Genetic techniques have become increasingly prevalent in livestock breeding, associated with new types of knowledge-practice and changes in the institutional and geographical relationships related to animal husbandry. This paper examines the value of Foucault's concept of 'biopower' to theorising livestock breeding and the implications of the rise of genetic knowledge-practices in agriculture, developing the concept to apply to nonhuman animals and to situations where humans and nonhuman animals are co-constituted through particular knowledge-practices and corporeal meetings. It focuses on the idea of 'population' as a central component of biopower, and relates this to conceptualisations of biosocial collectivity. Reacting to the inherent humanism of Foucault's outlining of biopower, the paper argues for its relevance in relation to nonhuman populations, and for heterogeneous conceptualisations of biosocial collectivity. Drawing on research with UK beef cattle and sheep breed societies, the paper explores how, in practice, populations are constructed in relation to the production of particular sorts of truths concerning, and particular modes of intervention in, the lives of nonhuman animals. It explores how heterogeneous biosocial collectivities are constituted around these interventions. The emergence of genetic techniques is shown to transform the processes constituting populations and heterogeneous biosocial collectivities, and this is discussed in terms of a new inflection of agricultural biopower associated with novel interventions in the lives of livestock animals. © Journal compilation © 2009 Royal Geographical Society (with the Institute of British Geographers).