I give all my light weight salebarn calves a shot of Draxxin when they are worked. The label says it lasts 7 days but my med rep says it lasts more like 14 days. Since starting this treatment several months ago, I have not had any sick ones. A little pricey ($3.50/ $4.00 cwt) but well worth it.
In addition to the antibiotic, I give a 7 way blackleg + pinkeye and an IBR, PI3, BVD, BRSV vaccine and a wormer.
These light calves need more energy than good hay can provide. I feed them a 14% creep ration that has Bovatec (for coccidiosis). Feed this free choice for the first 2 weeks and then limit feed ( don't want them to get too fat) 1% of bodyweight for 2 more weeks along with free choice hay. After 4 weeks they are on grass only as long as there is plenty of quality grass.
Just my 2 cents worth. Hope this answers your question.
