Seaweed supplement is included in diet of mammals and poultry to enhance immune response. In one embodiment, pasture forage is treated with seaweed supplement. When cattle or lambs are grazed on seaweed supplement treated endophyte-infected forage, immune function is preserved or depressed immune function is reversed. The enhanced immune function continues to the feedlot finishing phase even though no seaweed supplement is fed in that phase. In an independently inventive embodiment, seaweed supplement is administered to pigs exposed to PRRS disease to impart resistance to said disease and improve performance. In still another independently inventive embodiment, seaweed supplement is administered to lactating mares prior to weaning to mitigate the stress of weaning.