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.

High in vitro survival rate of sheep in vitro produced blastocysts vitrified with a new method and device

Background:To advance the use of embryo vitrification in veterinary practice,we developed a system in which embryo vitrification,warming and dilution can be performed within a straw.Ovine in vitro produced embryos(IVEP)were vitrified at either early(EBs:n=74)or fully expanded blastocyst stage(FEBs:n=195),using a new device named"E.Vit",composed by a 0.25-m L straw with a 50-μm pore polycarbonate grid at one end.Embryos at each stage(EBs and FEBs)were vitrified by either Two-step(TS)or Multi-step(MS;6 different concentrations of vitrification solutions)protocol.Non-vitrified embryos(n=102)were maintained in in vitro culture as a control.Warming consisted of placing the straws directly into 1.5 m L tubes containing a TCM-199 solution with three decreasing concentrations of sucrose.Blastocyst re-expansion,embryo survival and hatching rate were evaluated at2,24 and 48 h post warming.The number of apoptotic cells was determined by TUNEL assay.Results:Blastocyst re-expansion(2 h)after warming was higher(P<0.05)in FEBs group,vitrified with the MS and TS methods(77.90%and 71.25%,respectively)compared with the EBs group(MS:59.38%and TS:48.50%,respectively).Survival rates of vitrified FEBs after 24 h IVC were higher(P<0.001)in both methods(MS and TS)than vitrified EBs(MS:56.25%;TS:42.42%)and was higher(P<0.05)in the MS method(94.19%)compared with those in TS(83.75%).After 48 h of culture the hatching rate for FEBs vitrified in MS system(91.86%)was similar to control(91.89%),but higher than FEB TS(77.5%)and EBs vitrified in MS(37.5%)and TS(33.33%).Number of apoptotic cells were higher in EBs,irrespective of the system used,compared to FEBs.The number of apoptotic cells in FEBs vitrified with MS was comparable to the control.Conclusions:A high survival rate of IVP embryos can be achieved by the new"E.Vit"device with hatching rates in vitro comparable with control fresh embryos.This method has the potential for use in direct embryo transfer in field conditions.

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.