Unfortunately, many of these invasive broadleaves have toxins which, when consumed in enough quantity can sicken or kill livestock. Some poisonous species only need to have a few ounces consumed to sicken livestock, while the literature indicates that as much as 0.5 % of body weight, or more, must be consumed of other species. (0.5% of a 1,000 pound animal is 5 pounds).
Practicing good pasture management becomes very important during the summer. Walk your pastures and look for invasive species Learn to identify poisonous species and how to control and eliminate them. In many cases removing animals to rest the pasture, mowing, fertilizing and applying appropriate herbicides will help to keep your animals safe and prime the pasture for renewed growth this Fall.
Visit the Penn State Extension 'Poisonous Pasture Weeds' webpage for a listing of the most common poisonous pasture weeds we see here in the Northeast, and additional information.
As you walk your pastures looking for problem species, it is also an appropriate time to evaluate the pasture as a whole. If desirable species of grasses and legumes are difficult to find and you have growing weedy and bare areas, it may be time to renovate the pasture. See The Penn State Sustainable Agriculture website which has an excellent article to help you assess your pasture and take the appropriate action to meet your pasture productivity goals.