Reproduction efficiency of sheep is measured by fertility rate (the percentage of ewes lambing per ewe exposed to rams or artificially inseminated) and prolificacy (the number of lambs born per ewe lambing). The economic relevance of the two parameters depends on the production system, i.e fertility is crucial in dairy systems where milk yield is the most important source of income, whereas prolificacy is more relevant in meat systems. The worldwide acceptable rate of fertility is around 90%, whereas prolificacy depends very much on the sheep breed, varying from 1 up to 4 lambs per ewe lambing. Generally, meat breeds show higher prolificacy than dairy breeds.
It is well known that, within a sheep breed, reproduction efficiency can vary widely, with fertility rates lower than 70% and prolificacy equal to 1 lamb per ewe lambing. The improvement of these parameters is possible, with the best flocks reaching 95-100% fertility rate and 1.3-1.8 lambs per ewe lambing in non-prolific breeds and up to 3-4 in prolific meat breeds. To optimize the reproductive efficiency of a flock it is fundamental to consider the management of the flock during the reproduction cycle, according to the genetic potential of the breed.
Basically, reduced fertility rates may result from management factors affecting either male and/or female reproductive performances. Indeed ewes may fail to become pregnant because they are not mated or because they are unable to conceive after mating or Artificial Insemination (AI). In addition, ewes may not maintain the pregnancy or lose some or all of their fetuses during pregnancy (see the briefing on Gestation efficiency).
Ewes may not be mated for the following reasons: 1) mating occurs during anoestrous; 2) the ewe has a very poor body condition at mating ; 3) poor health conditions; 4) the ewe has lambed too recently; 5) inadequate libido of the male; 6) too low ratio of males to females in the flock at mating; 7) in the case of synchronisation of oestrus there could be errors (lost of sponges, too low dose of eCG); 8) presence of phytoestrogens or specific mycotoxins in the grazed herbage, which may temporarily or permanently suppress oestrus; 9) in the case of ewe lambs, they may not have reached the growth stage needed to accomplish a successful mating.
Ewes may not conceive because: 1) poor quality of the oocyte; 2) poor quality of semen; 3) pathology of the reproductive tract; 4) stressors such as shearing and other handling procedures; 5) heat shocks; 6) errors in the synchronization program (low doses of hormones, errors in the AI time).
Reasons for poor prolificacy in ewes include: 1) insufficient energy in the diet at breeding, when the animal has moderate to poor body condition score; 2) early death of embryos; 3) very young or very old animals; 4) insufficient dose of eCG if used in the synchronization program before AI.
To improve fertility rate is always possible when it is below the optimum. A good start is with adequate planning. There are two key factors to consider: management of the females, with particular attention to nutrition and an adequate number of fertile males in the flock during the whole period of mating. The implementation of these strategies will ensure the fertility rate is close to the optimum (say 90-95%). To this end, 6-8 weeks before the starting of the breeding season, the farmer should take into consideration the physiological (age, date of the last lambing, milk production level in dairy systems) and nutritional conditions of the ewes (body condition score) and their health status (parasitism, vaccinations). Nutrition should be adequate in order to favour the post-lambing recovery of body condition. A couple of weeks before mating or AI, ewes that have a body condition score below the optimal score for mating (2.75-3.25, for dairy ewes; 3.00-3.50 for meat ewes) should be submitted to a dietary flushing, based on concentrates, such as lupin or good quality herbage, offered at high allowance. Another important leverage to enhance fertility rate is the management of the male. Isolation of the males for 6-8 weeks before the breeding season will ensure a good response of the flock to the “ram effect”, especially if combined with an optimal number of males. The male needs to be prepared 6-8 weeks beforehand with good pasture and good quality concentrates, with an adequate level of minerals and vitamins, and good health status.
Each farm or flock may have different risk factors and prevalence for different causes of low fertility. Identifying the main reasons for fertility failure and the more important risk factors are the key steps in developing mitigation strategies. Recording fertility data and identifying when and why sheep do not lamb will help to improve reproduction efficiency.
Take home messages
- Low fertility is preventable, by tackling the main causes and risk factors at farm scale.
- Appropriate ewe and ram nutrition, especially in the last 6-8 weeks preceding breeding season, is the most important factor to optimize fertility rate.
- Providing suitable animal management, maintaining the flock in good health (control of diseases), contribute strongly to increased sheep reproductive efficiency.
- The use of an adequate ratio between fertile rams and ewes throughout the breeding season is crucial.
- Low prolificacy, depending on the production system and sheep breed, can be prevented by adequate nutrition around mating, such as flushing, and the use of effective crossbreeding programs.
Contact: This briefing has been prepared by Maria Dattena, Giovanni Molle, Sotero Salaris and Antonello Carta. Agris, WP2 for SheepNet. For further information discuss with your local coordinator of the Scientific and Technical Working Group.