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Predicting energy balance in pre-partum dairy ewes by ultrasound measurements of backfat and longissimus dorsi thickness
Termatzidou, S.A.; Siachos, N...
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Objective: The objective of this study was to investigate the association of ultrasound measurements of backfat thickness (BFT) and longissimus dorsi muscle thickness (LDT) with energy balance in pre-partum dairy ewes. The aim is to iden- tify practical, on-field applicable and non-invasive methods to assess energy status.
Material and methods: The study was conducted in four commercial dairy sheep flocks, each keeping one of the four main breeds reared in Greece. Fifty purebred ewes were pre-selected from each flock, based on ultrasound pregnancy diagnosis at day 60 after ram introduction; a total of 177 ewes (Chios, n=46; Frizarta, n=40; Lacaune, n=42 and Assaf, n=49), 3 to 5 years old, that remained pregnant and clinically healthy until lambing, were enrolled in the analysis. Body condition score (BCS) was assessed by palpation in the lumbar region and ultrasound measurements of BFT and LDT thickness were performed using a 5-MHz linear transducer, both at -30 days (-30d) and -15 days (-15d) prepartum.
The probe was placed perpendicular to the vertebral column between the transverse processes of the 3rd and 4th lumbar vertebrae. Each time, three measurements were performed and the mean val- ue (in mm) was calculated. The sum of BFT and LDT (TO- TAL_T), the LDT: BFT ratio (RATIO_T) on each timepoint and the difference in BFT (Δ_BFT) and LDT (Δ_LDT) between the two measurements (-30d minus -15d) were also calculated.
A blood sample was collected at -15d from each ewe and serum was separated within 2h of collection by centrifugation (15 min × 2500g) and stored at -20oC until analysis. Beta- Hydroxybutyrate (BHB) and non esterified fatty acids (NEFA) were measured with an automated chemistry analyzer (Vitalab flexor E., Vital Scientific N.V., Netherlands), using commercially availa- ble kits (BHB: Ben biochem. enterprise, Milano, Italy; NEFA: Randox Laboratories Limited, UK). Negative energy balance status was defined as serum BHB >0.8 mmol/L or NEFA >0.3 mmol/L and treated as a binary variable. The receiver operating characteristic (ROC) analysis was used to define thresh- olds for fat and muscle reserves and mobilization parameters to predict elevated BHB or NEFA status.
All variables with a significant area under the ROC curve were then entered in a binary logistic regression as categorical variables (using the thresholds obtained from the ROC curves) to assess the association of fat and muscle reserves and mobilization parame- ters with BHB and NEFA status.
Results: Δ_BFT and TOTAL_T(-15d) were significant predictors of BHB status (R2=0.272). Ewes with a BFT loss had 6.7 times (P<0.001) increased probability to have serum BHB >0.8 mmol/L; probability increased quadratically as Δ_BFT increased (R2=0.486; P<0.001). Moreover, ewes with TOTAL_T(-15d) ≤25.6 mm, which corresponds to an average BCS of 2.25-2.50, had 5.3 times (P<0.001) increased probability to have serum BHB >0.8 mmol/L; probability decreased quadratically as TOTAL_T(-15d) increased (R2=0.425; P<0.001). Δ_BFT and BFT(-30d) were significant predictors of NEFA status (R2=0.316). Ewes with Δ_BFT >0.65 mm had 3.2 times (P=0.002) increased probability to have serum NEFA >0.3 mmol/L; probability increased linearly as Δ_BFT increased (R2=0.441; P<0.001). Moreover, ewes with BFT(-30d) >6.86 mm, which corresponds to an average BCS of 2.75-3.00, had 6.7 times (P<0.001) increased probability to have serum NEFA >0.3 mmol/L; probability increased quadratically as BFT(-30d) increased (R2=0.915; P<0.001).
Conclusion: Ultrasound measurements of BFT and LDT during the last month of gestation can predict quite accurately the energy status of dairy ewes. As expected, ewes that start losing fat a month before lambing and with a poor BCS at -15d are at higher risk to enter in negative energy balance. Moreover, ewes with a high BCS during the last month of gestation are at greater risk of mobilizing fat reserves pre-partum. Ultra-sonography, a non-invasive technique, appears useful in monitoring energy balance during the critical pre-partum period.
Keywords: Energy balance, dairy sheep, ultrasound.
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Affiliation of the authors at the time of publication
Laboratory of Animal Husbandry, Faculty of Veterinary Medicine, School of Health Sciences, Aristotle University, Thessaloniki, Greece;
Companion Animal Clinic, Faculty of Veterinary Medicine, School of Health Sciences, Aristotle University, Thessaloniki, Greece.
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