. Pre-registration is $65; registration at the ranch is $75. Gourmet bison meat will be served for lunch. The ranch is south of Bassett or north of Taylor: take Highway 183 to mile marker 151, turn west and travel 5 miles to the blue mailbox and Hutchinson Buffalo Ranch sign, turn left and follow the road 2 miles to the ranch.

  June 19, Wednesday, 5 p.m.
  John and Susie Ellis' Libby Creek Farm, A Summer Solstice Celebration. Join the fun for the farm tour and picnic. The Ellis' are converting their operation to better fit their new opportunities and lifestyle. Visitors will be able to view new tree plantings, organic hay production, organic soybean seed production, a certified kitchen construction using hay bales, family food production and much more! To reach the far, travel from US 81 and I-80 south 1/2 mile to Himark Texaco, then 4 miles west to find the farm on the north side. For more information call 402-362-2630.

  July 10, Wednesday, 6-9 p.m.
  Liz Sarno and family are experimenting with multi-species grazing of cattle and poultry. They plan to graze turnips/oats, sudan grass/forage soybean and grazing alfalfa. They raise corn, soybeans, barley, spring wheat and alfalfa and practice strip cropping and ridge till. They have used an EQUIP grant to install a pasture watering system. Steve Huber, conservationist, will demonstrate how to test for compaction, infiltration, and organic matter. To reach the farm, travel from Highway 79 by the ballpark in Prague by paved road to Bruno/Abie west, six miles to stop sign, go right/north to the town of Abie the road turns gravel for two miles north and turn 1/2 east at W/43. If you’re starting from Schuyler, take Highway 15 south, take a paved road to Linwood for six miles until you see the sign to Abie. Follow the gravel road two miles south and 1/2 mile east. For more information, call 402-543-2217.

  July 27, Saturday, 8:30 a.m. to 4 p.m. The Grain Place, Inc., Marquette.
  Dave Vetter and staff are opening their organic farm and grain processing center for tours and workshops. In the morning you can tour the fields and visit about their extensive rotations using grass and cattle. In the afternoon there will be seminars and discussions on sustainable agriculture and local food systems. Lunch will be prepared with local foods. Be sure to register by July 19 to reserve a seat at the dinner table. For registration and/or more information call 402- 854-3195.

The following publications were recently released by UNL Cooperative Extension and are available from your local Cooperative Extension office.
• Western Bean Cutworm in Corn and Dry Beans, G98-1359, newly revised, addresses the life
history, scouting, and treatment of the western bean cutworm in corn and dry beans.
• Management of Phytophthora Diseases of Soybean, NF02-518, discusses the life cycle, extent
and management of the disease, which is most common in poorly drained or compacted soils, low-lying field areas and in soils with high clay contents; also discusses available seed treatments and application rates and variety selection for genetic resistance.
• Management after Wildfire in Central and Western Nebraska; EC02-160, with this year’s dry
conditions in western Nebraska, this is even more of an issue; in 2000, wildfires burned 235,425 acres; this pub addresses management changes needed after a wildfire to optimize the return of rangeland resources to pre-fire condition with minimal financial loss during the recovery process.
• Sampling Manures for Nutrient Analysis, G02-1450, guidelines for sampling manure for nutrient
content to improve crop and soil management, including recommended methods for solid, liquid and slurry manures and anaerobic lagoons; how to submit samples; and a list of some of the Nebraska laboratories providing manure testing services.
• If you’ve got a home garden or orchard, two new publications provide information on labeled fungicides for specific diseases and fruits, recommended application timing, and cautionary notes for various applications. The publications are: Fungicide Spray Schedule for Home Garden Tree Fruits, NF02-519, and Fungicide Spray Schedule for Home Garden Small Fruits, NF02-520.

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