Fifty years ago, predicting to a weed scientist that some cropland weeds would develop resistance to herbicides much in the way insects have developed immunities to certain insecticides probably would have invited a blank stare.

That's not the case today.

Within the last few years, weed control experts, much like their counterparts in insect control, are increasingly running up against weeds that can withstand repeated applications of glyphosate, one of the most potent weed killers on the market.

The biggest bugbear is horseweed, an unusually prolific, persistent weed widely found in the eastern United States. One biotype of horseweed appears to have developed tolerance to glyphosate, the herbicide of choice for the vast majority of Southern cotton producers and a mainstay of the agricultural industry in general.

Some weed experts, including Andy Kendig, a Missouri Cooperative Extension System scientist, believe this resistance constitutes a serious wakeup call to farmers concerning the growing threat of herbicide resistance.

Other weed scientists are less pessimistic, including Mike Patterson, an Alabama Cooperative Extension System weed scientist and Auburn University professor of agronomy.

“As weed scientists, we like to say, ‘Spray the weeds when they're the smallest because that's when they're the easiest to kill.’”

It's a simple, but valuable, maxim that was put to the test while Patterson and a fellow agronomist were working in north Alabama to address a persistent horseweed problem.

Working with Charles Burmester, a regional agronomist based at the Tennessee Valley Research and Extension Center in Belle Mina, Patterson set up a field trial in a farmer's field in March 2004.

What Patterson and Burmester discovered confirmed the truth behind the old saying. Glyphosate works on horseweed most effectively when enough of the chemical is brought to bear at the appropriate time.

“Basically what the field trial told us was that the farmer probably had not been timely with the burndown sprays,” Patterson recalls. “He let the horseweed get a little large before he sprayed with glyphosate.”

It's a problem complicated by unusually mild winters followed by rainy weather in the spring — factors associated with larger-than-normal horseweed before planting.

“A lot of what we're seeing, at least right now in Alabama, may be untimely spraying — spraying that is too late on weeds that are too big for the rate of glyphosate we're using,” Patterson says.

Research in Tennessee has shown it's still possible to kill the weeds with glyphosate after they've reached 8 inches, though producers will have to go up to the 4 or 5-quart range, far beyond the quart rate now considered routine.

In the future, Patterson says farmers may find it more economically feasible to begin adding other herbicides to their arsenal to control specific resistant weeds.

“You don't necessarily stop using glyphosate because it controls so many other weeds that aren't resistant, but you add another herbicide in the mix to control the specific resistant weed,” he says.

Some producers already are opting for 2,4-D, under several trade names, or small amounts of dicamba, sold under the name Clarity when used in cotton fields as a burn down product. Other options include Gramoxone, a burning foliar herbicide, plus Caparol, a residual herbicide to kill emerged horseweed to suppress further germination.

One of the problems associated with horseweed, including both resistant and non-resistant types, is that it continues to germinate even after the first flush of weeds is killed with glyphosate.

“The weed seeds are still on the ground, so you could be dealing with another flush if you don't have any residual herbicides down to suppress germination,” Patterson says.

That's why some growers have opted to use Caparol with Gramoxone.

“Caparol has residual activity and helps control the weeds that haven't germinated,” Patterson says.

One other option is Valor, which Patterson describes as a relatively new material that can be added with Gramoxone and glyphosate and that also has been shown to provide residual control.