To shear or not to shear in winter
David Miller, Murdoch University, Western Australia (WA)
Corresponding author: d.miller@murdoch.edu.au
Introduction
Shearing ewes in mid-late pregnancy in temperate environments has been shown to increase lamb survival rates under both pastoral conditions and winter-housed environments. However, it is unknown if this effect can occur under the relatively mild winter conditions of a Mediterranean climate. The hypothesis tested was that shearing ewes in mid-late pregnancy increases the vigour of lambs born in the Mediterranean climate of south Western Australia.
Materials and methods
Twenty pregnant Poll Dorset ewes were randomly stratified into two equal groups with similar liveweights and body condition score. Ten ewes were shorn and ten were sham-shorn (unshorn) on day 100 of pregnancy in July (mid winter).
The sham-shorn sheep underwent the same handling process of shearing as the shorn sheep, but without removing the fleece.
The mean minimum temperature for the ten days immediately following shearing was 3.9°C ± 1.3 — ± standard error of means (SEM) — and the mean maximum temperature was 17.7°C ± 0.5.
All the pregnant sheep were handled frequently prior to lambing so the ewes were accustomed to human presence for observation.
Three days prior to expected lambing the sheep were moved to a smaller observation area. Immediately after birth a blood sample was quickly collected from the lamb so as not to disturb maternal-newborn bonding, and then behavioural measurements related to lamb vigour were collected.
Results
Ewes
Analysis of blood samples collected from the ewes 24 hours after shearing showed that shearing resulted in raised plasma cortisol concentrations – 72 ± 12 nanograms per millilitre (ng/ml) versus 41 ± 8ng/ml – in the shorn ewes compared to the unshorn ewes (P<0.05).
Lambs
Lambs from shorn ewes took 25% less time to suckle after birth (P<0.05) and they also took 35% less time to stand after birth (P<0.05), compared to the lambs from unshorn ewes (Figure 1). This is despite average birth weights being the same for the two treatment groups.
From the blood sample taken from the lambs immediately after birth, analysis revealed no difference in circulating cortisol, glycerol, triglyceride or non-esterified fatty acid concentrations between the two treatment groups.
The lambs from the unshorn ewes had 91% higher circulating glucose concentrations than the lambs from the shorn ewes (P<0.05), which may indicate greater reliance on mobilisation of glycogen stores rather than brown adipose tissue for thermogenesis in the lambs from the unshorn ewes.
Conclusion
In conclusion, lambs displayed better vigour when the maternal ewe was shorn in mid-late pregnancy in the winter of a Mediterranean climate. It is possible that the changes in the vigour of the lambs from the shorn ewes, and the difference in circulating glucose concentration, may have arisen by foetal programming effects on thermogenic systems resulting from the cold stress (fleece removal).