05.10.2017  
 

Is Glyphosate in Vineyards Dangerous?

Napa County grape growers get advice about using herbicides

 
by Paul Franson
 
wines vineyard glyphosate roundup use
 
Weeds can harbor pests and provide disease pressure.

Napa, Calif.—Two recent news reports gave conflicting information about a popular herbicide. “Glyphosate is a likely carcinogen!” exclaimed one. The other said the opposite.

Yet other stories reported traces of the herbicide in some wines.

It was timely, then, that the Napa County Agricultural Commissioner’s office and the University of California Cooperative Extension already had a meeting scheduled about vineyard weed control using glyphosate and alternatives.

“It’s not my job to defend any product,” Napa agricultural commissioner Greg Clark told the large crowd. “But I do defend the right to use products approved in California.”

The first session featured Dr. Carl Winter, toxicologist at the University of California, Davis. He seemed almost exasperated as he said, “I’m amazed to be speaking to a large group about the risk of glyphosate. I’ve always regarded it as one of our most benign chemicals.”

Winter pointed out that as a toxicologist, whether a product is carcinogenic or not isn’t the most important factor. Occurrence, exposure and dose are critically important.

Though many crops have contact with glyphosate used to control weeds, they don’t receive a large dose because it could kill them. Any residue they are exposed to is very low. Glyphosate breaks down quickly both in nature and the body, he added.

Winter said he suspected that much of the hysteria about glyphosate was because it’s used with genetically modified crops such as corn, cotton and soybeans, which are engineered to be resistant to glyphosate while weeds aren’t. “Glyphosate is a lightning rod to GMO opponents,” he stated.

The crops’ GMO status can lead to higher levels of glyphosate in those fields, but this has nothing to do with wine grapes.

Winter said that the only known danger is eye contact. “It has very low mammalian toxicity,” he stated. “The EPA reference dose is 1.75 mg/kg/day. There is no health or safety impact at that level—true dangers have 1,000 lower maximum exposures (1 microgram/kg/day).”

In summary, “whether or not glyphosate is a risk, the dietary exposure is very low and of no health concern.”

Are other herbicides effective?
Following Winter’s debunking of alarm, weed advisor John Roncoroni of UC Cooperative Extension talked about the reasons for weed control and alternatives.

Weeds are direct competition for young vines, and if you’re trying to water-stress vines, weeds complicate measurement. Weeds also inhibit frost protection by inhibiting heat rising from the soil.

In addition, weeds can harbor pests like voles and gophers and provide increased moisture and disease pressure. They can interfere with harvest and can affect crop quality.

One classic way to remove weeds is mechanical cultivation. “It gives excellent weed control and good water penetration,” Roncoroni said. “It shuffles the deck, and weeds have to start from scratch.”

However, mechanical cultivation uses fossil fuel and can cause erosion. (Glyphosate has a low carbon footprint, it might be noted.) A wet year like this has been may not be a good year for cultivation, he admitted.

Roncoroni also likes sheep for weed control. “Sheep do a fantastic job in both organic and conventional vineyards.” He noted that if you raise your trellises, they can be used year-round, though most growers use them before the leaves appear. Irrigation tubing heights matter, however, and sheep can compact the soil. They might be expensive to rent, too.

Flaming works well, too, but timing is critical. It kills with desiccation.

Mulching can help but often hides voles.

Hand hoeing is excellent but can be expensive, and today’s labor shortages may make it impractical.

That brings us to glyphosate alternatives: Roncoroni dismisses gramoxone or paraquat since they are highly toxic to humans.

He finds that organic weed killers “work pretty well,” but require higher doses and are expensive. They also have to be applied early.

He noted that widely touted acetic acid is only registered in non-crop areas.

Using glyphosate effectively
In the final talk of the seminar, Dr. Kassim Al-Khatib of the University of California, Davis, gave advice about using glyphosate and other weed killers most effectively.

Glufosinate is another herbicide often considered an alternative to Roundup. It is sold as Basta, Rely, Finale, Challenge and Liberty. It is a contact herbicide, unlike glyphosate, which is systemic in action.

Many other herbicides are available for use around vines.

Formulations and adjuvants: Glyphosate was first registered in 1974 under the trade name Roundup. It is a foliar-applied herbicide that is mobile in both phloem and xylem.

It is tightly bound to phosphate sorption sites in soil, so soil activity is very rare.

The U.S. patent for glyphosate expired in 2000, and now glyphosate products are marketed by many companies.

Various formulations have different recommended application rates.

Adjuvants and glyphosate: Glyphosate is often combined with other materials to improve efficacy. An adjuvant is a spray additive used with a pesticide that enhances the performance or handling of that pesticide. There are many types of adjuvants, all having different effects.

Surfactants enhance the ability of an herbicide to penetrate a leaf or remain in an aqueous solution

Adding surfactant to the mix can increase herbicide droplet contact with foliage.

Surfactants cause greater droplet contact with the leaf surface and aid in movement into stomata, slow the evaporation of herbicide droplets, may increase rain-fastness of herbicides and assist in passage of hydrophilic herbicides through hydrophobic wax layers.

 

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