Weed Wisdom March 2018

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Green foxtail (Setaria viridis)

Green foxtail seed head

 By Charles Geddes

Since the 1970s, the #1 most abundant weed in the Canadian Prairies (Alberta, Saskatchewan, and Manitoba combined) is Green foxtail (Setaria viridis). 

It is an aggressive competitor and can deplete the amount of nitrogen available to crops, potentially resulting in yield loss.  The seed also is known to cause dockage issues in small grain cereal and oilseed crops like wheat, barley, canola or flax.

Recent prairie-wide weed surveys following POST-emergence herbicide application (sampled in late July/early August) indicate that green foxtail occurs in about 30% of sampled fields at an average occurrence density of about 24 plants per square meter.  It tends to be most dense in southern Manitoba, while a recent survey showed that green foxtail is the 7th most abundant weed species in Alberta.

Green foxtail is a monocotyledonous (i.e., grassy) summer-annual weed species found in cultivated areas, field pastures, lawns, turf, roadsides, railroads and waste sites. It is erect, can branch at the base and grow up to one meter tall. The ligule has a fringe of hairs up to 2 mm long, auricles are absent, and the sheath is open and overlapping at margins.  The leaves are light green in color, flat, smooth and generally 0.5 to 1.5 cm wide and up to 30 cm long. The seed head is a cylindrical cluster that resembles a small bottle brush and is composed of a dense group of spikelets. 

Germination of green foxtail occurs from shallow depths at warmer temperatures, often causing this weed to emerge in-crop following spring tillage or PRE-emergence herbicide application. This weed has a prolonged emergence period that allows it to emerge in multiple flushes throughout the growing season. Additionally, green foxtail can produce a prolific amount of seed only six weeks after germination. This allows multiple generations of green foxtail to exist and reproduce within a single growing season.

In the Canadian prairies, people found populations of green foxtail with resistance to herbicide mode of action groups 1, 2 or 3.  While certain populations of it exhibit resistance to herbicide groups 1 and 2 or groups 1 and 3 in Canada, we haven’t found a triple-resistant population yet. 

A couple of management suggestion are increased crop seeding rates or decreased row spacing to take advantage of green foxtail’s shade sensitivity. Also, green foxtail does not emerge well from soil depths below about 5 cm, so an occasional tillage pass also help manage it.

Mature green foxtail