To evaluate the importance of non-linear relationships between Relative net income per day of productive life (RNI/DPL) and individual traits, 921,915 potential offspring were simulated from all possible matings of 20,487 Jersey cows and 45 active AI sires. Predicted milk yield, fat yield, and 13 linear type traits of potential progeny were used to predict RNI/DPL of all potential progeny. Five methods of mate selection and pairing were evaluated for their effectiveness in choosing mates and the amount of computer time required to choose those pairings. Results of a linear programming (LP) method were used as a comparison for the other four more easily applied methods. Two of the other four methods were not significantly (P > .01) different from the LP method. Although the random pairing method was significantly different, similarity of results, for this method indicated non-linear relationships between RNI/DPL and individual trait scores are of minor importance. A11 four methods used considerable less computer time than the LP method. Analysis of variance for predicted RNI/DPL (all possible offspring) indicated herd, dam within herd, sire, and inbreeding class to be significant (P < .01) variables in determining RNI/DPL. However the sire by dam within herd interaction did not significantly affect RNI/DPL, again indicating non-linear relationships between traits and RNI/DPL were not very important. Regressing PD's, Cl's, and their crossproducts for milk yield, fat yield, and 13 linear type traits showed the relative importance of crossproducts to be minimal in comparison to the linear effects of parental genetic evaluations. / Master of Science

Dairy farming is one of the most important agricultural industries in South Africa, and thus improving the performance of dairy cows, with respect to economically important dairy traits, would be beneficial. Selection of dairy cows has traditionally been phenotypic, but new molecular techniques have made it possible to evaluate phenotypic dairy traits at the DNA level, providing the possibility of more accurate selection. The economically important dairy traits, milk production and reproductive performance, are quantitative traits, and are therefore controlled by many genes and the environment. A number of genes have been identified that have been shown to influence economically important dairy traits, including the lep gene. This gene encodes the hormone leptin, which has been proven to regulate feed intake, energy balance, fertility and immune function. A polymorphism has been identified in the lep gene, which may be associated with economically important dairy traits. This study on a South African Jersey herd investigated the possible association of the polymorphism, RFLP-Kpn 21, with milk production and reproductive performance. The lactation records of fifty Jersey cows that completed their first lactation between 1997 and 2004 were collected, and these cows were genotyped for the RFLP-Kpn 21 polymorphism, located at exon 2 of the lep gene. This involved the extraction of DNA from venous blood, using a salting out technique. The extracted DNA was amplified using PCR primers; the reverse primer included a purposeful mismatch. The role of the purposeful mismatch was to create a recognition site for a restriction enzyme (Kpn 21), thus allowing the alleles of the polymorphism to be identified through a restriction digestion protocol. Two alleles were identified, the C- and the Tallele. The genotype of each cow was identified using PAGE. The significance of the genotype effects on the milk production traits and the reproductive performance traits were estimated using the F-statistic provided by a GLM Univariate analysis. In conclusion, no significant effect of the RFLP-Kpn 21 polymorphism was found for milk yield, butterfat and protein percentage, ICP and SPC (p > 0.05), but a possible association with lactose percentage was suggested by the statistical analysis (p < 0.05). Further investigation of South African Jersey cows will be necessary in order for conclusive results to be obtained. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2005.