Over 90% of Canadian kochia populations are resistant to acetolactate
synthase (ALS)– inhibiting herbicides. We questioned whether the target
site–based resistance could affect plant growth and competitiveness.
Homozygous F2 herbicide-resistant (HR) kochia plants with an
amino acid substitution at Trp574 (sources: Alberta [AB],
Saskatchewan [SK], and Manitoba [MB]), or Pro197 (MB, AB with two
populations) were grown in replacement series with homozygous F2
herbicide-susceptible (HS) plants from the corresponding heterogeneous
population (total: six populations). In pure stands, growth of HR plants
from AB and SK was similar to that of HS plants, regardless of mutation;
conversely, MB2-HR plants (Trp574Leu) developed more slowly and
were taller than MB2-HS plants. Final dry weight of HR plants in pure stands
was similar across all six populations, whereas that for HS plants in pure
stands and HR–HS plants in mixed stands (50–50%) varied with population.
Results for AB and SK populations suggest little impact of either ALS
mutation on kochia growth, whereas those for MB lines would suggest an
unidentified factor (or factors) affecting the HS, HR, or both biotypes. The
variable response within and between lines, and across HS biotypes
highlights the importance of including populations of various origins and
multiple susceptible controls in HR biotype studies.