Effects of metabolizable protein on intake and milk production of dairy cows independent of effects on ruminal digestion

The role of protein in food intake regulation is complex in ruminants. Previous research has shown that a deficiency in degradable nitrogen (N) could affect microbial activity and decrease intake. On the other hand, an increase in metabolizable protein content of the diet seems to stimulate food intake in lactating dairy cows. The aim of this experiment was to determine whether metabolizable protein supply plays a direct role in the stimulation of food intake. Treatments comprised two infusions of soya protein isolate (800 g/day) either into the rumen (RP) or into the duodenum (DP), which were compared with two iso-energy infusions of glucose (880 g/day) either into the rumen (RG) or into the duodenum (DG). Four ruminally and duodenally cannulated cows producing 36.5 kg/day of milk were assigned to a 4 X 4 Latin-square design with periods of 4 weeks. Duodenal infusions of protein (DP) significantly increased (P<0.05) dry-matter intake (DMI) (+ 1.9 kg/day), rate of intake (+ 8.2 g DMI per min), milk yield (+ 4 kg/day), protein content (+ 2.3 g/kg) and protein yield (+ 191 g/day) compared with the glucose infusion in the duodenum (DG). No significant effect was observed with ruminal infusion of protein (RP) compared with the glucose infusion in the rumen (RG). The protein infusions had no effect (P>0.05) on the apparent digestibility of dry matter, organic matter, neutral-detergent fibre or acid-detergent fibre and also no or only small effects on ruminal fermentation variables. Plasma concentrations of most of the essential amino acids increased significantly with the duodenal infusion of protein, whereas ruminal infusion of protein had no significant effect. It is concluded that direct supply of metabolizable protein stimulates intake independently of ruminal digestion effects.