Paraquat fights glyphosate resistant Palmer amaranth

This season has seen a redoubling of efforts to fight the spread of glyphosate resistant Palmer amaranth (Amaranthus palmeri) in US crops. The key to resistance management is to use herbicides which have different modes of action. When it comes to achieving a broad-spectrum effect like glyphosate, the choice of alternatives is extremely limited. Paraquat’s unique mode of action1 together with its broad-spectrum weed control and fast action, make it an invaluable tool in the fight against weed resistance to glyphosate.

Glyphosate resistant Palmer amaranth infested cotton, Georgia, USAPalmer amaranth is one of several important weed species in the same genus (Amaranthus) commonly known as pigweeds or waterhemps. It is a major problem in many southern states, particularly in Georgia, where it is considered to be the number one weed problem in cotton.

Weed scientists at the University of Georgia estimate that an average of just two Palmer amaranth plants in every 6 m (about 20 feet) length of cotton row can reduce yield by at least 23%. Furthermore, when a single plant can produce an alarming 450,000 seeds, you can bet the problem will be worse the next season.

Already, as much as 400,000 ha (one million acres) in Georgia and South Carolina are estimated to have become infested with glyphosate resistant Palmer amaranth since it was first noticed in 2005. The genes for glyphosate resistance are also spreading by pollen, so new generations may also be resistant, and many fields in Arkansas, Tennessee, New Mexico, Mississippi and, most recently, Alabama are also infested.

The University of Georgia has produced guidelines for tackling glyphosate resistant Palmer amaranth in cotton. These include programs to manage the problem, but the best approach is to delay the appearance of resistant weeds for as long as possible. Paraquat is recommended for use in conservation tillageDescription Any tillage and planting system that covers 30 percent or more of the soil surface with crop residue after planting to reduce soil erosion by water. Authoritative On-line References and Resources Core4Conservation is part of the Purdue University-based Conservation Tillage Information Centre. programs mixed with up to three other herbicides each with a different mode of action and adding a soil residual effect. In serious situations where glyphosate resistant weeds still infest the growing crop, paraquat can be applied between the crop rows using a hooded sprayer which shields the crop from the spray.

Scientists at the University of Tennessee have seen cases where Palmer amaranth has been resistant not only to glyphosate, but also to the sulfonylurea herbicide trifloxysulfuron-sodium. They have been stressing how crop rotation can play an important role in preventing and managing resistance. Different crops will mean different herbicides will come into play, widening the use of alternative modes of action. Particularly effective is to double crop wheat and soybeans in one year. This needs a fast turnaround between crops, so paraquat is the ideal choice of burndown herbicide to combine with a pre-emergence residual.

In several southern states, the leading manufacturer of glyphosate products has been recommending pre-emergence weed control programs for glyphosate tolerant GM soybeans based on the Gramoxone Inteon brand of paraquat. Monsanto suggest that farmers apply paraquat mixed with a soil-acting herbicide, such as metolachlor or alachlor, which will add a residual effect. These mixtures will give fast burndown and delay the emergence of new flushes of weeds. Glyphosate mixed with a soybean selective herbicideDescription A chemical product used for eliminating certain types of weeds only (ie either grasses or broadleaved weeds) and not affecting specified crops. Authoritative On-line References and Resources An invaluable source of contemporary information about herbicides and weeds from Iowa State University. like fomesafen can then be sprayed later to control weeds in the growing crop.

Syngenta have developed an on-line resource for helping to fight all weed resistance problems. can be used to develop customized solutions to problems depending on crop, weeds and location. The site also includes articles, expert views and a blog with weekly news reports. Recently, an item on Palmer amaranth (pigweed) and its control in soybeans in Arkansas was posted.

This article also illustrates the effectiveness of a paraquat-based burndown application.
For more information on herbicide mode of action and why it is so important click here.

1 Paraquat shares its mode of action with the closely related diquat which is usually used as a desiccant.