The production of information on feed characteristics is increasing rapidly and numerous data bases are now built in various organizations (research, universities, private companies…). From all these data, several methods can be applied to predict nutritive and feeding values of resources. These methods are more or less easy to apply, accurate, repetable, costly, and there is a concern on the way to combine all this heterogenous information into a consistent frame to obtain ultimate reference values . The present feed tables, which were built through various ways, can differ largely for the referenced values of the same feed. The present communication focuses on some of the issues which appear in predicting feed values of tropical resources for ruminants. Other important aspects, such as chemical analysis, have been discussed in the plenary session.
Are in vitro and in vivo data equivalent? A first aspect adresses the level of equivalence between information from in vitro and in vivo data. In vivo digestibility of organic matter (OMD) is the key information to assess the feed nutritive value. However, it is fairly difficult and costly to carry out OMD, therefore in vitro methods are more and more frequently applied. From pooling data of the literature, it appears that in vitro and in vivo results are rather similar for feed having NDF content and OM digestibility in the ranges of 40–50%DM and 55–65% respectively. For higher NDF contents, in vivo OMD values become higher than in vitro ones with differences that can be more than 20 points. This bias is probably the consequence of the adaptation of animals to rough feed (longer transit and chewing times, more N recycling…). Consequently, in vitro data must be used cautiously to predict in vivo OMD for feeds rich in the cell wall.
Usefulness of in sacco data? A second issue concerns the usefulness of values of in sacco, or in situ, degradation of feed constituents. This method has proved to be interesting to predict protein or starch digestion in the rumen and by-pass flows of the corresponding fractions of feed. Thus, a challenge is to pool the published in situ data to extract main values allowing the building of tables including reference values of in sacco effective degradability of N, starch…. Moreover, since dietary indigestible NDF is the major determinant of OMD, the in sacco method can also be applied to predict NDF undigestibility. Comparison between in sacco and in vivo NDF undigestibility of rations is very encouraging. Thus in sacco can also be used to rank concentrate and by-product feed according to their (un)digestibility of NDF. This approach distinguishes feeds in 2 extreme groups according to their NDF undigestibility: more than 50% (cereal straws, hulls of rapeseed, peanut and sunflower…) and less than 20% (palm products, corn grain products, soybean hulls, citrus and beet pulp…). Only a few feed have an intermediary position between these two groups.
From feed to diet evaluation? A third aspect deals with the fact that tabulated nutritive values (NV) of feeds are assessed in standard conditions while, in practice, the target is to evaluate NV of diets. Diet NV is not the sum of the associated feed NV due to influences of various factors: feeding level (FL in terms of DMI%LW), percentage of concentrate (%CO), level of N supply to microbes in the rumen… Moreover for some items such as CH4, the influence of FL and %CO are interacting and complicated, demonstrating that CH4 production cannot be a tabulated feed attribute. Thus, the prediction of dietary NV from feed is a complicated task requiring response functions to key factors of not only OMD or energy digestibility, but also energy flow like CH4 and urine. Estimation and standardization of these functions of response is an important issue for the future.
From nutritive to feeding values? Feeding value is generally assessed with the level of spontaneous DOM intake per kgLW0.75, thus it is approximatively the product of DMI (kg/LW0.75) and OMD which are also mutually linked. For forages, since the DMI values referenced in tables are measured into cages, an ultimate concern is the prediction of the actual value of DMI at pasture which could be somewhat different.
In conclusion, the contexts presented demonstrate that nutritive and feed value evaluation from the numerous available data has to be rigourously conducted and must be carefully traced to allow further improvements without having to re-start from zero.