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

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 × 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 × 2, isolation × 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.

[1]  T. Keady,et al.  Effects of shearing at housing, grass silage feed value and extended grazing herbage allowance on ewe and subsequent lamb performance. , 2009, Animal : an international journal of animal bioscience.

[2]  P. Morel,et al.  Mid-pregnancy shearing of autumn-lambing ewes in New Zealand , 2008 .

[3]  K. Stafford,et al.  The effect of mid-pregnancy shearing and litter size on lamb birth weight and postnatal plasma cortisol response , 2007 .

[4]  K. Stafford,et al.  The effect of mid-pregnancy stressors on twin-lamb live weight and body dimensions at birth , 2007 .

[5]  D. Revell,et al.  Mid-pregnancy shearing can increase birthweight and survival to weaning of multiple-born lambs under commercial conditions , 2006 .

[6]  M. Oliver,et al.  The effect of mid-pregnancy shearing or yarding stress on ewe post-natal behaviour and the birth weight and post-natal behaviour of their lambs , 2006 .

[7]  Hugh T. Blair,et al.  The effect of pasture allowance fed to twin- and triplet-bearing ewes in late pregnancy on ewe and lamb behaviour and performance to weaning , 2005 .

[8]  T. Parkinson,et al.  The effect of maternal shearing and thyroid hormone treatments in mid pregnancy on the birth weight, follicle, and wool characteristics of lambs , 2005 .

[9]  A. Lawrence,et al.  A review of the behavioural and physiological adaptations of hill and lowland breeds of sheep that favour lamb survival , 2005 .

[10]  M. Kuran,et al.  Shearing Pregnant Ewes to Improve Lamb Birth Weight Increases Milk Yield of Ewes and Lamb Weaning Weight , 2004 .

[11]  P. Morel,et al.  Effect of liveweight and condition score of ewes at mating, and shearing mid-pregnancy, on birthweights and growth rates of twin lambs to weaning , 2004, New Zealand veterinary journal.

[12]  A. Boissy,et al.  Effects of repeated stress during pregnancy in ewes on the behavioural and physiological responses to stressful events and birth weight of their offspring , 2004 .

[13]  S. Morris,et al.  Shearing during pregnancy — review of a policy to increase birthweight and survival of lambs in New Zealand pastoral farming systems , 2003, New Zealand veterinary journal.

[14]  T. Parkinson,et al.  Metabolic changes in ewes shorn during mid-pregnancy , 2003 .

[15]  A. Jobe,et al.  Differential effects of maternal betamethasone and cortisol on lung maturation and growth in fetal sheep. , 2003, American journal of obstetrics and gynecology.

[16]  D. Revell,et al.  Birth weight responses to shearing ewes in early to mid gestation. , 2000 .

[17]  D. Smeaton,et al.  The effects of shearing Finnish Landrace × Romney ewes in mid-pregnancy on lamb survival, birth weight and other weights. , 2000 .

[18]  D. Lay,et al.  Effects of prenatal stress on suckling calves. , 1997, Journal of animal science.

[19]  S. N. Mccutcheon,et al.  Selective enhancement of growth in twin foetuses by shearing ewes in early gestation , 1997 .

[20]  J. G. Cunningham.  Textbook of veterinary physiology , 1997 .

[21]  Geoffrey Hinch,et al.  The Behaviour of Sheep: Biological Principles and Implications for Production , 1992 .

[22]  F. Blecha,et al.  Repeated restraint and isolation stress in lambs increases pituitary-adrenal secretions and reduces cell-mediated immunity. , 1991, Journal of animal science.

[23]  A. Hargreaves,et al.  The stress response in sheep during routine handling procedures , 1990 .

[24]  K. Pierzchala,et al.  Lack of adaptation to repeated emotional stress evoked by isolation of sheep from the flock. , 1987, Zentralblatt fur Veterinarmedizin. Reihe A.

[25]  N. Jay,et al.  Ewe maternal behaviour score and lamb survival , 1985 .

[26]  K. Pierzchala,et al.  The effect of shearing on the concentration of cortisol and thyroid hormones in the blood plasma of sheep. , 2010, Zentralblatt fur Veterinarmedizin. Reihe A.

[27]  B. Y. Tang,et al.  Ultradian and circadian rhythms in the plasma concentration of cortisol in sheep. , 1979, The Journal of endocrinology.