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Net reproduction rate

Definition of net reproduction rate

Net reproduction rate (Ro) is the number of ewe offspring reaching joining age and produced by each ewe during her lifetime in the breeding flock.

  • Ro = (L x S x J) ÷ 2L = number of lambs born per ewe joined (half are assumed to be ewes).
    S = survival rate from birth to 18 months (age at first joining).
    J = average number of times a ewe is joined in her lifetime.

This term is a clearly defined statistic used in population studies for all species, including humans. It is used to define changes in population size. Therefore it should only be used in its strictest sense and not as a vague term for reproductive performance.

What net reproduction rates tell us

Values for net reproduction rate may be from zero to more than two for merino sheep depending on the flock and area for which it is estimated. This is because number of lambs born, survival rate and number of joinings per lifetime will vary considerably.

If net reproduction rate is equal to one, the population (or flock) will maintain its size. If it is greater than one, the flock is increasing in size and if less than one, it is decreasing.

Using net reproduction rates

If sheep are to survive in an area, each ewe must produce a ewe offspring that reaches joining age during her dam's lifetime. Net reproduction rate can, therefore, provide information on the replacement rate required to maintain flock size under the particular conditions of the flock.

Alternatively it can be used to determine the rate of increase possible if certain proportions of the ewe offspring join the breeding flock. In areas where reproduction rates are low, it can also be used to predict the decline in flock numbers - since flocks where net reproduction rate is less than one cannot replace themselves.

Estimates of net reproduction rate

Table 1. Various estimates of net reproduction rates (Ro) for different merino flocks

Location Flock Ro Reference

Trangie, NSW

Plain (fold minus)


Dun (1964)

Wrinkly (fold plus)


Central western Qld

25 flocks


Moule (1966)

Julia Creek, Qld



Rose (1978)



Net reproduction rates for each of the Julia Creek flocks was below one and showed that neither group could maintain its numbers in the long term by natural increase. The plain Julia Creek sheep were 43% above wrinkly ones in net reproduction rate. This showed that even under extreme environmental conditions, plain sheep have much better survival than wrinkly ones.

The net reproduction rate for ewes in central western Queensland of 1.1276 was much higher than that of the Julia Creek flocks, a reflection of better environmental conditions and hence reproduction and survival rates.

Using net reproduction rate to determine where losses are occurring

In calculating net reproduction rate, it becomes apparent where losses in flocks occur (see Table 2 below).
Table 2. Comparisons of net reproduction rate for plain and wrinkly sheep from Julia Creek, Queensland and Trangie, New South Wales.

Julia Creek






Net reproduction rate (Ro)





Average number of lambs born per ewe joined (L)





Survival rate from birth to breeding age (S)





Average number of times a ewe was joined (J)





The average number of lambs born per ewe joined (L) in each flock at Julia Creek was only half that of the respective flock at Trangie. These low figures were a reflection of losses at all stages of reproduction.

The survival rate from birth to breeding age (S) was less for Julia Creek flocks. The high weaner losses at Julia Creek account for these differences.

The average number of times a ewe was joined (J) was very similar for flocks at both centres. However, at Julia Creek all ewes who reached breeding age entered the flock, no culling was practised and ewes were retained in the flock until they died. Ewes in the Trangie flocks were culled when their health and condition deteriorated to such an extent that they could not reasonably be expected to produce another lamb.

Practical application of net reproduction rate

Using net reproduction rate, a breeder can define many of their flock problems and find where losses occur. The breeder needs to know average lambing percentage and the survival of ewes to first joining, as well as assess death rates within age groups in adult ewes, in order to calculate the average number of joinings obtained from a ewe.

Such an exercise at Julia Creek showed that many ewes which survived to marking were not present at first joining. This large loss of ewe weaners (up to 24% for the spring joined flock) had been previously unrecorded.

Further information

  • Dun, RB (1964) Australian Journal of Experimental Agriculture and Animal Husbandry, 4:376-385.
  • Dun, RB and Eastoe, RD (1970) Science and the Merino breeder, Victor CN Blight, Government Printer, Ultimo, New South Wales.
  • Moule, GR (1966) Australian Veterinary Journal, 42: 13-18.
  • Rose, M (1978) Wrinkle score selection and its influence on the reproductive performance and mortality of merino sheep in north west Queensland, Master of Science Thesis: University of New South Wales.
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Last updated 26 September 2012