Relationship between passage rate and extrinsic diet characteristics derived from rumen evacuation studies performed with dairy cows

A meta-analysis based on experiments with dairy cows was conducted to study the effects of extrinsic diet characteristics on passage rate (Kp) of indigestible neutral detergent fibre (iNDF). A data set was collected that included 108 dietary treatment means from 29 studies. Dietary treatments consisted of different forages supplemented with varying amounts of concentrate feed. Minimum prerequisite for an experiment to be included in the analysis was that Kp was calculated using the flux/compartmental pool method, and that live weight (LW), total and forage dry matter intake (DMI), neutral detergent fibre (NDF) and iNDF diet concentrations were given or could be estimated. Total DMI and intake of NDF (NDFI), proportion of concentrate in diet (CProp), and milk yield in the data varied from: 8.71 to 39.0 g/kg LW, 3.76 to 16.6 g/kg LW, 0 to 0.693, and 0 to 35 kg/d, respectively. A mixed model regression was used to analyse responses of fixed factors on Kp. Root mean square error (RMSE) for the models are given. The 3 best equations were: (1) Kp = 0.0159 + 0.000790 x NDFI (g/kg LW) (RMSE = 0.00295/h); (2) Kp = 0.0129 + 0.000755 x NDFI (g/kg LW) + 0.0189 x CProp (NDF basis) (RMSE = 0.0031/h); and (3) Kp = 0.00661 + 0.000938 x NDFI (g/kg LW) + 0.0220 x CProp (NDF basis) + 0.0114 x iNDF/NDF (RMSE = 0.00328/h). Equation 3 expressed on DM basis was: Kp = 0.00816 + 0.000400 x DMI (g/kg LW) + 0.00401 x CProp + 0.00908 x iNDF/NDF (RMSE = 0.00379/h). Model performance suggested that Kp is more closely related to NDFI than DMI. The positive coefficients for CProp on NDF basis and iNDF/NDF is related to the faster Kp of concentrate particles compared with forage particles and that Kp increases when NDF potential digestibility decreases.