Weed of the Week: redroot pigweed

It is a pig when it comes to using water, nutrients and land.

Redroot pigweed, or amaranthus retroflexus, is a dicot weed in the amaranthaceae family. It was first discovered to have developed resistance to Group 2 herbicides in Western Canada in 2010.

Group 2 herbicides are known as ALS inhibitors and are popular in cereal production in Western Canada.

Research in Saskatoon has shown that the resistant biotypes are able to tolerate thifensulfuron and tribenuron, DuPont’s Refine SG, Arysta’s Deploy and Cheminova’s Nimble. They might also be cross-resistant to other Group 2 products.

The annual pest has also developed Group 5 resistance in Ontario and the United States, where atrazine has been used extensively in corn production.

Group 5 is less common in Western Canada, with the only major product used in the region being metribuzin (Bayer’s Sencor and UPI’s Tricor), which is popular with lentil, pea and chickpea growers.

An uncontrolled pigweed plant can produce more than 100,000 seeds a season, making it a threat to soil where flooding has kept producers from carrying out weed control.

Manitoba Agriculture said the seed prefers warm temperatures of 20 C or higher for germination.

As a result, the seeds will tend to germinate after the crop is up and potentially after post emergent applications of herbicide, provided there is enough moisture.

Seeds can remain viable for up to five years, which means flooding in 2010 and 2011 have prepared many prairie fields for prolonged problems with pigweed.

Pigweed can significantly reduce yields in less competitive crops such as flax and pulses.

Scouting and controlling redroot pigweed is best done early in the season, once the soil warms.

The leaves are oval and have a notched tip when the plants are young and later develop a diamond shape.

The weed’s large, reddish taproot can scavenge moisture from deep in the soil’s profile. Lower stalks are thick and smooth, while upper areas and branches are rough and hairy.

Flower spikes carry densely packed green blossoms in July and August. The tiny seeds are round, black and shiny.

When the weather is hot in June, the weed can rapidly advance past growth stages where it can easily be controlled in crops that aren’t herbicide tolerant.

Pigweed is also toxic to livestock, including free ranging swine, despite being considered an edible plant by humans when it is in its early stages of growth. The crop is cultivated in parts of China and the seed is ground into flour.

However, it also considered a weed in that country.

Redroot pigweed is a poor competitor, which means crops that have a solid head start because of early seeding can perform some of their own weed control by shading out the pest.

Controlling this native plant can be accomplished with multiple mode of action herbicides and tank mixes.

Herbicides with residual action will also help reduce infestations in appropriate crops. Herbicide tolerant crops can be an effective tool in dealing with the pest because the weed has no resistance to those herbicide groups.

Delaying post emergent applications of broadleaf products until soil temperatures are higher than 20 C allow for pigweed seed germination and control, but this creates issues with other weed staging.

As a result, crop scouting is essential to ensure that other pests aren’t getting a leg up on the crop.

Shallow tillage or heavy harrowing in late fall will encourage earlier spring germination as the soil warms, creating an earlier window of control opportunities.

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