Advantages of a dynamical approach to rumen function to help to resolve environmental issues

Simulation studies demonstrated the difference between current practice (static representation) and a dynamic representation of rumen function. Where current feed evaluation indicated absent or linear responses to changes in dry matter intake (DMI) and exchange of grass and maize silage, simulated responses were essentially non-constant and non-linear. Besides diet characteristics, rumen fermentation conditions also strongly affected simulated degradability of organic matter (apparent), neutral detergent fibre (NDF), starch (apparent) and feed protein; simulated outflow of feed starch and protein; quantity of fermented organic matter (apparent), efficiency of microbial yield and rumen nitrogen (N) balance. In addressing environmental issues, the argument was made that seemingly good empirical relationships between N excretion or ammonia emission and practical values of milk urea or rumen protein balance might be useful for the purpose of legislation, but are far less useful for individual farmers with the need to respond to (new) constraints put to them by legislation, economy and control of farm management. The error made with these empirical relationships is almost as large as the difference among individual farms and, therefore, a dynamic mechanistic approach seems more promising.