I recently received this query. “I have a client who wants to mix a ‘crop enhancer’ with glyphosate to control weeds in glyphosate-tolerant soybeans. According to the manufacturer’s website, this product ‘can be combined with other applications (e.g. fertilizer, herbicides, insecticide, etc.).’ However, when I read the composition of the product, it appears to contain a lot of cations, which I think would tie up the glyphosate and reduce weed control. I don’t want my client to tank mix these two products. Am I being overly cautious? Enclosed is a small amount of product, could you test its effectiveness in the greenhouse?”
Your instincts were correct. This product contained a high amount of cations which can reduce the performance of glyphosate (Hall et al., 2000). Lamb’s quarters control was noticeably reduced when the “crop enhancer” was included with glyphosate (Figure 1 at top). Measuring the dry weight of harvested lamb’s quarters in each treatment revealed that an application of glyphosate provided a 90 per cent reduction in lamb’s quarters biomass, but the “crop enhancer” and glyphosate tank mix only reduced lamb’s quarter biomass by 40 per cent (Figure 2 at bottom of page).
Tank mixing a herbicide and fertilizer is not an illegal practice. In 2009, the Pest Management Regulatory Agency (PMRA) issued a memorandum known informally as the “PMRA tank mix policy” which gives the go-ahead for an applicator to mix products that are both registered for use on a given crop. However, the memorandum does say that “the addition of a fertilizer to the spray carrier may result in a greater chance of host-crop injury so initial use should be limited to a small area to confirm results prior to widespread use.” In other words “you’re on your own” in terms of any performance guarantee whether it be crop safety or weed efficacy.
The bottom line is, make sure there is a body of evidence and experience to indicate compatibility of products in a tank mix. Otherwise the results could be costly.