Applying a novel method for the analysis of beef ultimate pH in the detection of quantitative trait loci : a thesis presented in partial fulfilment of the requirements for the degree of Master of Applied Science in Animal Breeding and Genetics at Massey University

Abstract

Beef ultimate pH (pHU) is an economically important trait related to meat quality. Values of pHU higher than the normal 5.5 have a detrimental effect on tenderness, colour and keeping quality. The amount of lactic acid that is produced by the conversion of the glycogen stored in the muscle at time of slaughter (G0) determines pHU. A novel biochemically-based approach for pHU analysis was evaluated in the detection of quantitative trait loci (QTL) affecting this characteristic. The procedure proposed by Pleasants et al. (1999) transforms pHU to the underlying glycogen generating a new variable, named predicted glycogen(PGU). This model may overcome the limitations in pHU investigations derived from its typical skewed distribution, characterised by a peaked primary mode at 5.5 and a long tail that comprise high pHU values. In addition to PG0, G0, pHU and the logarithmic transformation of pHU (LpHU) were analysed in:a simulated back-cross involving two inbred lines based on a model including a QTL and polygenic effects influencing G0 and thus pHU; and in experimental data from a reciprocal back-cross between Jersey and Limousin implemented by AgResearch. The significance levels achieved by LpHU did not differ from pHU, indicating that there was no advantage of using this transformation.Evidence of QTL was clearer for PG0 than pHU in the simulation. A better performance of PG0 compared to pHU was observed when there were more elevated pHU values. Results from the experimental data did not confirm the superiority of PG0 in QTL detection. With the exception of one value of 6.2, pHU data obtained in the experiment were close to 5.5. It is concluded that PG0 may improve the significance in QTL searching compared to pHU when pHU include high values that lead to the typical skewed distribution. The new procedure can also be exploited in other investigations utilising pHU. Additional research work involving the characterisation of G0 and pHU is recommended to reevaluate the parameters assumed in the implementation of this innovative approach. Key words:beef, ultimate pH. muscle glycogen, quantitative trait loci