It is rare for cattle over two years of age to get blackleg. It is most important that calves receive vaccine. If the vaccine was given according to the label directions to cattle as calves and yearlings, then they should have good (permanent) immunity.
In special circumstances, veterinarians may recommend giving a blackleg vaccination to older cows to increase the protection they offer the calf in colostrum. Also, some choose to give the 7-way clostridial vaccines (which include blackleg) to older cows for additional protection against the other clostridial agents (such as C. perfringens).
