Dan Bryant

October 26, 2009

5 Min Read

There’s much more to effective postemergent weed control in California vineyards than identifying the weed species, selecting the best herbicide, and timing the application. Kurt Hembree, Fresno County farm advisor, says selecting the proper nozzle type, pressure, and spray rig speed for the job will also go far in giving good coverage and control, while avoiding drift that can damage the crop or sensitive areas around it.

Using a spray table designed by the Pesticide Applicators Professional Association to demonstrate the spray patterns of various nozzles, Hembree and his colleague, Steve Vasquez, reviewed the do’s-and-don’ts in herbicide applications during the 2009 Grape Day at the Kearney Agricultural Center, Parlier.

For nozzles, they recommend using those with an orifice size of less than 02 that operate at a pressure of 15 to 40 psi, and, unless the chemical manufacturer specifies otherwise, using a spray volume of at least 10 gallons per acre.

Noting that TeeJet brand nozzles to reduce drift potential are readily available, Hembree said, “Standard flat-fan nozzles work fine, but consider using ‘extended range’ types that produce different droplet sizes under different operating pressures.”

For example, he added, operating an XR10003VS nozzle at 3 mph and 20 pounds psi will deliver about 18 gallons per acre of medium sized droplets, and ideal form for a systemic herbicide such as glyphosate.

Operating the same nozzle at 30 psi will deliver about 26 gpa of fine-sized droplets best suited a contact product like Gramoxone.

However, he added, rather than increasing pressure for more volume, you can select a larger nozzle size from types that can reduce drift by 50 percent to 95 percent.

A variety of nozzle tips are available, and Hembree said those made of brass wear the most quickly and are not economical. “If you’re going to spend the money, get ceramic tips. They’ll last a lifetime,” he suggested.

It may seem obvious, but Hembree and Vasquez put reading the herbicide label instructions at the top of their list of best practices for spraying. The label describes the ways to get the most from the product, reduce drift, and prevent crop injury.

Windy days are an example of conditions when applications can go off-target. Don’t spray when winds are greater than 7 mph unless appropriate spray shields are used. Temperatures above 80 F cause many products to become volatile, so in such conditions, consider spraying later in the day when it is cooler, taking into account any possibility of an inversion layer.

On foggy days, or when the air is still, fine droplets can remain suspended in the air and reach off-target areas.

The product label will indicate buffer zones to be used to protect nearby sensitive plants. Spraying should not be done when the wind is blowing toward a sensitive area.

Spray rigs designed to apply herbicides at low volumes of less than 5 gallons per acre produce very fine droplets of less than 100 microns that can drift for long distances. Such equipment should be operated with shields to confine the droplets and set to operate close to the weed canopy to minimize drift. Shields over booms and nozzles can reduce drift by 35 percent to 75 percent.

Keeping the spray rig in proper shape and accurately recalibrating it as needed will help in saving herbicide, time, and labor.

Although the label indicates the best time for treating for the target weed stage, the growth stage of the vines must also be considered. “Vines that are showing signs of bud swell are extremely sensitive to postemergent herbicides,” Hembree said.

Treating when weeds are less than 4 inches high means the boom can be lower for less risk of drift and damage, while raising it for taller weeds does the opposite.

“Operate booms and nozzles at their lowest recommended height. For 80-degree tips, this is 18 inches, and for 110-degree tips it is 12 inches. Orienting nozzles forward allows for further height reductions,” he said.

Since spray rig speeds greater than 6 mph can create a wind-shear effect on droplets and greater drift risk, a speed of 3 to 4 mph is adequate for most vineyard work. The close row spacing and trellis systems usually limit the speed for safety reasons anyway.

In a final reminder, Hembree said spray adjuvants can improve the efficacy of postemergent sprays, but can also alter their viscosity and droplet size. Testing the spray pattern with water-sensitive paper beforehand will show if droplet size is appropriate.

Roger Baldwin, UC IPM wildlife pest management advisor at KAC, reported his research on controlling pocket gophers, one of the most significant vertebrate pests in grapes.

His trials near Sebastopol showed, he said, “trapping plus fumigation worked far better than the other approaches we looked at.”

This method, using Gophinator traps and aluminum phosphide, met the minimum standard of 70 percent control and was also the most economical. It was compared with baits treated with strychnine and with gas-explosive Rodentator devices.

Baldwin pointed out that in all trials, substantial improvement in control was achieved with a second treatment.

Matt Fidelibus, UC Extension viticulturist at KAC, detailed his research in collaboration with Hildegard Heymann, professor of sensory science in the Department of Viticulture and Enology at UC Davis.

Evaluating dried-on-the-vine Fiesta and Selma Pete variety raisins with a 118-member, consumer tasting panel, they learned that raisins made from more mature grapes were preferred.

Panelists, defined as persons who ate raisins at least once a week, were asked to indicate their preferences among samples.

Fiesta at 19 Brix and Selma Pete at 21 Brix are the industry minimum recommended maturity levels for making raisins. “These are adequate for growers; however, consumers might prefer the grapes be left longer before the drying process,” Fidelibus said.

“The raisin industry assessment for quality focuses on physical characteristics like air stream sorter grade, size, moisture content, and cleanliness, but consumer preference is affected by hedonic characteristics of color, flavor, and texture.”

As food for thought, he posed the question whether future raisin varieties and production methods would be more profitable if the industry was guided by consumer sensory satisfaction rather than the physical traits of the raisins.

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