Controlling weeds in pastures and hayfields seems like a constant task at hand. Not only do you have to battle weed control in the summer, you also have to battle winter weeds as well. The result of not controlling weeds in pastures can lead to a higher density of weeds than desired forage species which results in a loss of grass production and quality. With that being said, not only do you have to control weeds, you also need to be able to identify toxic weeds that can result in death of your cattle. Last week at a meeting, I was informed that a producer lost a cow to respiratory issues from ingesting Perilla mint, see picture below. All plant parts of perilla mint are toxic, especially the flowering structures. Dried plants in hay can be toxic, but the greatest risk is associated with consumption of fresh plant material, especially if flowers and fruit are present. Perilla mint contains ketones that cause acute respiratory distress syndrome in cattle (ARDS), also called panting disease. Treatment is often ineffective. Although these plants can occur anywhere in a pasture or feed lot, they typically favor semi-shaded environments, and are most frequently located around farm structures, edges of woods and along fence rows. Cases of poisoning from these weeds are a concern during the late summer and early fall when other grasses and forages might be in short supply and the perilla mint is flowering. Cattle will normally not feed on these toxic weeds unless there is a shortage of other feed. Therefore, it is crucial to have a ready supply of quality feed available for farm animals during this time of the year. Control of perilla mint in pastures, barn lots and forage fields is very important. It can be 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. Perilla mint is most susceptible to control during the spring months when the plants are young and actively growing. Once they come to bloom, the plants become hardy and more difficult to control. As an annual, it can be removed mechanically by taking advantage of its shallow root system. Herbicides, such as 2,4-D (low-volatile ester formulations) or tank-mixtures containing both 2,4-D and dicamba (several formulations) along with a surfactant, provide good control when applied in early spring. Herbicides containing aminopyrlid (Milestone, Grazon Next) also provide effective control, but may persist in treated hay and in manure derived from animals that were fed with treated forage or hay.