A:   Accorning to Weeds of the Great Plains by Stubbendieck et al., marestail ( horseweed) "contains volatile oils, tannic acid, and gallic acid that may cause skin and mucosal irritation in humans and livestock, especially horses." I am unaware of any actual animal deaths caused by the plant. Its nutritional value is unknown but likely to be very low as well as unpalatable. If offered to cattle, other more desirable feedstuffs most likely will be needed to provide most of the intake and nutritional requirements of the animals.