This is not entirely true, as even very good quality birds will likely undergo stress when they are transported to your pens from the farm, if the weather happens to take a change for the worse, or they are harried by predators, these high levels of stress can lead to the birds immune systems kind of shutting down, allowing illnesses to effect them more.
Therefore it is a very wise precaution to have a general madication added to the feed for about the first 10 days.
I never bothered with the flubenvet, as I treated my birds with a much cheaper but very effective alternative.
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