Plasma metabolite and hormone concentrations in Friesian calves of low or high genetic merit : effects of sex and age : a thesis presented in partial fulfilment of the requirements for the degree of Master of Agricultural Science in animal science at Massey University
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Over the last 30 years the ability of dairy cows to produce milk has been improved considerably. Although this partly reflects improvements in nutrition, health and management programmes, genetic improvement through selection programmes is no doubt one of the major contibutors to this improved milk yield. In New Zealand, selection programmes based on artificial insemination started in 1955 and emphasised milk fat production. Since then (1955 - 1987), a 24% improvement in the average genetic merit of cows has been achieved (Anonymous, 1986/1987). This genetic progress appears to have contributed about 80% of the total increase in milk yield over the same period (Holmes, 1988). Animal breeding programmes for dairy cattle worldwide have been based on the principles of quantitative genetics. Although these programmes are the most reliable and accurate methods available at present, they are expensive and genetic improvement is very slow. In New Zealand, contracts are made each year by the Dairy Board with individual farmers to purchase about 150 newborn bulls (mainly consisting of Holstein - Friesian and Jersey) for testing. All these bulls are kept in the breeding centre until they are 5 years old, waiting the assessment of their daughters' performance. Thereafter, only a few of the proven bulls are used extensively in the AI scheme while the others are culled. The long generation interval involved in this progeny testing is a major limitation to high rates of genetic gain, despite the improved accuracy of selection possible (as opposed to selection on ancestry information alone). In an attempt to overcome these problems of the conventional breeding system, there is growing interest in developing new techniques which will lead to faster genetic responses. One of these techniques is to attempt prediction of genetic merit using physiological characteristics. The basic concept of this approach is that milk production reflects the net effect of numerous biochemical pathways which are under genetic control. Thus variation in genetic merit might be reflected in different plasma levels of metabolites or regulatory hormones involved in these metabolic pathways. If it were possible to identify "physiological markers" which were strongly associated with genetic merit but independent of age and sex, then the efficiency of genetic improvement of dairy cattle could be enhanced considerably by increasing the accuracy of selection and/or reducing the generation interval. Additionally, an understanding of the physiological basis of dairy merit might lead to the development of exogenous promoting agents for milk production and, in the long term, to the identification and manipulation of specific genes controlling lactational performance.
Dairy cattle, Genetics, Breeding, Milk yield