THE INRA "FILL UNIT" SYSTEM FOR PREDICTING THE VOLUNTARY INTAKE OF FORAGE-BASED DIETS IN RUMINANTS: A REVIEW 1

A vast amount of data on forage "ingestibility", particularly in sheep, and on the feed intake capacity of cattle and sheep is available. However, it cannot be fully utilized in feeding practices to predict the intake of forages when they are fed alone or in forage-based diets. This is mainly because the voluntary dry matter ingested (VDMI) that is given in tables is not an adequate intake unit for diet formulation. It does not deal with the problems of intake in the same simple additive manner as with those of energy and protein feeding. In the system presented here this objective has been reached by converting the observed VDMI into a new unit: the "fill unit". By definition, 1 kg dry matter (DM) of a reference young pasture grass has a "fill value" (FV) of one fill unit both in sheep (I FUS) and in cattle (1 FUC). Its VDMI (in g/kg "Ts) amounts to 75 by the standard sheep (SS) and 122.6 by the standard lactating cow (SLC; 600 kg live weight, 17 kg milk). Each forage in the feed tables has two FV: one for sheep (75: VDMI by SS) and one for cattle (122.6: VDMI by SLC). The values range from .8 FUS or .9 FUC for some young green legumes to 2.6 FUS or 1.9 FUC for poor quality straw. Each category of animals, as defined by sex, live weight, level of production, stage of lactation or fattening, etc. has a feed intake capacity (IC) expressed by one single value in fill units (15 FUC for the SLC), regardless of the food given. The forage FV are based on extensive measured VDMI in sheep and the animal 1C on measured VDM1 in feeding experiments. The expected VDMI of a forage can be predicted easily when it is fed along (IC:FV) or with a moderate amount of concentrates (less than 30 to 40% of the diet DM). Equations have been developed to account for the interactions between forages and concentrates. The advantages, shortcomings and future improvements of the system are discussed using field observations and selected references.