It's only a matter of time before any weed becomes resistant to a specific herbicide. But by using good resistance-management practices--including tank mixing, sequential applications and rotating different modes of action--growers can prolong a chemistry=s usefulness.

Extension specialists in both California and the Mid-South hope that new products in the pipeline as well as Section 18s will also help growers stay one step ahead of resistance problems.

Using sound resistance strategies has taken on new meaning in recent years as barnyardgrass throughout the Mid-South developed resistance to propanil. Few producers currently use the product as a stand-alone treatment because of the widespread problem.

Within the past few years, one small pocket in Mississippi and two in Louisiana have shown resistance to Facet, a grass herbicide from BASF.

In Mississippi, the one known case developed after the grower used Facet as a stand-alone herbicide for several consecutive years, says Mississippi State University Extension Specialist Joe Street.

Recently, a pocket of barnyardgrass near Jonesboro, Ark., has shown multiple resistance, meaning it can tolerate more than one product. The herbicides are propanil and Facet.

The problem developed after a producer applied Facet for three years in a rice-soybean rotation. AWhen they started using Facet on rice, they noticed a patch [of barnyardgrass] that wasn=t controlled,@ says Dr. Ron Talbert, a University of Arkansas agronomy professor in Fayetteville. AThey used propanil, too, and this particular sample is resistant to both.

AThere are reports of Facet resistance around the world. It seems to be a particularly susceptible situation in the barnyardgrass and maybe some of the other species.@

Talbert has additional seed samples collected during the 1999 season from fields where growers suspect resistant barnyardgrass, although he has yet to finish the testing.

But there may be other reasons the weeds survived, too. AYou could have poor herbicide performance due to the weather, stage of growth and so forth,@ Talbert says.

If you suspect a resistance problem this summer because treated barnyardgrass survives, Talbert recommends collecting seeds from the weed and sending them to your county agent.

BASF acknowledges the problem and this season unveiled a resistance-management program to slow its spread.

AIt=s only been in a couple of locations, but it=s our intent to keep it that way,@ says Scott Gibson, Facet product manager in Research Triangle Park, N.C. AFacet has been very successful at the label rate for an early application, either soil-applied or very early post for maintaining resistance control.@

Key to the program will be moving growers away from late post-emergence applications of Facet, Gibson says.

Rotating herbicides also will be a component of BASF=s plan. By going on early with Facet, you=ll be able to tankmix Prowl or Command with it to provide different modes of action.

The earlier timing allows you to come back early post with another application--either another product with a different mode of action or a tank mix containing Facet and another product.

Developing a strong resistance management program may not be as easy for California producers, who have fewer chemistries from which to chose and several herbicide-resistant weeds, including one that appears to tolerate several products.

In California, many growers have already lost Londax to four resistant broadleaf weeds. They are redstem, ricefield bulrush, smallflower umbrellaplant and California arrowhead.

Within the past few years, one biotype of watergrass appears to have become resistant to several products, says Dr. Albert Fischer, a University of California, Davis, assistant weed science professor. Watergrass belongs to the same genus as barnyardgrass.

The problem appears centered around the Princeton area. Two forms of watergrass are found in California rice fields, an early and late form. It is the late form that in some cases can tolerate Ordram, Bolero, Whip and possibly even Regiment, a not-yet-registered product from Valent.

Farmers have dubbed this resistant late watergrass Arice mimic.@ An aggressive weed, it doesn=t take too many plants to reduce rice yields, Fischer says.

Growers who believe they have resistant weeds should contact their pest control advisors (PCAs) or farm advisor to have seed samples collected and sent to Fischer. He receives 50 to 60 samples a year.

Fischer and his team plant the samples in test pots and spray them with various rates of different herbicides to see whether they=re resistant, and if so, to what products at what rates. So far, he=s found late watergrass resistant to all five products but not to propanil.

He and a post doctorate researcher are trying to develop a quicker method that could perhaps provide growers with herbicide options during the season. The current test takes about a week.

Fischer also is studying whether herbicide combinations may provide better control than single products.

To help manage resistance, experts recommend rotating different modes of chemistry, rotating crops and using mechanical cultivation or hand roguing where feasible. Fischer says California producers= hands are tied with late watergrass control because they have only one herbicide alternative--propanil. And the California Department of Pesticide Regulation restricts propanil use around prune orchards.

AWith few herbicides and few options for rotation, we are obviously in a bind,@ Fischer says. AAnd we all know resistance to propanil can develop also.@

The California Rice Commission as well as several Mid-South state Extension Services are exploring applying to the U.S. Environmental Protection Agency for Section 18s for a handful of products that could help manage resistance. As of press time, the EPA had not acted on the requests. To find out their current status, check with your PCA, crop consultant, county agent or UC farm advisor.

Profit Tip: To slow resistance, rotate or apply sequential treatments of herbicides with different modes of action, tankmix products with different modes of action and mechanically cultivate where possible.

Hotlink: Herbicide Resistance Action Committee