To the uninitiated, it might look like farmers are trying to kill their crops by spraying an undiscriminating herbicide before harvest.
But pre-harvest applications of glyphosate-based herbicides are actually aimed at killing perennial and winter annual plants that take advantage of standing stubble. If properly applied, the crops show little effect.
“Newer approaches to tillage have left a gap in the area of weed control for many producers. Pre-harvest applications will help to control weeds that tillage used to help take care of,” said Dennis Gardisser, a spraying system engineer and consultant at the University of Nebraska. “When you stop tillage rotations you have to move on to other weed control measures.”
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Research at the university has shown winter annuals and perennial weeds that survive late into the fall can use up to 75 millimetres of moisture before the first snow. Weed control before and after harvest can aid moisture conservation.
Weeds that get an early start in spring can further deplete moisture. More moisture loss occurs between weed germination and pre-seeding or post seeding applications of herbicides.
Researchers say the pre-harvest timing is nearly perfect in crops where dandelion, Canada thistle, perennial sow thistle, toadflax, milkweed, narrow-leaved hawksbeard and quackgrass pose a problem.
Neil Harker has been studying the effect of pre-harvest applications of glyphosate-based herbicides for several years at the Lacombe, Alta. Agriculture Canada research centre.
Research into wheat, barley, flax, canola, solin and lentils suggests careful application of the herbicides will not affect seed viability or cause residue buildups in grain.
“It’s a cheap way to get control of perennial weeds. In some cases it may be the only way,” said Harker.
As well, in Harker’s 1994-95 study of field peas, glyphosate applications when seed moisture has fallen below 30 percent caused no reduction in germination or seedling vigor.
Western Canadian farmers can choose from two glyphosate salt herbicides, Zeneca’s Touchdown and Monsanto’s Roundup.
“As a method of weed control these products are great when applied in August. I wouldn’t recommend them solely as a desiccant, though. There are better products for that. If they do desiccate the crop it would be a side benefit and shouldn’t be counted on,” said Harker.
