|Waghorn, Garry - AGRESEARCH GRASSLANDS, NZ|
Submitted to: Journal Dairy Science Supplement
Publication Type: Abstract Only
Publication Acceptance Date: February 25, 1997
Publication Date: N/A
Technical Abstract: Using digestion kinetics to estimate ruminal response may provide useful information for the formulation of dairy rations, but techniques must be developed that give accurate and precise kinetic parameters. The objective of this project was to evaluate the effects of the animal donor and its diet on the measurement of digestion kinetics. Four cows in midlactation were used in a balanced 4X4 Latin square design with a 2X2 arrangement of treatments. Diets containing alfalfa or corn silage with either 24 or 32% aNDF (amylase-treated NDF) were fed twice daily during four week periods. Ruminal contents were collected from each cow, blended with chilled buffer to detach bacteria, and used to inoculate flasks containing either alfalfa or corn silage. After average pH during 12 h post-feeding was determined for each cow, a second in vitro trial was conducted in each period with the pH of the in vitro system adjusted to reflect that of the donor. Kinetic parameters were determined for a single first-order model with discrete la time using nonlinear regression. Ruminal pHs were different for each donor and diet. Average pH for the standard in vitro method was between 6.46 and 6.56. Average pH of the in vitro trial in which pH matched that of the donor was 5.89, 5.76, 5.61, and 5.54 for diets AS32, AS24, CS32, and CS24, respectively. Digestion kinetics differed between forage sources and between in vitro system pH. In addition, both cow donor and its diet affected digestion kinetics. Potentially digestible aNDF decreased, fractional rate of fiber digestion decreased, and indigestible aNDF increased as the ruminal pH of the donor or diet decreased. Our results suggest that accurate modeling of ruminal pH and its affects on digestion kinetics must be a component of any dynamic model of ruminal fermentation.