2. Materials and methods
2.1. Sampling
Raw milk samples were collected each month from farm vats in five of the main milk collection regions of New Zealand (Northland, Waikato, Taranaki/Manawatu, Canterbury and Southland). The aim was to test 300 samples overall and approximately equal numbers from each region. Sampling began in April 2007 (end of the 2006/07 dairying season) and ceased in May 2008 (end of the 2007/08 dairying season). Each month, individual samples (250–400 ml) were collected aseptically from five randomly selected farm vats within each region. An effort was made to avoid testing any supply more than once during the study. The outcome was that 297 samples were actually collected during the study: Northland — 54 samples from 53 suppliers; Waikato — 54 samples from 48 suppliers; Taranaki/Manawatu — 60 samples from 60 suppliers; Canterbury — 64 samples from 64 suppliers; Southland — 65 samples from 65 suppliers. Only seven farm vats were sampled more than once during the entire study.