Comparison of omasal and reticular sampling methods on ruminal nutrient outflow and digestion in lactating dairy cows
An experiment was conducted to compare omasal and reticular sampling methods on ruminal nutrient outflow and digestion in lactating dairy cows fed normal- or reduced-starch diets. Eight ruminally-cannulated multiparous Holstein cows (96 ± 8 DIM at trial initiation) were randomly assigned to a 2×2 factorial arrangement of treatments in a replicated 4×4 Latin square design with 21-d periods. Treatments were finely (F; mean particle size = 552 µm) and coarsely (C; 1270 µm) ground dry shelled corn in normal (NS) and reduced (RS) starch diets fed as TMR. The NS and RS diets contained 27 and 18% starch (DM basis), respectively, by partially replacing corn grain with soy hulls. Continuous infusion of flow markers Cr-EDTA and YbCl began on d 15. Spot samples of omasal digesta were collected 4 times daily every 2 h on d 18-20, with a 6-h interval between sampling days to represent a 24-h feeding cycle. A 250-ml digesta sample was taken from the reticulum immediately after omasal digesta collection. Indigestible NDF, determined after a 288-h ruminal in situ incubation, was used as a large particle marker and digesta samples were reconstituted using the triple-marker system. Data was analyzed using Proc Mixed of SAS. Dry matter intake was 23.2 ± 1.6 kg/d across all treatments (P > 0.43). Marker concentrations were greater in omasal samples compared with reticular samples (P < 0.001), resulting in increased (P < 0.001) estimates of apparent ruminal digestibility of NDF (39.1 vs. 37.8%), and starch (83.5 vs. 78.9%) for omasal sampling. Outflow of starch from the rumen was greater (P < 0.01) for reticular sampling (1.0 vs 0.8 kg/d), however outflow of NDF was similar (P = 0.82) between sampling methods (4.1 kg/d). Ruminal NDF digestibility was greater (P < 0.001) for RS compared to NS for omasal samples (43.4 vs. 34.9%, respectively). Unexpectedly, the diets containing C resulted in greater ruminal starch digestibility (P = 0.02) compared with F for omasal (85.0 vs. 82.2%, for C and F, respectively) or reticular samples (81.4 vs. 76.5%, for C and F, respectively). Although differences for ruminal nutrient digestibility estimates between sampling methods were observed, they were relatively minor. Therefore, reticular sampling appears to be an acceptable method to estimate ruminal nutrient outflow and digestibility.
dairy cow, omasal sampling, reticular sampling