Effect of duodenal perfusion of protein on the intake of dairy cows with or without incomplete milking

The protein content of the diet modifies feed intake in lactating dairy cows, but the mechanisms are unclear. The objective of this trial was to determine whether a duodenal perfusion of protein, with or without the expected increase of milk production, plays a role in the feed intake response. Treatments were (C): control perfusion (500 g . d -1 of glucose + 145 g . d -1 of urea); (P): duodenal perfusion of soy-protein isolate (500 g . d -1 ); and (Pim): the same treatment as (P) but with milk yield limited to the control by incomplete milking. Six rumen and duodenal-cannulated cows producing 35 kg . d -1 of milk were assigned in a double 3 3 Latin square design with periods of 4 weeks and inter-periods of 2 weeks. The intake, number and size of the meals, intake duration, chewing activities, digestibility (DM, OM, CP, NDF and ADF), molar proportions of volatile fatty acids and ammonia were not significantly affected by protein perfusions (P and Pim) (P > 0.05). Duodenal protein perfusion (P) significantly increased (P < 0.05) milk yield (+ 4.7 kg . d -1 ), protein production (+ 160 g . d -1 ) and protein content of milk (+ 1.8 g . kg -1 ) in comparison to the control perfusion (T). With incomplete milking (Pim), the milk production was the same as in the control but protein content increased (+ 1.9 g . kg -1 ) and fat content decreased (-3.2 g . kg -1 ). Incomplete milking did not reduce intake, suggesting that mechanisms controlling intake and milk production were partly dissociated.