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Metabolic response to ketosis mimicking challenge in dairy heifers as marker for genetic merit Binder, Heinrich


A challenge procedure was developed aimed at simulating a ketotic stress situation in dairy heifers. Parameters of the metabolic response were analyzed for their potential as metabolic markers for milk production. The challenge consisted of 5 IU insulin per 100 kg b.w. followed by an infusion of 2.0 g D(-)betahydroxybutyrate (BHB) per 100 kg b.w.. Eight blood samples were drawn within 120 minutes and analyzed for BHB, glucose and free fatty acids (FFA). A total of 125 challenge tests were performed on 93 Hoistein heifers of 6 to 13 months in 2 groups. In group 1, 26 heifers were challenged twice within 4 weeks, forming challenge series 1 and series 2, respectively. In group 2, 38 heifers were challenged in series 3 and 34 heifers in series 4 which included 6 repeated heifers from series 3. BHB concentrations, immediately after the infusion, were 11.75 ±0.43, 11.92 ±0.44, 10.96 ±0.36 and 11.93 ±0.37 mg/100ml (mean±s.e.) in the 4 series. Plasma concentrations of glucose decreased for 30 to 35 minutes as well as free fatty acids for 25 to 30 minutes following the insulin injection. Additional analyses of 10 biochemical plasma compounds in a subset of 14 heifers showed that no other metabolites or enzymes changed significantly subsequent to the challenge. Weight of the heifers was the most important factor affecting response parameters. Parameters of the response in plasma concentrations of BHB, glucose and FFA were estimated using regression models fitted to the concentration curves and were analyzed for repeatability, heritability and correlations with breeding value for milk yield (BV). The repeatability of BHB and glucose response varied between the two groups but the different age structure did not systematically affect the results after correction for weight. Repeatability estimates were: Clearance of BHB 0.215 (group 1) and 0.758 (group 2); Half-life time of BHB 0.478 (1) and 0.578 (2); Volume of distribution of BHB 0.537 (1); Time to lowest concentration of glucose (t[sub min]) 0.485 (1) and 0.112 (2); ratio of clearance to t[sub min] 0.290 (1) and 0.253 (2). FFA parameters were not repeatable. The repeatability estimates were higher than 0.5 for amylase, alkaline phosphatase, creatinine, cholesterol and COT. The repeatability was zero (negative) for protein and urea. Genetic parameters were estimated using an iterative M1VQUE procedure with an additive sire model. Twenty-five sires with an average of 3.1 daughters were included. Heritabilities were estimable for clearance of BHB (h² =0.42) and minimum concentration of glucose (h² =0.47). The genetic correlation between the two parameters was r[sub g] =-0.41. Correlations between response parameters and BV were generally low and not consistent in heifers for which BV was estimated using pedigree information of expected transmitting ability (ETA) of sire (62 heifers), dam (75) or both (47). In stepwise multiple regression with ETA of sire as dependent variable, clearance (C[sub M]), the ratio of C[sub M] with time to minimum concentration of glucose (t[sub min]), biological half-life time of BHB (t[sub ½]) and minimum concentration of glucose (A[sub min]) were the regressors chosen in the best 4-variable model, accounting for 11.4 % of the variability of ETA. It was concluded that the challenge caused short-term changes similar to subclinical ketosis in plasma concentrations of BHB and glucose, but failed to provoke a generalized metabolic reaction. The repeatable parameters of BHB and glucose concentrations were considered possible indicators for the prospective performance of the heifers.

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