1. A series of experiments with adult sheep were carried out in an attempt to place on a quantitative basis the generalisation that the voluntary food intake of ruminants increases with the quality of the fodder they are given.2. Methods of determining voluntary intake free of subjective bias were developed. It was shown that voluntary intake varied with a fractional power of body weight close to 0·734. The length of time necessary to establish stable intakes was 12–15 days and the number of times fresh food was offered each day had no apparent effect on intake.3. It was found that voluntary intake of long fodders was related to the apparent digestibility of their energy, increasing rapidly as digestibility increased from 38% to 70% and thereafter more slowly.4. The giving of concentrated food resulted in a drop in the voluntary intake of fodder. With high quality hay 100 g. concentrates replaced 100 g. hay. With poor quality hay, 100 g. concentrates replaced 47 g. of hay.5. The passage of three widely different hays through the gut was measured and the poorest passed most slowly. Calculations showed that the dry matter content of gut contents was the same for all three materials irrespective of their quality.6. It was shown that an increase in digestibility of 10 units in the range 40–60% resulted in considerable increases in the total amount of energy apparently digested and in equivalent increases in daily gain.7. The digested energy consumed/day/kg. W0·734 (E) can be related to voluntary intake (I) g./day/kg. W0·734 by the equation:—E=4·(I—31)which provides a rapid and easy method of assessing fodder quality under conditions of ad libitum supply.8. The results are discussed and it is shown that under ad libitum feeding conditions an increase in the nutritive value of unit feed from 50 to 55, i.e. by 10%, increases body gain by 100%.

SUMMARY1. The energy value of three hays cut at three stages of maturity was. measured by calorimetric methods and, in addition, the amounts of the hays consumed voluntarily by sheep were measured. Voluntary intake was alsa measured when 200, 500, 800 and 1,200 g. of pelleted concentrates were given.2. The metabolisable energy values of the three hays measured at the-maintenance level were 2·45, 2·26 and 2·16 kcal./g. for cuts 1, 2 and 3-respectively (cut 1 being the earliest). The net energy values for fat production were 0·96, 0·85 and 0·68 kcal./g. respectively.3. The apparent digestibility of the hays fell with increasing intake, but supplementation with concentrates increased their apparent digestibility.4. The voluntary intake of the hays given as the sole food was 70, 62 and 57 g./kg. W0·73 for the three cuts, respectively. When concentrates were given in increasing amounts, the intake of cuts 2 and 3 increased to maxima of 65 g. and 64 g./kg. W0·73 respectively and thereafter declined. With cut 1, intake of hay declined even with the smallest intake of concentrates. Voluntary intake of the hays was maximal when the protein content of the whole ration was 8·5%.5. When no supplement was given the digested energy per sheep per day supplied by cut 1 was 64% greater than that supplied by cut 3. When 800 g. concentrates were given, however, the ration including the early cut of hay provided only 2% more energy than that including the late cut.6. It is shown that if hay were given as the sole food cut 3, which produced 57% more weight of crop per acre, would provide 38% more metabolisable energy/acre and 11% more net energy (starch equivalent)/acre than would cut 1. The late cut would also enable 92% more sheep to be kept feeding to maximal appetite on an acre of produce. However, the total live-weight gain per acre would be only 25% of that obtained with thefirstcut.7. Comparable calculations have been made for the experiments in which concentrates were given. It is shown that here too optimal times of cutting can only be assessed in terms of estimates of animal production.

1. Four roughages were given ad lib. to each of eight steers. The amount consumed and the apparent digestibility of the energy of each roughage were determined together with the weight gains of the steers.2. The mean voluntary intake of dry matter was related to the mean apparent digestibility of the energy of the roughages in the same quantitative way for the steers as had been noted with sheep. Mean weight gains were proportional to the mean number of calories of energy apparently digested.3. Significant differences between individuals occurred in their voluntary food intakes, and those animals which consumed most digested it least efficiently. The variation in intake from individual to individual expressed as a standard deviation was ±7·5% of the mean.4. The quantitative similarity of sheep and cattle as far as the relationship between their voluntary consumption of food of different apparent digestibility is concerned, is contrasted with their dissimilarity with respect to fasting metabolism. It is suggested that sheep are likely to prove more efficient convertors of the energy of roughages to body gain than are cattle.Further work is clearly necessary to explore the extent of these differences.