The greater the time with the dam, the greater the pathogen load in the calf gut
We have known for some time that the longer newborn calves spend in the company of adult cows the greater the risk for scours in the next week or two of life.
In a study just published in the Journal of Dairy Science researchers analyzed fecal samples from 382 calves from 100 farms. (JDS 96:1203-1210 D. Klein, and others "Prevalence and risk factors for shedding of thermophilic Camppylobacter in calves with and without diarrhea in Austrian dairy herds.")
"To define control strategies to reduce Campylobacter in calves, we identified on-farm risk factors." One of the four risk factors identified was "time of cow-calf separation following birth."
Findings? "Herds in which calves were left with their dam for more than 1 hour had a 2.6 higher risk of being Campylobacter positive than herds in which calves were separated from the cow immediately after birth." (p<.05)(p1205). This finding held up regardless of whether or not the herd had been identified as having a calf diarrhea problem.
Bottom Line? The longer calves spend in the company of adult cows (including the dam) the higher the exposure to all pathogens (bacteria, viruses, parasites).
My rule of thumb is to let the calf remain with the dam as she licks it off. As soon as the calf can stand and starts to walk, out she comes! I feel comfortable using behavior as a guide to remove the calf rather than using a specific time.