School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemLactation in mice as a model to study genetic improvement of dairy cattle in the tropics( 1989)In chapter one of this thesis aspects of reproduction and production, mainly milk production were reviewed. Components of reproductive efficiency such as the fertility components: age at calving, service periods, days open, calving interval and non return rate; were also investigated. The physiological relationship between those traits and milk production and persistency, in both temperate and tropical environments, was also considered, as well as longevity or stayability as other alternatives for evaluating reproduction. Attention was put on first and second lactation performance. With a view towards understanding physiological aspects of reproduction and production in dairy cows, the nature of lactation in mice was reviewed from the relatively few works that have been done. The papers reviewed included the role of pre-natal and post-natal maternal effects and the relationship between littersize and milk-yield. Daily yield and lactation from a number of studies were compared. Theories of genotype-environment interaction (GM) and adaptation, as well as their implications, were reviewed in the subsequent part of. chapter 1. Evidence of GEE in dairy cattle (Friesian, Holstein or Friesian-Holstein, crossbred with Friesian, and native) in both reproductive characters and milk production throughout the tropic regions were examined particularly closely. Mating systems and selection responses in genetic improvement programmes also were reviewed. Constraints and shortcomings for the achievement of genetic progress through natural mating, AI and progeny-testing in the tropics were examined in conjunction with attainment of high milk production in the temperate countries. The use of new technology (MOET) for manipulating reproductive efficiency was also introduced in the hope that it can be used to increase breeding efficiency in the tropics. The milk-yield and reproductive . characters, of two genotypes of mice (one genotype had been selected for high early reproductive and milking performance and the other was a random bred control) were measured in the normal and a hot environment to check if GEI were present. Both genotypes had been developed in the normal temperature. Evidence of interaction was found in the second parity and only in reproductive traits, e.g. fecundity, interval between mating and birth of second parity. These results are supported by many studies of GEI for both milk-yield and reproductive characters in dairy cattle in the tropics. Milk production does not show GEI, if the environment is defined merely as temperature. The previous selection of the improved genotype had resulted in positive correlated responses in: litter size and litter weight at birth in the first parity, production characters (growth characters and preweaning weight of the litters) in both parities, and on milk production but not on persistency. The selected line did maintain its superiority across environments. However females from the improved genotype performed worse in almost all characters in the hot environment compared to the normal environment. In an analogy with dairy cattle, the high production capacity of the selected line was not exhibited in the adverse environment. Problems of genetic improvement in the tropics were discussed. They included the use of appropriate selection criteria based on . more knowledge of both physiological and genetical relationships between milk-yield and other-characters, with the aim of increasing adaptability and productivity in the target environment. Other possibilities for genetic improvement of dairy cattle in the tropics and the use of synthetic breeds for overcoming the problems in the long term were also discussed.