Proceedings of the World Congress on Genetics Applied to Livestock Production, 11.827 Genome-wide association study and genetic parameter estimates for blood β-hydroxybutyrate concentrations during early lactation in pasture-grazed dairy cows V. Bonfatti 1 , S-A. Turner 2 , I. M. Macleod 3 , S. Bolormaa 3 , B. Kuhn-Sherlock 2 , C.V.C Phyn 2 ,& J. Pryce 3,4 1 Università degli Studi di Padova, Dipartimento di Biomedicina Comparata e Alimentazione, Viale dell’Università 16, 35020 Legnaro, Italy valentina.bonfatti@unipd.it (Corresponding Author) 2 DairyNZ Ltd., Private Bag 3221, Hamilton 3240, New Zealand 3 Department of Economic Development, Jobs, Transport & Resources, 5 Ring Road, 3083 Bundoora, Australia 4 La Trobe University, 5 Ring Road, 3083 Bundoora, Australia Summary This study aimed to identify genomic regions associated with hyperketonemia, and to estimate genetic parameters for β-hydroxy-butyrate (BHB) concentration in blood in pasture- grazed, early-lactating dairy cows. Blood samples were collected from 876 cows, 3 times per week for the first 5 weeks of lactation, and BHB concentration was measured using a cow- side device. A genome-wide association study, performed using a total of 37,309 SNP markers, revealed no associations with the average, maximum and variance of BHB concentration in any wk of lactation, the proportion of events with BHB ≥ 1.2 mmol/L (positive event), the d of lactation corresponding to the first positive event, and the occurrence (yes/no) of hyperketonemia positive event. However, the majority of traits exhibited genetic variation, with values of heritability ranging up to 0.34 (for the average BHB concentration in the 3 rd wk of lactation). Genetic variation in blood BHB concentration might be exploited in future breeding programs aiming at improving the metabolic health of dairy cows during early lactation. Keywords: hyperketonemia, β-hydroxybutyrate, heritability, GWAS Introduction Hyperketonemia (HYP) is defined as an abnormally high concentration of circulating ketone bodies, mainly β-hydroxy-butyrate (BHB), and can occur during extreme negative energy balance during early lactation in dairy cows (Duffield et al., 2009). A circulating BHB concentration of 1.2 mmol/L or greater is the generally accepted reference threshold to diagnose HYP, and is associated with increased risk of subsequent displaced abomasum, metritis, and clinical ketosis, as well as decreased probability of pregnancy at first insemination and decreased milk production (Duffield, 2009). A possible strategy to reduce the risk of hyperketonemia is through selective breeding. However, estimates of genetic parameters for BHB concentration in blood are scarce, because of the difficulty of regular sample collection on large numbers of cows. To date, limited attempts have been made to calculate genomic predictions for various health disorders and the