Actually...... either may work...or not... for a while....alot of the 'mycins' no longer offer effective treatment, as the invading bacteria's have become resistant to them.. this is a geographic issue. But , as you likely already know, a portion of the tetracylines, and mycins bind them selves to milk proteins and are expelled with little benefit to the animal anyway.
Plus, most milk replacers have sub-therapudic amounts of neomycin, and ocytetracycline in them, and is great to some extent as a preventative, but when the animal does get sick, again. the levels are so low that there is no benefit, and may infact have helped set up the invading bacteria to be resistant to these two drug types.
This of course does not mean that they will not work in many or most cases, just that resistance is out there.
What I read, SMZ, a sulfa drug, has much less resistance to it, and an added benefit is that it is considered therapudic treatment for coccidia.
we use it extensively..and it's relatively cheap to purchase in bulk. and mixes well in milk or other liquids if you don't want to bolus it...it is tasteless as well.
IF..and I am speaking theraticly here...IF ......gentamycin was still legal to give to cattle, I would use it orally, by putting 3 ml in the bottle of replacer or better yet... a scour electrolyte treatment........for a few days to cleanse the gut......start with a fresh palate so to speak... and then add yogurt or probias right after you finish the antibiotic treatment t oget the good bacteria going before some more bad boys come back to colonize it again.
Disclaimer.....I might not know what I am talking about...but it seems to work here. And I am not a Vet. nor do I play one on TV...but the wife is a physician and some of her wisdom and knowledge has been absorbed. thus........... :cboy: