Legume re-establishment in northern temperate grasslands following application of aminocyclopyrachlor and aminopyralid

Abstract

Herbicides with residual properties are often used for broadleaf weed control in pastures but can remove desirable legumes such as alfalfa (Medicago sativa L.) and white clover (Trifolium repens L.). Field trials at five sites within central Alberta, Canada were used to assess legume population dynamics during recovery after spraying. Medicago sativa and T. repens populations were monitored for three years following application of aminopyralid (AMP) at 120 g a.i. ha−1, or aminocyclopyrachlor (AMCP) at 60 g a.i. ha−1, in combination with repeated mowing and legume over-seeding. Bioassays were performed twice a year (spring and fall) using M. sativa and T. repens seeded into soil sourced from each plot up until 26 months-after-spraying (MAS) and grown in a greenhouse. Medicago sativa and T. repens densities in the field remained below non-sprayed control levels throughout the study (26 MAS). In contrast, bioassay results indicated a shorter time than the field trial for legume recovery, with recovery occurring in soil from treated grassland as soon as 14 MAS. There were no differences between AMCP and AMP in the time required to facilitate legume establishment. Critically, legume establishment in the field depended on intermittent mowing, particularly for T. repens, which failed to persist in the absence of mowing at 22 MAS regardless of herbicide application. Medicago sativa had a high potential to establish from the natural seed bank, while T. repens depended on over-seeding. These findings indicate it may be possible to reintroduce legumes to temperate pastures as early as the second growing season after herbicide application, though field trials indicate legume population recovery will depend on seeding and secondary disturbances to reduce competition.