How many tank mixes are too many?

The federal Pest Management Regulatory Agency (PMRA) no longer enforces off-label tank mixes as long as each of the tank mix components are:
—registered on the crop of interest,
—within the registered staging window for the crop of interest,
—not adjuvants unless one or both of the other components require the addition of an adjuvant not provided by any of the other components,
—able to provide additional value to the user,
—used according to the most limiting restrictions found on of the labels of any of the individual components of the mix,
—not specifically restricted on any of the component product labels. Note: General label statements such as “Do not mix with any other product not specified on this label” constitute a prohibition of off-label tank mixes,
—only for commercial class pesticides.

All risks are borne by the user.

For more, read “Use of Unlabelled Tank Mixes of Commercial Class Pest Control Products Used for Crop Production or Vegetation Management – Frequently Asked Questions, Pest Management Regulatory Agency, November 3, 2010”
However, growers must take extra precaution with any tank mix that is not identified and approved on the product label.

The risks associated with off-label tank mixes include but are not limited to:

—Damage to the crop
—Antagonism or other reduction in control of one or more of the labelled pests
—Residues in excess of the limits of those set for the components of the mix in the harvested grain
—Creating a clogging lump in the spray tank
—Violent reactions – explosion

The jar test may give the producer an indication of the last two problems (chemo-physical compatibility), but won’t tell them a thing about the biological compatibility of the components or whether toxic vapours are being produced. The best source of information about all of the above points is going to be the manufacturers of each of the components in the mix and getting written recommendations from them to the compatibility of the off-label mix. Many of the manufacturers of pesticides no longer apply to PMRA for the addition of new tank mix registrations beyond the original label, but rather provide a list of products that they support being mixed with each of their products.

Technically an off-label tank mix is still an unregistered use. PMRA has just chosen through policy to not actively enforce compliance of this portion of the Pest Control Products Act (Canada). 

Click here for more on pesticide mixing.