Factors affecting milk yield and composition were reviewed and it was concluded that feeding level and diet type (notably unwilted pasture silage) may have independent effects on yield and composition of milk. The objective of this research was to investigate the separate effects of forage type and feeding level on milk yield and composition and to study some of the mechanisms that may effect the efficiency of utilization of DE in these diets for milk synthesis. Initially effects on milk yield and composition and nutrient utilization were studied in a series of experiments in which dairy cattle were individually fed in stalls on varying levels of forage diets. Unwilted pasture silage was compared with pasture, wilted silage, formalin treated silage, and unwilted silage supplemented with pasture, maize silage or protein concentrates. These experiments showed that both level of feeding and the type of forage diet offered to dairy cows can affect milk yield and composition. Increasing the intake of pasture, increased the yields of milk, fat, protein and lactose, and increased milk protein percentage and decreased milk fat percentage. Similar results were obtained with unwilted silage except that no relationship was found between silage intake and milk protein concentration. Cows offered unwilted pasture silage produced less milk containing a lower concentration of fat and protein than cows offered pasture at the sane intake of DE. percentage. Neither ration affected milk lactose percentage. The efficiency of utilization of DE for milk synthesis by cows fed silage was improved by either reducing protein degradation during ensiling, or by providing pasture or protein concentrates as supplements. Maize silage as a supplement had no effect. These studies indicated that the amount of protein entering the duodenum of cows offered unwilted silage was limiting milk protein synthesis. Subsequently cows were surgically prepared with abomasal cannulae to test this hypothesis. Consistent and significant increases in milk yield, milk protein concentration and milk protein yield by cows fed unwilted silage were obtained when abomasal infusions of sodium caseinate were given in a series of studies. Further studies showed no responses were obtained with abomasal infusions of glucose which indicated that the response in milk protein synthesis was due to amino acids per se. The magnitude of the responses in milk protein synthesis to abomasal infusions of casein were higher for cows offered silage in comparison to those fed pasture at similar intakes of DE. The difference in the responses of the cows on the two rations was due apparently to an inadequate supply of essential amino acids for cows fed silage in comparison with those fed pasture. Abomasal infusions of L-methionine increased milk yield, milk protein percentage and milk protein yield to the same extent as casein suggesting that methionine might be the major essential amino acid that was limiting milk protein synthesis of cows fed unwilted pasture silage.