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Ewe reproduction is measured by the capacity to conceive and rear lambs to weaning each season following puberty. This is influenced by the timing of joining and lambing and paddock allocation.
Once these factors have been decided, ongoing ewe management, particularly ewe body condition score, plays an important role in determining reproductive success.
Timing of joining
Oestrus activity in ewes is stimulated by shortening day length. Fertility increases after the longest day of the year (22 December), peaking between March and May.
A rising plane of nutrition in the lead up to joining will also lead to a higher ovulation rate when ewes are between condition score 2 and 3.
Ewes cycle every 17 days and this should be considered in determining the length of joining. Joining flocks for a winter/spring lambing is usually timed to span two or three cycles and match feed demand at lambing with feed supply.
Timing of lambing
In a Merino breeding enterprise, lambing should begin 3-4 months before the end of the growing season to encourage desirable weaner weights. In prime lamb enterprises, the timing of lambing should be influenced by market specifications.
When selecting a target market, lamb producers must decide on the most profitable compromise between:
- Number of ewes joined per hectare.
- Likely reproductive rate.
- Lamb sale weight.
- Timing and market price.
Producers may forego the ideal joining time to change the timing of lambing on the basis that, although fewer lambs may be produced, the premium paid by the market compensates for the reduced production. Cost of production calculations can determine this.
Several factors need to be considered when selecting lambing paddocks including:
- Pasture availability -Twin bearing, lower condition score ewes and maidens should be allocated best pasture and shelter.
- Paddock shelter - Wind speed can be controlled through using naturally sheltered (higher than 10cm) lambing paddocks. This can help reduce lamb mortality rates by up to 10%.
- Low worm risk - In high rainfall regions, gastrointestinal parasites are a major cause of production loss in ewes and poor growth in lambs. Pregnant ewes should be allocated to paddocks with low worm contamination during lambing.
- Paddock and flock size - Lambing paddocks should be stocked to match pasture availability with ewe demand.
- Predator control - Predators can cause up to 10% loss in lambs and need to be managed.
- Supervision during lambing - Disturbance of the lambing flock should be minimised, however, if supervision is warranted, the best time to enter the paddock is in the afternoon.
Managing ewes at joining
The most important factors of ewe management influencing reproductive performance are:
Managing nutrition in the lead up to joining and throughout pregnancy will help ensure optimal reproductive performance. Ewes in good condition score will conceive and rear more lambs. Producers should aim to have ewes in condition score 3 at joining and maintain this until lambing. Supplementary feeding may be required to maintain condition score.
Issues such as a worm burden, liver fluke or footrot will slow weight gain or cause weight loss resulting in lower reproductive rates. Care should be taken to ensure that ewes are healthy and contented during joining and pregnancy.
Ewes should not be joined in full wool as this can affect joining rates, however, shearing should not be carried out within two weeks of joining. Shearing during late pregnancy or during cold wet weather while ewes are pregnant is also ill advised as this can lead to metabolic problems and pregnancy toxaemia as well as an increased feed requirement of 25-30%.
Ewes are better able to withstand crutching before joining and in the last four weeks of pregnancy as this tends to be less disruptive and results in less time off feed - depending on the type of enterprise and location of the property.
Severe stress when joining and during pregnancy can cause abortion or result in pregnancy toxaemia or hypocalcaemia. Potentially stressful situations, such as dramatic changes in diet and large paddock moves, should be avoided.
Similarly, if ewes require a pre-lambing drench, this should be administered close to lambing to maximise the benefit.
Potentially toxic plants and pastures should be avoided during joining, even if sheep appear well and unstressed. High endophyte perennial rye grass pastures, for example, may result in a decrease in conception rate of 20%.
Producers should consider the costs and benefits of pregnancy scanning ewes. Scanning can be used to identify dry ewes and multiple pregnancies. This allows twin bearing ewes to be managed through the allocation of appropriate paddocks to meet feed demand and the removal of dry ewes.