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The objective of this study was to identify QTL affecting susceptibility to Mycobacterium paratuberculosis infection in US Holsteins. Twelve paternal half-sib families were selected for the study based on large numbers of daughters in production and limited relationships among sires. Serum and faecal samples from 4350 daughters of these 12 sires were obtained for disease testing. Case definition for an infected cow was an ELISA sample-to-positive ratio >/=0.25, a positive faecal culture or both. Three families were selected for genotyping based on a high apparent prevalence (6.8-10.4% infected cows), high faecal culture prevalence (46.2-52.9% positive faecal cultures) and large numbers of daughters tested for disease (264-585). DNA pooling was used to genotype cows, with an average of 159 microsatellites within each sire family. Infected cows (the positive pool) were matched with two of their non-infected herdmates in the same lactation (the negative pool) to control for herd and age effects. Eight chromosomal regions putatively linked with susceptibility to M. paratuberculosis infection were identified using a Z-test (P < 0.01). Significant results were more rigorously tested by individually genotyping cows with three to five informative microsatellites within 15 cM of the significant markers identified with the DNA pools. Probability of infection based on both diagnostic tests was estimated for each individual and used as the dependent variable for interval mapping. Based on this analysis, evidence for the presence of a QTL segregating within families on BTA20 was found (chromosome-wide P-value = 0.0319).

Increased twinning incidence in beef cattle has the potential to improve production efficiency. However, phenotypic selection for twinning rate is difficult because of the trait's low heritability and the long time interval necessary to collect phenotypic records. Therefore, this trait and the correlated trait of ovulation rate are ideal candidates for marker-assisted selection. The objective of this study was to complete a genome-wide search for ovulation rate quantitative trait loci (QTL) in two related sire families. The families (paternal halfsib sires 839802 and 839803) were from a population of cattle selected for ovulation rate at the USDA Meat Animal Research Center, Clay Center, Nebraska. Putative ovulation rate QTL have previously been identified in the 839802 family on chromosomes 7 and 19; however, marker coverage in the original scan was not complete. This study fills the gaps in marker coverage of the earlier study by adding approximately 60 informative microsatellites to each sire family. Each family was genotyped using selective DNA pooling. Sons and daughters were included in either the high or low pool based on their estimated breeding value deviations from the mid-parent average (EBVMD) for ovulation rate. Approximately 40% (839802) and 26% (839803) of available progeny comprised the high and low pools combined. Pooled typing revealed possible associations (nominal P < 0.05) between ovulation rate and marker genotype for 11 and 15 microsatellites in the 839802 and 839803 families, respectively. Subsequent interval mapping strengthened support for the presence of an ovulation rate QTL on BTA14 (chromosome-wise P < 0.02).