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Exposure of ewes to stressors in mid- and late- pregnancy : postnatal effects on the ewe and lamb : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science at Massey University, Palmerston North, New Zealand
This thesis set out to examine the effect of maternal undernutrition and exposure to
stressors between pregnancy day 50 to 100 and 100 to 147 of pregnancy on the ewe and
her lamb. The long-term effects of these stressors during pregnancy on lamb growth,
plasma cortisol response to a stressor, metabolism, behaviour and future reproductive
success were examined.
The component of mid-pregnancy shearing that causes the increase in lamb birth weight
is unknown. It was hypothesised that the increase in lamb birth weight was due to the
stress response of the ewe to shearing. This work examined the effect of a range of
stressors at approximately day 80 of pregnancy. These stressors included yarding,
crutching and sham-shearing that may be components of the shearing procedure that
produce a stress response. In addition, repeated stressors between day 74 and 106 of
pregnancy including isolation, sham-shearing and exogenous cortisol injection were
used to examine the role of a longer-term stress response on lamb birth weight.
Mid-pregnancy shearing has consistently resulted in an increase in lamb birth weight,
however all the other stressors investigated had no effect. Therefore, the hormonal stress
response of ewes to shearing was unlikely to be the cause of the increase in lamb birth
weight. Mid-pregnancy shearing also resulted in minor changes in ewe and lamb
behaviour 12 to 24 h after birth. Shearing during pregnancy had no effect on the cortisol
response of ram lambs to handling or castration however differences were observed
between singleton- and twin-born lambs.
Ewe nutrition during pregnancy
Ewes mildly undernourished in between days 70 and 107 of pregnancy that were then
provided with adequate nutrition between days 108-145 of pregnancy gave birth to
lambs with similar birth weights as ewes well-fed during both periods. Therefore the
effects of undernutrition on lamb birth weight can be minimised if undernutrition can be
limited to the earlier period (day 70-107 and 108-147 of pregnancy). Mild
undernutrition in both periods (day 70 – 145 of pregnancy), resulted in lambs that were
lighter and that exhibited behaviours associated with a greater ‘drive’ to maintain
contact with their dam than lambs born to ewes well-fed during the same period. This
suggests that even mild undernutrition in both mid- and late- pregnancy should be
The effect of maternal nutrition and lamb litter size on the behaviour of female offspring
was examined at 1 and 2 years of age. Litter size had no effect on ewe behaviour at 1
year of age however at 2 years of age twin-born ewes had higher maternal behaviour
scores than triplet-born ewe lambs. Nutrition of the maternal grand dam during
pregnancy had only a minor effect on the behaviour of female offspring at 1 year of age
and no effect on maternal behaviour at 2 years of age.