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

Abstract

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 °C) 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 °C) 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 °C) 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.

Reproductive Management of Rams and Ram Lambs during the Pre-Breeding Season in US Sheep Farms

“…Heat stress can be induced with temperatures above 32 degrees Celsius for long periods of time or greater than 37 degrees Celsius for short periods of time. This results in an increased number of abnormal spermatozoa in the ejaculate, a phenomenon that can be observed 35 days post heat stress exposure [ 49 , 50 , 51 ]. Heat stress during the summer months can greatly affect the production of viable sperm and increase the percentage of unsatisfactory spermatozoa morphology, motility and concentration.…”

“…Some preventative practices include shearing the wool 4–6 weeks prior to the hot summer months (depending on the breed of the male) and providing adequate shade and clean, fresh, high-quality water. The negative effects of heat stress can potentially affect BSE evaluation results; therefore, it is recommended to perform the BSE four to eight weeks post exposure to extreme hot conditions [ 52 , 53 ].…”

“…Transmission of the disease occurs from ram to ewe or from ram to ram, causing infected rams to produce lower quality semen, resulting in sub-fertile or sterile rams, depending on the severity of the disease presentation. This disease can be initially diagnosed through scrotal palpation, assessing the epididymis and testicular tissue, and a blood test that must be submitted to a certified laboratory for diagnosis [ 52 , 53 , 54 , 55 ].…”

Causes and Remedies for Non-Infectious Infertility in Sheep and Goats

“…Long/severe cold or extremely hot/dry summer seasons can adversely affect fertility (Uçar and Parkinson, 2003;Uçar, 2006;Sönmez, 2012;Manstny, 2015;Sejian et al, 2021;van Wettere et al, 2021). Keeping breeding animals in a sheltered place in extreme cold/heat in shady cool environments has a positive effect on fertility (Edwards-Callaway et al, 2021).…”

Alternative models for transgenerational epigenetic inheritance: Molecular psychiatry beyond mice and man

“…However, research based in the primary production industries may be an unexpected source of early clues as to how these occur. Afterall, developing the knowledge to control the effects of heat stress has been crucial to the field of agriculture for maximising crop yield[ 36 , 37 ] and maintaining livestock fecundity and fitness[ 38 , 39 ].…”