To determine the efficacy of pre-harvest herbicides in reducing weed seed production in kochia and wild mustard.
Strategies are needed to reduce weed seed production in order to reduce populations of herbicide resistant weeds in the long term. Application of herbicides to weeds with a weed wiper is being examined as a strategy to reduce seed production of herbicide resistant weeds in lentil. Control of wild mustard seed production with weed wiping was evaluated at the University of Saskatchewan’s Kernen Crop Research Farm at one site in 2015, and two sites in 2016. Herbicides tested included 2,4-D amine, glyphosate, and dicamba, and were wiped on weeds at three weekly timings beginning when 80% of wild mustard inflorescences emerged above the lentil crop (weeks 1, 2, 3), as well as a combined week 1 & 3 treatment at both sites in 2016. In 2016, a low volatility formulation of dicamba was used, as lentil yield reductions and seed damage had been observed with weed wiping of dicamba in 2015. Herbicides applied at different timings were compared with untreated controls.
All products reduced weed seed production by more than 50%. Weed wiping applications were most effective at reducing wild mustard seed production when done earlier (week 1 or 2), and became less effective as weed wiping was delayed. In 2015, weed wiping at 80% wild mustard inflorescence emergence above lentil resulted in >90% reduction in wild mustard seed production. A second application (weeks 1 & 3 treatment) did not give better weed control than a single, well-timed application at either of the 2016 sites. Weed wiping with glyphosate in 2015 reduced wild mustard seedling emergence the following spring by 54%, while other herbicides had similar spring wild mustard populations to the untreated control. Dicamba reduced lentil seed yield and caused seed discolouration at all sites, regardless of product formulation or timing of application. 2,4-D also lowered lentil yield at one of the three sites, but not the others. Weed wiping with glyphosate resulted in similar or higher yield to untreated. Future studies will focus on refining rates of 2,4-D and glyphosate to increase efficacy and lower risk of crop injury, as well as screening additional herbicides and mixtures.
In Experiment 1, applying dicamba, 2,4-D amine, and glyphosate to wild mustard reduced seed production by 51, 54, and 72%, respectively. Glyphosate also reduced mustard seedling emergence by 52% in the following spring, and resulted in similar or better lentil yield than no weed wiping. In contrast, dicamba caused >50% yield loss, and therefore is not recommended for weed wiping in lentil. The optimal timing of weed wiping varied from year to year. In general weed wiping was most effective if it was conducted in the first or second week of wild mustard bolting. Weed wiping twice, in the first and third week of mustard flowering, was at least equally effective to wiping once at the optimal timing. In Experiment 2, different herbicides and concentrations reduced weed seed production inconsistently, and to a lesser degree, than in Experiment 1. Increasing the concentration lowered weed seed production for all herbicides on tame mustard, and five of seven herbicides on volunteer canola. Glyphosate, mecoprop, 2,4-D amine, 2,4-D amine + glyphosate, and mecoprop + glyphosate reduced mustard seed production by an average of 35%. Strong winds (approx. 50 km/hr) on the date of herbicide application may have played a role in the lower efficacy of weed wiping in Experiment 2 than Experiment 1.