Watch out for Poisonous Plants this Summer

Watch out for Poisonous Plants this Summer

Mark Renz Extension Weed Scientist, University of Wisconsin-Madison

As pastures are not productive in much of Wisconsin due to the drought and the supply of hay remains low throughout the US, the potential for animal’s to eat and be poisoned by toxic plants will likely increase over the next several months.  To address this issue we will be posting a poisonous plant factsheet for livestock in Wisconsin.  In this factsheet we overview what poisonous plants are and under what conditions they can be toxic to livestock. In addition, we have provided a list of common poisonous plants found in Wisconsin along with a description of the level of toxicity and resulting symptoms from ingestion. Below I will briefly address with how to manage poisonous plants in fields animals graze as well as purchased hay.

Grazed fields

Poisonous plants: When animals are hungry, their selectivity decreases and they may eat plants typically avoided. This is common especially under drought conditions. The only options to prevent animals grazing poisonous plants in these conditions are to 1) provide ample forage so animals avoid them, 2) remove the animals from the infested pasture until ample forage regrows, or 3) control the poisonous plants in the pasture prior to grazing (recommended option). If controlling these plants with an herbicide don’t forget to follow any grazing restrictions present on the label.  If no grazing restrictions are present for the herbicide we still recommend not grazing for at least a 14-day period as often treatment can temporarily increase palatability of these plants.

Nitrate Accumulating Plants: Fields with an abundance pigweeds, common lamb’s quarters, and common ragweed are common in drought stricken pastures and other fields that could be grazed.  These weeds can result in animal toxicity as these species take up excessive nitrogen and convert it to nitrate which can be toxic. This situation is common under drought conditions we are experiencing, especially if areas were previously fertilized. If enough of these weeds are eaten animals can be poisoned from excess nitrate. Thus we recommend controlling these weeds prior to grazing when at least 20% of the feed is composed of these species.  An herbicide application is the most economical control method and several options exist that are registered for use in pastures (consult A3646: Pest Management in Wisconsin Field Crops for detailed information).

Poisonous plants in harvested forage

As demand for hay continues to be high we expect fields that typically are not harvested to be baled and sold (e.g. CRP fields).  While much of this hay will not have poisonous plants, I expect some will and will lead to livestock poisoning. While animals often avoid poisonous plants in a pasture they lose much of their ability to not select these plants when they are hayed and mixed with desirable forage.  While some plants lose their toxic properties as they dry, many remain toxic. Before buying hay I suggest inspecting several bales thoroughly to make sure that the majority of the material is palatable forage and not poisonous plants.  While all bales can’t be inspected, spot checks on select bales will avoid the potential for poisoning from species that require large amounts ingested (e.g. nightshade species). Unfortunately several species are toxic at much smaller doses (<5% of feed ingested), and these are essentially impossible to find in hay when purchased.  These include milkweed species, jimson-weed, poison hemlock, black locust and white snakeroot. In these cases the best scenario is to know the field where your feed is coming from and check to see that these weeds are absent before harvesting and or purchasing.  Otherwise the only other option is to closely monitor the ingestion of the feed and removal of forage that appears to be weedy and/or avoided by animals. While this option is far from ideal, it is the best solution we can offer in these difficult times. If animals start to show any symptoms of poisoning, contact a veterinarian as soon as possible.

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