The control of Johne's disease requires the identification of Mycobacterium avium ssp. paratuberculosis (MAP)-positive herds. Boot swabs and liquid manure samples have been suggested as an easy-to-use alternative to sampling individual animals in order to diagnose subclinical Johne's disease at the herd level, but there is a need to evaluate performance of this approach in the field. Using a logistic regression model, this study aimed to calculate the threshold level of the apparent within-herd prevalence as determined by individual faecal culture, thus allowing the detection of whether a herd is MAP positive. A total of 77 boot swabs and 75 liquid manure samples were taken from 19 certified negative and 58 positive dairy herds. Faecal culture, three different polymerase chain reaction (PCR) methods and the combination of faecal culture with PCR were applied in order to detect MAP. For 50% probability of detection, a within-herd prevalence threshold of 1·5% was calculated for testing both matrices simultaneously by faecal culture and PCR, with the threshold increased to 4·0% for 90% probability of detection. The results encourage the use of boot swabs or liquid manure samples, or a combination both, for identifying MAP-positive herds and, to a certain extent, for monitoring certified Johne's disease-negative cattle herds.