Previous work by this group (Metcalf et al 1996) has shown that milk protein concentration can be increased by jugular infusion of different mixtures of amino acids (AA). The current experiment was designed in part to investigate which groups of AA had the greatest effect in stimulating milk protein content.

Milk protein concentration can be influenced by dietary manipulation, but the efficiency of amino acid transfer into milk and the predictability of the response are both poor. Previous reports from this research group have demonstrated that milk protein concentration can be repeatedly increased in response to vascular infusions of essential amino acids (EAA) in animals fed on a low crude protein (CP) diet (140 g CP/kg DM) in mid-lactation (Metcalf et al., 1996). The present study was designed to compare milk protein content responses to vascular EAA infusion in animals fed a high protein diet (180 g CP/kg DM) at peak lactation.

Metabolisable energy (ME) and absorbed protein appear to be more efficiently utilised for tissue gain in ruminants fed cereal compared to forage diets but the mechanisms involved are not understood. Forage and cereal diets differ in their pattern of rumen fermentation and hence the relative proportions of absorbed nutrients such as ammonia (NH3) and VFA (Seal et al, 1991). Splanchnic bed metabolism has been demonstrated to extensively modify both the quantity and form of absorbed nutrients (Reynolds et al, 1991) and therefore is an important site for the regulation of nutrient supply to the peripheral tissues. The present study was thus designed to examine the effect of forage versus cereal diet upon splanchnic nutrient metabolism in growing cattle.