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Greg McCann, Senior District Veterinarian, Central West LHPA

Posted Flock & Herd March 2011


The ewe is a short day breeder, which is to say that she normally has her strongest reproductive performance when daylight is decreasing.

The optimal breeding period for the ewe is during March, April and May. For some breeds, the tendency to breed in autumn is very strong. This includes Border Leicester, Wiltshire Horn and Romney Marsh to name a few. For other breeds, the tendency is not as strong and they will exhibit a level of reproductive performance away from the autumn period. Such breeds include the Merino, Dorper and Poll Dorset.

A normal cycling ewe will come into oestrous about every 16 days.

The majority of sheep breeds have a similar ability to conceive and produce offspring. The exceptions are Booroola strain Merinos, Finns & East Friesians. This means that Merinos are on par with Border Leicester x Merino ewes, and all others.

The difference between the breeds is in the losses.

Based in long term scanning results, the average ewe flock has the potential to produce about 135% of lambs (Kilgour, 1992). The average Merino flock marks about 80% of lambs which represents a loss of about 35% of potential lambs, while the average marking percentage of other breeds is about 115% which represents a loss of about 15% of potential lambs.

Reproductive wastage (that is, not enough lambs marked) is often due to multiple factors. These are discussed in this paper.

Ewe factors involved in reproductive wastage

1. Time of Joining (out of season breeding)

Many sheep breeders choose to join their ewes out of season. Their reasons for doing so include having lambs available for sale at strategic times of the year, keeping them out of major grass seed infestations, or to ensure that all sheep husbandry procedures are well away from peak cropping work load. Stud breeders use out of season joining to achieve a marketing advantage that can come with having older and bigger rams for sale.

Many use out of season breeding without having full knowledge of the cost in terms of lower reproductive performance. Traditionally there is a drop of around 35% in reproductive performance, depending on the breed involved. Occasionally there are some major crashes with out of season breeding with losses that can be well over 50%. These crashes are, for the most part, unpredictable and the only way to alleviate the loss incurred would be to routinely scan all ewes that have been joined out of season and to be prepared to rejoin if necessary. This split joining can have major ramifications for routine farm planning.

Using the 'ram effect' can alleviate the reproductive loss associated with out of season joining. This relies on a natural synchronising effect of introducing males to non-cycling, out-of-season ewes that have been away from sight, smell and sound of rams for a period of one month. On introduction of males to the group, two ovulation peaks occur at day 2 and 8 after introduction. However, these ovulation events are normally NOT associated with oestrous (heat) due to a lack of progesterone. Teasers should be used initially to encourage the ewes to cycle without wasting ram power on silent oestrous cycles. By replacing the teasers (hormoned wethers or vasectomised rams) after a period of 10 days, functional rams are able to serve ewes on the return to ovulatory oestrous events. The teasers must be removed or they will compete with the functional rams – the ewes cannot tell the difference. The Ram Effect is ineffective after the summer solstice.

Another loss that is associated with out-of-season joining is that the lifetime wool production of lambs born in autumn is often reduced due to lack of feed bulk and/or quality.

2. Age of Ewe

Mating activity is instigated by the ewe. Maiden ewes lack experience.  Ewes that have been joined at an early age as maidens will need a protein spike prior to next joining to regain reproductive potential (similar to heifers).

Maiden ewes have a short oestrous (heat) length of between 0.5 to 6 hours. The suggested joining percentage for maiden ewes is 2.0% of non-maiden rams.  Increase this by 1% for out-of-season joining.

Adult ewes have an oestrous length of between 12 to 36 hours.  The ssuggested joining percentage for adult ewes is 1.5%. Increase this by 1% for out-of-season joining.

Maiden ewes conceive less readily, produce fewer twins and are less likely to raise their lambs than older ewes (Kleemann & Walker, 2005).

3. Body Weight of Ewe

The minimum body weight of maiden ewes at joining is 85% of their adult weight. This is a better guide than a set weight. The range is from 35kg to 45kg – depending on breed.

Regardless of the body weight of the ewe, an increasing body weight is vital. The heavier the ewe, the higher the ovulation rate (more likely to get twins).

4. Breeding History of Ewe

One study (Lee, Atkins and Sladek, 2009) mapped the reproductive performance of a flock of ewes, and slotted the ewes into 1 of 4 production quartiles. Ewes from the highest quartile annually produced one more lamb than ewes from the bottom quartile and reared 90% of lambs born, while ewes from the bottom quartile only lambed every second year and only reared 50% of lambs born.

Reproductive performance has low heritability.

Fastest gains in reproductive performance are made from culling 'Lambed & Lost'. This group includes repeat offenders as a result of udder/teat damage, and those with poor maternal instincts.

5. Temperature

High ambient temperatures (>35° C) during mating can cause a reduction in fertility. A drop in fertility of 10% was recorded in work by Kleemann & Walker (2005). This effect is thought to be via early embryonic mortality.

Available shade becomes critical to the ability of rams to thermoregulate. However, given adequate shade, rams are efficient in thermoregulation and do maintain normal fertility during high ambient temperatures.

6. Stress/Handling

The most sensitive period affecting early embryonic mortality is within the first 3 weeks from conception. Anecdotal evidence suggested losses in the order of 15% can occur due to stress/handling during this time.

Stress/handling includes shearing and any other management procedure like drenching that involves mustering and yarding.

Pregnancies past this point are generally robust.


1. Ram Joining Percentage

The suggested joining percentage for maiden ewes is 2.0% of non-maiden rams.  This should be increased by 1% for out of season joining.

The suggested joining percentage for adult ewes is 1.5%, increased by 1%, again, for out of season joining.

2. Age of Rams

Maiden rams that have been raised in a single sex, single age group environment lack mating skills, and have been the cause of some disappointing joining results. Many of these joinings involve a breeder trying out a new blood line or breed of sheep, and the poor joining results do nothing to endear the breed or line under trial to the producer.

To overcome the problem of ram immaturity, join maiden rams 50:50 with mature rams to non-maiden ewes. Maiden rams only require one joining to gain the required experience.

3. Single Sire Joinings

Single sire matings, as practiced by some studs, have a tendency to suffer from poor results as a result of fertility problems. These are invariably expensive losses due to the genetic merit of the ewes involved.

Single sire matings with maiden rams are routinely a disaster.

Managerial factors involved in reproductive wastage

1. Paddock considerations

Mating is a social event, and to augment the process, use small paddocks for joining to keep rams and ewes in close contact.

Multiple watering points & bore drains require a higher joining percentage.  Use a paddock with a single watering point where possible.

2. Predation

Losses due to predation are often underestimated. One study in north western NSW found lamb losses of approximately 20% due to feral pig predation (Plant et al., 1978).

Regional activities as a group produce the best results against predation. Being the only sheep producer in the area lambing down at a particular time is not a good feel. Your lambs are the only easy item on the menu, and you don't have the assistance of your neighbours baiting at the same time, for the same reason.


  1. Kilgour RJ (1992) Lambing potential and mortality in Merino sheep as ascertained by ultrasonography Australian Journal of Experimental Agriculture 32(3):11-13
  2. Kleemann DO and Walker SK (2005) Fertility in South Australian commercial Merino Flocks: sources of reproductive wastage Theriogenology 63(8):2075-2088
  3. Kleemann DO and Walker SK (2005) Fertility in South Australian commercial Merino Flocks: relationships between reproductive traits and environmental cues Theriogenology 63(9):2416-2433
  4. Lee GJ, Atkins KD and Sladek MA (2009) Variation in the lifetime reproductive performance of merino ewes Cooperative Research Centre for Sheep Industry Innovation NSW Department of Primary Industries, Orange Agricultural Institute, Forest Road, Orange, NSW, Australia
  5. Plant JW, Marchant R, Mitchell TD and Giles JR (1978) Neonatal lamb losses due to feral pig predation Australian Veterinary Journal 54:426-429


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