Ruminal in situ disappearance and whole-tract digestion of starter feeds in calves before, during, and after weaning.

Ten bull calves (n = 5/diet) were cannulated at 3 wk of age and used in a 2 × 2 factorial design with repeated measures over time to compare rumen and whole-tract degradability of 2 calf starter diets and to describe an in situ technique for estimating ruminal degradability of diets in calves at different ages. Calves received milk replacer and 1 of 2 starter diets through wk 7. Mean birth weight was 38.7 ± 1.3 kg. Weaning occurred in wk 8, and calves received only starter (up to 4,500 g/d) through wk 15. Starter diets were a complete pellet (PEL; 42% starch, 13% neutral detergent fiber, NDF) or texturized feed (TEX; 31% starch, 22% NDF). Portions of each diet were dried and ground through a 2-mm screen, and 1.25 g was inserted into concentrate in situ bags (5 cm × 10 cm, 50-µm porosity). Each calf received duplicate bags of each diet for a total of 8 bags/calf (2 diets × 2 time points). All bags were inserted at the time of starter feeding. Half of the bags were removed at 9 h, and the other half were removed at 24 h. After removal from the rumen, bags were rinsed, dried (55°C), and composited by diet and by calf within week for NDF, nitrogen (N), and starch analyses. This process was repeated over 3 d during wk 5, 7, 9, 11, 13, and 15. Daily starter intake and total fecal excretion were recorded during the same 3-d periods. Diets, refusals, and feces were subsampled, dried, ground, composited by calf by week, and analyzed for NDF, N, and starch content. Apparent digestibility coefficients, total intake, and fecal excretion were calculated and analyzed with a mixed models procedure. Intake and fecal excretion of all measured nutrients increased from wk 5 through wk 15 of age and were greater for calves fed TEX, whereas the proportion of dry matter (DM), N, and starch apparently digested through the total tract decreased from wk 5 to 15 and was greater in calves fed PEL. Ruminal disappearance of DM, N, and starch after 9-h incubations increased linearly with age. Likewise, DM, NDF, and N disappearance after 24-h incubations also increased. Ruminal disappearance of DM and NDF was greater for PEL than for TEX. Ruminal disappearance was estimable for DM, NDF, N, and starch. In addition, changes over time and changes due to rumen environment were clearly demonstrated. Based on these data, there is potential to design specific rations and feed processing methods for calves based on their ability to utilize nutrients.