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SUMMARYPartly on the basis of their investigation and of synthetising data of the professional literature the authors have analyzed the correlation and effects of the body size and of some other grading characters of cattle in relation to the production of products by whole populations.According to their examinations the efficiency of size of the individual production is not always characteristic for the trend in the productivity of the total population. Thus e.g. the selection aiming at a larger daily gain, which is advantageous for the feed conversion of the individual, may lead to the development of a cow stock of a large body weight, which may decrease the quantity of beef (calfs) and of milk which can be produced on the identic quantity of nutrients.Examining the relation of body size and milk-yield in populations producing under the same environmental conditions -in contradiction with most earlier examinations— within the breed there is no conspicuous correlation between the two characters according to the more recent results of researches. Similarly a substantial reciprocity -within the breed— between the body size of parents and beef-production of their progenies cannot be demonstrated either. The probable cause of this is that within the breed the different live weights are not separated genetically. On the ground of all these over a certain limit no meritorious and rational increase of beef—, nor the milk production can be expected from the increase of body size.While in relation to milk-production the influences of the individual characters (milk quantity, butterfat and protein contents etc.) on the individual and population production are relatively clarified, the situation is more intricate ( * ) Paper presented in the Study

Authors worked out a breeding scheme in order to utilize the yearly genetic progress in milk and beef production as well as "hybrid vigor" and "type heterosis". Latter term is proposed to be introduced into the terminology of animal production. This scheme which integrates milk and beef production, keeps space with the international genetic progress and attains a heterosis effect over 6 0 per cent in milk production and at the same time assures a heterosis effect (hybrid vigor as well as type heterosis) of i oo per cent in the single suckler cow stock as well as in the population for direct fattening. In this scheme pure breeding is practiced only in herds meant for producing sires of high genetic quality whereas in milk and beef production only crossbred populations are involved. The scheme is already in practice within a large scale experiment of 6 ooo cows in criss-cross breeding by using the best genetic material of USACanadian and Danish f e y sey breeds and of three types of beef breeds. In this conception breeds are no more the objects of production but the building bricks of the production systems. (TxoNtsorr, 1977 ) showing that a possible source of linkage disequilibrium may be the hitch-hiking effect of a selected locus on another closely linked neutral locus.

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