This thesis described the role of raw bovine colostrum feeding programs and natural nursing practices in the transmission of Mycobacterium avium subsp. paratuberculosis (MAP), and the efficacy of commercially available colostrum replacement products in preventing MAP transmission, additional to their effect on production and longevity performance outcomes. Incidences of fecal excretion of MAP by calves following natural exposure were also evaluated.

Calves fed CR (vs MC) had a lower risk of MAP infection when the serum ELISA (HR = 0.474, P = 0.081), bacterial fecal culture (HR = 0.572, P = 0.076) or both test combinations (HR = 0.559, P = 0.056) were used to define MAP status of study cohorts, suggesting that MC could be an important vehicle by which calves become exposed to MAP within hours following birth, and that CR feeding programs may be an effective management tool for use in dairy herds in a Johne's disease control effort.

From birth-to-54 months of follow-up, risk of death (HR = 1.22, P = 0.17), culling (HR =1.01, P = 0.95), and death and/or culling (HR = 1.1, P = 0.61) event outcomes did not significantly differ between groups (CR vs MC). Similarly there were no significant differences between groups (CR vs MC) with respect to the risk of death (HR = 1.22, P = 0.46), culling (HR =1.01, P = 0.98), and death and/or culling (HR = 1.05, P = 0.85) event outcomes when only heifers that entered the lactating herd (period from first calving date to 54 months of age) were considered. Feeding CR (vs MC) had no significant effect on age at first calving (P = 0.34), number of breedings per conception in the first (P = 0.83) and second (P = 0.32) lactations respectively, and calving-to-conception intervals in the first (P = 0.7) and second (P = 0.21) lactations, respectively. Considering the milk yield outcome, feeding CR (vs MC) significantly (P = 0.02) decreased first lactation milk by 429 kg, although there were no significant effects of feeding CR (vs MC) on second lactation (P = 0.18) and lifetime milk yields (P = 0.5), respectively.

Risk of MAP infection was not significantly different between groups of calves that ingested MAP DNA positive colostrum (vs MAP DNA negative colostrum) when the serum ELISA (HR = 0.74, P = 0.65), bacterial fecal culture (HR = 0.92, P = 0.85) or both test combinations (HR = 0.82, P = 0.65) were used to define MAP status of study cohorts, suggesting lack of an added risk of MAP infection associated with ingesting MAP DNA positive raw colostrum by Holstein calves. This finding contradicted several other reports which seem to provide evidence in support of colostrum as a possible early vehicle by which calves get exposed to MAP in infected herds.

Cows that were fecal culture positive were significantly more likely to have detectable MAP in their colostrum (OR =2.02, P < 0.001) and teat skin (OR =1.87, P =0.008) compared with fecal culture negative cows with the population attributable fraction estimates for exposure for each of the latter outcomes being 18% and 19.5%, respectively.

Finally, MAP was not recovered from fecal samples collected between 1-to-90 d of age and tested using the sedimentation bacterial culture method suggesting that the calves studied did not excrete detectable levels of MAP in feces following natural exposure.