I'm not terribly worried about my goats eating any of these plants to the point of death or even serious illness. None of them are immediately toxic (like Japanese Yew would be) and none of them are particularly attractive to goats, either. (For fun reference: Plants that Goats Won't Eat.) So long as I continue to feed my goats a good mix of grain and hay, and provide pasture that is of the tasty varieties, they should mostly avoid the bad plants on their own. My real problem is that the weeds crowd out the grass. I want nutritious, pretty pastures.
Here are the three plants in particular which are making my pastures less than edible for the goats, and one more that amuses more than worries me.
Perilla Mint otherwise known as Beefsteak Plant - The best time to scout for and control perilla mint is late April to early June. It is very difficult to control in late summer and early fall when it also becomes the most dangerous to livestock. If control measures are not taken early, it becomes even more crucial in late summer to maintain an adequate supply of quality feed for cattle and other farm animals so they will not feed on these toxic weeds. Grazing in infested pastures should be limited during late summer when perilla mint is flowering. Avoid harvesting forages in areas infested with these weeds. Mowing perilla mint plants before seed is produced will help prevent further reproduction and spread.
Livestock poisoned by perilla mint have respiratory problems from lung lesions, with symptoms similar to pneumonia.
Creeping Charlie (also a mint) - Although it has been used as a salad green and in herbal medicines for thousands of years, the safety of Glechoma hederacea has not been established scientifically, and there is sufficient evidence to warrant caution with its use. Glechoma hederacea is toxic to cattle and horses. Glechoma hederacea is known to contain terpenoids; terpene-rich volatile oils are known to irritate the gastrointestinal tract and kidneys. The volatile oil also contains pulegone, a chemical also occurring in pennyroyal, that is a known irritant, toxic to the liver, and also an abortifacient. The total yield of volatile oil in Glechoma is less than 1/30th the concentration that of pennyroyal.
Creeping Charlie can cause respiratory distress and excessive sweating. All parts of the plant are poisonous when fresh, but less so when dry (such as found in hay).
Horse Nettle (actually a nightshade) - Carolina horsenettle is considered a noxious weed in several US states. It can spread vegetatively by underground rhizomes as well as by seed. It is resistant to many post-emergent herbicides and somewhat resistant to broad-spectrum herbicides such as glyphosate and 2,4-D. In fact, herbicide use often selects for horsenettle by removing competing weeds. It is an especially despised weed by gardeners who hand-weed, as the spines tend to penetrate the skin and then break off when the plant is grasped. The deep root also makes it difficult to remove. If you trim horsenettle in the presence of other weeds, the toxic substances from the horsenettle will cause the other weeds to die.
Toxicity is highest in green berries, followed by red or black berries, leaves, stems and roots. It is estimated that one to ten pounds of ingested plant material is fatal for horses. Some symptoms of solanine poisoning include dilation of pupils, diarrhea, loss of appetite, and loss of muscular coordination. Some other signs of poisoning are a sudden state of depression, apparent hallucinations, and convulsions.
And the amusing and easily eradicated bonus plant that Sancho likes to eat: Pokeberry -
Pokeweed (Phytolacca americana) is a commonly found weed in horse pastures and around fence lines. It grows erect, resembling a tree, and can reach up to 10 feet in height. The stem is often purple or red in color and can reach a diameter of 4 inches. The leaves are long and elliptical, growing between 12 and 20 inches long. Pokeweed produces clusters of green berries, which mature to a dark purple color. The roots are the most toxic part of the plant, but horses can also be poisoned from the leaves and stems. Pokeweed is not very palatable, so horses with access to plenty of good quality forage should avoid it.
A toxic compound, called phytolaccotoxin, can cause a burning sensation in the mouth, low grade chronic colic, and diarrhea.