Several key articles from Bovine Veterinarian on why parasites affect performance; and importance of parasite control prior vaccination:

1. Vaccination and Parasite Control - by Bovine Veterinarian (jan 17, 2011) -click here 

2. How and why parasites affect performance - by Bovine Veterninarian (Sept 2, 2014) -click here 

3. Effective Parasite Treatment & Monitoring - by Bovine Veterinarian (Feb 13, 2015) -click here