Saturday, September 22, 2018
   
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Pennycress, tansy mustard showing up in area fields

Increasing temperatures this spring have allowed for winter annual weeds to become more apparent in local fields and pastures.

A few days ago, a local producer asked Extension officials to identify some plant samples from a grass pasture. The two weeds were identified as field pennycress and tansy mustard.

Field pennycress (Thlapsi arvense) is a member of the mustard family (Brassicaceae) and is classified as an annual or winter annual forb. It can grow in alfalfa, pastures, gardens, small grain fields or roadsides.

This plant has a taproot system, non-hairy stems and can grow anywhere between 1 and 2 feet tall. Leaves are alternate with the lower blades shaped like spatulas, containing petioles (stalks that attach the leaf to the stem) and having leaf margins that are either entire (not toothed) or coarsely toothed.

Leaves near the top of the plant are oblong, narrower than the bottom leaves, and have coarsely toothed margins with no petioles. The leaves on the upper portion of the plant clasp the stem at the leaf base.

White flowers are produced between April and June. Seed pods appear to have wings on their margins with a small notch at the top. Approximately 2-8 seeds are produced per pod.

Small mammals and some birds can eat the seeds without experiencing any poisoning. However, field pennycress seeds may cause gastric distress in livestock. It has also been known to cause hemoglobinuria when heifers eat contaminated hay.

Since this plant can be found in areas commonly used for grazing or haying, it is important to scout for this winter annual weed during the optimal time periods. These weeds typically emerge during fall and seedlings overwinter, grow, flower and produce seeds during the spring.

If plants cannot be controlled during the fall months with herbicides, it is best to spray them before flowering in early spring. If plants have begun to flower, some herbicides can still be used, but their control may not be as good as when the plants are smaller.

Tansy mustard (Descuriania pinnata) is also a member of the mustard family that is an annual or winter annual herb found in rangeland, roadsides, cereal crops and no-till fields.

The plants have a taproot system with hairy stems (0.3-2.9 feet tall) and alternate, dark green leaves.

These leaves are deeply divided or bipinnately compound on the lower half of the stem with smaller, pinnately compound leaves towards the top of the stem.

Yellow, yellow-green or white flowers are produced between March and August. Seed pods are small and club-like that produce seeds in two distinct rows.

Forage quality is fair for sheep, but poor for cattle production. Some states have recorded tansy mustard poisoning in cattle as the plants contain neurotoxins that cause staggering, anorexia, tremors, or tongue paralysis when consumed. However, large amounts must be eaten before any of these symptoms become apparent.

Being a winter annual weed, it is important to scout for and manage this plant in the fall as plants are emerging or during early spring, preferably before flowering.

If these weeds are problematic in grass pastures, growth regulator herbicides such as 2,4-D or dicamba may be helpful to control these weeds in your operation.

When considering herbicides to control other weeds in a grass pasture, consult the label to determine if field pennycress or tansy mustard would also be controlled.

If found in a field of alfalfa, multiple herbicides could be applied depending on if the alfalfa is glyphosate tolerance or not. Herbicide efficacy against these weed species and others when applied in alfalfa fields can be found in the 2016 Guide for Weed, Disease, and Insect Management in Nebraska.

Also, if these weeds and other winter annuals were not caught in time this year and herbicides were not effective this spring, record where these weeds were the most prevalent and be sure to scout and make herbicide applications this fall when they emerge.

Sarah Schlund

UNL Extension Educator