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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Date
1990
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Massey University
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Abstract
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.
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Keywords
Dairy cattle, Genetics, Breeding, Milk yield