Of the many advances in ruminant nutrition made in recent times the change from dilute roughage-based feeds to highly concentrated grain-rich diets has probably had the greatest single impact on ruminants and on their rate of production. This change has been associated with many new and unexpected nutritional problems. This article deals with the role and uses of the ionophores, the B-vitamins, buffers and branched-chain fatty acids in such concentrated production diets for ruminants. It is evident from this review that intensified animal production and factors such as the use of new drugs, feed additives and recent developments such as the greater use of bypass proteins have an important bearing on the ruminant’s requirement for nutrients. There is therefore a need to re-examine established nutrient standards and principles continually as further intensification of ruminant production will influence these requirements and interrelationships in currently unforeseen ways.