Herbicides are more effective when sprayed on young weeds, particularly when the weeds are stressed by drought.
A study by the University of Saskatchewan and Agriculture Canada shows early spraying is best in most situations.
The study was carried out from 1994 to 1997 by Rick Holm of the U of S plant science department and by Ken Kirkland, manager of Agriculture Canada’s research farms at Scott and Melfort, Sask.
It evaluated the application of several herbicides on wild oats in wheat at early, middle and late stages of the weed.
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“In the Saskatoon trials, with light populations of wild oat, there was little difference between the two- and four-leaf stage of herbicide application,” said Clark Brenzil, weed control specialist with Saskatchewan Agriculture.
“When these applications were delayed to the six-leaf stage of the weeds, however, there was an average of $10.70 per acre loss in reduced yields.
“Under heavy wild oat populations at Saskatoon, treatments sprayed at the two-leaf stage yielded about $31 per acre more than those left until the six-leaf stage. All results were based on wheat at $3 per bushel. The highest yields were, on average, 40 to 60 bushels per acre for clean treatments.”
Weeds weren’t as bad at Scott but yields were still higher on plots that were sprayed early.
Most herbicide labels state the chemicals are less efficient when weeds are under stress such as drought, said Brenzil.
From a herbicide performance perspective, late spraying could also be seen as a stress factor.
“The accumulation of stress factors increases the risk of a herbicide failure, so late spraying of plants that are already stressed can lead to failure of the herbicide.
“My advice to farmers, especially this year, is to try and target the early to mid range of the application stages recommended on the product label.”
The few weeds that appear late and are missed by early application won’t hurt yield as much as weeds that remain unsprayed while the farmer waits for stragglers.
