Transit time ultrasound measurement of portal blood flow in cattle

Portal blood flow (PF) is central to the quantitative characterisation of dietary nutrient uptake. Dilution of PAH (p-aminohippurate) is unsuited to rapidly changing flow and visceral studies often use frequent-feeding to encourage “steady-state” digestion and absorption and minimise postprandial PF variation. Such data is of limited value to understanding nutrient flux and visceral responses to conventional feeding (once/twice daily) eg. insulin levels did not differ on similar ME intakes of frequently-fed (12x2h) forage or concentrates (Reynolds and Tyrrell, 1991) whereas in steers fed once daily, insulin was higher (P<0.10) postprandially on a concentrate diet (Thorp et al., 1996). Transit-time ultrasound (TTU) gives real-time, continuous flow but the steer portal vein was regarded as anatomically unsuitable for TTU (Huntington et al., 1990) with PF of less than half those by PAH (20 vs 42ml/min/kg LW). The current study was initiated to monitor temporal PF changes by TTU in steers fed once-daily.