The effects of oral drenching with Co or vitamin B12, drenching frequency and Co via rumen bolus on plasma vitamin B12 concentration in weaned lambs

•Drenching with Co is more effective than drenching with vitamin B12.•Weekly drenching with Co maintained adequate vitamin B12 status in weaned lambs.•Weekly drenching is more effective than drenching every 2 weeks.•Rumen bolus failed to deliver the specified sustained release over a 12 week period.•Rumen bolus was as effective over the first 6 weeks as drenching with Co every 2 weeks.