Genetic studies of reproduction in four species of domestic animals

The objectives of this study are to determine the most effective technique of assessing reproductive performance, to analyse factors influencing the performance and the problems involved in analysing data from records of performance from commercial farms, and to make recommendations for improving the reproductive performance, in meat-producing species of farm animals using genetics methods. The study consists of a review of the present status of knowledge on the genetics of reproduction, a series of analyses of reproductive performance using records of performance of three meat-producing species of farm animals (beef cattle, pigs and sheep) and one species of laboratory mammal (mice) and concludes with some suggestions of steps that may be taken for recording performance and improving breeding programs in meat producing farm animals to raise reproductive performance. The review discusses the concepts of reproduction and growth, identification and measurement of reproductive traits, factors affecting reproductive performance and alternative strategies for improving the level of reproduction in domestic animals. Reproduction is considered to be a function of fertility, survival and growth and the success of reproduction depends on the total contribution made by the sire, the dam and the progeny resulting from specific matings. Studies on the reproductive performance in beef cattle cover two chapters. The first (Chapter 2) describes an analysis of reproductive performance of beef cattle from 13 properties in Victoria. The overall data gave a mean age at first calving of 863.5 days, mean calving interval of 361 days, calving rate of 88.7%, mortality rate at birth of the calves of 4.3%, preweaning mortality of 1.5%, postweaning mortality of 0.3% and twinning rate of 1.5%. At this level of performance the rate of reproduction of beef cattle from commercial properties in Victoria is better than that in northern Queensland and comparable if not better than those reported in the United States. The heritability estimates of reproductive traits calculated from the records of performance in many cases gave values beyond the theoretical limits (from 0 to 1). This was attributed to confounding effects of management practices. The next chapter (Chapter 3) consists of studies on the effects of crossbreeding between British breeds of beef cattle (Angus, Hereford and Shorthorn) and the crossing of these breeds with Friesian sires and Friesian-cross heifers. Traits analysed include growth rates from birth to yearling age. Factors affecting these traits are examined and their heritability is estimated. Calving performance of the dams and twinning are also studied. Crossbred calves generally showed superior preweaning growth rate than the purebred calves and even better performance occurred if one of the parents was a Friesian. There were no significant differences among the crossbred calves produced by the crossbred dams and the purebred dams when both were mated to Shorthorn sires indicating that in the material available maternal heterosis had little influence on growth compared to F1 heterosis as expressed in the crossbred progeny. Birthweight and postweaning growth rate generally were moderately heritable but preweaning growth rate generally gave negative estimates of heritability indicating greater influence of environmental variations on the latter trait. Data obtained from crossbred progeny tended to give lower estimates of heritability than those from the purebred progeny. It was not possible to analyse traits related to fertility and postnatal survival because of imcomplete recording of performance from the farm. The analysis of reproductive performance of Corriedale sheep (Chapter 4) gave an overall conception rate of 91%, lambing rate of 112%, twinning rate of 37%, mortality of lambs at birth of 5%, mean period between start of mating and lambing of 166 days and lambing interval of 369 days. This level of reproductive performance is slightly higher than generally reported by other workers for various breeds of sheep. Selection for wool and twinning made in the flock did not seem to have produced any effect on reproduction. Ewes born twins were lighter at birth, had high rate of mortality, lower rate of preweaning growth and wool production, had a lower rate of conception, rate of lambing and rate of twinning at first mating, and reached their peak level of reproductive performance at an earlier age than those born as singles. Females born co-twin to a male in the same litter showed effects of being at a disadvantage in their prenatal and preweaning development. Preweaning body growth had fairly high estimates of heritability while fertility of the dam had very low estimates of heritability. Measurement of overall reproductive performance gave the highest estimate of heritability suggesting greater accuracy of this measurement as a measure of reproductive performance of female ruminants. The analysis of reproductive performance in pigs gave an overall estimate of 78.8% success at mating, total litter-size at birth of 9.4 of which 6.5% were stillbirths, total litter-weight of 12.2 kg., average weight of piglet of 1.43 kg. and gestation length of 115 days. Except for the rather low conception rate, the level of reproductive performance is comparable to those reported for various breed in Britain, Canada and the United States. The reproductive performance was mainly affected by parity of sow while traits related to fertility of sows and survival of progeny were improved by using crossbred dams. Development of a new breed through selection for high litter-size did not result in significant improvement in reproductive performance. The analysis also indicated that there was little to be gained by having a litter-size of over 12. Most of the reproductive traits had low estimates of heritability except for average conception rate of the individual boar which was shown to be moderately heritable and should therefore be included in a selection program. The analysis of the effect of selection for high litter weight at 9 weeks of age in two lines of mice when compared with a control line indicated that there was very little direct or correlated response except perhaps for some initial improvement in the average bodyweight of the individuals selected. On the other hand there was an overall decline in production towards the end of the selection culminating in the extinction of both the selected lines. This was associated with a rapid rise in inbreeding coefficient in the small populations. Heritability estimates of traits related to litter production in the control line indicated that these traits have low heritability, particularly if measurement was based on litter-size rather than on litter-weight. Estimation of heritability using data from the selected lines produced incredibly high estimates. Possible genetic implications of the results are discussed. The difficulties involved in analysing data recorded in commercial farms were discussed, particularly, the insufficiency of information being recorded in the performance records. Genetic factors influencing the various components of reproduction and the overall reproductive performance in domestic animals based on the results of the various analyses were also examined. The feasibility of applying the different methods for improving the reproductive performance was considered and some recommendations were put forward for, the improvement of reproductive performance in meat-producing species of farm animals in southern Australia. The recommendations may be summarised as the encouragement of mating of crossbred females derived from breeds of superior maternal ability and efficiency of reproduction to males derived from breeds of superior growth rate and selected for good records of success at mating