3 Answers

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If the cow is a dairy cow, and her she is treated with medications, it must be discarded.  If the calf is on the cow, first few days for colostrum, then medicate and keep calf on her, too important to insure calf gets that colostrum.  After that period of time, she shouldn't be nursing, but calf fed from a pail or nurse bottle.  If that is case, then use milk from a non-treated cow to feed the calves, as you do not want them to build immunity to antibiotics.   By the way, there are also other treatments needed for an abscess, depending if it is on the foot, leg, etc.  It may be lanced, drained, and washed with surgical scrub daily.  Also some topical medications other than antibiotics may be used. 

Not if the calf is nursing. You will have to clean the abscess by hand with a sterilization formula. Or store her milk (before you start her on the antibiotic) and hand feed the calf while she is on the antibiotic.