The effects of mild prenatal stress during mid- and late-pregnancy, on lamb vigour traits
Shearing during pregnancy has been shown to increase lamb birthweight but the underlying mechanisms remain unknown.
This thesis aims to examine the effects of prenatal husbandry-like and cold stressors during mid- (pregnancy day 85–95) and late-pregnancy (pregnancy day 120–130) on lamb vigour traits. The long-term effects of prenatal mild stress on lamb vigour traits were assessed using lamb growth and body dimensions, neonatal behaviour and reactivity after a cold challenge and the lamb’s thermoregulatory abilities when exposed to cold. Maternal glucose response to the stressors and lamb metabolic responses were also examined.
It was hypothesized that increased birthweights were a result of a maternal stress response to the husbandry-like stressors involved with shearing, particularly handling, and/or the subsequent cold exposure of shearing. It was also hypothesized that this maternal stress response would impact on lamb vigour traits such as behaviour and thermoregulation. This thesis examined the effects of a range of mild stressors commonly experienced by ewes during husbandry procedures during mid-pregnancy (pregnancy day 85–95) or during late-pregnancy (pregnancy day 120–130) such as yarding, handling, shearing, transport as well as cold exposure. The effects of these stressors on lamb vigour traits were assessed using a separation test for reactivity, before and after a 1 hr separation and cold challenge and thermal imaging during the cold challenge to determine the ability of the lamb to maintain body surface temperature.
Shearing during mid-pregnancy increased lamb body dimensions and weight at birth while other stressors did not, which suggests that the causal mechanisms are something other than psychological and cold stress. Cold exposure during both mid- and late-pregnancy (cold exposure and/or cold exposure resulting from shearing) negatively impacted the reactivity levels of newborn lambs by increasing their latency to bleat, stand and return to ewe during a separation test. Cold exposure combined with acute stressors in keeping with husbandry procedures resulted in improved body surface temperature maintenance in newborn lambs when exposed to cold. The presence of effects of mild stressors for both stages of pregnancy on all three lamb vigour traits tested, suggests a number of alternate mechanisms are triggered by maternal-pregnancy stress impacting foetal development. Understanding how to modulate these effects could allow flock management to be modified to improve lamb survival.