Biological methods to predict the nutritive value of tropical and mediterranean feedstuffs: a critical review

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.