Milkweed is relatively well-known because it’s the primary food source for monarch butterfly larvae. It is, however, toxic to livestock. Dogbane, while lesser-known, is a milkweed relative that is also toxic.
There are actually over 140 species of milkweed plants that collectively make up the genus Asclepias. Only about 25 of these species grow in the United States, and 13 are found in Maryland. All milkweeds are similar in appearance, although some observable variation does exist between species. Most but not all species exude a milky white sap when broken.
Common milkweed is the most widely disseminated species found in Maryland. It is an erect, perennial plant with hairy, simple stems. The leaves are waxy-looking with a prominent white midvein and are pointed at the tip. It blooms in early summer, and the flowers are arranged in spherical clusters. Flowers are usually lavender to light pink. After flowering, common milkweed produces one to several seed pods that begins as green, then dry and split open to release its seeds. Each seed is attached to hairs that allow it to be carried with the wind.
Dogbanes are within a separate genus, Apocynum, but both milkweeds and dogbanes belong to the family Apocynaceae. Dogbane also contains a milky white sap. It looks very similar to milkweed and is often difficult to distinguish. Dogbane, however, is more branching than milkweed, and the stems are smooth rather than hairy. The flowers typically have a flatter, disc-like arrangement, whereas the flowers on milkweed are arranged in a spherical pattern.
The toxic elements in both milkweed and dogbane are cardiac glycosides. Cardiac glycosides are chemicals that inhibit function of the heart muscle by inhibiting the sodium-potassium pumps in cardiac muscle cell membranes.
The lethal dose of milkweed or dogbane, for most animals, is about 0.05% of body weight. For a cow or a horse, that’s about half a pound of plant material; for a sheep or goat, it’s just a few ounces. Fresh leaves are the most toxic, but dried leaves also contain the toxic compounds. Death from poisoning usually occurs within 12 to 24 hours of ingestion.
Signs of poisoning include rapid, weak, or otherwise abnormal pulse; depression, weakness, staggered gait, and lack of muscular control; dilation of pupils; difficulty breathing; and colic and/or bloat. If caught early enough after ingestion, a veterinarian can administer drugs that help prevent the toxic compounds from being absorbed by the gut.
Like with most toxic plants, livestock do not readily consume milkweed or dogbane unless it is the only forage available. The risk for poisoning increases when fields are overgrazed or dormant, such as during mid-summer. Perhaps the highest risk for poisoning is from contaminated hay simply because it can be difficult to tell that toxic plants are present. Hay producers should take precautions to prevent milkweed and dogbane from being present in hay. Livestock producers who feed hay should ensure they buy from a reputable hay dealer and be educated about how to select quality hay. It’s also generally advisable to ask your hay dealer what weeds are problematic in his or her field and what measures are taken to control them.
Early control is important with milkweed and dogbane because the seeds travel easily with the wind which can result in these weeds spreading quickly. Mowing can prevent the plants from going to seed, but because they have a well-established root system they will often recover. Herbicides are usually the most effective way to control milkweed and dogbane. Spot spraying with glyphosate is recommended for small areas of infestation. Larger areas may need to be treated with an herbicide selective for broadleaf weeds, such as 2,4-D. The best time to apply herbicides is during the bud stage, just before the flower blooms, which usually occurs in early or mid-summer. At that time of year, the plant has depleted its root energy stores in order to produce the flower and will readily translocate a systemic herbicide to the roots.