Lamb’s quarters has consistently been identified as the most abundant and problematic weed affecting field crop production in Ontario even though the Pest Management Regulatory Agency database contains hundreds of herbicides that list lamb’s quarters as a susceptible weed. There is an arsenal of products available to farmers for the control of this weed, so let’s focus on maximizing their effectiveness.
Soil applied herbicides improve consistency of control with glyphosate
A management strategy that only relies on non-residual, post-emergence herbicides for lamb’s quarters control is more likely to fail. One is hopeful that the entire lamb’s quarters population will emerge within a small time period and be at a sensitive stage at application. This never happens.
There is usually a wide range in seedling emergence and plant stage. Consequently when growers want to make only one post-emergence herbicide application, understandably they tend to wait until the majority of weeds are up. Inevitably there will be a wide range in growth stage, with a portion of the population being too big to control.
Not only will this cause reduced weed control, but increased grain yield losses from weed competition. When glyphosate-tolerant lamb’s quarters populations were targeted in Indiana and Ohio, the use of soil-applied pre-emergence herbicide improved season-long lamb’s quarters control with glyphosate by as much as 30 per cent (Figure 1 below).
Big lamb’s quarters are hard to control
Calcium (Ca2+), a divalent cation, is known to antagonize glyphosate activity and reduce weed control. As a lamb’s quarters get larger, the concentration of calcium within the plant increases (Figure 2 below).
Control is most consistent with glyphosate when lamb’s quarters are about 2.5 cm tall at application, although studies have observed good control with glyphosate when lamb’s quarters was less than 10 cm tall at application. Regardless, once lamb’s quarters is taller than 10 cm, higher rates of glyphosate are needed to get acceptable control.