The biological value of the proteins of lucerne and trefoil and of an earthnut meal, as determined by the Thomas-Mitchell method.

Grass undoubtedly constitutes the major source of protein for stock-feeding in this country, and it is a surprising fact that our knowledge of the quality of this protein is so limited as to be almost negligible. Crampton (1934) and Crampton & Finlayson (1935), in the course of investigations into the nutritive value of young pasture grass and of certain pure species, in which they used the rabbit as their experimental animal, claimed to have obtained some indication of protein quality. The cow has been used by Morris, Wright 8; Fowler (1936) in a study of the protein value of pasture grass for milk production at different periods of the growing season. Bartlett, Henry, Kon, Osborne, Thompson & Tinsley (1938) successfully employed rats in determining, by the Thomas-Mitchell method (Mitchell, 19z3-4), the biological value of the proteins in artificially dried and sun-cured pasture grass of high protein content. The same experimental procedure was used by Smuts (1938) in determining the biological value of lucerne protein for both rats and sheep. Turk, Morrison & Maynard (1934) had previously applied the method to sheep fed lucerne hay. Chibnall(1939) and Lugg & Weller (1944, 1948a, b) have used amino-acid analysis on extracted grass proteins as a means to the study of quality. The method of Osborne, Mendel & Ferry (1919), with the guinea-pig as experimental animal, has been used by Thomas & Armstrong (19jo) for the investigation of protein quality in swards and herbage species. It may be said that most attempts to assess the quality of proteins have followed one of two lines of approach, namely ( a ) a study of the nitrogen balance, and (b ) the measurement of body-weight increase under defined conditions. The first of these is exemplified by the method of Thomas and Mitchell, and the second by that of Osborne et al. (1919). In contrast to the latter procedure, which has been discussed at some length by Thomas & Armstrong ( I ~ S O ) , the Thomas-Mitchell method affords a direct means of assessing the adequacy of dietary protein for supplying the amino-acid requirements of the body, measuring both loss during digestion and the extent to which absorbed nitrogen is utilized within the body, i.e. the biological value of the protein. The guinea-pig would appear to be the ideal experimental animal for biological value work on grass; it can exist on a diet comprised wholly of grass, whether fresh or dried, or of high or low protein content. I t has so far been the experience of the authors, however, that the maintenance of guinea-pigs in thriving condition, and without loss of weight, on a diet low in nitrogen is so difficult as to be almost impossible. Of thirteen