Optimizing glyphosate efficacy to successfully control alien invasive species

Optimizing glyphosate efficacy to successfully control alien invasive species
Hestia Nienaber
Agricultural Research Council – Small Grain Institute, P/Bag X29, Bethlehem, 9700 Email:

Glyphosate is a broad spectrum, non-selective systemic herbicide. It is effective in killing all plant types including grasses, perennials, woody plants and alien invasive species. An invasive species refers to those non-indigenous species which have evolved elsewhere and have been purposely or accidentally relocated into an environment where they persist, proliferate and cause harm to the environment as well as to the inhabitants of that environment. Glyphosate is one of only a few herbicides registered for use on alien invasive species. However, when glyphosate is mixed with carrier waters that contain high salt levels, the activity of glyphosate is reduced as a result of chemical modification of the herbicide. This effect is known as antagonism in the spray mixture. Adjuvants, such as ammonium sulphate, are added to such spray mixtures to overcome antagonism. Although adjuvants do not have any herbicidal effects of their own, they play an important role to increase herbicide efficacy. Several adjuvants are registered for use with pesticides, however there is a misconception that adjuvants are alike and that one adjuvant may be substituted for another. This study  was  conducted  to  determine  which  specific  adjuvants,  including  ammonium  sulphate, optimise glyphosate efficacy.

Multiple greenhouse trials were conducted at the Agricultural Research Council Small Grain Institute, Bethlehem. Roundup®  (glycine) was used as representative of a glyphosate herbicide for all trials. Four registered and five unregistered adjuvants, representative of seven different adjuvant classes, were applied to cultivated oats, cv. Potberg at the recommended rate. A treatment without adjuvant served as control. All experiments were set up as randomized complete block designs. Distilled water was used to eliminate the effect of poor water quality since glyphosate is very sensitive to poor water quality (eg. hard and brackish water). The five unregistered adjuvants that were applied were included in the trials to determine their effect on glyphosate efficacy. The experiments were evaluated after fourteen days by means of weighing fresh shoot mass and data was analyzed using Genstats 15th ed.

Antagonism still occurred when a spreader/sticker adjuvant was mixed with Roundup® in the spray mixture. Low percentages control was also recorded with both buffer/wetter adjuvants (lower than

45%); both stickers (lower than 77%) and the plant oil (lower than 80%). Percentages control of higher than 85% were recorded with when ammonium sulphate, mineral oil or a wetter on its own was mixed with the spray mixture. Additionally, the addition of ammonium sulphate to spray mixtures most often leads to greater herbicide efficacy and weed control.

Results from this study indicate that various adjuvants always have an impact on the efficacy of glyphosate herbicides. This interaction however does not necessarily increase the efficacy of the herbicide and therefore it remains important that correct choices are made and most appropriate spray mixtures be used for optimal weed control.