FIG 1.

Experimental design. Twenty 5-month-old Holstein steers (T1 to T20) were randomly assigned to groups (A and B) and pens for four trials conducted at 6-month intervals (HS [high dose, summer], HW [high dose, winter], LS [low dose, summer], and LW [low dose, winter]). Steers within the groups were randomly assigned to pens of two, indicated by boxes. In both years 1 and 2, the summer trials began in August and the winter trials began in February. In the HS and HW trials, group A cattle were challenged orally and group B cattle were challenged rectally with 10⁹ CFU (high dose) STEC O157. In the LS and LW trials, group A cattle were challenged rectally and group B cattle were challenged orally with 10⁷ CFU (low dose) STEC O157. Cattle were pastured following clearance of STEC O157 in each trial until 1 week prior to the start of the next trial.