There is a high potential for inadvertent herbicide injury to crops in western Canada on an annual basis because of the diversity of crops grown in close proximity to each other, although accurate data regarding the annual number of injury incidents is not available. A field study was conducted at two locations in southern Manitoba, Canada, in 2001 and 2002, to investigate the effects of a range of dosages of MCPA ester, glyphosate, and thifensulfuron:tribenuron (2:1) applied to the seedling growth stage of conventional (nongenetically engineered) canola and white bean on subsequent shoot dry matter and crop yield. Similar to other studies that utilized sublethal herbicide dosages, results between site-years were variable, particularly for crop yield. Where possible, a nonlinear log-logistic model was fitted to the data. Generally, canola was more sensitive than white bean to the herbicides used in this study. Based on the fitted regression equations and recorded mean values for canola, 10% of the commercial herbicide dosage normally applied in other (possibly adjacent) crops caused greater than 10% canola yield loss for 9 of 12 unique combinations of herbicide-site-year. For white bean, 10% of the commercial herbicide dosage caused yield losses greater than 10% for only 4 of 13 unique combinations of herbicide-site-year. Spray drift is probably the most common source of inadvertent application of herbicide to sensitive crops; generally, only a fraction of the herbicide dosage applied on an adjacent crop drifts off-target. The results of this study indicate that for any of the three herbicides investigated on canola and white bean, it is difficult to accurately predict eventual crop yield loss based on early season sublethal herbicide injury symptoms due to site-year variability and the potential for crop recovery and compensatory growth. This response was particularly true for white bean in this study.

Nomenclature: Glyphosate, isopropylamine salt; MCPA, iso-octyl ester; thifensulfuron, methyl ester; tribenuron, methyl ester; canola, Brassica napus L. ‘46A65’; white bean, Phaseolus vulgaris L. ‘Envoy’.

Additional index words: Herbicide drift, herbicide injury, sublethal dosage, dose response.