ANIMAL BREEDING IN RELATION TO NUTRITION AND ENVIRONMENTAL CONDITIONS

Summary 1. The action of the gene on the characters of an animal is limited to those manifestations which are physiologically possible if the gene is to survive. Environmental conditions affecting the physiology of the animal will therefore have a selective action on those genes which give rise to characters in harmony with the environment concerned. The main problem in animal breeding is to be able to direct the course of evolution and to select genes giving rise to increased development of certain characters. 2. Where the development of a character is limited by environmental conditions it is not possible to select for genes which have the capacity of manifesting themselves in excess of these environmental limitations. If, therefore, the physiological factor limiting this manifestation of the gene is removed by placing the animal under optimal conditions for the development of the character in question, then it is possible to select for genes giving a higher level of production. 3. Owing to conditions affecting survival rate at birth, genetic control of reproduction has, under natural conditions, often become linked to variation in daylight hours. Egg production in hens depends on increasing hours of daylight and in sheep on decreasing hours, varying in degree according to the latitude of origin of the breed. Before progress can be made in the selection of genes concerned with this, the survival rate at birth has to be regulated by the control of environmental conditions. 4. Most commercially valuable characters in animals are quantitative rather than qualitative, so that it is the degree to which the character is manifest that is important. In breeding for high milk yield various physiological factors (letting down of milk, food intake before and after parturition, milk pressure, etc.) have in turn become limiting factors for further increase in yields, and further genetic selection for increased yields has not been possible until appropriate action has been taken to eliminate these limiting factors. 5. A disadvantage of the method of selection within a specially controlled environment is that the environmental conditions limit the scope of the genetic characters that can be selected for effectively. Thus selection for high milk yields in temperate climates is not directly applicable to tropical climates, for factors regulating body temperature in high air temperatures have not been selected for, and in tropical climates combination with genes for this latter character must be made before the genes for high milk production become effective. 6. In the selection of genes for meat production and growth the control of the plane of nutrition during the different phases of growth is important, for by this means control of the body proportions is obtained. The natural nutritive conditions under which the specialized breeds of pork, bacon, lard and native pigs have been produced, when repeated under experimental conditions on inbred lines, give the same types of individuals as before. Genetic selection would appear to be most effective if carried out in a nutritional environment favourable to the development of the character in question. Nutritional conditions affect post‐natal to a greater extent than pre‐natal development; on the other hand, genetic conditions are of greater importance than nutritional ones during pre‐natal development because the uterine is more stable than the external environment. 7. It is suggested that by controlling the environment in which the next generation is reared, i.e. the uterus, man could direct the future evolution not only of his domestic animals but also of himself by selection of genes within this environment.