Evaluation of backfat and longissimus dorsi thickness ultrasound measurements as predictors of negative energy balance in pre-partum dairy ewes

Abstract

Non-esterified fatty acids (NEFA) and β-hydroxybutyric acid (BHBA) concentrations in blood serum are the most used indicators of energy status in dairy sheep. Ultrasound measurement of backfat thickness (BFT) and longissimus dorsi muscle thickness (LDT) is a non-invasive and on-field applicable technique, with no established critical thresholds for assessing energy balance in dairy sheep. The objective of the present study was to evaluate the use of ultrasound indices of BFT and LDT as image biomarkers of negative energy balance in pre-partum dairy ewes. The study was conducted in four commercial dairy sheep flocks in Central Macedonia Region of Greece, that each one reared a specific breed of purebred sheep. The final number of ewes with full records used in statistical analysis was 177 (flock A-Assaf.E, = 49, flock B-Chios = 46, flock C- Lacaune = 42 and flock D-Frizarta = 40). All animal measurements were performed one month (−30 ± 4d) and two weeks (−15 ± 4d) prior to estimated lambing date. Body condition score was assessed by palpation at the lumbar region. BFT and LDT were determined with ultrasonography after body condition scoring, to avoid bias on BCS estimation. The sum of BFT and LDT (TotalT), the LDT: BFT ratio (RatioT) and the difference in BFT (Δ_BFT) and LDT (Δ_LDT) between the two measurements were calculated. Blood sampling of ewes was performed on − 15d for serum BHBA and NEFA measurements. Negative energy balance status was defined as serum BHBA > 0.8 mmol/L or NEFA > 0.3 mmol/L. The receiver operating characteristic (ROC) analysis was used to define thresholds for fat and muscle reserves and mobilization parameters to predict elevated BHBA or NEFA status. All variables with a significant area under the ROC curve were transformed into binary variables and used in 2 multifactorial binary logistic regression models to assess the association of fat and muscle reserves and mobilization parameters with BHBA and NEFA status. Ewes with Δ_BFT > 0.65 mm had 3.2 times (P = 0.002) increased probability to have serum NEFA > 0.3 mmol/L. Moreover, ewes with BFT(−30d) > 6.86 mm, had 6.7 times (P < 0.001) increased probability to have serum NEFA > 0.3 mmol/L. Ewes with a BFT loss had 6.7 times (P < 0.001) increased probability to have serum BHBA > 0.8 mmol/L. Moreover, ewes with TOTAL_T(−15d) ≤ 25.6 mm, had 5.3 times (P < 0.001) increased probability to have serum BHBA > 0.8 mmol/L. The results showed that ultrasound measurements of BFT and LDT the last month of gestation can be used as a reliable alternative to BCS or metabolic profiling for the assessment of energy status in dairy sheep. Measuring BFT one month prior to lambing or TotalT two weeks prior to lambing can both provide reliable estimations.