Detection of quantitative trait loci for clinical mastitis in crossbred Holsteins in the tropics

Clinical mastitis (CM) is a major health problem causing profit loss in the dairy industry in tropical hot/humid countries. The aim of this study was to detect quantitative trait loci (QTL) influencing CM. The evaluation was carried out in two steps. First, clinical mastitis was measured and the breeding values (EBV) were estimated. Second, animals with known EBV were genotyped, and their predicted probabilities for mastitis occurrence were used in QTL analysis. Five sires of 437 Holstein crossbred cows from a commercial farm in Thailand were genotyped and the QTL affecting CM studied using a daughter design. A total of 7411 clinical mastitis records and 8624 animals in the pedigree from 1978 to 2007 were analyzed. The genetic parameters for CM were estimated using a multi-variate threshold-linear model (TLM) using Bayesian/Gibbs sampling. The results showed that setting CM data as a binary (model I) or categorical trait (model II) affected the genetic correlation between milk yields (MY) and mastitis occurrence. Thirty-eight microsatellite markers along six chromosomes (2, 4, 6, 7, 18 and 23), with an average interval of 6.88 cM, were used in the analysis. Predicted probabilities for CM occurrence scored as a binary or categorical trait were compared to find an appropriate data set for QTL detection. The QTL associated with CM (P=0.0487) using binary data was found on chromosome 4 at 2 cM close to RM188. For categorical data, suggestive QTL were found on chromosomes 2, 7 and 18 near DIK1155 (P=0.0648), BMS2258 (P=0.0590) and BMS2213 (P=0.0750), respectively. In addition, putative QTL for MY in Thai crossbred Holsteins were found on chromosomes 23 and 4 near DIK5408 (P=0.0579) and MAF70 (P=0.0420), respectively.