R.A. Campbell, Shu Hua LiU, Jian Zhong Huang and J.A. Studens

SERG Project # 1996/02


Executive Summary

This project addressed the need, identified in the SERG 1989-94 five-year plan, for more information on the herbicide/target plant biochemical interface. Forestry herbicide efficacy has tended to be erratic for reasons which were not obvious. It was felt that more physiological information was needed in order to optimize efficacy and make it more consistent. This project looked specifically at the influence of drop size, drop number and spray concentration on uptake and translocation of glyphosate and triclopyr.

Probably the most important conclusion from this project is that uptake and translocation are markedly different for these two herbicides. The practical implication of this is that there will not be one set of application and environmental parameters which optimize forestry herbicide application in general.

With triclopyr ester, absorption (as % of applied) was greater than translocation (as % of absorbed or % of applied). With glyphosate, the opposite was true. This is consistent with the expectation that uptake of a non-polar compound would be greater than a polar one which would be limited to transport via aqueous pathways. The fact that triclopyr ester was taken up faster but translocated away from the point of uptake slower than glyphosate probably accounts for the fact that the former exhibited greater contact injury. Increasing concentration (ai) of the droplet while decreasing droplet number or droplet size resulted in decreased total translocation (translocation as % applied) for triclopyr ester but increased total translocation for glyphosate. When concentration was kept constant while droplet number was decreased and droplet size increased, total translocation decreased for triclopyr but remained constant for glyphosate (a decrease in translocation [as % of absorbed] was compensated for by an increase in absorption [as % of applied]). The following conclusions can be drawn. Total translocation was greatest: 1) for glyphosate when droplet concentration was high; 2) for triclopyr ester when droplet number was high.

The trend to increase operational efficiency by reducing spray volume for aerial application of herbicides for forest management (the minimum legal spray volume for glyphosate as Vision' was reduced from 30 L ha-1 to 20 L ha-1 in 1998) may not be appropriate for triclopyr. As spray volume decreased for a given droplet size, glyphosate uptake and translocation increased, whereas triclopyr uptake and translocation decreased. In order not to reduce uptake and translocation of triclopyr as spray volume decreased, droplet size also had to be decreased. So, the gains in operational efficiency achieved by reducing spray volume for triclopyr could be more than offset by decreases in the length of the spray window during which off-target movement is minimized.