Specific Escherichia coli isolates lysogenised with prophages that express Shiga toxin (Stx) can be a threat to human health, with cattle being an important natural reservoir. In many countries the most severe pathology is associated with enterohaemorrhagic E. coli (EHEC) O157 isolates that express Stx subtype 2a. In the United Kingdom, phage type (PT) 21/28 O157 strains have emerged as the predominant cause of life-threatening EHEC O157 infections and this phage type commonly encodes both Stx2a and Stx2c toxin types. PT21/28 is also epidemiologically linked to super-shedding (>103 cfu/g of faeces) which is significant for inter-animal transmission and human infection as demonstrated using modelling studies. The importance of Stx2a for transmission of E. coli O157 between animals had not been assessed in an experimental system. In this study, wild type and isogenic strains that differed in their capacity to produce Stx2a were selected for experimental oral colonisation of calves to assess the significance of Stx2a for both super-shedding and transmission between animals. It was demonstrated that PT21/28 was excreted at significantly higher levels than a PT32 isolate, irrespective of its capacity to produce Stx2a, and therefore other characteristics of this PT must contribute to the higher excretion level. By contrast, Stx2a significantly increased animal-to-animal transmission and therefore the number of animals that can become super-shedders. In addition, we demonstrate that Stx2a-encoding prophages generally are induced faster and at higher levels than those encoding Stx2c, meaning that for strains that contain both subtypes, Stx2a is the main toxin produced. This rapid induction combined with its role in transmission are likely to account for the successful emergence of Stx2a+ E. coli isolates in cattle.