Reports of more severe waterhemp infestation in Midwest soybean acres continue to roll in as we head into harvest, and that means more herbicide-tolerant weed seed going into the ground to cause larger future problems.
That future challenge gets worse when these weeds have stacked the deck, as some waterhemp resists multiple herbicides — glyphosate, triazines, ALS inhibitors, PPO inhibitors and HPPD inhibitors. Weed scientists harp about the importance of controlling this weed seed bank. Even just a few plants here or there can rapidly grow into a large problem that won’t go away easily because combines and tillage spread the seed all over. There are no simple herbicide solutions. It takes a serious, multiple-year, integrated resistance management plan, by field and by crop.
If you’re thinking more tillage is the answer, you should read our Page 24 story, “Herbicide-resistant weeds: the tillage dilemma,” because spring tillage causes more waterhemp to emerge. In Illinois trials last year, tillage in May to knock down emerged waterhemp caused a sixfold spike in waterhemp emer- gence in the following two weeks compared to no tillage. And an early June tillage caused a 14-fold increase in waterhemp emergence. The same thing happened with its more dangerous pigweed cousin, Palmer amaranth.
The thing about this story that made me cringe was the deep tillage recommendation — which destroys all conservation and soil health benefits. But as some research is proving, in fields with severe problems, a one-time moldboard plowing can bury about 80% of weed seeds below the germina- tion zone. A Missouri trial showed that deep tillage combined with crop rotation and herbicide diversification proved effective.
In Arkansas, a similar reduction was found with a one-time deep till. Scientists say it also takes pressure off herbicides by burying high weed seed populations. And that allows other weed control measures to work, as well. For example, Palmer amaranth emergence was reduced by 93% over two years when deep tillage was combined with a cereal rye cover crop.
But be aware, this is a last-resort tactic that is not 100% effective. Run- ning the plow more than once will bring the seed back up into the germina- tion zone. Plus, the challenge that remains after deep tillage will be killing the remaining herbicide-resistant weeds so they don’t re-establish a severe prob- lem again.
If you still have an old cultivator, it could provide some help, but it won’t save the day.
And no-tillers should take heart, because seed mortality is high on the soil surface thanks to help from birds, mice and insects. Plus, no-tillers often use more diversified herbicide programs that increase effective weed control. Read on to learn more about the pros and cons of tillage, starting on Page 24.
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