Weed of the Week: wild mustard

Most wheat crops used to have a yellow phase in late June or early July when wild mustard came through the crop.

Before herbicide tolerant canola, the weed was a yield robber and a contamination threat to canola and rapeseed growers.

The aggressive plant, formally known as Sinapis arvensis, can be found in most of the grain growing regions of the northern hemisphere.

It can be short or tall, reaching up to 100 centimetres in some cases. It has branches and leaves that are hairy, unevenly toothed and end in a rounded lobe.

Unlike canola, wild mustard’s leaves are supported by short stalks.

Branches at their base often have violet spots. The stem hairs are stiff and pointed toward the ground.

Seeds begin germination when soil temperatures reach 2 to 4 C and will continue with fresh rain and as the soil temperature rises.

Seeds prefer to start shallow in the soil, but can hang around for up to a decade, until conditions are right for it to germinate.

Seeds can survive animal feeding, which means hay from fields where the mature plant is present can help spread the weed.

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Farmers who scout for the weed early in the season should look for broad, kidney-shaped, cotyledon leaves.

The yellow flowers can be mistaken for other mustard family plants. The seedpods are also known as siliques.

It differs from other mustards by having some seeds in the flat terminal end of the seedpod.

The pods narrow toward the last third of the terminal end and are held to the plant at the other end by a short, thick piece of stalk.

The weed discourages many producers from growing mustard because of its ability to contaminate fields of tame mustard.

Contamination in canola can reach a point where the crop is graded sample, resulting in steep financial losses. The threshold is about 20 wild mustard plants per sq. metre.

A Saskatchewan farmer once won a judgment against the Saskatchewan Crop Insurance Corp. for denying his weed damage claim.

The farmer had taken an agronomist and a custom spraying contractor’s advice to apply a common herbicide that controls wild mustard in a Polish rapeseed crop.

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A significant weed population persisted despite the herbicide application and the crop was lost. The weed was identified as wild mustard, but in fact the wild mustard in the field had been killed. It was a look-alike brassica, wild radish, that survived the spray and did the damage.

Wild mustard plants act as hosts to a variety of disease-causing organisms, which can damage tame brassicas, such as canola.

It can activate spores and become infected by disease when growing in soil containing clubroot.

Herbicide tolerant canola varieties and hybrids have helped reduce the weed’s presence in those fields.

The spring annual has little or no resistance to tillage, but tillage can also disturb ungerminated seeds, causing a new crop of problems and creating other issues related to moisture loss, erosion and increased production expense.

Ten plants per sq. metre in cereals can result in a 10 to 20 bushel per acre yield loss, while 10 plants per sq. metre in flax can cut yields by up to 50 percent and 33 plants per sq. metre can reduce yields by 65 percent in peas.

The good news is that a variety of herbicides are available to control the weed in a pre-seed burnoff and in-crop situations. The only control challenges are in non-herbicide resistant brassica crops and when the weed’s growth stage is well advanced before application.

Some of the weed’s population has developed tolerance to Group 2, ALS inhibitor herbicides including ethametsulfuron-methyl, imazethapyr, thifensulfuron-methyl and tribenuron-methyl.

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As with many weeds, the most effective methods of control are multiple mode of action herbicide approaches, crop rotation and limiting seed production.