If the calves are weaned Corid in the water is easiest, otherwise you have to drench them daily for five days, unless you want to let the cows drink it too, which won't hurt them but a greater expense. The boluses I thinkare probably long-acting sulfas, but there are a few drawbacks to this. Sulfa boluses aren't a labeled use for coccidia (the feds are getting very picky about this if there is an available product that is labeled for the intended use) and they only they are coccidiostats, not coccidiocidal like Corid, meaning that Corid kills the bugs instead of slowing them down. Static products usually need to be administered over a longer period of time to be effective against coccidia, so for it to really work you would have to re-bolus the calves several times anyway, again off-label. So I personally would go with Corid.