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Breeding goals for merinos

Written by  Dr Joan Kleynhans
| in Veearts
| February 4, 2016

For optimum success as a Merino farmer it is most important to set long-term breeding goals for your Merino flock. A good Merino is an excellent dual-purpose animal.

In many cases Merino farmers make up to two thirds of their income from mutton.

Ewes should be highly fertile with good mothering qualities. Merinos are hardy animals, adaptable to all climates. They produce the finest quality wool of all sheep breeds. They are the most economic converters of grazing to mutton and wool. Ewes should produce 5kg of fleece (18 tot 22 micron) annually.

“They produce the finest quality wool of all sheep breeds.”

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Measure 'to know'

The first step towards genetic improvement is to identify, measure and record individual animals. Qualities to measure include:
• Weight - at birth, at weaning, after weaning and in adulthood.
• Fertility - indicated by the time it takes to get pregnant, inter-lambing period, number of lambs born and number of lambs weaned.
• Productivity - fleece weight, staple length, fibre thickness, and carcass qualities.
• Age - the age of the ewe should be taken into account.
Additionally genetics and environmental factors will influence results. For comparisons to be meaningful enough to guide selection of genetically superior animals, animals of similar ages under similar nutritional and environmental conditions must be compared. Do not compare single lambs to twin lambs for instance.

It is important to record parentage of individual lambs to build up a pedigree record.

The best animals within each group are usually genetically superior. If this system has been in place for several generations of sheep, results are increasingly meaningful.

To select for fertility, ewes that have not lambed should be culled and ewes with twin lambs and their offspring should get preferential treatment.


The main components of a ewe’s reproductive performance are the total of lambs born and weaned and the quality of these lambs - measured as individual weaning weight of these lambs, relative to others of the same age in the same flock.

Research in Sweden has indicated that positive genetic relationships exist between ewe weight and maternal effects on lamb weight. Therefore, selection for larger lamb weights increases ewe weights and  will also improve the maternal ability of the ewe. The positive genetic relationships that exist between direct and maternal effects on lamb growth and mature ewe weight, imply that selection for higher lamb weights will increase the size of the ewes and also improve maternal ability.


When selecting rams, factors  to be considered include the following:
• Conformation and body structure. Conformation may be difficult to evaluate if wool is long. Use hands as well as eyes.
• Consider size and constitution, as well as body weight. Stand back to assess overall build.
• A ram should have a strong head and muzzle, a good spring of rib and wide leg spacing.
• Examine the feet carefully. Look for faults, such as a dipping topline, poor shoulders, bad feet, bad mouths and skin pigmentation.
• Evaluate the wool quantity and quality. A good ram should have a long stapled, free growing, bright white fleece, with a high wax ratio to handle dust and rain. Staple length should carry down the leg and belly.  Skin must be soft and pliable. A large framed ram will have more wool to shear.

See www.merino.co.za/culling-faults/ for a comprehensive list of faults to look out for.

“Rams must be physically examined to detect any abnormalities which may interfere with successful mating. “

Rams must be physically examined to detect any abnormalities which may interfere with successful mating. Examine the scrotum and its contents. Testicles should be of normal size, symmetrical, firm and oval, with no lumps. Palpate firmly to detect consistency and resilience. Any lesions, especially of the epididymis, should be regarded as potentially contagious due to Brucella ovis or Histophilus somni/Actinobacillus seminis. In this case a veterinarian will need to perform tests to determine whether a flock is infected and then embark on a test-and-eradicate programme for infected rams.  The prepuce and penis should be checked for lesions or abnormalities. Also evaluate mating performance with a ewe if possible.

Rams should be tested for fertility by a veterinarian, especially when purchasing expensive rams, but even home bred rams may have lowered fertility and may compete for ewes with fertile rams, lowering lambing percentage, or drawing out the lambing period.

Rams should be screened 6-8 weeks prior to the mating season to allow time to change the ram team and/or purchase additional rams in time for the breeding season to replace defective rams.

A reputable breeder should be able to produce records which were evaluated by a breeders' society.  

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The best way to protect your flock against worms over the longer term is to breed for resistance. Resistant sheep have a lower level of worm infection, because their immune response is better. Worm resistance can be improved by using Australian Sheep Breeding Values (ASBVs) for fecal worm egg count (WEC). Worm resistance is a heritable quality.  

Resistant sheep are not always the best producers. Sheep that grow and perform well despite worm infection, are resilient. Although resilience is a desirable trait, it has a very low level of heritability. Resilient sheep, with high worm burdens, deposit worm eggs onto pasture, which will attack susceptible sheep.

In poor grazing conditions resilience decreases and these sheep struggle to cope with the additional stress of parasites.  Resistant sheep won't support significant worm burdens and will cope better with poor conditions.
See www.wormboss.com.au

Rams for breeding should first be selected based on the productive qualities required. From this group, purchase or select those with the most negative WEC ASBV. By doing so, one should acquire sires that are both productive and worm resist-
ant, but exclude those that are resilient, but not resistant.

Resistant sheep are likely to do better in dry times, compared to resilient sheep. Over time, breeding for resistance leads to a dramatic decline in worm populations and a huge saving on drenches.

ASBVs for WEC are expressed as a percentage rather than eggs per gram (EPG). The lower the value, the more resistant the ram. The purpose of the value is to express the relative difference between animals at any time. The egg count per animal can vary considerably over time, depending on recent worm challenge and time since last drenching. However, the percentage difference between animals is more consistent than the actual difference in eggs per gram.

If a sheep is resistant, fewer ingested worm larvae will manage to establish themselves within the gut and complete their life cycle to become adults. In fact, the establishment rate can be as low as 5 to 10%. In sheep with poor immunity, on the other hand, 50 to 60% of worm larvae reach adulthood and start laying eggs.

In resistant sheep a good immune response will reduce the numbers of eggs laid by established adult worms. The immune response may also expel established adult worms from the gut over a period of days to weeks. As a result pasture contamination is significantly reduced in the case of resistant sheep.

A lot has been said about measuring and selecting scientifically. However, I do believe sheep breeding is also an art. It takes dedication, passion and fine skills of observation, as well as knowledge of the breed, combined with scientific facts, to achieve optimum results.