Ovine Observer

The potential for dual energy x-ray absorptiometry to predict lamb age

Fiona Anderson, Murdoch University, WA
Corresponding author: F.Anderson@murdoch.edu.au

Introduction

Lamb maturity, as indicated by age, has been identified as a factor that contributes to eating quality. The current Meat Standards Australia (MSA) lamb model utilises lamb dentition to classify carcases as lamb or hogget. However, the accuracy of lamb age prediction using dentition is poor therefore a single categorical description of age such as teeth eruption is not ideally suited to the marketplace. 

Dual energy x-ray absorptiometry (DEXA) has been used for the accurate determination of body composition in production animals including sheep. R values are obtained from the analysis of high and low energy DEXA images, and reflect the atomic mass and mineral content of the tissue being scanned.

DEXA has been used for the measurement of bone in humans, and changes in mineralisation with age/maturity are detectable using DEXA. In lamb, bone mineral content has also been shown to change over time, with older animals having decreased concentrations of cortical bone magnesium. Therefore DEXA images of lambs are likely to reflect the changing bone mineral content and subsequently lamb age and/or maturity. Thus we hypothesise that DEXA R values will associate with lamb age, reflected through changing bone mineral content.

Materials and methods

A total of 595 lambs representing six slaughter ages (kill groups) underwent DEXA scanning using a commercially-installed online DEXA scanner. Of these lambs, 544 had information regarding sire type (122 Maternal, 166 Merino, 256 Terminal), sex (250 wether, 294 female), litter size (196 single, 348 multiple) and dam breed (164 Border Leicester x Merino, 380 Merino).

The first and the last kill groups had bone magnesium, phosphorus and calcium concentrations of the 12^th rib determined to examine the extremes of lamb age in the experiment.

DEXA images were obtained using a single emission from a 140 kilovolt (kV) x-ray tube, with a set of two images captured by two photodiodes with separate specificities for low and high energy photons. The DEXA images were analysed to determine R values for every bone pixel and the mean of the R values for each carcase image.

Results

Whole carcase bone DEXA

Bone DEXA R varied between kill groups, sire type, and dam breeds (P<0.01). There was no consistent trend between kill groups (age of slaughter) and bone DEXA R. Merino sired lambs had the highest bone DEXA R values, which were 0.02 and 0.03 units greater than the Maternal and Terminal sired lambs (P<0.01). Dam breed was only able to be compared within the Terminal sired lambs, and within this group of lambs, the Merino dams had a bone DEXA R that was 0.01 greater than the Border Leicester x Merino dams (P<0.05).

The bone DEXA R value decreased (Figure 5) and bone DEXA R standard deviation (SD) increased as lamb age increased. The ability to predict age using Bone DEXA R and SD was moderate (R² = 0.19, RMSE 29.6). R² is a measure of the variation explained by the model, with 1 being a perfect prediction, and root mean squared error (RMSE) a measure of the error in the predicting model, with a smaller RMSE indicating the prediction based on the bone DEXA R is close to the actual lamb age. 

Figure 5 Predicted age at slaughter (days) using bone DEXA R value. Markers represent residuals from the predicted means (solid line) ± standard deviation (dotted line) 

Rib mineral content

Only rib magnesium concentration in milligrams per gram (mg/g), wet weight, demonstrated any association with predicting lamb age in days with moderate precision (R2 = 0.27, RMSE = 42.7).  Lamb rib magnesium content was predicted with poor precision by rib DEXA R Mean (R2 = 0.10, RMSE = 0.57).

Discussion

In support of the hypothesis DEXA has been shown to differentiate lamb age, albeit with relatively poor precision. In this experiment the relationship between bone DEXA R value and lamb age was negative, which suggests that DEXA R values decrease with lamb age. Given the association of DEXA R value with mineral content and atomic mass the opposite was expected, as bones mineralise with maturity. There is a sigmoid relationship between bone mineral density and age in children, so it is possible that the decrease in density associated with age seen in this experiment may be in the time period when there is relatively small change in bone mineral content and bone density.

There was a limited association between bone mineral content and lamb age. Contrary to previous studies in lamb, bone magnesium content increased with age. The magnesium concentration result is difficult to explain but may reflect environmental, dietary, or health differences between the two kill groups.

Another application of DEXA in addition to age determination would be to predict eating quality of lamb meat. Cattle ossification has been shown to be a better indicator of eating quality in beef prior to skeletal maturity, therefore bone mineral profile, and DEXA R mean and SD values may be useful in describing eating quality in lamb.

Conclusion

A rapid post-slaughter method for determining age would provide assurance that lambs were being correctly classified, better satisfying the requirements for inclusion in the MSA pathways program. Future work will focus on improving the relationship between DEXA R value, lamb age and mineral content over an extended lamb age range and diverse genetics. Establishing a link between age, bone mineral content and eating quality has obvious benefits to the lamb industry through its input to the MSA grading system and may lead to better utilisation of older carcases if eating quality can also be predicted.