It is a dilemma faced by most growers at one time or another: spray early and get ahead by killing the first weed flush — those that can compete against the newly seeded crop. Or spray later and control more weeds with one pass.
Do the yield losses add up for later season spraying or can second post-emergent applications be more costly?
Dan Holman of the Holman the farming group at Luseland, Sask., says his family has been trying to spray their post-emergent weed control as early as possible and there have been financial rewards from higher yields.
He said later season applications tend to favour killing larger numbers of weeds, but the earlier ones tend to produce more grain.
“If you have stress (later in the season) at the time you are spraying, you can hurt tillering and heading,” said Holman.
“When you apply a pesticide you are trying to protect the yield that is there,” he said referring to the improved tolerance most cereals have to an earlier season application of post-emergent herbicides.
“A second flush might come, but the crop usually canopies in and that prevents them from doing much damage,” he said.
Neil Harker has spent his career researching weeds and suggests that the clean look of a late-sprayed crop is often misleading.
The Agriculture Canada scientist from Lacombe, Alta., said the damage to yields has been mostly shown to take place earlier in the crops’ life cycles, rather than later.
Holman said rains might bring some significant second flushes, but that moisture is helping the crop get further ahead as well.
“If it gets bad enough, you can still make a smaller application later on, but often it isn’t necessary,” he said.
Every year is different and Harker said producers can gauge their own situations, “but if there are no weeds, you can wait.”
Holman chose to do that last year and said it worked out well, “due to the lack of moisture.”
Chris Mansiere of Bayer Cropscience said it comes down to a farmer’s individual situation.
“It’s all about your field. No weed pressure? No worries.
“But ask yourself the questions. ‘Did you get the pre-burn on?’ If things were so dry you didn’t, getting in there early is a very good idea.”
Mansiere said there once was a time when post-emergent herbicides could only be used effectively on weeds shortly after their emergence.
“The products have gotten better and can be used in wider windows (of application),” he said.
Bayer carried out plot trials last season to examine the yield differences between early and late herbicide applications and found that the earlier applications produced greater yields.
“If you were going to have to spray the same chemicals at the same rates, at the same cost and get 10 percent more yield you might consider that,” he said about the two timings.
In addition to the earlier timing, weeds and crops at later stages are harder to cover with spray. As the canopies tighten up it takes higher water volumes to reach those little plants on the soil’s surface and properly cover the larger weeds that need to receive a proper dose of spray.
Harker said research has repeatedly show earlier applications tend to provide bigger yields, often at the equal or lower costs.
“No two years are the same, so you can’t always use a recipe to be the most efficient or effective,” he said.
“In a dry year, there may not be weeds up to control, and that’s OK,” he said.
Harker said that at the same time, producers who wait can find themselves behind in weed control if the weather turns and keeps them out of the field after letting the crop and weed stages advance to more critical levels.
There are some well-respected theories about how plants perceive their neighbourhoods. Having higher weed populations in direct proximity to the young, growing crop is thought to contribute to competitive behaviours that don’t result in greater yields for the producer.
“While those are just theories, they tend to prove (to be true),” he said.
Federal researchers Harker, John O’Donovan, George Clayton, with the Canola Council agronomy head John Mayko, now of Apex and Agritrend, found that in field-scale trials early season applications of herbicides produced significantly higher yields.
Experiments were conducted at several western Canadian locations over variable landscapes.
“In eight of 10 cases, imidazolinone resistant canola yield decreased linearly as herbicide application (Odyssey, Tensile or Absolute) was delayed beyond the one- to two-leaf stage. In two of 10 cases, canola oil content also decreased … if treatment was delayed,” the research reported.
Assuming canola prices range from a low of $250 per tonne to a high of $650, growers could lose $20 to $53 per acre, respectively, by delaying herbicide application from the one to two leaf stage, to the three-or five-leaf stages. It could also translate into losses of $42 to $111 per acre by delaying herbicide application from the one-to-two stage to the six-to-seven leaf stages
If weeds are up before the crop and are not controlled, they can compete with the crop, absorbing water and nutrients that were would have been available for the intended plants.
When it comes to cereals, peas, chickpeas, lentils, soybeans and canaryseed, Danielle Eastman of BASF says products like Heat or saflufenacil that can provide extended control after a spring burn off with glyphosate.
“There are options, provided it rains (to activate the Heat),” she said.
“But there are a lot of products available and it is creating some confusion out there, so producers have to choose carefully,” she said.
Some producers use a longer term approach to control in-crop weeds ahead of seeding and apply granular products such as Edge, ethalfluralin, to canola, pea and sunflower fields. Another granular approach, for cereals, other than oats, and canola that helps to retain control after the crop goes in the ground is Avadex, triallate.
Mike Grenier of Gowan Canada said producers can improve margins by gaining the advantage of the residual effects of the products.
“Plus you can use the (Avadex) Group 8 and Group 3 (Edge) products to control Groups 2 and 4 resistant weeds,” said Grenier.
Holman said that “whatever approach you use, getting those weeds out early has been important to our yields. We see it time after time.”