Protein-energy interrelationships in growth and lactation of cattle and sheep.

Introduction Flexibility is needed in planning rations for livestock to meet the fluctuations in the economic situation, highlighted for instance by the recent scarcity and high price of protein-rich foods. Tables of requirements tend to treat protein and energy separately, whereas their joint effects on production are becoming more apparent. The ‘requirements’ concept could with advantage include that of ‘responses’, i.e. change in output with change in input of either protein or energy or both, The Agricultural Research Council (1965) made some progress in this direction, but a thorough-going analysis of the independent and joint effects of protein and energy intakes on production is still lacking. This paper sketches some of the main relationships involved. The level of energy intake affects protein utilization in the monogastric animal (Munro, 1964), and Miller & Payne (1964) distinguished three types of response curve: response to (a) increment of food at constant protein: energy ratio, (b) additional protein at constant energy intake and (c) additional energy at constant protein intake. A similar approach is required for the ruminant. Type (a) responses constitute broadly the total plane of nutrition effects and are of considerable agricultural interest (Thomson & Aitken, 1959, for sheep; Allden, 1970, for young stock generally; Broster, 1972a, for lactating cows), but a wide range of independent variation of protein and energy intakes (type (b) and (c) responses) is necessary to analyse fully protein-energy interrelationships.