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SPARTA, Wis. - Besides stunting crops and burning pastures, the drought of 2012 created another problem. It made cattle on pasture more likely to eat any poisonous plants they encountered.
Wisconsin is home to at least 21 poisonous plants that are quite common, according to University of Wisconsin-Madison Weed Scientist Mark Renz. He talked about what these plants can do and how to lessen the problem during a recent pest management update meeting at Sparta, Wis.
Just what makes a plant poisonous? Renz said it depends on the amount of the plant that's eaten, the size and species of animal, the animal's overall health, and the amount of toxin in the part of the plant that's consumed.
How do you know whether a dairy animal has nibbled on a bit of nightshade, for example? It's not always easy, Renz said.
"Symptoms can vary from an inability to perform to the fullest potential to more serious manifestations," Renz said.
These more serious symptoms can include slobbering, tremors, lack of coordination, erratic behavior, convulsions and sudden death.
What's more, the amount of toxicity can change over time. That can happen because of irregularity in ingestion - eating a little of the poisonous plant one time and then eating a bit more later.
Complicating matters more is the fact that the amount of toxin in a plant can change. There might be more poison present when environmental conditions change, Renz said.
Pasture management can affect the amount of toxin in a plant, too. The severity of poisoning can also hinge on the part of the plant that's eaten. In some plants, it's the leaves that are the most poisonous, while in others it's the stems, roots, fruit or seeds.
"Toxicity is the result of many factors that can make diagnosis and a determination of the seriousness (of the poisoning) difficult to determine," Renz said. "For simplicity, we separate poisonous plants that are common to the Upper Midwest into three categories."

Three categories
The categories are mildly toxic, moderately toxic and highly toxic. Renz said mildly toxic plants are poisonous under certain environmental or management conditions.
By contrast, moderately toxic plants can result in injury or death if they're eaten in moderate amounts - five to 25 percent. Then there are the highly toxic plants. If they make up even less than five percent of an animal's feed, these can cause serious injury or death.
Not all poisonous plants are a threat to all types of livestock, Renz said. For instance, certain species of buttercup can harm cattle, pigs, sheep and horses, while red maple affects only horses.
Renz offered lists of mildly, moderately and highly poisonous plants. Those that are mildly poisonous and affect cattle include: buttercup, bracken fern, lamb's-quarters, certain kinds of pigweed, and wild parsnip.
The list of moderately poisonous plants that can harm cattle includes eastern black nightshade, horse nettle, climbing nightshade, oaks, and saint-johns-wort. Highly poisonous plants that can harm cattle include certain species of chokecherry, black locust, cocklebur, jimson weed, some milkweeds, poison hemlock, and white snakeroot.

Conditions matter
"Fortunately," Renz said, "the concern about toxicity is often a result of specific situations. Understanding what conditions can lead to plant poisoning can help reduce the risk of harm or death in susceptible herds."
Renz mentioned seven situations when livestock might encounter poisonous plants:
• Spring's first grazing. Animals put on pasture the first time in spring will find poisonous plants that are young and palatable. If other forage, such as grasses, has not started to grow, the animals might turn to the poisonous plants. Renz said it's best to regularly control poisonous plants and not let stock into certain areas until there's enough desirable forage for them to eat.
• Limited desirable forage is available. "When animals are hungry, their selectivity decreases, and they might eat plants they'd otherwise avoid," Renz said. "Make sure adequate forage is available, especially when poisonous plants are present."
• After an herbicide application. Many weeds don't normally taste good to livestock. But, Renz said, after an application of an herbicide, the palatability can increase "dramatically." If poisonous plants are sprayed, don't let livestock graze in the area for at least 14 day. He recommended reading the herbicide label for more recommendations, and following label directions.
• After an application of nitrogen. Some plants tend to accumulate nitrates easier than others. These include pigweed, common lamb's-quarters, and common ragweed, Renz said. These plants can become toxic after an application of nitrogen, or during and after a drought. If these weeds make up at least 20 percent of the feed in a fertilized pasture, they should be controlled before grazing begins.
• Yard waste and clippings from many ornamental plants are palatable and highly toxic to livestock. Renz said to not dump yard waste or clippings in pastures or holding areas.
"This is one of the most common scenarios for livestock poisoning in the Upper Midwest," Renz said.
• New area. Animals in a new area can be susceptible to poisoning simply because they do not know which plants are there. Renz said to introduce animals gradually to a new grazing area or a newly seeded pasture, and keep an eye on them for physical and behavioral changes.
• In harvested forages. Remember that poisonous plants can end up in harvested feed like dry hay and haylage, Renz said. Trouble is, it's tough to tell whether or not toxic plants are in the feed. And, it's difficult for animals to avoid poisonous plants when they're mixed in with desirable forage.
Renz said, "A knowledge of the source of the hay (or haylage) is the only realistic way to prevent this situation."[[In-content Ad]]

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