In Mediterranean and tropical areas, the specific characteristics of the plants and
herbivores are the result of co-evolutive mechanisms and the interactions between feed and
animal are more important than under temperate conditions. The biological methods available to
predict nutritive value have been proposed and developed under temperate conditions and
simulate or describe mainly feed utilisation in the rumen. In general, these methods do not produce
unique and comparable results, because they are capable of measuring either the whole rumen
digestion, or the fermentation process, or the degradation process or microbial synthesis
individually. In addition, the biological methods have different levels of simplification of the feed
utilisation process and take into account, in different manners, the level of intake, the association
between feeds, the recycling of Nitrogen (N) in the rumen and the rate of rumen passage. In order
to predict the nutritive value of tropical and Mediterranean feedstuffs, biological methods have
been compared in terms of their general and specific approach, level to robustness and sensitivity,
specific requirements and level and type of predictability. In addition, the main differences between
tropical and temperate forages are described and discussed.
