The effect of different types of stressors during mid- and late pregnancy on lamb weight and body size at birth

Abstract

Mid-pregnancy shearing has consistently been shown to increase lamb birth weight, which can lead to an increase in lamb survival rates. However, shearing ewes during the winter months and under outdoor pastoral farming conditions can expose the recently shorn ewe to a greater risk of hypothermia. The aim of this study was to determine if exposure of ewes to repeated stressors, in mid-and late pregnancy, would result in an increase in lamb birth weight. This information may assist in the elucidation of the mechanism for the birth weight response to mid-pregnancy shearing, which in turn could assist in the design of management options to increase lamb birth weight without placing the ewe at risk. One hundred and forty-four twin-bearing Romney ewes were allocated to one of six mid-pregnancy treatments: control, isolation on 2 or 10 occasions, sham-shearing on 10 occasions, intramuscular cortisol injection on 10 occasions or shearing. Isolation, sham-shearing and cortisol treatments were conducted twice a week beginning, on average, day 74 of pregnancy and shearing occurred on day 76. During pregnancy, ewe treatment had no effect on ewe live weight. However, average ewe body condition scores were higher in the shorn group than in the sham-shorn or cortisol groups ( P , 0.05). Intramuscular injections of cortisol had a greater effect on ewe plasma cortisol concentrations than all other treatments ( P , 0.05). Shearing produced a greater plasma cortisol response than isolation 3 10 and sham-shearing ( P , 0.05). Ewe plasma cortisol responses decreased during the 5 weeks of isolation and sham-shearing but cortisol injections produced a greater response during the fifth treatment than the first or ninth treatments ( P , 0.05). Lambs born to shorn ewes were heavier and had a longer crown rump, forelimb and hind limb lengths than all other lambs ( P , 0.05). In addition, lambs born to ewes in the cortisol treatment were lighter than lambs born to control, isolation 3 2, isolation 3 10 and shorn ewes ( P , 0.05). The plasma cortisol concentrations observed for ewes injected with cortisol were far greater than those observed in all other groups, which is likely to explain the low birth weights of lambs born to ewes in that group. These results indicate that the mechanism by which mid-pregnancy shearing increases lamb birth weight is unlikely to be repeated stressors.

The importance of the gestation period for welfare of lambs: maternal stressors and lamb vigour and wellbeing

“…The ewe must first have the potential to respond (lambs born to unshorn ewes destined to have low birth-weight) and secondly, the ability to respond to shearing (adequate maternal body reserves or nutrition). The absence of an increase in birth-weight in response to maternal shearing observed in singleton lambs by Corner et al (2007Corner et al ( , 2010b and Jenkinson et al (2009) could be ascribed to the first of the above criteria. However, Kenyon et al (2004) failed to find any interaction between ewe body condition score (BCS) (over a wide range) and response to shearing.…”

“…Other responses Corner et al (2006bCorner et al ( , 2007Corner et al ( , 2010b focussed on the effects of shearing ewes in mid-pregnancy in conjunction with other treatments. In the first experiment (Corner et al 2006b), ewes were gathered on GD80, fasted for 24 h and then shorn or not shorn.…”

Pregnancy and Litter Size, But Not Lamb Sex, Affect Feed Intake and Wool Production by Merino-Type Ewes

“…Any metabolic stress therefore affects wool production [4], particularly if the diversion of body reserves to the fetus and new-born lambs leads to deficiencies in trace elements or sulphur amino acids [5,6]. The relationship between pregnancy and wool production is illustrated by the enhancement of fetal growth and birth weight when ewes are shorn in mid- or late pregnancy [7,8]. Brown et al [9] and Rose [10] have suggested that the competition for nutrients is greater during pregnancy than lactation, although litter size was not assessed in either of these studies.…”

Measuring wool cortisol and progesterone levels in breeding maiden Australian merino sheep (Ovis aries)

“…It is not possible to stipulate pregnancy as the only variable affecting wool cortisol concentrations. Other environmental factors include access to water, nutrition, climate and predation have also been shown to affect long-term cortisol concentrations [10, 17, 22] and cannot be excluded as potential extrinsic influences within the present study.…”