Optimising sheep flock structure

Page last updated: Wednesday, 12 October 2022 - 12:35pm

Please note: This content may be out of date and is currently under review.

Selling or changing breeding numbers can have a profound impact on the on-going structure of the flock.  It is important to determine the changes to the flock over many years to understand the impact of those decisions.  The sheep flock structure tool allows producers to model the impact of retaining more wethers, increasing marking rates, selling maiden ewes or retaining ewes for longer.

Many factors affect a producer's decision to sell or retain sheep, including profitability of other enterprises, changes in prices for meat or wool, increasing or decreasing stocking rates and rebuilding flocks. These decisions will change the composition of the flock and its ability to maintain the flock over time. Changing lamb marking rates and shifting the balance betwen producing Merinos or more cross bred lambs will also have far reaching effects on the flock size and structure in the longer term.

The flock structure calculator predicts changes in the flock structure over time. Inputs include ewe and wether numbers and their ages at the start of the first year and expected sales. Other inputs include the ratio of merino ewes mated to merino rams and terminal sires and your weaning percentage. The calculator then estimates the numbers of ewes and wethers over six years. Outputs include a summary line graph of stock numbers over time, yearly stock numbers by age and commentary on the impact of sales on your breeding flock.

The calculator also determines the impact of keeping wethers, changing the ratio of merino ewes mated to merino rams and terminal sires and changing your weaning percentage on your breeding flock numbers. The tool can be used to compare two scenarios side by side. You also have the ability to alter stock numbers and sales in any year.

The Sheep flock structure calculator will show the impact of changes on the:

  1. age structure of the breeding flock
  2. potential number of lambs at a given weaning percentage 
  3. culling or classing percentage
  4. proportion of wethers retained.

View the Sheep flock structure calcutator.

Case study

Farmer Susan has a pure merino flock of 8800 sheep and a weaning percentage of 100%. She retains the majority of wethers until six years old and sells culled lambs, maidens after classing and does an age cull each year until ewes are six years old and then sells them as full mouths.

This has given her a balanced flock structure which retains similar numbers in the longer term.


Susan decides that she wants to sell all Merino ewes at hogget age so she can take advantage of good prices. By changing the inputs in the additional sales after joining section to reflect this scenario, and viewing the flock structure outputs in year 1, Susan can see that her flock will stabilise after about six years but she will have lower annual sales and a declining wether flock over that time.

selling hoggets


This calculator:

  • does not deal with two flock types, for example Merinos and composite flocks, at the same time
  • assumes that all sheep will be dispersed at six years old
  • does not account for changes in stocking rate due to more ewes being run than wethers
  • assumes that all cross bred lambs will be sold
  • shows the changes in year 1 reflected across all years.

Enter the calculator here or use the link to the right.

Additional information

DPIRD has a number of studies on the finacial impact of increasing flock size at both the farm and state level, as well as considerations around selling or retaining stock in poor seasons.

Contact information

Perry Dolling