Last spring Mike, a grower in southern Saskatchewan, called me with a problem that was stumping him.
He’d followed all the best management practices when planting his fall rye crop the season before, and for the most part, he’d been rewarded. About 90 per cent of the crop looked great. But there were a couple patches, totalling about 10 acres, where the crop was a virtual loss.
In those parts of the field, grassy weeds were doing what both of us thought was fairly unlikely. They were out-competing a fall rye crop. It was so bad, you might think the field had never been planted in the first place.
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Mike’s field records didn’t show an obvious problem, and a field visit showed most of the weeds in question were downey brome. It was Mike’s casual remark that the field “looked like a lawn” that reminded me of a presentation I’d seen at the Crop Diagnostic School the previous year by provincial weed specialist Dr. Bruce Murray.
“If I’m right, the problem is that your weed spectrum is adapting to your new production practices,” I explained.
Mike had switched to zero till several years earlier. Like many growers in his area, he found the system fit well with fall-seeded crops like fall rye and winter wheat, which he had started growing more often over the past several seasons.
Now, agronomists are finding these production system changes are driving changes to the weed spectrum by applying different selection pressure, sometimes with unexpected results -such as an out-competed fall rye crop.
“A new weed spectrum is emerging that’s suited to this new ecology,” I told Mike. “The weeds that we’re now selecting for germinate in fall stubble. They can be really tough to control the following spring because they’re so well-established.”
When these weeds are combined with a winter annual crop that’s sown at the same time the weeds are emerging, the combination can be dynamite.
At this point there wasn’t a lot that Mike could do to limit the damage to this crop, other than treating the patches to kill the invaders before they set seed. Because they mature and set seed in late May or early June, it was important to deal with them immediately to stop them from adding even more seeds to the seed bank.
“At least you caught it before it affected the whole field,” I said. “But you’re going to want to make some management changes.”
Mike will likely have to refrain from planting winter annual crops on this field for a while to deplete the seed bank. He may also consider adding spring tillage back into his system on a limited basis in order to change up that selection pressure.
Downey brome and the closely-related Japanese brome are the two most virulent winter annuals. However, growers should also be on the lookout for night flowering catchfly, cleavers, chickweed and a few others. Generally speaking, they’re all much easier to control in the fall, when they’re smaller. If you let them overwinter, you’ve got a much tougher challenge on your hands the following spring.
Brian Hellgard manages Richardson International’s Kelburn Farm research site near Winnipeg and was recently named Prairie region Certified Crop Adviser of the year for 2008. He is also the company’s manager of agronomy
