Current feed evaluation systems aim to match supply and requirement for various nutrients. These systems are largely empirically based and fail to address the underlying mechanisms causing variation in feed digestion and nutrient absorption. Modeling exercises were undertaken to evaluate these mechanisms with a distinct representation of rumen, small intestine and large intestine functioning. Volatile fatty acids (VFA) are the main source of metabolizable energy and propionic acid the main glucose precursor. Their accurate estimation is a prerequisite to understanding variation in ruminant performance. Published VFA prediction methods differ in approach, type of information used, and level of detail represented. Substrate fermented (or bypassing rumen fermentation) is estimated from rates of outflow and degradation. The type of rumen VFA produced is mostly associated with the type of substrate fermented or some general dietary characteristics. But, details of rumen fermentation processes or intraluminal conditions are rarely taken into account. Also, the concepts used and presumptions made in rumen modeling efforts may restrict the possibilities to apply these representations of VFA production. The large intestine delivers a minor fraction of total VFA production (on average some 10%) but variation in hindgut fermentation is large and should be taken into account to obtain accurate estimates of the total tract VFA production. The absorption of VFA depends on the amount of VFA produced as well as on intraluminal conditions and rumen wall characteristics. Intraluminal state and VFA absorption rate are mutually dependent, rumen epithelia strongly adapt to intraluminal conditions, and intraluminal state affects rumen fermentation as the source of VFA. This means that for an understanding of the contribution of enteric fermentation to feed digestion and energy absorbed as VFA, these aspects need to be considered simultaneously.
|Journal||Journal of Dairy Science|
|Issue number||E-Suppl 1|
|Publication status||Published - 2010|