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For the Mpwapwa cattle breed in Tanzania, the efficiency of various breeding schemes including an open nucleus was investigated by model calculations. Artificial insemination and intensive recording of production are assumed to be applied in a nucleus which is the main breeding unit. As a pre-nucleus, associated herds with less intense data recording serve as the basis to screen superior cows for nucleus replacements, provide the capacity for progeny testing, and operate as bull multipliers for commercial herds.The criteria of efficiency were genetic gain and profit from selection for a dual purpose breeding objective (milk and beef) in a total population of 10 000 cows. Introducing a two breeding tier scheme through separating all recorded cows into a nucleus and pre-nucleus leads to an increase of the genetic gain rather than the profit. Further improvement is obtained by introduction of artificial insemination in pre-nucleus herds. The nucleus size should not exceed about 5% of the cow population and an optimum size of the pre-nucleus is about 15%. Opening the nucleus to replacement cows coming from the pre-nucleus affects the aggregate genetic gain very little although it can be recommended if milk yield is to be mainly improved or if the total profit is taken into account. Furthermore, the nucleus should be opened if there is little difference between the heritabilities in the nucleus and pre-nucleus and also in order to avoid detrimental effects of inbreeding and genotype × environment interaction.
For the Mpwapwa cattle breed in Tanzania, the efficiency of various breeding schemes including an open nucleus was investigated by model calculations. Artificial insemination and intensive recording of production are assumed to be applied in a nucleus which is the main breeding unit. As a pre-nucleus, associated herds with less intense data recording serve as the basis to screen superior cows for nucleus replacements, provide the capacity for progeny testing, and operate as bull multipliers for commercial herds.The criteria of efficiency were genetic gain and profit from selection for a dual purpose breeding objective (milk and beef) in a total population of 10 000 cows. Introducing a two breeding tier scheme through separating all recorded cows into a nucleus and pre-nucleus leads to an increase of the genetic gain rather than the profit. Further improvement is obtained by introduction of artificial insemination in pre-nucleus herds. The nucleus size should not exceed about 5% of the cow population and an optimum size of the pre-nucleus is about 15%. Opening the nucleus to replacement cows coming from the pre-nucleus affects the aggregate genetic gain very little although it can be recommended if milk yield is to be mainly improved or if the total profit is taken into account. Furthermore, the nucleus should be opened if there is little difference between the heritabilities in the nucleus and pre-nucleus and also in order to avoid detrimental effects of inbreeding and genotype × environment interaction.
In the design of breeding programs the consideration of open nucleus breeding systems may result in a useful alternative. The available theory dealing with these systems assumes constant parameters, but a more exact approach would take into account the possible changes in genetic variance. Numerical evaluation of formulae allowing for loss of genetic variance due to linkage disequilibrium and increase of variance due to mixing groups with different breeding values, suggests that optimum transfer rates, as predicted from the constant parameter theory, are in close agreement with the actual values; however, genetic gain is overestimated by about 20 % in common cattle and sheep nucleus breeding schemes for traits with high heritability, but less for traits with low heritability. The advantage of opening the nucleus is approximated well by constant variance theory.
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