Growth curve and body weight in Göttingen minipigs - a phenotypic and genetic study

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The main focus of this thesis was the examination of a possible body weight reduction in Goettingen minipigs and the detection of an optimal selection strategy for this trait. The aims were the analysis of body weight development of the Goettingen minipig and the estimation of genetic parameters for body weight. Further, different selection strategies had to be examined to build up a new breeding scheme. As a first step, the growth pattern of the Goettingen minipig was studied. The body weight development of minipigs was investigated by modeling growth curves with different non-linear and linear functions and by comparing their body weight development with the de¬velopment of normal fattening pigs. Data of two Danish sub-populations were used. In total 189,725 body weight measurements of 33,704 animals collected from birth (d 0) to 700 d of age were analyzed. The different growth models were compared by using the Akaike s Information Criterion (AIC), whereas a small AIC value indicated a good fit. Regarding the whole growth curve linear polynomials of third and fourth order of fit had the smallest AIC values for the two sub-populations. Among the nonlinear functions the Logistic had the highest AIC value indicating the poorest fit. A comparison with fattening pigs showed that the minipigs have a nearly linear body weight development in the time period from birth to 160 d. Fattening pigs have very low weight gains in the first third of the examined time period in relation to a specific end weight. Later fattening pigs have increasing daily weight gains resulting in a growth curve that is more s-shaped than the growth curve of the minipig. After analyzing the body weight development of minipigs, genetic parameters for body weight were estimated to discover the potential for breeding on low body weight. In a first study, genetic parameters were estimated for a time period of 30 to 400 days with a ran¬dom regression model (RRM). The RRM included random animal, common litter envi¬ronment and permanent environment effects, respectively. Regressions for the random effects in the RRM were modeled using Legendre polynomials of second of fit. Eight age classes were built to consider heterogeneous residual variances. The heritabilities were moderate and ranged from 0.21 (375 d of age) to 0.25 (275 d of age). Genetic and phenotypic correlations between body weights in different age classes decreased with increasing distance between age classes, a circumstance which has to be focused on when building up a new breeding scheme. The analysis of eigenfunctions showed that a selection for low body weight has positive effects on this trait throughout the whole range of time. To get more into detail and with a focus on practicability genetic parameters were esti¬mated for a time period of 30 to 700 days to have an overview also over later ages. Further, the parameters were estimated using a multiple trait model (MTM) and a random regression model (RRM) to detect which model is more appropriate for routinely working breeding value estimation. Another aim was to find out the best selection strategy to get a genetically smaller minipig in the future. Therefore, 19,505 body weight measurements of 3,461 Goettingen minipigs were analyzed. Heritabilities were moderate with slightly higher values estimated with the RRM. Genetic correlations between body weight measurements at different ages were decreasing with increasing time lag between the measurements as it was already shown for the time period of 30 to 400 days. An operational breeding goal for relative weight reduction (RWR) was suggested in which the weight reduction in each age class is expressed as per cent of the actual body weight and is weighted according to the proportion of animals sold in this age class. Expected genetic progress was calculated for two different selection ages (80 and 150 d), whereas the selection at 150 d of age resulted in a higher genetic progress for RWR. Further, several aspects related to selection like decrease of genetic variance, marker-assisted selection or inbreeding depression were discussed in this thesis.