Green foxtail is still the province’s top weed, yellow foxtail is on the rise and wild oats have declined, according to last year’s Manitoba Weed Survey results.
Wild oats, usually the second-most-abundant weed in the province, slipped to fourth in 2016, overtaken by both wild buckwheat and barnyard grass.
Dr. Jeanette Gaultier, principal investigator of the study, called the shift in wild oats one of the most interesting 2016 results in terms of crop impact.
Wild oats were found in 20.7 per cent of the 659 fields tested with an average 4.4 plants per square metre, compared to 52.7 per cent of fields that reported wild buckwheat and 23 per cent that reported barnyard grass.
Among fields with wild buckwheat, stands averaged two plants per square metre, compared to 4.8 plants of barnyard grass, while fields with wild oats found the weed in 5.6 per cent of 20 tested quadrants, compared to 13.1 per cent of fields with wild buckwheat and 6.4 per cent of fields with barnyard grass.
“I’m not sure if it’s just because those two species have been increasing or I kind of wonder, and this is just my opinion, if just the fact that we’ve been using more glyphosate in our cropping system, which works fantastic on wild oats but not so great on wild buckwheat, I wonder if that kind of played into some of those shifts as well,” Gaultier said.
Wild buckwheat, according to Manitoba Agriculture, can decrease wheat yields by 12 per cent with five plants per square metre, or 10 to 20 per cent in flax with stands of five to 15 plants per square metre. At the same time, the department adds, yield impact can vary greatly depending on emergence timing.
Ten wild oat plants per square metre, meanwhile, may decrease wheat, barley and canola yields by 10 per cent and flax by 20 per cent, according to Manitoba Agriculture.
“Wild oats tend to be, or used to be, one of our major weeds just because before we had really good herbicide, it was really hard to control and, because it emerges so early and it grows so quickly, it’s always been more problematic,” Gaultier said. “Wild buckwheat too does fit into that category. It does emerge quite early, but it grows a little slower and then can be less competitive relatively to wild oats but can still be a problem in crops. Then, barnyard grass, I find it’s a warm-season grass, so it starts emerging later but it can definitely be a competitor, especially now that we’re not just growing wheat and canola, that we’ve shifted to some of the crops that have a similar life cycle to barnyard grass.”
Yellow foxtail, meanwhile, climbed into the province’s top 20 weeds for the first time in 2016.
That growth is of concern to the Manitoba Corn Growers Association, which highlighted the weed in its most recent newsletter. Among the 41 cornfields tested, yellow foxtail ranked 10th.
Relatively uncompetitive with cereal and canola (a stand may reach 50-100 plants per square metre before yields drop five per cent), the association has warned that as few as five yellow foxtail plants in the same space may be enough to cause a similar yield loss in corn.
Pam de Rocquigny, Manitoba Corn Growers Association general manager, said the weed could be confused with green foxtail or barnyard grass. The association hopes to stress correct weed identification to optimize herbicide management.
“Obviously, part of when weed surveys do occur it’s providing information not only to producers but to the agricultural industry in terms of, are we seeing shifts in weed species? Are we seeing a shift in the distribution of the weed species? Are we seeing herbicide resistance cropping up in weed populations and how do we quantify that?” she said.
Barnyard grass was the most common weed in cornfields, according to the report, followed by wild buckwheat, canola, lamb’s quarters and round-leaved mallow, among others.
“Depending on the year, different weed species can be of concern each year, just depending on not only the region but the growing season as well,” de Rocquigny said.
While Gaultier acknowledged yellow foxtail’s jump in the rankings, she said weeds such as wild oats are a greater concern.
“The only reason I would say it’s a concern is because people might overlook it, just assuming it’s green foxtail,” she said. “Despite having those characteristics — silky hairs where the leaf blade meets the stem, we’re not always getting down on our knees and taking a look.”
While yellow foxtail jumped through abundance rankings to sixth in 2016, Gaultier noted that populations vary widely from region to region. The weed is less common in the western part of the province, she said, while populations concentrate in south-central Manitoba.
Annual spiny sow thistle showed the highest relative increase across decades, although increases plateaued from 2002 to 2016. Since the 1970s, spiny annual sow thistle has jumped 62 points on the relative abundance scale, although, with 11.1 per cent of tested fields reporting the weed in 2016, it remains the 15th most abundant overall.
Green foxtail kept its historical spot at the top of the list, present in only 24.4 per cent of tested fields, but at stands of an average 14.6 plants per square metre and present in 10 per cent of quadrants tested within a foxtail-positive field.
“You find it in a lot of fields, it’s relatively uniform throughout fields and then you also find it in really high numbers,” Gaultier said.
Gaultier stressed that weed survey results are not representative of all fields and do not replace field scouting.
“We did go into over 600 fields and it gives a really good snapshot of what’s out there, but, of course, every field is unique,” she said.
Manitoba Agriculture will now compare weed abundance data with farm management practices, although Gaultier does not expect that report until at least 2018. Data on herbicide resistance is also being analyzed.