FIG. 5.

Uterine C. abortus inoculation experiments at high and low C. abortus cohort challenge levels corroborate the influence of cohort challenge on bovine fertility. Validation experiments for the effect of cohort challenge on fertility were performed with two groups of heifers. Group A received a cohort challenge estimated to be similar to that of the first experiment. Group B received 20% of the estimated original cohort challenge. (A) Pregnancy data for Holstein heifers (n = 29) with or without cohort challenge to C. abortus. The uterine inoculum dose was either 3 × 10⁷ IFU of C. abortus strain B577 or a placebo challenge. In this experiment, the estimated relative cohort challenge of the initial experiment was duplicated with a first-round uterine inoculum of 3 × 10⁷ IFU C. abortus strain B577 administered to 15 heifers. The relative cohort challenge value was estimated as 52.7 (compared to that of the initial experiment [48.5]). (B) Pregnancy data for Holstein heifers (n = 32) with or without an estimated 20% of the relative level of cohort exposure of members of group A. The relative value of cohort exposure to C. abortus was reduced to 10.4 (approximately 20% of the cohort challenge value in the initial and the first validation experiment [experiment A]) by reducing the number of heifers (n = 7) that were uterus inoculated with C. abortus and by reducing the inoculation dose (to 6.67 × 10⁶ IFU) per heifer. The uterine inoculum dose was either 6.67 × 10⁶ IFU C. abortus strain B577 or a placebo challenge. The results of these experiments are consistent with a reduction of bovine fertility subsequent to cohort challenge. Reduction of the estimated relative cohort challenge by 80% appears to have abolished the cohort-induced depression of fertility in heifers that did not receive uterine challenge, but this cohort challenge still negatively influenced fertility in heifers that also received a uterine challenge.