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Head Shepherd
Heat Stress: Impacts on Sheep Reproductive Performance with Dr William van Wettere
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This week on the podcast we’re discussing heat stress during mating and pregnancy with Dr William van Wettere.
Heat stress is classed as anything over 32°C, which can disrupt fertility in ewes, semen quality in rams and embryonic development in lambs, so it has an overarching effect on all aspects of reproduction.
Mark and Will start by discussing the effects of heat stress on ewes. Trials show that heat stress affects ewes most in the five days leading up to ovulation and the first five days after ovulation; while heat stress after day eight of ovulation did not affect fertility. Heat stress also reduces the duration of oestrus, with the impact depending on the timing of the heat stress. Trials show that heat stress can shorten the length of oestrus by five to eight hours. Heat stress can also impact the cycle length, lengthening it by nearly two days in some cases.
One trial showed that for every additional day above 32°C, lambing percentages decreased by around 3.5%. A lot of this is comes down to disruption of the ewe’s follicle development. Because of this, and poor placental development, lamb birth weight and survivability can be affected. Heat stress at different times throughout the pregnancy can also have an effect.
Mark and Will also discuss the impact on ram fertility, which can be affected at any time in the 60 days leading up to mating. Heat can cause not only a reduction in the amount of sperm but also cause issues with motility and along with structural abnormalities.
Fortunately there are solutions to mitigate heat stress. Research has shown that sheep that have to look for feed are more likely to succumb to heat stress, so confinement feeding with adequate shade could be an option. Another solution is to increase the ram per ewe ratio. There is also research looking into the role of melatonin in mitigating heat stress effects.
This is a great factual podcast that will arm you with knowledge about what goes on with your sheep when you’re mating during the peak of summer so that you can make a positive change for future matings.
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Understanding Heat Stress in Livestock
Speaker 1Welcome Dr Will Van Wetterer to Head Shepherd .
Speaker 2Thank you , Hi Mark . How are you ?
Speaker 1Good mate and thanks very much for your time . Yeah , we stumbled across you recently in one of the papers you published . I thought we'd get you on for a yarn . But before we get into heat , stress and shit , we might cover some other aspects of your career . You're there at the University of Adelaide . What have you done prior to that to get to where you are today ?
Speaker 2I've done a huge amount . I did my undergrad in Edinburgh , scotland , and Ag . Came out to New Zealand , did some dairy work , came over to South Australia , did some dairy work , got offered a PhD in pigs , worked with pigs intensively until about I don't know 2017 , 18 , and then transitioned to sheep . Still do a bit of pig stuff , but a bit of a transition more to trim down survival , no heat stress as well . It's my career . In a few seconds .
Speaker 1In a few seconds . Excellent , so yeah , previous . So you're now all sheep , or you do a bit of pig stuff still , or all sheep now .
Speaker 2Pretty much mostly sheep . I still got a couple of big projects going in the background . There are similar areas um not heat stress so much , but more um piglet survival . It's probably more recent stuff . Done a fair bit of work around um trying to make sure piglets basically sows have more piglets and making sure the piglets they have a better quality and have increased capacity to cope because , just like sheep , there's a proportion that are born a bit less viable , less vital . So we're trying to reduce , making sure the piglets have a better quality and have increased capacity to cope because , just like sheep , there's a proportion that are born a bit less viable , less vital , so we're trying to reduce that . So , fairly similar .
Speaker 1I was reading a little bit of I don't know , the post-grad projects or something around trying to get more sales per or trying to , I guess , group them up rather than having them in single stalls and stuff . Is that work ongoing or that's all finished up now ?
Speaker 2uh , we're not doing it well , so most say well . I'd say the majority of sales , especially in australia , are not , are housed in groups . Now , um , we did a bit of work in that space . Um , the next transition probably is to look at firing pens and firing crates . We did a bit of work around that as well , trying to look at how we can reduce confinement of sails during the end of pregnancy and lactation .
Speaker 1Yeah , right on , right , I will swing the sheep . How did you end up interested in heat stress ? It's obviously fairly topical , but where did you get into that ?
Speaker 2Well , to be honest , I'm Scottish so I struggle as soon as it gets 20 degrees . So I guess I had a bit of interest . In fact it's just hot and I figured that all production animals are starting to struggle . Um , I think initially it was around pigs , because there's some really good work in pigs now , especially out of the states that shows that there's an intergenerational impact of heat stress during gestation . Um , so pigs are exposed to heat . Pig fetuses are exposed to heat . They likely have long-term effects on them as adults . I transitioned into sheep based on the work I was doing in piglet survival to look at twin lamb survival , and then obviously the heat impacts on fetal growth sparked my interest , and then that basically moved me into heat stress and started thinking about ways to alleviate or at least understand the problem .
Speaker 2Because , I figured it is an issue . It's a significant issue for everyone . I mean , you just look in the paddocks looking at sheep wandering around , obviously struggling a bit at times , and it's hot .
Speaker 1So yeah , yeah , for sure , and I always find it a little bit interesting that when people worry about heat stress in new zealand compared to some of the conditions that I grew up with in um at home , but it , I mean , we'll get into the detail I guess , but is it kind of what they're used to as well , like , if you're used to do they adapt to it .
Speaker 2I think potentially . I mean I'm assuming that they must do to an extent . I think we , passively , are probably selecting to an extent as well with animals that do breed and do well . So yeah , I think they do get used to it . Animals that probably are born and live in hotter parts of the country probably cope with it better than those which have to be moved there . So yeah , I think so .
Speaker 1Yeah , yeah , yeah , fair enough . So we'll start off with a clear definition of what heat stress is , because , as you said , for you it's over 20 and for me it's under 20 . But as in , if it's that cold and then I'll get cranky , which is most of my life at the moment , but yeah , I guess I was a little bit surprised at how low the sort of cutoff is . I think 32 degrees is sort of the number that is used . Yeah , you can correct me on that .
Speaker 2No , no , I think that's fair . I mean , if you think about traditionally the critical temperatures for sheep , the lowest ones , around 12 , and things start to become a problem , anything between about 25 and 31 , they start to struggle with the temperature . I guess you'd say Our data does suggest , and I think other data does suggest , that around 32 degrees , at least in a Mediterranean climate , is when animals start to move , to become a bit hypothermic , so they can see their temperature starts to move outside their normal . We've done a bit of that work with indwelling temperature sensors in rams and ewes . You can see their temperatures arising outside what's normal .
Speaker 2So I'd stick with the 32 . It's probably fairly good . That's under a Mediterranean climate , under a more tropical environment . I guess you've got to start taking humidity into account as well . And then it becomes more complex because you have the whole temperature humidity index thresholds , which I mean I think we probably accept around 75 for severe heat stress and around maybe 79 for extreme . But it is , this is done . I'm sorry I'm not giving a very definitive answer , but it's complicated by the fact that everyone's different , like you're different to me . Sheep are very different , so some sheep can maintain a lower temperature when it's hot compared to others , et cetera . So there's variation within populations , so within genotypes , and also variation between genotypes as well . So it's kind of a I'd say 32 is probably pretty sound , but there is variation .
Speaker 1Yeah , does that help you ? Yeah , I guess the best almost a constant joke on this well , maybe not on this , but generally when I'm presenting it it's kind of my best answer is it depends , and that's like the best scientific answer . It's a fairly start most answers with it depends and go from there .
Speaker 2I think we'd stick with 32 , but then have a bit of a grey around it .
Speaker 1Yeah , yeah , and so a lot of people listening are going well , 32 is a pretty low temp for big chunks of Australia in summer . When we're going to get through the different aspects , is it kind of minimum temperature for the day or is it maximum , or is it how long they stay at that temperature , or how does that work ? That's another it depends answer . I can tell by the smile yeah , it depends answer .
Speaker 2I can tell by the smile yeah , um , yeah , it depends , um , I think it their ability to okay , it's a long answer . So in terms of the actual physiology of the animal , it depends on their ability to cool down . I think as well . And how long the heat is regular stayed up , stays up for . Traditionally , when we look at the stuff on reproduction , we've looked at the max temperature on the day . But these are the kind of questions that we're asking ourselves all the time at the moment , because we have a lot and lots of data coming in from temperature . So I'd say it's the max of the day . I'd work on that . I would say that the longer the heat is elevated for , the bigger the problem , especially if it's elevated overnight and it cannot return to normal at least by the same day . I don't think it would stay up ?
Speaker 1Yeah , and we do . I guess the reason I'm interested and lots of people are interested , just same as you . I guess like we see people having windows in their scanning results where the scanner says there's just no lamps in this gap . Or we had a conversation the other day and a guy from southern Queensland was talking about light lamps and stuff and so when we were actually literally on that call and I was Googling away and found your review paper and some of the numbers are big . So if we do run through kind of some of the scale of the impact , so if we talk I guess we'll just go through ewes and lambs and rams , but if we start with ewes and that sort of hate around the reproductive event itself , as in mating , what's the sort of impacts on that ?
Speaker 2So I mean , if we work on so there's some good data , that was some good studies that were done in South Australia and WA which , when you put them together , indicate that about every day above 32 degrees during that week of joining each day , so each day reduces the number of lambs born by about 3.5% .
Speaker 2So if every 100 ewes joined you get three and a half fewer lambs born for each day . So obviously if you then add them on top of each other you can get to quite a significant impact . The reason why that happens you then have to go back to some of the more detailed studies which show it's a reduced oestrus expression on the ewe , so she has a shorter oestrus or oestrus behaviors are shorter or less strong . There's probably a decrease in osup quality and therefore there is increased embryo loss and then obviously less fewer , lower pregnancy rates as well
Impact of Heat Stress on Livestock
Speaker 2. So it's that week around joining that seems to have the biggest impact around lambing rates and it's quite a lot . I mean we did the maths on it . We reckon it's worth around costs about 2.1 million lambs a year potentially that are not born not lambs that aren't born .
Speaker 1Yeah , yeah , yeah . No , it's big numbers , which is what got me interested as well . And so how much is oestrus reduced , or is that a significant chunk ?
Speaker 2It's a fair amount . I'd have to go back to the original literature and have a look at it . It is all in the review though .
Speaker 1Yeah , yeah , it is yeah , which we'll put a link to that review on the in the show notes that anyone who's a bit , who's keen enough to read a paper , can jump on there and get the , get the tree date , the full details , the , uh , all they can watch at lamex and I'm sure you'll be explaining that , the so , yeah , so obviously I should write time of time they're in estrus , uh , not ovulation , right . But I would say I would say quality is there an ovulation rate impact as well , or less ? That sort of less clear right , okay , yeah , right . So anyway , we end up with less , less slams on board and and obviously , if it's I don't know what we're saying anecdotally then if it's really bad , obviously you just get nothing conceived because it seems to be , and maybe that's a ram . Well , that wouldn't be around anyway , I'm not but there seems to be windows in the where there's just nothing happening . There's two things going on there as well .
Speaker 2I mean , if you want to just focus on the use for a little bit . The other problem we have , of course , is that if you go on , heat stress during pregnancy as well affects blood supply to the fetus , nutrient supply to the fetus . You end up with significantly lighter lambs and then obviously impacts on lamb survival . And I think maybe for the producer point of view the interesting thing there is it's the heat stress during pregnancy in the second and third trimester that has the biggest issue . But even if they heat stress , heat events during the second trimester , even if it gets cool in the third trimester , the lambs don't recover , the fetuses don't recover fully . So you still have a problem .
Speaker 1So it's a placental effect ? Yeah , the placenta is basically reduced in size .
Speaker 1Yeah , those numbers are massive . In terms of lamb birth weight . Obviously , if anyone can visualize the lamb birth weight survival graph in their head which , if you're on that , well , most twin lambs are on a slippery slope . If you drop every 100 grams , 200 grams drops your survival chances down a fair bit . So if you're getting into those sort of scales it's massive . I guess we are a little bit fortunate . Well , no , that's not true . I was going to say if you're a winter-spring lamb , then it might not be as critical . But a lot of people lambing in April , it's going to be most of pregnancies in summer .
Speaker 2Yeah , and I think that's the issue . Your joining is either impacted or your pregnancy is impacted if you're lambing in autumn or late summer .
Speaker 1Yeah , so lamb , it's all birth weight , or is there other ? Are they at given birth weight ? Are there any other impacts ?
Speaker 2uh , well , I haven't we . There's not a lot of work on the impacts of heat during pregnancy on the lambs ? Um , uh , some of them , so I'm only laughing .
Speaker 1There's noise in the background um , yeah , yeah , I can hear it here .
Speaker 2Yeah , I know um , yeah , those are my postdocs that you referred to earlier . Um , so basically what we would assume is that you can work on from dairy cows . We see that , um , dairy cow fetuses that are gestated when their mom is heat stressed , their milk production is reduced as adults . In pigs their ability . The fetus that is gestated when their mom is hot has impacts on their growth , their heat tolerance as well . So there probably is going to be impacts on the lamb going forward and obviously I think if the ewe is giving birth towards the end of summer or early autumn , when it's still hot , there could be potential impacts on lactation as well , which will affect the lamb as well .
Speaker 1Yeah , right on . So if we swing to rams and we've've sort of most of us have heard the yarn that whatever it is 56 days or whatever the number is from from start of like cell differentiation through to viable sperm , and obviously any interruption during that process is going to not be good , for if a ram can't can't thermoregulate , the test is and there's going to be trouble . What's the sort of scale of that impact ?
Speaker 2um , well , I haven't , we haven't quantified that as much , but what we can basically see is the first thing to think about is variation in rams again , the ability to cope , um . So the variation can be so much that the sperm is completely knocked out , or it can be reduced by 30 percent , roughly right , there's a variation on that scale . Um , you're right , it's about six , six to two weeks . So a single heat event , so a single day of 40 degrees , six to two weeks before they ejaculate or join , will have an impact on semen quality , however there probably may be . There's likely to be an impact of heat events on the day of join as well , at least on the epididymal sperm as well . Potentially which probably hasn't been looked at quite as much , but it's also thinking about is why there could be a direct impact on the ram at the ram joining period as well . Yeah , fair enough .
Speaker 1What we typically see , though , is decreased fertility , lower sperm numbers and then , if they're joined , lower pregnancy rates yeah , as we're talking , I can't help but think about my phd project where I my last , most important experiment where I had them all synchronized in the middle of March in WA and we managed to get three days above 45 degrees in a row for the exact same period where my years were synchronized , but amazingly they pulled it
Managing Heat Stress in Livestock
Speaker 1off .
Speaker 1There was a lot of shade in that paddock and stuff and it did cool down a bit at night and you get the easterly coming through at night . But yeah , it was panic stations for sure for me , wondering what the hell was going to be , whether I have any pregnant ewes to work on for the rest of the year . But I guess the question in most people's heads and you'll be no doubt investigating this is sort of what do we do about it ? We know that it's hot , we know it gets hot . Do we like if you have a heatwave ? Do you then leave the rams in for a couple of extra weeks ? Or what are the ways that we're thinking about negating these impacts ?
Speaker 2I mean these are the good questions , aren't they ? I mean you touched on one which I think is logical , although I mean I don't think the benefits is the shade . I mean I don't think the benefits is the shade . I mean the benefits haven't been quantified necessarily , but I think the shade is there . Some people talk about extending the joining length , so joining period , I guess other things you can maybe look at if there is I mean I'm just putting things forward right , and a lot of this hasn't been tested you can increase your RAM ratio as well , potentially to make up for that variation .
Speaker 2A lot of the times we see , maybe , if we assume that maybe 30% are really affected by the heat , maybe you need to put more out to compensate for that . Potentially , shade diet probably has an impact too . The biggest , bigger issue we see is contradictory well , not contradictory , it's conflictory is the fact that high-fiber diets increase metabolic heat , which further decrease the ability of the ewe and the ram to maintain body temperature . But during summer they're mostly fed a high forage diet . So that probably I would look at potentially reducing making a more concentrate diet potentially looking at that as an option .
Speaker 2Again , it's just a suggestion . Other things you can do is really simple , things like maybe don't make the sheep walk as far to get to resources so they can settle down for the day . They don't have to walk around looking for their feed , if that's possible . Again , limitations . There's other stuff . There's some work that we're doing at the moment which I think probably does stand up . You can look at things that actually traditionally increase fertility . I'm hoping that they'll work during the summer anyway .
Speaker 2Some benefits , I think , around melatonin seems to be working reasonably well for ewes and rams . Jason says I'm going to plug it . If you come to my talk you can hear more about that , but I think more research now is needed on variation individuals . So I think probably shade , look at your diet , reduce the roughage content at your diet , potentially change your ram ratios , possibly um , and maybe vary your joining length as well if necessary so it's fair to say we're at the , we're probably at the start of a research journey , really in terms of working out things that definitely work I think , for things that definitely work .
Speaker 1Yes yeah , fair enough , and we've got a anything more definitive than that , but I know there's a lot of research going on at the moment . So , yeah , some answers coming through . Yeah , for sure , and we'll remember . Yeah , as you said , the ruminants have a massive inbuilt heat , are sitting there generating heat , so it's not just like they obviously got a the room and it's is kicking away making them warm at the best of time .
Speaker 2So , um , yeah , and , anecdotally , some of our producers we work with reckon they're okay if they can settle in for the day and just find something cool . Um , If they're being forced to walk around a lot , that's when they see a lot of problems too , which comes down to the same thing exercise and heat , not good , yeah , so things like joining in confinement under shade is going to be a combination that might work .
Speaker 1Yeah , excellent , excellent .
Speaker 2The melatonin on Repro , or melatonin on Survival we're doing a fair bit of work through the SA Drought Hub and with the sheep industry funds around melatonin for rams and ewes . We're showing benefits for fertility of ewes , which we guess , but I guess we're doing it over summer , which is probably outside , when you would normally do it , which is late winter , early spring or spring , and also working the rams is quite good . It looks like it's affecting thermal regulation , improving thermal regulation , and it's definitely improving semen quality as well , and so that's just from collecting semen all through summer the semen quality's improved , so it shows some promise .
Speaker 1Yeah , righto . I mean , a lot of breeds are still aren't all spontaneously ovulating until sort of Feb , March , are they ?
Speaker 2So you're operating before that , or We've been giving them to the ewes all the way through , from early December all the way through , till the last one , I think is end of Feb . Yeah right , all the way through . But we're generally doing it at joining , as opposed to 30 days before as well , yeah right .
Speaker 1And we've also seen some changes in thermal regulation as well , so the ability to put brands and use , which I think is quite useful , Excellent , so Will the toughest question for the day or depending on how good at changing your mind are is what's the last thing you changed your mind about ?
Speaker 2I have no idea . I asked my postdocs to help me with this one , but we couldn't decide . Is that because there's a lot of ?
Speaker 1them , or there's a lot of them .
Speaker 2Yeah , I don't know , probably about lunch , I've no idea yeah , fair enough what's the last thing you change your mind about ?
Speaker 1oh so it's difficult one no , it's not an easy one . The um , I think I did . I get the odd person bouncing it back to me , but it's a tough one to answer .
Speaker 2Yeah , I can't . I tried , I have . Literally . It's the one I've been racking my brain about all morning , since you've done it and I can't do it .
Speaker 1Yeah , no , all good . Anything else you want to cover , or are we good to go ? No , I think that's it . I mean , is there anything ?
Speaker 2else you want to cover , or are we good to go ? No , I think that's it . I mean , is there anything else you want to know ? I don't know if I provide any definitive answers . That's all . I don't think there are a lot . It has a huge impact . Common sense , I guess , some of the things I've suggested . I think there's a lot of research across the country being done to try and solve this problem for producers .
Speaker 1Yeah , I guess it'd be one of those things that'll keep kind of getting away from us as well . As things keep , yeah , warming up , we'll get like there'll be no status quo on this . It's going to keep , or ? Yeah probably a bit of a moving moving feast yeah , or this year's a good example . We seem to have a hotter autumn than normal yeah , yeah spreading out of it yeah , yeah , excellent , right , I will let you get back to those postdocs . What's the most exciting postdoc project going on ? That's a tough question .
Speaker 2That's a tough question . Yeah , we're doing some really good work around understanding variation in thermal regulation , so I've got one postdoc working on that she's just finishing work at the moment how animals vary in their heat tolerance and how that affects their fertility , so that should be out soon . And another one doing some really good stuff around different amino acids and twin lamb survival as well , which I think will be interesting too . And of course , we have a big project at the moment on shedding sheep . It's trying to quantify reproduction of shedding sheep , which I think is a good project too , and that will be interesting for heat stress because there should be more heat tolerant .
Speaker 1Is there a chance that , like if we do select for , uh , just people right now , but if it's like for animal , that is better , dissipating heat some some way , if that's one of the ways they tolerate does that mean we end up with a lower lamb survival , if they they're kind of joining in summer and then lambing in winter , and yeah , I wonder how antagonistic those two things are going to be .
Speaker 2It's a really good question I think um gordon was wondering on that one day as well which basically , yeah , you're right , we're selecting animals when we make them during summer , and we select the people more fertile during summer . We're also selecting for animals that are preparing their progeny for a hot environment too , so they are antagonistic . I think I read somewhere that you can't select for heat tolerance and cold tolerance at the same time .
Speaker 1I don't know .
Speaker 2So potentially the answer is yeah .
Speaker 1Leave . The Mediterranean Half of southern Australia has just been wiped off the map . Yeah , basically All shit can move to the tropics . Yeah , excellent , righto , we'll leave you to it and I look forward to catching up in Adelaide .
Speaker 2Yeah , excellent , thank you .
Speaker 1Thanks , mate .
