Resistance of a Sorghum halepense biotype to glyphosate is confirmed in Tartagal, Salta

 
(Buenos Aires, August 16, 2006) – As a result of researches initiated by companies represented by CASAFE (Argentine Association for Agricultural Health and Fertilizers) and CIAFA (Association for the Argentine Industry of Fertilizers and Agrochemicals), related to the failure to control Johnsongrass (Sorghum Halepense) using glyphosate in the Northern region of Argentina, both associations confirmed the existence of a Johnsongrass biotype resistant to glyphosate in Tartagal, Province of Salta, to the Northeast of Argentina.

In this case it is highly important to provide growers with recommendations for the management of the resistant biotype and its prevention from spreading, top executives from CASAFE and CIAFA stated.

Johnsongrass is a perennial grass native of Southern Eurasia. It reproduces from seeds and from thick scaly underground rhizomes and may reach a height of 1.5 meter tall. It was brought into Argentina as a forage and due to its excellent adaptation to the environment, it was transformed into a weed nationwide.

Private follow-up of the case began in 2004 after a grower from the Province of Salta submitted an inquiry to Monsanto on failure to control Johnsongrass using glyphosate. Monsanto took the initiative in this research and, like other associated companies, has worked to provide information on the advances of the trials as well as preliminary recommendations to the growers from the area.

Results of researches

The studied area has nearly 63 Km. from North to South and nearly 35 Km. from East to West, taking Campichuelo as the South end and Tartagal as the North end. A preliminary assessment of the affected area determines a range from 7,000 to 10,000 hectares.

Trials were conducted, both field and greenhouse trials, confirming failure to control Johnsongrass using glyphosate at the label rate.

Resistance is the natural hereditary ability of a biotype within a certain population, enabling it to survive and reproduce following treatment using a herbicide which, under common use conditions, effectively controls all other individuals within the same population. In that context, the species as such is affected by the herbicide application, however, a plant or plant group (biotype) survives and completes its reproductive cycle, although the herbicide was applied.

All weed species have the probability to have biotypes with may turn out to be naturally resistant to herbicides. The mutation giving rise to a resistant biotype is not related to the herbicide application. Actually, agronomic experience shows that one of the potential consequences of using herbicides to control weeds in modern agriculture is the selection of the naturally resistant biotype.

CASAFE and CIAFE’s recommendations for the control of the johnsongrass biotype resistant to glyphosate:

So far, reported cases of resistance to glyphosate have been managed effectively through Management Programs including a number of factors. In these Programs, chemical control is carried out by means of mixtures with other herbicides.

Recommendations are given on a case-by-case basis and they provide the most practical and economical solution to growers according to their needs.

These are the current general recommendations for glyphosate-based herbicides in the treatment of plots of land in the northwest region of Argentina where a johnsongrass biotype resistant to glyphosate might have been verified:

Plant RR soybean in a plot of land free from emerged weeds by previously applying a recommended herbicide. In the event plants of the resistant biotype are observed, this biotype must be cut using mechanic means in order to prevent the production of seeds and their spread.

Plant certified seeds, free from any other weed seed. Using seeds from infested plots of land might spread the weeds to other plots of land.

During the crop harvest, start from the plots of land that are not infested and leave infested plots to be harvested last. Once the harvest is finished, clean the harvesters thoroughly within the plot of land, gather all material obtained as a result of this procedure and burn it, observing all safety measures.

Monitor plots of land constantly before and after each application of herbicides (*) in order to detect any defects in the control procedure as soon as possible.

One of the main purposes of the procedure consists in preventing johnsongrass plants from blooming and producing seeds. In the event seeds are produced, johnsongrass panicles will have to be cut and destroyed, using a biosafe method, in order to prevent seeds from spreading.

(*) In case of doubt on the application, the active principle chosen or the possibility of tank mixtures of different herbicides, we strongly recommend to ask for the opinion of technicians from the area.

Selection of glyphosate-resistant weeds – Early actions

For the purposes of preventing and delaying the selection of glyphosate-resistant biotypes in other weed species, CASAFE and CIAFA recommend:

* Always apply the recommended dose of glyphosate at the right time. The use of quality formulations is of essence.
* Achieve the most comprehensive weed control prior to planting.
* Achieve the most comprehensive weed control during the cycle of RR crops.
* Avoid monoculture.
* Check for defects in the effectiveness of the treatments with glyphosate. Do not confuse resistance with defects in the application.
* Plant certified seeds, free from weed seeds.
* Prevent weed seeds from spreading through agricultural equipment (e.g. harvesters) that may introduce / spread the seed of a resistant biotype.

CASAFE and CIAFA recommend growers to follow the main agronomy principles and to inform the sales representative of their associated company of cases of insufficient weed control through glyphosate for the purposes of conducting an appropriate follow-up of the case.

Contacts: Carlos Capparelli (4381-2742/6418; ciafa1@ciudad.com.ar)

Guillermo Cal (5779-4046/7; gcal@casafe.org)