The apparent digestibility of crude protein by cattle and sheep of different breeds and types, both temperate and tropical, has been studied by means of an analysis of the results of feeding trials with herbages and mixed feeds in many parts of the world. No matter whether herbages or mixed feeds are used the digestibility coefficient (y) is shown to be related to the percentage crude-protein content of the feed (x) by means of an expression of the form y = a + b log x. The particular equation y= 70 log x − 5 is shown to give a reasonable fit of all the available data.The digestibility of crude protein in the feed is shown to increase very rapidly at low protein levels (from about 2 to 9%), and there after it rises more slowly as the crude-protein content increases.There is close agreement between the equations for herbage fed alone and for mixed feed which suggests that the total percentage of crude protein in the feed, irrespective of its nature, determines the digestibility of the protein.

The nutritive ratio provides a direct link between the digestible protein and digestible non-protein components of a feed, and this relationship may be used to study various aspects of food digestibility, not only in ruminants but also in non-ruminants. It may be written as Nr= %Tdn-%dig.P % dig. PWhere Nr is the nutritive ratio and % Tdn and % dig. P are the respective percentages on a drymatter basis of total digestible nutrients and digestible crude protein.Since a ratio is invariant with respect to multiplication or division by a common factor it does not matter whether the values of dig. P and Tdn are expressed as percentages of dry matter, as in common usage, or if dig. P is expressed as a percentage of Tdn (i.e. dig. Pj). The latter form is perhaps simpler to use when considering the relationship between Nr and dig. P and the above expression (1) reduces to * , = _ ™ _ -l .(2)Thus, the nutritive ratio is a function of the digestible protein only, irrespective of the form in which the latter is expressed; it is entirely independent of the nature of the animal, whether ruminant or non-ruminant, and the relationship between Nr and dig. P will always be a curve of the above form.The nutritive ratio must also be related to the crude-protein content (c.p.) of a feed if the digestible protein is found to be related to the crude protein.Conversely, there must also be a relationship between the digestible and crude-protein contents if nutritive ratio is found to be related to crude-protein content. This paper deals with the form of these relationships as derived from the results of digestibility trials with pigs and horses as well as with cattle, sheep and goats. 15 RUMINANTS (CATTLE, SHEEP AND GOATS)It has earlier been shown for these ruminants, that there is a simple general relationship between the total crude protein of a single or compound feedingstuff and its apparent digestibility coefficient (Glover, Duthie & French, 1957). This also means that there is a relationship between the crude-protein content of a feed and the amount of digestible protein. Thus, if the original general relationship is real there should be an intimate relationship between the crudeprotein content of a feed and its nutritive ratio. This was tested by plotting the values for crude protein and nutritive ratio from the data published by Schneider (1947). Only those from direct digestibility trials were used and those determined 'by difference' were excluded. Further, the 392 records for cattle (Fig. 1 a) were plotted separately from the 902 records for sheep and goats (Fig. 16) in case there were marked differences between them. For convenience of reproduction the few records of nutritive ratio which exceeded 50:1 are excluded from the figures; thirteen were not included in Fig. l a and fifteen in Fig. 16. These records were, however, included in the calculation of the relevant curves.Both figures confirm that the relationship is substantially of the form of log Nr = a + b log c.p., and the relevant regressions are Cattle log Nr= 2-61 -...

The apparent digestibility of crude protein by the non-ruminants, pigs, horses, rats, man and the rabbit, is shown to be related to the crude protein content of the feed, and the form of the relationship is similar to that for ruminants. With non-ruminants the apparent digestibility of crude protein is markedly depressed by the crude fibre content of the feed, whereas with ruminants the depression is only slight. The relevant equations show that pigs are much more sensitive to crude fibre than horses and rabbits, and both the latter react more markedly to crude fibre than do the ruminants.Despite the apparently significant differences between the equations for the ruminant and nonruminant herbivores, it is shown that over the normal range of crude protein and crude fibre content in feedingstuffs suitable for herbivores, the apparent digestibility coefficient of crude protein is similar for all. In other words, despite different abilities to cope with crude fibre, the herbivores as a class digest crude protein in normal feeds to much the same extent. On the other hand, the pig, an omnivore, is shown to be very markedly affected by the crude fibre content of such feeds.

The role of crude fibre in modifying the general equation relating the digestibility coefficient of crude protein to the percentage of crude protein in the dry matter of a feed is described.It is shown that for normal average food-stuffs the effect is slight and that for all practical routine purposes the original equation will provide satisfactory estimates of the digestibility coefficient when only the crude-protein content of the dry feed is known.On the other hand, when exceptional feeds are encountered, particularly those in which the crudefibre/crude-protein relationship is abnormal, then more accurate estimates of the digestibility coefficient of crude protein can be obtained from the modified equation which is described above.

It is shown that reasonable estimates of the average amounts of total digestible nutrients and gross digestible energy of ruminant feeds can be derived from knowledge of only the crude-protein and crude-fibre contents of such material as fed. These estimates do not appear to be markedly affected, if at all, by differences in class of feed or species of ruminant whether cattle, sheep or goat.A table of the average estimates of digestible crude protein and total digestible nutrients, to be expected at different levels of crude protein and crude fibre, is presented for use in the estimation of the average nutritive value of feeds whose digestibility has not been studied in detail by means of numerous animal trials.Inherent in the relationship between either total digestible nutrients or gross digestible energy and the proximate partial composition of the feed is a suggestion that if a low plane of protein nutrition is prolonged, that is when the crude protein of the dry diet lies persistently below some 5% in the dry feed, there is likely to be a sharp fall in the total digestibility of the feed.