Weed of the Week: kochia

Kochia, despite being identified as one of the most dangerous weeds in the West, remains a growing problem in the region.

Droughty conditions have allowed this tough competitor to flourish, even if it is only a few plants making it to maturity at the field corners or on the fence lines, where it often ends its life on a windy day.

The pest also gets a root-hold in saline spots because it has developed a strong tolerance to salt. Those few plants, carrying up to 25,000 seeds, love a good tumble across the hay and the grain field, spreading their progeny.

The tumbleweed has one of the most effective seed-spreading systems in agriculture. Those who discover glyphosate-tolerant versions often see the tumbleweed’s travel pattern as a green, casually-meandering stripe through their field, especially in chem-fallow situations where the resistant pest stands out.

It can be a serious issue for pulse crop growers because peas and lentils can become tied up in the weeds and even after desiccation, creating a challenging harvest. The plant is often greener than the surrounding crop, potentially filling the combine’s return and adding to plugging issues in difficult conditions or forcing slow progress through harvest.

Kochia’s ability to resist Group 2 herbicides — the ALS inhibitors —including imidazolinones, sulfonanilides, sulfonylureas and others, means it has become hard to control in pulse crops.

Group 4 resistance means cereals have a serious enemy with the loss of 2, 4-D, dicamba and fluroxypyr to control the pest. Group 5 products, such as atrazine and metribuzin, won’t work on some populations. Group 6 — triazines — in some cases aren’t effective, and now Group 9 — glyphosate — no longer works.

The weed can also be resistant to multiple products in Saskatchewan, Alberta and Manitoba, where there are incidents of ALS inhibitor and glyphosate combined resistance. Groups 2 and 4 stacked resistances also occur in Saskatchewan. In Kansas and Colorado, populations resistant to Groups 2, 4, 5 and 9 in one plant have been found.

Like humans, the plant is a serial out-crosser, meaning that each of a mature plant’s 25,000 seeds are individuals with unique characteristics, accommodating multiple mutations and future natural selection to a variety of perils.

Kochia formally called kowhai scoparia, is also known as summer cypress, burning bush and goosefoot.

In drought, roots will extend as far as three metres into the soil, hunting water and nutrients. It is capable of growing up to two metres high when competing with tall crops such as corn or durum.

When small, the weed has an erect, multi-branched stem and can have purple stripes. The hairy, grey-green leaves look dusty.

Kochia flowers lack petals and are made up of small, green sepals. Each of the tiny flowers becomes a brown, oval seed with a groove on the sides. The burning bush term comes from plants that turn bright red after seed-set.

There is some good news. Kochia’s seeds have short lives in the soil, so control can be achieved in a single year, either through control of germinated seeds or through seed degradation. Seeds germinate quickly in the spring, and seedlings are cold tolerant.

Just 21 plants per sq. metre reduce wheat yields by one-third. The plant can be devastating in flax and pulse crops, choking out broadleaf crops for sun and moisture.

Kochia has evolved to survive drought, salt, cold temperatures, competition and now many popular herbicides. | Mike Raine photo

As a livestock feed, the plant is high in protein and carbohydrates and produces well under the toughest conditions.

It does contain saponins, alkaloids, oxalates and nitrates in amounts that can be toxic to livestock, so it must not exceed 50 percent of the diet.

Due to the Group 2 resistance, using other groups of broadleaf herbicides is critical to its control. Multiple mode of action products or tank mixes are important to any control strategy.

Glyphosate, when applied pre-harvest, gives good control of sub-mature plants and seeds, other than those that are tolerant.

In cereals and flax, bromoxynil with MCPA can be effective or tank mixed with other products. For flax, Fortress is an option. However, applications need to be made as early as the crops can stand the herbicide.

Adding tillage and mowing are also effective should the pest get away from herbicides.

The tiny kochia seeds can end up in and are tolerated to some degree in most seed lots. Luckily, there are a wide variety of herbicides to control the pest in cereals.

In peas, solutions are harder to come by. Edge and Authority, with or without Charge, are the label options for pre-seeding, soil applied herbicide. Clearfield options in lentils provide control, and Edge can be used in fall. Herbicide tolerant canola is an effective strategy for that crop. Edge can used with canola for glyphosate tolerant hybrids.

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