Continued genetic improvement for milk production has been associated with decreased fertility in US Holstein cattle. A previous study (Ashwell et al., 2004) identified a putative quantitative trait locus (QTL) affecting daughter pregnancy rate at 54 cM on chromosome 18 in one Holstein grandsire family. The goal of this research is to determine the validity of the putative QTL using additional markers and an extended pedigree. Thirteen microsatellite markers located throughout the chromosome were genotyped in 973 animals that were descendants of the original grandsire in which the QTL was identified. Sons of the grandsire as well as six grandson and six great-grandson families of the original grandsire were selected for this study (range of 16 to 169 sons per family). In analysis of the sons using QTL Express, the same putative QTL affecting daughter pregnancy rate was detected and placed at 45 cM. In a joint analysis of thirteen of the largest families, each containing 10 or more sons, a significant QTL for daughter pregnancy rate was detected at 27 cM. QTL affecting daughter pregnancy rate was detected in two additional sub-families (Family II-5 and Family III-2), indicating a putative QTL affecting daughter pregnancy rate is most likely segregating within this pedigree. Across-family analysis also detected putative QTL on chromosome 18 affecting productive life at 35 cM, somatic cell score at 33 cM and percent difficult births at 72 cM. Analysis of individual families identified eight significant putative QTL and six suggestive putative QTL at the chromosome-wise level affecting somatic cell score, productive life, calving ease, percent difficult births, milk yield, fat yield, protein yield and fat percent. A complex pedigree analysis is underway to make full use of statistical power to refine the QTL region affecting fertility. Further verification of the QTL effects identified in this study will allow identification of positional candidate genes to be applied i.

Selection programs aiming at improving health and survival in cattle are complicated by low heritability estimates, the fact that true herd life and carcass quality is not known until the end of an animal’s life, and that many health conditions manifest late in life. Young animals are now heavily favored in breeding programs because low generation intervals accelerate the rate of genetic progress, which means selection decisions must be made before phenotypic observation of health and survival is feasible. Moreover, profitability is compromised when livestock producers raise animals that fail to produce due to health failure or that do not meet quality standards. Telomere length (TL) was hypothesized as a biomarker that could be recorded early in life, be associated with health and survival, and have higher heritability than other measures of health. Thus, our research aims were to: 1, determine associations of TL with health, wellbeing and production in Holsteins raised for dairy or beef purposes; 2, determine TL heritability, genetic variance, and genetic correlations with cow health and performance; and 3, map quantitative trait loci affecting TL and provide TL genomic predictions to industry partners and breeders. There were not significant changes made to the research plan during the project, but the timeline of the project was not met. Laboratory processing of samples was significantly delayed due to Covid along with some sample collection. TL measurements from >1100 animals across the US and Israel are available to date. TL declines modestly with age, in agreement with observations from other species. A genomic analysis was conducted using a single-step approach and TL had a moderate heritability estimate of 20% across age groups. The initial genome-wide association-analysis indicated that TL is a quantitative trait whose expression is influenced by effects across the genome. Moreover, there is a strong association of calf and dam TL at birth. Genetic relationships with health and survival were ascertained through correlations of genomic estimated breeding values (gEBV) for TL with gEBV for other traits routinely recording in national genetic evaluations. Higher TL is genetically associated with longer herd-life, a greater likelihood that cows will avoid premature on-farm death, and reduced disease incidence. The relationship appeared to be strongest when TL was measured during the first two years of life. Based on genotyping different cell types, there was evidence that maternal and colostral derived cells are present in newborns, which could bias TL measurements during the first weeks of life to a small degree. The implications of this research are that TL is a promising trait to include in multiple trait selection programs because it is heritable, available early in life, and correlated with longevity and health. Our TL reference population is currently being expanded, and genomic estimated breeding values will be disseminated to industry partners upon completion of the reference population so that they can evaluate the utility of incorporating TL into their breeding programs.

Keywords: quantitative trait locus, pregnancy rate, Bos taurus chromosome 18.

Quantitative Trait Loci (QTL) is a topic of major agricultural significance for efficient livestock production. This advanced-level textbook covers all the statistical methods that have been used or proposed for detection and analysis of QTL and marker- and gene- assisted selection in animal genetics and breeding, as well as new advances that have revolutionized the field since the first edition.