Sheep Farm Husbandry - Reproduction: The ewe and mating management

By Dr Clive Dalton

The breeding season


Sheep are seasonal breeders and in latitudes away from the equator, they are stimulated to cycle by the declining daylight pattern in autumn. If you move ewes between hemispheres they change their breeding season to suit the new light pattern. Sheep that are farmed near the tropics where there is equal daylight and dark tend to breed all year round. You can also modify the breeding season by putting sheep on artificial lighting indoors.

Puberty

• Female sheep reach puberty around 6 months old, depending on breed and live weight.
  
• Reaching puberty depends more on getting to a critical weight rather than being a specific age. Highly fertile breeds like Finn sheep have earlier puberty than meat breeds.
  

Oestrus and cycling

• Ewes are on heat (show oestrus) for around 24 hours, but there is enormous variation around this mean. It can vary from 4 hours to 72 hours depending on a range of factors such as age of the ewe, the breed, and especially contact with a ram.
• Ewes cycle (return to oestrus) every 17 days but again this can range from 14-20 days.
• If a viable embryo is not established, the ewe will start cycling again and return to oestrus (see conception below).
  

Signs of oestrus
Signs of oestrus in the ewe are not as obvious as in cattle. Look for these:

• The ewe will seek out a ram.
• She will sniff him and chase after him.
• She will crouch and urinate when a ram sniffs her side or genital area.
• She will fan her tail, perhaps defaecating when the ram sniffs her.
• When the ram is preparing to mount, she will turn her head to look at him.
• Ewes do not mount other ewes like cattle do.
• A ewe may bunt a ewe away from the ram seeking his undivided attention.

Mating sequence
The following pictures show the sequence of joining a ram with some ewes that had been totally isolated from a ram, but at a time in the mating season when they most likely would have started cycling. This was confirmed when a ewe was mated about 15 minutes after the ram was introduced.

Ram joins ewes and immediately starts sniffing their rear ends for any
that may be on heat. Note the ewes start sniffing the ram attracted by his sight and smell.

The ram has found a ewe that is either on heat or coming into heat.
She is standing still, starting to look back at the ram and tail fanning when sniffed.

The ram has chased the ewe around for a while (not his mouth open panting).
She stops and looks back at the ram waiting for him to mount.


The ram keeps checking, building up his libido.
The ewe keeps waiting


Ram still resting - ewe still waiting for action


Action at last. Note ram gripping ewe with his front legs while thrusting.

Conception

• The ram’s semen moves up through the ewe’s cervix, into the uterus and up the Fallopian tubes to meet and fertilise any eggs (ova) that have been shed from the ovary. Viable embryos will result that float around freely and are fed from the uterine fluids.
• By the third week, the embryo attaches to the wall of the uterus and the placenta starts to develop. On the dam there are buttons (caruncles) and on the lamb cotyledons grow and they are joined rather like Velcro through which nutrients and waste products flow.
• Week 4-10 sees great placental growth and it’s size is critical – the bigger it is the better for the embryo, the foetus and the eventual lamb.
• Each growing foetus has its own placenta but the number of caruncles on the uterus wall is limited to around 100-120 by the end of pregnancy, so they have to be shared among however many lambs will be present.
  
• So multiple lambs with fewer effective buttons will consequently get fewer nutrients than singles – and even if a foetus is reabsorbed, it’s too late for others to use the spare buttons to get more nutrients.
• From 20-30% of ova shed from the ovaries don’t end up as lambs – it seems as if nature keeps a close eye on possible overpopulation this way.
• The first 30 days after conception and in particular up to day 18 seems to be the worst time for embryo losses.
• A viable embryo at 13 days after conception is the signal for the uterus to establish a pregnancy which is done by complex hormone interactions – a major one being the growth of a “Corpus Luteum” (CL) or “yellow body” on the ovary which produces progesterone that tells the ovaries that the ewe is pregnant so no more ovulation is needed until further notice!
• The age at which the embryo is lost affects the ewe’s return to oestrus. If it’s lost after day 13 then oestrus will be delayed, but if lost before day 13 then the ewe will cycle again normally after 17 days.
• The information above may sound a bit academic but it has important practical implications for flock management immediately after joining.
  
• While these delicate hormone-controlled processes are going on, you would be most unwise to stress ewes in any way if it can be avoided (e.g. by shearing and yarding for any reason), and certainly not treat them with any internal or external chemicals – e.g. drench or dips chemicals float around in the blood stream.
• There may be no scientific evidence to confirm these warnings, but it’s not worth the risk to flock fertility – as it’s the main component of profit. We just don’t know enough about potential problems and disasters can be very expensive. Remember that consultants and veterinarians don’t pay farmers compensation for wrong advice!
  

Pregnancy
Pregnancy in the ewe averages 147 days but expect a wide range from 137 to 161 days. Ninety percent of pregnancies range from 145-155 days. The number of lambs carried will affect this with multiple births usually not going over time as much as singles do.

Flushing

• This practice is as old as sheep farming. The shepherd feeds the ewes on a rising plane of nutrition using specially saved feed for 2-3 weeks prior to joining with the rams.
• This stimulates the ovaries to shed more eggs and increase the chances of not just the ewe taking the ram early, but also increasing the number of multiple births.
• How good a result you get depends on a couple of things – benefits from the “static” effect and benefits from the “dynamic” effect.
• The old shepherds didn’t use this jargon but what it means is that there are benefits from having heavy ewes to start with as opposed to skinny ones (the static effect), and then benefits from increasing their weight (the dynamic effect) on top of this.
• The total benefit if things work out right can be 15-20% better lambing percentage, from better feeding 2-3 weeks before joining and another 3 weeks afterwards.
• Flushing doesn’t seem to affect the pattern of onset of oestrus and has no effect on the number of barren ewes in the flock.
• If you start off with skinny ewes off hard hill country at around 40kg, then don’t expect much of a flush. If you have heavier ewes of 50kg or more, then the result should be better.
• With high-fertility ewes these days it’s probably not worth bothering about flushing as they will shed plenty of eggs in any case. In fact, not flushing them may be attempted to cut down the number of multiple births (e.g. quads) but it’s not very effective.
• And you have to consider the costs involved. Some farmers may grow a special-purpose pasture or crop for flushing which can add greatly to the costs.
  

Mating different age groups of ewes

• Two-tooth ewes that have never met a ram before have slightly different mating behaviour. This may result in lower fertility if they are mated in a flock with older ewes that know the game so most farmers run their two-tooths separately to their older ewes – and with an older experienced ram.
• Similarly hoggets should be mated on their own with an older experienced ram.
• Old ewes seem to have more patience with a young inexperienced ram lambs, probably because they have a stronger heat that lasts longer.
  

Mating management
It’s a good idea to muster the ewes up with the ram at least once a day incase any ewes get isolated. This is not likely to happen in small flat paddocks but is certainly possible on large steep hill country blocks.

Shearing and dipping

• Shearing before joining depends on the shearing programme for the farm.
  
• Even with annual shearing, it’s been the normal practice based on old research with Romneys to shear two-tooths 3-4 weeks before joining to improve their fertility. With modern breeds this may not be necessary but the practice remains on some farms.
• Shearing should not be done very near to joining or in the 4-6 weeks after joining. The concern is the effect it has on ovulation before mating and embryo implantation after, so in the light of no conclusive evidence – don’t take the risk.
• Dipping should certainly not be done for 6 weeks before and 6 weeks after joining. Farmer experience with certain dips has shown bad effects on fertility so again in the light of no official research results – don’t take the risk
  

Out of season lambing

• In New Zealand this has to be achieved with intra-vaginal devices delivering hormones to stimulate the ewe to cycle after treatment.
• Even with the best of management, results are never as good as in the natural mating season in terms of the number of ewes that take the ram and ovulation rates.
• There are also substantial extra costs for the exercise which could not be recouped from the commercial lamb market in the past.
• Dorper rams have been claimed to stimulate ewes to cycle after weaning their lambs, and Poll Dorset rams have had the same effect on ewes – if they have been fed well.
• The price premium for out-of-season lamb certainly seems to be growing for supermarkets that want lamb all year round.
  

Disclaimer This material is provided in good faith for information purposes only, and the author does not accept any liability to any person for actions taken as a result of the information or advice (or the use of such information or advice) provided in these pages.