There are a number of coloured dyes, such as those used for foam markers, that can be added to water or a tank mix to show where a spray has deposited. Such dyes are commonly added to herbicides for spot spraying to show which plants have been sprayed.

Coloured dyes may also be useful for assessing the functioning of auto section control, to ensure the ‘look ahead’ function is working correctly, or as a rapid check for assessing when target-selectable sprayers are turning the nozzles on and off.

This can be very effective for assessing applications into standing stubble, where the coloured dye is easy to see. However, many of the commercially available coloured marker dyes may not be suitable for determining where individual droplets have landed, or provide a useful guide to assessing the number of droplets that have deposited onto the target or the droplet density.

Some of the commercially available coloured dyes that are designed to show where herbicides have been applied may not be suitable for use on food crops as they may contain toxic substances. It is important for the applicator to read the directions for use for each product and to access the safety data sheet before using a marker dye for assessing in-crop spray applications.

To assess actual spray deposits onto crops using a dye or pigment it is best to use water-soluble, non-toxic products. Even for non-toxic dyes the label instructions must be followed to avoid potential contamination of plant foliage intended for human consumption.