Risks and Reliability of Using Herbicides at Below-Labelled Rates

Risks and Reliability of Using Herbicides
at Below-Labelled Rates
By Jianhua Zhang, Susan Weaver and Al Hamill, Greenhouse & Processing Crops Research Centre,
Agriculture and Agri-Food Canada, Harrow, Ontario

There is increasing interest in reducing the amount of herbicides used in agricultural production systems, both for economic and environmental reasons. Labelled rates of herbicides are intended to provide reliable weed control over a wide range of conditions. However, many published studies and observations of individual growers support the idea that herbicide rates lower than the labelled rate can provide adequate weed control in some situations. Caution should be exercised, though. The risk of weed control failure increases as rates are reduced, and farmers using below-label rates bear the responsibility for potential failure. By examining a large number of scientific studies, we hoped to identify some of the factors that influence the success of reduced rate applications.

A total of 76 published studies (71 per cent conducted in North America, 26 per cent in Europe) were located, each documenting information on herbicide rate, weed control efficacy, cultivation, method of application, use of adjuvants, tillage system, soil texture, and main crop at two or more sites or over two or three years (resulting in a database of 1,901 entries). Of these studies, 79% were published in the 1990’s, and 70% focused on corn, soybeans or cereals. Herbicide rates, expressed as a percentage of the labelled rate, were divided into six rate groups (less than 20%, 20-39%, 40-59%, 60-79%, 80-99%, and 100% of full label rate). From our analysis to determine the influence of inter-row cultivation, timing of application, tillage system, soil texture, and main crop on the efficacy of reduced herbicide rates, we were able to identify the following trends:

greater potential for rate reduction was observed in corn than in soybeans (Fig. 1). With inter-row cultivation, weed control efficacy in corn was over 80% in more than 90% of the cases at herbicide rates as low as 20% or more of the labelled rate. This level of weed control was not achievable even at the full labelled rate in soybeans.
a given herbicide rate group provided a similar level of control under conventional and conservation tillage systems.
greater potential for rate reduction was observed when herbicides were applied PRE (pre-emergent) than when applied PPI (pre-plant incorporated) or POST (post-emergent), whether with or without cultivation. PRE applications provided 80% or greater weed control in at least half of the cases (and better than 70% weed control in three-quarters of the cases) at herbicide rates of 20% or more of the recommended rate. Variation in the size of weeds at the time of POST applications may partially account for their lower efficacy at reduced rates.
a given herbicide rate group normally maintained a higher weed-control efficacy in coarse-textured soil than in fine-textured soil. The database was not large enough to test for interactions between herbicide rate, soil texture and timing of application.
The vast majority of studies in the database were conducted at research sites where weed pressure was very high. Greater potential for herbicide rate reduction may exist at locations or in cropping systems where weed pressure is low.

It is worth noting that substantial variation in weed-control efficacy existed in each of the herbicide rate groups. Even at the label rate, weed control was as low as 20-40% in a few cases. These results are not surprising, considering herbicide performance depends not just on rate, but on many factors such as temperature, rainfall, and growth stage. Under drought stress or when weeds are beyond the recommended growth stage, unsatisfactory weed control can be expected regardless of rate. Overall, reductions in herbicide rate resulted in poorer weed control and greater variability, unless backed up by other weed control measures such as inter-row cultivation. Nevertheless, this study has identified some potential for reduction in herbicide rates with minimal risk, particularly for corn grown with inter-row cultivation, on coarse-textured soils and/or with pre-emergence herbicide application.

ACKNOWLEDGEMENTS
Funding for this project was provided by the Food Systems 2002 Pest Management Research Program of the Ontario Ministry of Agriculture, Food and Rural Affairs.