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COUNTRY LIFE IN BC • NOVEMBER 2016 Preparing for next year’s weeds in corn and other crops by RONDA PAYNE


ABBOTSFORD – Weeds are one of the most expensive issues in a vegetable field. Spraying or hand-picking takes time and money so the method chosen has to be as effective as possible. Dr. Darren Robinson, horticultural crop weed management associate professor with the University of Guelph, has been studying the control of weeds in vegetable crops for a number of years. He spoke about his most recent findings at the Pacific Agriculture Show last January.


“I work on weed management systems that work on the production systems the growers currently have,” he says. “The goal is to start with a clean field and to keep it clean.”


Having spent a considerable amount of time studying weeds in corn and other crops, Robinson advises growers to choose the best pre-emergence herbicide for the field by taking into account the strengths and weaknesses of post-emergence herbicides. The pre-emergence products he looked at were Primextra II Magnum and Integrity. The costs for both products were found to be similar for 10 to 12 week controls in a similar range of weeds like annual grasses and broadleaf weeds. While Robinson notes corn is fairly tolerant to the ingredient atrazine, he also says certain conditions can increase the potential for crop injury. Re-cropping can be an issue depending upon farm practices.


“There are several vegetable crops that can be injured the year after Primextra II Magnum is applied and that definitely is a consideration,” he says. “We still get significant yield reductions.” Those yield reductions can be more than 50%


and Robinson has found it’s even worse when Callisto is mixed with it. Another downside is the fact there is some control over pigweed and lamb’s quarters, but not full season control. There is no control from Primextra II Magnum in triazine- resistant weeds.


While Integrity is slightly cheaper, the price difference is minimal for about the same weed control. Robinson notes there is control of triazine-resistant weeds with Integrity, but it’s not as effective on yellow nutsedge as Primextra II Magnum.


There were no crop rotation or


recropping issues with Integrity but Robinson notes, “we can’t use early post-emergence in corn,” meaning Integrity is not as timing-flexible as Primextra II Magnum. “Integrity causes injury if applied post-emergence. It can go to as severe as plant death.”


Knowing the difference


For post-emergence options, Robinson points to Accent and Callisto. Like the pre-emergence treatments, with these tools, it’s necessary to know what weeds are the most problematic, such as understanding the difference between crabgrass and fall panicum.


Accent did not offer any residual control and Robinson found crabgrass and yellow foxtail difficult to control due to continual emergence. This product can also impact recropping but the risks are much less than that of other products, except in the case


of hybrid corn.


“There are some hybrids that are really, really sensitive to Accent,” he says.


DR. DARREN ROBINSON


These cases of injury were quite pronounced when Accent was mixed with Basagran causing what Robinson notes as “significant injury,” while Pardner mixed with Accent caused a lesser degree of injury. Pardner is good in controlling lambs quarters but not pigweed. It can also injure corn, possibly setting growth back if applied in hot, humid conditions.


Callisto offered an excellent extended period of control to


crabgrass if applied post-emergence but, again, hybrid varieties are at risk.


“If applied post-emergence, we can see significant hybrid sensitivity,” Robinson says.”


It also can cause impacts on growth the following year. While the plant matter above ground is healthy, below ground (as in carrot or onion) growth may be an issue.


Permit has been known to impact some hybrid varieties as has Basagran Forte. The latter can also cause injury in non-hybrid varieties but is generally temporary. Robinson also notes Impact mixed with atrazine will provide good post-emergence control of lambs quarters, pigweed, wild buckwheat, wild mustard and yellow foxtail.


“You can’t just relay on pre-emergence or post- emergence [products] to control these weeds,” Robinson summarizes. “You have to have a strategy that looks at both.”


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