PHYSIOLOGY Institute of Veterinary Medicine and Animal Sciences, Estonian University of Life Sciences, Tartu, Estonia Field Trial on the Relationship of Blood Metabolites and Body Condition Score with the Recurrence of Luteal Activity in Estonian Holstein Cows K. Ling 1,4 , A. Waldmann 1,2 , J. Samaru ¨ tel 1 , H. Jaakson 1 , T. Kaart 1 and A. Leesma ¨e 3 Addresses of the authors: 1 Institute of Veterinary Medicine and Animal Sciences, Estonian University of Life Sciences, Kreutzwaldi 1, 51014 Tartu, Estonia; 2 Institute of General and Molecular Pathology, University of Tartu, Ravila 19, 50411 Tartu, Estonia; 3 Piistaoja Experimental Station, 86801 Piistaoja, Pa¨rnu County, Estonia; 4 Corresponding author: Tel.: +372 7 313 480; fax: +372 7 313 477; E-mail: katri.ling@emu.ee With 3 tables Received for publication November 2, 2006 Summary Associations of body condition scores and blood metabolites, measured before calving and at different periods during early lactation, with recurrence of luteal activity were investigated in a 250-head commercial dairy farm during a 4-year period (1999– 2002). The study was conducted on 48 dairy cows (60 lactations) with average 305-day milk yield of 8149 kg per cow. Blood samples taken 1–14 days before calving and 1–14, 28–42 and 63– 77 days after calving were analysed for aspartate aminotransf- erase, glucose, ketone bodies, triglycerides, non-esterified fatty acids and cholesterol. Milk progesterone (P 4 ) profiles (samples collected twice a week, P 4 levels measured in whole milk by enzyme immunoassay) were used to evaluate the interval from calving to first luteal response, P 4 >5 ng/ml, and the interval from calving to first normal cycle. The MIXED procedure of the sas system was used to study the association of investigated parameters. A higher concentration of ketone bodies before calving was associated with shorter interval to recurrence of first normal cycle (P ¼ 0.007) and tended to be related to shorter interval from calving to first luteal response (P ¼ 0.071). A lower prepartum aminotransferase activity showed a tendency to be associated with shorter interval from calving to first luteal response (P ¼ 0.084).Results suggest metabolic status up to 2 weeks prepartum to be related to the resumption of postpartum luteal activity in Estonian Holstein dairy cows. Introduction Several factors and the interactions of these factors, such as nutrition, metabolic and body condition state, production, health and management affect fertility in dairy cows, as reported in a number of studies (Kruip et al., 1998; Clark et al., 2000; Mwaanga and Janowski, 2000, review). Early return to cyclicity postpartum has been considered to be beneficial to overall reproductive performance of the dairy cow, fertility has been better in cows that have displayed oestrus once or more before the start of breeding compared with cows inseminated at their first oestrus (Thatcher and Wilcox, 1973; Beam and Butler, 1999). Already in the dry period and even more so in early lactation, changes take place in the endocrine system of cows, intensifying gluconeogenesis, lipolysis and ketogenesis. To compensate the difference between energy uptake and energy requirements in a state of negative energy balance, high-producing cows intensively utilize their fat deposits. Ketone bodies (KB) synthesized in the liver in the course of incomplete oxidation of released fatty acids, particularly acetoacetic and b-hydroxybutyric acids, are essential energy sources in many tissues and save glucose (GLC) for lactose synthesis (Herdt, 2000). In addition to the blood metabolites that indicate the metabolic status of dairy cows, the body reserves of cows, expressed as the body condition score (BCS) and BCS change, are used to monitor the amount and mobilization of body adipose tissue. It has been reported that health and fertility parameters are related to the BCS at calving or BCS change during early lactation or both (Broster and Broster, 1998, review). The relationship between repeatedly measured blood metabolites and interval from calving to resumption of luteal activity has been evaluated in several studies. Taylor et al. (2003), Shrestha et al. (2005) and Mann et al. (2005) evaluated metabolite differences between progesterone (P 4 ) profile groups at specific time points post-calving; Clark et al. (2000) trial ran from calving to 10 weeks post-calving; Zurek et al. (1995) and Reist et al. (2000,2003) focused their analyses on early lactation. The objective of the present study was to relate metabolic and energetic status of the cows, measured through blood metabolites and BCS before calving and at different periods during early lactation, to recurrence of luteal activity. To our knowledge, the relationships between blood metabolites meas- ured up to 2 weeks before calving and interval to resumption of luteal activity postpartum have not yet been studied. Materials and Methods The study was conducted in a 250-head commercial tie- stall dairy farm during a 4-year (1999–2002) period. The www.blackwell-synergy.com J. Vet. Med. A 54, 337–341 (2007) Ó 2007 The Authors Journal compilation Ó 2007 Blackwell Verlag ISSN 0931–184X