Review of the impact of heat stress on reproductive performance of sheep

Heat stress significantly impairs reproduction of sheep,and under current climatic conditions is a significant risk to the efficiency of the meat and wool production,with the impact increasing as global temperatures rise.Evidence from field studies and studies conducted using environmental chambers demonstrate the effects of hot temperatures(≥32℃)on components of ewe fertility(oestrus,fertilisation,embryo survival and lambing)are most destructive when experienced from 5 d before until 5 d after oestrus.Temperature controlled studies also demonstrate that ram fertility,as measured by rates of fertilisation and embryo survival,is reduced when mating occurs during the period 14 to 50 d post-heating.However,the contribution of the ram to heat induced reductions in flock fertility is difficult to determine accurately.Based primarily on temperature controlled studies,it is clear that sustained exposure to high temperatures(≥32℃)during pregnancy reduces lamb birthweight and will,therefore,decrease lamb survival under field conditions.It is concluded that both ewe and ram reproduction is affected by relatively modest levels of heat stress(≥32℃)and this is a concern given that a significant proportion of the global sheep population experiences heat stress of this magnitude around mating and during pregnancy.Despite this,strategies to limit the impacts of the climate on the homeothermy,behaviour,resource use and reproduction of extensively grazed sheep are limited,and there is an urgency to improve knowledge and to develop husbandry practices to limit these impacts.

Neonatal lamb mortality:major risk factors and the potential ameliorative role of melatonin

High incidences of pre-weaning mortality continue to limit global sheep production,constituting a major economic and welfare concern.Despite significant advances in genetics,nutrition,and management,the proportion of lamb deaths has remained stable at 15–20%over the past four decades.There is mounting evidence that melatonin can improve outcomes in compromised ovine pregnancies via enhanced uterine bloodflow and neonatal neuroprotection.This review provides an overview of the major risk factors and underlying mechanisms involved in perinatal lamb mortality and discusses the potential of melatonin treatment as a remedial strategy.Supplementing pregnant ewes with melatonin enhances uterine bloodflow and fetal oxygenation,and potentially birthweight and neonatal thermogenic capacity.Melatonin freely crosses the ovine placenta and blood-brain barrier and provides neuroprotection to the fetal lamb during periods of chronic and acute hypoxia throughout gestation,with improved behavioural outcomes in hypoxic neonates.The current literature provides strong evidence that maternal melatonin treatment improves outcomes for lambs which experience compromised in utero development or prolonged parturition,though to date this has not been investigated in livestock production systems.As such there is a clear basis for continued research into the effects of maternal melatonin supplementation during gestation on pre-weaning survival under extensive production conditions.