Metabolic fuels of the ruminant consist mainly of volatile fatty acids produced in the rumen by microbial digestion of dietary polysaccharides and amino acids derived from dietary protein. As almost no glucose is obtained directly from its food, any specific requirements for this sugar are met by synthesizing it from non-carbohydrate sources: principally propionate and the glucogenic amino acids.

The two main types of kale grown in Britain, marrowstem and thousandhead, both belong to the species Brassica oleracea L. Marrowstem is a thick-stemmed plant, 1–1·5 m tall, whereas thousandhead has a higher ratio of leaf to stem and is a shorter plant. Kale is sown in late spring and normally harvested from October to February, thousandhead being the more resistant to frost. The crop yields about 50 000 kg green matter per hectare (7000 kg dry matter) and is either consumed in situ or cut and carried to housed animals. Cattle are rarely fed on kale alone, but sheep may be; when kale is grazed its consumption is not closely controlled. Kale is grown widely in Britain and north Europe generally, and also in New Zealand.

Although the importance of calcium and magnesium homoeostasis has been recognized for many years the precise mechanisms by which this is achieved are still not clearly understood. Over recent years interest in blood calcium regulation has been stimulated by the discovery, isolation and synthesis of calcitonin. Its significance in normal adults, however, remains in some doubt (vide infra). In this paper is outlined our present knowledge of the endocrine factors which help to regulate the control of plasma calcium and magnesium concentration in ruminants through the operation of negative feedback mechanisms. Mention will also be made of those hormones which, although capable of altering the plasma concentrations of either calcium or magnesium, are secreted independently of these concentrations. It will be seen that in many instances the chemical similarity between calcium and magnesium is reflected in hormone effects or in hormone secretion rates. However, this is not always found to be so, e.g. the control of calcitonin secretion appears to be affected in different directions by plasma calcium and magnesium concentrations.

There has been an active interest in the toxicology of copper since the middle of the 19th century and a review by Davenport (1953) covers the early work on the hazard to man and animals of both the acute and chronic forms. Review material specific to chronic copper poisoning in farm livestock has also been published by Broughton & Hardy (1934), Eden (1940), Todd (1962) and Bull (1964).

The first description of true chronic copper poisoning in farm animals would seem to be that of Mallory (1925) who produced the condition experimentally in sheep. Beijers (1932) described similar symptoms in sheep grazing orchards which had been sprayed with a copper fungicide, and 2 years later Broughton & Hardy (1934) published their detailed experimental investigations showing the dangers of excessive copper intakes to sheep. The similarity between the symptoms described in these reports and those of ‘yellows’ or ‘toxaemic jaundice’ in Australia was recognized (Bull, 1964) and the experiments of Albiston, Bull, Dick & Keast (1940) confirmed that this naturally-occurring condition was of similar origin. The importance of chronic copper poisoning as a nutritional hazard was, therefore, fully established.

Urolithiasis is not at present an important problem for the livestock industry in the United Kingdom, but the probable introduction of more intensive systems of management may change this position. The rapid fattening of male animals with high-energy diets predisposes to formation of phosphatic calculi and obstructive urolithiasis in the feed lot system of the USA and it is a form of this type of management which is most likely to be introduced to this country. For this reason, this review will be limited to the problem of phosphatic calculus formation, although other calculi, especially siliceous forms cause economic loss in extensive areas of the world.

Mycotoxins have for long been associated with disease and death in animals, but because of the great variety of fungi present in animal feedstuffs and the detailed investigations needed to establish a causal relationship between mycotoxin and a clinical disease, few disease syndromes have been identified.

In this paper, it is proposed to consider three confirmed clinical diseases caused by mycotoxins, to review briefly their history and identification and to consider their biological effects in ruminants.

An international group (Advisory Committee on the Application of Science and Technology to Development, 1968) has stated that ‘The protein gap in the nutrition of the population of our planet is becoming a most important scientific, technological and health problem and a national and international policy issue. The Governments of developing countries should make every effort feasible to supply their populations as quickly as possible with sufficient high quality food and to develop and apply the necessary supporting science and technology in the fields of nutrition, food science and technology’.

This paper was prepared at very short notice and no written version will be published.

The metabolism of the animal is equipped to adapt to changes in both the internal and the external environment. Among internal factors are activity versus rest and sleep, and the menstrual cycle in the case of the female. Metabolism must also respond to variations in the external environment, such as heat and cold, and notably the availability of food. Metabolic adaptation to nutrient supply is of two kinds. First, there are transient physiological adaptations to the intermittent intake of nutritionally adequate meals. These short-lived adaptations account for a large part of the diurnal variations that have been observed in the protein metabolism of mammals (Wurtman, 1969). Secondly, long-term adaptive reactions occur when there is a decrease in availability of an essential nutrient in the diet. Under such circumstances, tissue constituents are lost to varying degrees from different parts of the body. It is proposed to discuss here mainly short-term physiological adaptations to variations in amino acid supply and their relevance to the needs of the body for dietary protein. Adaptive changes resulting from long-term protein deficiency are considered in the paper by Waterlow & Stephen (1969).

The earliest estimates of protein requirements, such as those of Smith (1863) and Pavy (1874) who recommended about 125 g per day for the average working man, were based on studies of the diets of individuals or of groups of subjects who were considered to be healthy and leading normal active lives. Such estimates were simply a reflection of the dietary habits of those under study, but helped to establish minimum standards of feeding in institutions. About the turn of the century, similar recommendations ranging from 118 to 150 g were made by Voit (1881), Atwater (1895) and Rubner (1903) (Fig. 1), and these came to be vigorously defended as representing not merely feeding standards but as irreducible minima below which a proper state of health and vigour could not be maintained.

This symposium is concerned with the assessment of protein nutritional status in man. The question we have to examine is whether measurements of enzyme activity are, or may be in the future, of any value for this purpose. There are several concepts which may be useful in considering this problem, such as the nature of enzymes and the regulation of their amount in the body, the effect on activity of a biochemical lesion at the cellular level, and the adaptive changes that reflect an altered balance of metabolic pathways.

In normal young men, in whom the calorie requirements are fully met, an intake of about 50 g protein per day promotes nitrogen balance, and little additional storage results even from much larger protein intakes (Galloway & Spector, 1954). Chittenden maintained that an adult who adopted the intake of 120 g per day suggested by Voit, or who advised others to do so, was ‘encouraging individual and racial suicide’ (Cathcart, 1912), and the animal experiments of Ross show that by all objective criteria, except that of body-weight, the rats whose dietary intake is restricted fare better than those allowed unlimited quantities of a diet of good quality (Ross, 1961; Ross & Bras, 1965).

Analysis of the effects of amino acid starvation in reticulocytes is comparatively simple compared with similar analysis in other tissues of whole organisms. This is mainly because of the absence of RNA synthesis in reticulocytes, but also because the bulk of the protein being synthesized is haemoglobin, a protein whose structure is completely known. The absence of RNA synthesis eliminates complications that would otherwise arise through RNA-mediated control mechanisms which in turn might mask the effects of amino acid starvation on the protein synthetic machinery in the cells (Munro, 1969). Consequently reticulocytes have been used to study the effect of amino acid starvation on the actual process of protein synthesis and assembly.

Before considering the various possible protein sources, we must make some sort of estimate of the possible protein need by—say—the end of the century. That estimate controls the amount of effort we should put into work on protein supplies and the degree of novelty that we should envisage. The estimate depends on the number of people to be fed and the amount of protein that each needs. Both quantities are uncertain: the first because it must be guessed, the second because it is hotly disputed.