Cotton weed control has changed radically due to resistant pigweed


Farmers many times get the bad rap that they’re resistant to change, but that’s not true, especially when their survival depends upon it.

Surveys conducted by the University of Georgia reveal that monumental changes have occurred in a relatively short amount of time as it relates to cotton weed control practices in response to glyphosate-resistant Palmer amaranth pigweed.

During this same time, the cost of weed control in cotton production has roughly doubled.

Based on conservative estimates gathered by UGA researchers, approximately 50 percent of upland cotton in the United States is infested with glyphosate-resistant Palmer amaranth. This resistance developed in response to the widespread planting of glyphosate-resistant cotton varieties.

Early on, as growers were adopting these varieties, surveys were conducted to examine weed control practices. These studies showed that glyphosate use increased — both the number of applications applied and the area to which glyphosate was applied — but there also was  a corresponding decrease in the use of other herbicide classes.

Looking at the widespread nature of glyphosate-resistant weeds — particularly Palmer amaranth pigweed — and the subsequent resistance management strategies, UGA researchers expected there would be another shift in crop production practices.

Understanding these grower practices and how they change, according to the survey, will help fill in the gaps in research and Extension. It also helps to identify potential areas of abuse and prevent additional resistance.

The objective of the study was to determine if cotton weed management in Georgia has changed with glyphosate-resistance Palmer amaranth.

Two surveys were conducted — one was of growers in Georgia about their individual farming practices and the other was of county Extension agents who provided third-party information on a county-wide basis.

Comprehensive survey

A written survey was developed and administered in order to characterize Georgia cotton growers’ production practices before and after the development of glyphosate-resistant Palmer amaranth.

The survey specifically asked growers and Extension agents from across the state to describe the chemical, cultural and mechanical weed control practices that were used on their farms prior to and then following 2005.

Additional questions queried farmers about the costs associated with weed control and about the most significant weeds occurring in cotton.

The surveys looked at the commodities being grown, herbicide use, additional weed management practices, and weed pressure, both before glyphosate-resistant Palmer amaranth — the years 2000 to 2005 — and afterwards, from 2006 through 2010.

Respondents included 65 growers in 16 counties in Georgia and 10 county Extension agents. It encompassed the major row and forage crop areas in the state.

The responding growers were responsible for 13 percent of the state’s cotton, and the total acreage from the county agents responses represented 24 percent of Georgia’s cotton.

Growers produced cotton, peanuts, soybeans and corn, with some livestock, forage and vegetables.

Prior to the development of glyphosate-resistant Palmer amaranth, morningglory was listed as the most troublesome weed pest in row-crop farming.

The growers in the survey said about 78 percent of their acres were infested with glyphosate-resistant pigweed, and the county agents’ number was close to 90 percent of total acreage.

Looking at herbicide use patterns relative to cotton planting and emergence and growth, the survey questioned respondents about the herbicides applied pre-plant and at planting, postemergence over-the-top, and postemergence layby and directed applications.

As expected, with preplant, burndown and at-plant herbicides applied, there was a significant decrease in the acres treated with glyphosate. The use of 2,4-D stayed relatively the same, but there were significant increases in the use of paraquat for controlling Palmer amaranth. The agents saw a significant increase in the use of 2,4-D.

There were significant increases in the amount of diuron, flumioxazin and fomesafen being used, and agents and growers also saw an increase in the amount of pendemetholon being applied in Georgia.

In postemergence treatments, there was a decrease in the amount of glyphosate being applied with respect to the treated acres.

Increase in herbicide use

At the same time, there was a corresponding increase in the amount of glufosinate and metolachlor being used. Similar trends were seen by the county Extension agents who responded.

Growers said post-directed and layby herbicides applied included a decrease in the use of glyphosate while MSMA and diuron stayed about the same.

But there were significant increases in the amount of flumioxazin and metolachlor being used. Agents’ responses were similar, including significant increases in the amount of MSMA and diuron being applied to Georgia cotton acres.

The survey also asked growers how many applications of glyphosate per year were being made. Producers said they were making approximately 2.3 to 2.4 applications of glyphosate per year, both before and after the development of resistant Palmer amaranth.

There was a significant increase in the use of glufosinate for controlling Palmer amaranth. Agents also saw a static number of glyphosate applications but a sizeable increase in the number of glufosinate applications.

Growers are still putting out two applications of glyphosate per year within a crop cycle. However, they might not be placing them on every acre, indicating they may be treating certain fields differently according to the weed pressure.

Looking at costs, growers went from $32 per acre for herbicide weed control to almost $63 per acre. The agents saw an increase of $28 per acre to $68 per acre for herbicide costs.

In 2000 to 2005, 17 percent of growers were hand-weeding 5 percent of Georgia’s cotton acreage at $2 per acre. According to the UGA survey, that has increased to 52 percent of cotton acres being hand-weeded at $24 per acre.

Agents also saw an increase in the amount of acreage being hand-weeded and the associated costs.

The number of acres being subjected to in-row cultivation during the growing season also is increasing.

The survey summarizes that glyphosate-resistant Palmer amaranth is the primary weed problem among Georgia row-crop farmers.

Growers and county agents are reporting similar trends in weed management practices, hand-weeding and cultivation are increasing, glyphosate use — though still high — is decreasing, and at the same time there are increases in the use of other herbicides.

The survey recommends that particular attention be paid to the use of glufosinate, flumioxazin and fomesafen, because of the need to conserve and not over-use, causing more resistance problems.