DHI data suggest a 45- to 70-day dry period is optimal with no loss of milk production (thus, your cow's one and a half months should not affect milk yield significantly). But, if body condition is too low, expect a loss of six pounds of milk for every one pound of body fat that was not replaced. New research with shortened dry periods suggest that a 30-day dry period could be acceptable, so there probably is little effect on milk yield. If the cow received dry cow mastitis therapy, there is a good possibility of an antibiotic residue in her milk after a month and a half. Before adding her milk to the farm bulk tank, test her milk with an antibiotic residue detection test that can be used for that antibiotic. Mike Hutjens, Extension Dairy Specialist, University of Illinois - Illini DairyNET

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