Research shows plant growth regulators (PGRs) are most effective — and least likely to cause damage — when they’re applied with precise timing, and that stage can pass quickly as a cereal crop develops.
There are three questions to answer before applying a PGR, explains Anne Kirk, cereal specialist with Manitoba Agriculture, in this video recorded at the 2022 Manitoba Crop Diagnostic School at Carman.
The first question: is the crop under stress? “We know there can be negative effects from PGRs, whether it’s a yield decrease or some injury on the plants, if we have PGRs applied when the plants are under stress, whether that’s drought, excess moisture, heat, insects, or diseases,” she notes.
Second, is the crop at risk of lodging? This will depend on fertility and whether conditions are conducive for growth and good yield potential.
The third question, and the focus of this Western Canada-focused Wheat School episode: is the crop at the right stage?
“We see the best effects from plant growth regulators when they’re applied at the ideal timing, which is between growth stage 31 and 32,” explains Kirk.
To determine which stage wheat or other cereals are at, Kirk suggests removing the leaves or tillers from a few primary stems, then use a knife to slice down the stem lengthwise to find the tillering node, the first node, the second node if it’s present, and the developing spike or panicle.
Growth stage (GS) 30 — the early side of ideal timing — occurs when the first node is less than 1 cm above the tillering node.
“When we’re at growth stage 31, the first node is at least 1 cm above the tillering node, and within growth stage 31, you’ll actually see the second node start to develop. We only hit growth stage 32 when the second node is 2 cm above the first node,” demonstrates Kirk.
“Depending on the environmental conditions the plant can go through those stages pretty quick, so I really recommend people cut into those plants and identify which stage they’re at if you want to get the most benefit out of a plant growth regulator,” she says.
The two PGRs that are approved for cereal crops in Western Canada — under the brand names Manipulator and Moddus — are registered for application until growth stage 39, notes Kirk, however “that’s when people see the most negative effects, when they’re applying it right at the end of that window.”
If applying a PGR, Kirk also recommends considering an on-farm trial, ideally with replicated strips, to assess the benefit of the treatment.