[Snoopy]

I think it works on the principle related to hens, but not too sure

i.e . Hens can either make feathers or eggs - not both.

When a animal is a baby say, they can either grow, or fight infections - the reason why
hens are given antibiotics in their water every week, is so that they grow muscle (meat) instead of fighting infections.

Therefore giving something antibiotics could help it to grow 

[Hilarysmum]

Thanks that makes sense.