At a time when soil erosion is recognized as one of the biggest threats to the world’s ability to continue feeding itself, it’s disturbing to see weed scientists advising tillage to address invading “superweeds.”
There is no question that addressing the lengthening list of weeds that have developed resistance to glyphosate must be a top priority for researchers and extension agronomists advising conventional farmers.
As reported by Reuters, resistance to glyphosate has now reached the point where row-crop farmers in the Midwest are unable to control weeds with herbicides. University of Missouri and University of Kansas weed scientists are proposing a return to tillage.
Researchers say Palmer amaranth is the No. 1 weed to watch, because of its aggressiveness — it grows one to two inches per day and produces millions of seeds — and its ability to develop resistance to herbicides.
These weeds were a problem limited to the deep south a few short years ago, prompting farmers in Arkansas to resort to hands and hoe to bring them under control. They are now “exploding” in the Midwest, Kansas State University weed specialist Dallas Peterson told Reuters.
The U.S. Department of Agriculture says 70 million acres of U.S. farmland had glyphosate-resistant weeds in 2013.
You have to admire these plants’ remarkable ability to adapt. It took scientists decades of effort to figure out how to modify crops to have resistance to herbicides and less than two decades for nature to accomplish the same thing, albeit helped by farmers through intensive selection pressure.
It is only a matter of time before more of these invasive plants make their way to the Canadian Prairies. Stacking of traits for resistance to other herbicides as well as glyphosate will help, but is a short-term solution. So is tillage.
But does it have to be either/or of two bad choices?
What if there was a third option, not just in theory, but with a proven track record through more than two decades of research and monitoring through several crop rotations?
The Glenlea long-term crop rotation study, now the longest-running study of its kind in Canada, compares four-year crop rotations for annual crops and annual crops mixed with forages under organic and conventional production systems.
Those trials, which started in 1983, have shown that the annual grain-based conventional rotation is the most productive by way of yield, although not the most economic. But it is susceptible to resistant weed problems because of its reliance on herbicides.
However, researchers have found that adding a perennial alfalfa crop into the rotation for two years virtually eliminates the risk of herbicide-resistant wild oats. It’s also very good at reducing other annual grasses, thistles and quack grass.
“Alfalfa does not reduce small-seeded broad-leaved weeds (like pigweeds) so effectively. But by not spraying these weeds during the alfalfa phase, we reduce resistance development to herbicides in these populations. In other words, we make the herbicides last longer,” said the rotation study’s overseer Martin Entz.
Farmers accustomed to growing annual crops under conventional management are quick to argue the economics; they can’t afford to “lose” two years of production to a crop such as alfalfa, for which they have no use unless they have livestock.
However, the economics of including perennial forages in an annual cropping system are changing as the cost of controlling some of the unintended consequences of conventional systems, such as superweeds, rises and more becomes known about how alfalfa can benefit the total farm.
Alfalfa reduces the nitrogen needs by 40 per cent over the rotation cycle and it provides other soil-building qualities, such as biological drainage — its deep tap roots aerate the soil and draw out excess moisture.
There are other reasons for considering this as an option, such as the role of forage-based diets in sustainable livestock production and the production of healthier food products, which at this point sell for a marketplace premium. Or the growing demand for cover crop seed in the U.S.
No one is suggesting that alfalfa is the panacea for invasive weeds or any other issue plaguing a farming system. Picking one approach as the answer is a mistake that has been made before. But it’s an option — another tool that farmers can consider. And thanks to long-term trials such as these, they have accurate, scientifically valid metrics to help them assess how it fits on their farm.