Can genetic propensity for lambing difficulty be predicted by pelvic and body shape dimensions measured by X-ray computed tomography (CT) scanning of ram lambs?

The ability of sheep to lamb unaided is important for both financial and welfare reasons. Current recording of lambing ease is subjective and unreliable for breeding purposes. The aim of this study was to investigate genetic control of measurements that could be taken from routine x-ray computed tomography (CT) scanning of ram lambs, within UK breeding programmes, to predict lambing difficulty of their progeny, or their daughters’ progeny. Measurements of 6 CT-derived lambing ease predictor traits (hip width, shoulder width, pelvic area, pelvic height, pelvic width, pelvic angle) were taken from archived CT images from 437 Texel ram lambs from 58 flocks (average age ~20 weeks) scanned over 15 years, as part of the UK national terminal sire breeding programme. Heritabilities, after adjusting for live weight, ranged from 0.16 to 0.65, with the highest estimates for the pelvic traits. Lambing difficulty scores (17705 records over 16 years), recorded on a six-point scale of increasing severity, were available from lambs born within the same flocks. Lambing difficulty was lowly heritable when expressed either as a trait of the lamb (h2 = 0.05) or the ewe (h2 = 0.02) with large common litter effects, low maternal effects and low repeatability in the ewe. Genetic correlations gave some indication that wide hips and shoulders, at a fixed live weight, may be associated with increased lambing difficulty of the lamb (rg = 0.28 and 0.47) and that lower pelvic width and angle (more horizontal) may be associated with increased lambing difficulty of the ewe (rg = −0.22 and −0.39), although standard errors were high. Moderate genetic correlations between body width and pelvic measurements suggest scope to select for optimal combinations of these measurements. Further research in this area could lead to incorporation of robust breeding values for lambing ease into sheep breeding programmes to improve animal welfare.