Grassland farming on the lowlands of New Zealand is a system in which grass-clover pasture is grazed all year, management practice having as its objective close equation of animal requirements with herbage production whilst maintaining maximum sustained herbage yield from the pasture. The combination of ryegrass with white clover provides the basis for a sustained supply of high quality herbage and comparative flexibility in animal husbandry practices enables their requirements to be approximated to the seasonal pattern of herbage production. Accepting the sown species as the basis for maximum herbage production, then consideration must be given to the role of any unsown species which may replace them. Ingress of unsown perennial species is probably of greater concern than ingress of ephemerals. The reason is that their presence may prevent the sown species from recolonizing the invaded niches, thus resulting in some degree of permanent alteration to the species composition of the pasture. The presence of unsown perennial species has provoked considerable speculation about, but little proof of, their agronomic disadvantages. Assuming that the presence of these species is limiting the potential of herbage production that could be obtained from ryegrass-clover pasture ; if proven, consideration can then be given to the means by which ingress of unsown species occurs and to the means of preventing this, or alternatively to the means of pasture; if not proven, then species indicates deficiencies in the capacity of the rye-grass-clover association to fully exploit the environment.