I tractor-pastured mine on heavy white dutch clover all last year, with free access to layer ration. They seemed the picture of health. Under certain conditions, clover can grow a mold or fungus that can cause toxicity to livestock, e.g., causing "slobbers" in horses. Certain types of clover like alsike can also cause photosensitivity / liver problems in equines, and perhaps chickens also have some specific reactions to different types.