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Modelling to investigate ewe wastage

Updated: Mar 8

Ewe wastage has been identified as a factor limiting flock productivity on New Zealand sheep farms.


Ewe wastage includes on-farm mortality and premature culling of ewes before the potential end of their productive life.


We have asked Lydia Farrell to write a summary of her recent publication on ewe wastage. Lydia is in the third year of her PhD studies in the Farm Management department at Massey University where her research project is “Bio-economic modelling of New Zealand sheep farming systems”. Originally from Northland, Lydia has a background in dairy farming but was excited to take on a project where she learns more about sheep production systems and develops her own model to explore scenarios not previously examined at a farm systems level in New Zealand.


Current estimates of wastage rates (WR) across New Zealand commercial flocks range from 2.8% of breeding ewes to approximately 20%, with large variation between farms and between production years (Griffiths et al. 2017).



Key Points:


  1. Greater rates of mature ewes leaving the flock during their most productive years reduced flock average age. A younger flock had a lower reproductive rate, thus produced fewer lambs for sale.

  2. Feed consumption decreased with increasing ewe wastage due to reduced production and fewer stock on-farm.

  3. Lower income from lamb sales lead to lower sheep enterprise profitability and sheep enterprise profit decreased $1,069 per 1% increase in ewe wastage rate for the farm modelled.

  4. Reducing ewe wastage from 21% to 5% could increase sheep enterprise cash profit by 33%.


It is not known the proportion of wastage in New Zealand flocks accounted for by culling vs. deaths, however, ewe flock mortality rates have been reported to range from 2.8% to 16% (Griffiths et al. 2017, Anderson and Heuer 2016) and ewe reproductive performance is typically the main driver of premature culling.

New Zealand breeding ewe flocks are typically self-replacing, which means that a higher WR will result in more replacement ewe lambs needed, reducing the number of lambs available for sale. Further, ewe reproductive performance peaks at approximately 4-6 years of age, therefore, greater WR results in a greater proportion of younger, less productive ewes in the flock.



Study design


This study modelled a representative New Zealand North Island Hill Country breeding ewe flock with operating profit estimated as Cash Operating Surplus (COS).


The ewe flock size, etc. was based on average values from Beef + Lamb New Zealand survey data for a flock in the Manawatu region in the 2016/17 production year.


This farm had 1,879 Romney-type mature breeding ewes with a base lambing rate of 123%. The lambing rate was adjusted for each age of ewe according to their relative reproduction performance shown in Figure 1.


Figure 1: Relative reproductive performance of ewes aged two to seven years.

Pasture growth and quality data for North Island sheep and beef farms were used to estimate feed supply on the 423 ha farm. Sheep make up 63% of farm stock units, so this proportion of feed and working expenses were assumed to be for sheep.


Wastage of the mature ewe flock was varied from 5% to 21% and the ratio of mature ewe deaths to culls was maintained at 19:81.


What happened when wastage rate increased?


Flock average age was 4.18 years with WR5%, decreasing to 3.54 years with WR21% (Figure 2).

As flock average age decreased the lambing rate also decreased, from 1.33% (weaning 2,487 lambs) with WR5% to 1.22% (weaning 2,265 lambs) with WR21% (Figure 2).



Figure 2: Average flock age and numbers of lambs weaned with increasing ewe wastage.

As replacement requirements increased with increasing wastage, the proportion of lambs kept as replacements and therefore, not available for sale increased from 12% to 21% (purple) of total lambs weaned across the WR range (Figure 3).



Figure 3: Proportion of weaned lambs that are available for sale or kept as replacements with increasing ewe wastage.

Due to fewer lambs sold and more kept as replacements there was lower income from stock sales, resulting in lower total income from sheep operations (Figure 4).


Farm expenses were relatively constant with increasing WR, therefore, sheep COS deceased. COS decreased from $256/ha to $192/ha when WR increased from 5% to 21%, respectively. COS for the total sheep enterprise (63% of 423ha) decreased from $68,221 with WR5% to $51,166 with WR21% (Figure 4).


For the representative farm modelled, sheep COS reduced $1,069 per 1% increase in WR for the representative sheep enterprise where sheep consume 63% of feed on 423ha.



Figure 4: Cash income, expenses and sheep cash operating surplus (COS) with increasing ewe wastage.

Maximum total annual flock energy requirements occurred with WR5% (13.43 million MJ ME). Total annual energy requirements decreased to 12.45 million MJME with WR21%, as greater numbers of ewes left the flock and fewer lambs were produced leading to reduced feed requirements for adult ewes, sold lambs, and gestation and lactation for all lambs (Figure 5).


Total energy requirements for young stock (black, replacement lambs from weaning until two years old) increased with higher replacement rates and their energy requirements accounted for an increasing proportion of total flock energy requirements (Figure 5).



Figure 5: Annual energy requirements for adult ewes; reproduction (pregnancy and lactation); young stock (replacement lambs from weaning until turning 2 years old); and sold lambs (non-replacement lambs from weaning until sale) with increasing ewe wastage.

Efficiency of pasture use in this study decreased from 162 lambs sold per million MJME consumed annually by the flock with WR5% (2170 lambs sold in total) to 145 lambs sold per million MJME consumed annually with WR21% (1800 lambs sold in total), indicating that pasture is used less efficiently with higher WR.





Higher ewe WR and subsequent lower total flock energy requirements lead to greater monthly energy surpluses, resulting in a greater positive end of production year energy balance (Figure 6).


Closing energy balance at the end of the production year in June was 24,107 MJ ME with WR5% and 939,017 MJ ME with WR21%. In this study, the largest energy surplus occurred in mid- to late-summer (Figure 6) when pasture is most vulnerable to maturing, with reproductive growth reducing pasture quality.



Figure 6: Monthly cumulative energy balance with ewe wastage rates of 5, 15, and 21%.

Possible alternative uses of the feed surplus for the system are to conserve and sell excess feed. This can be difficult on hill country as there is less area that can be mechanically harvested. Another option is to farm at a higher stocking rate, either increasing the breeding ewe flock or leasing pasture for grazing.


Greater profitability gains and more efficient use of feed could likely occur from using this surplus feed to better feed existing stock to improve their performance and reduce WR, as improved pasture utilisation and adequate feeding of breeding ewes have been found to increase farm profitability (Kenyon et al. 2014, Young et al. 2011).



What are the implications?


Average ewe death rate from the Beef and Lamb New Zealand Farm Survey Analysis for New Zealand North Island Hill Country farms is 4.2% and flock replacement rate is 21.7%.


These values best match the modelling scenario of WR15% used in this study which had an annual sheep COS of $219/ha, where reducing ewe WR to 5% from 15% would increase the sheep enterprise COS by 17% (COS = $58,547 with WR15% and COS = $68,221 with WR5%).


For farms of this type losing a large proportion of ewes prematurely, reducing flock WR from 21% to 5% would increase COS by 33% (COS = $51,166 with WR21% and COS = $68,221 with WR5%).


Lamb prices have risen since the 2016/17 production year that was modelled, suggesting that the potential profit gains for reducing ewe wastage have also increased.


These findings indicate that strategies to reduce ewe wastage should have a positive impact on flock productivity and farm profitability provided the costs do not exceed the profit gains identified in this study.


Such strategies may include changes to ewe management to improve reproductive performance and reduce deaths around lambing or due to climatic or disease events.


Research currently underway aims to accurately determine the rate of ewe wastage on commercial New Zealand sheep farms (Griffiths, 2016).


These findings, combined with those of this study, will provide clarity for sheep production industries around the productive and economic impact of premature on-farm ewe losses.



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Full Paper:

Farrell, L., Tozer, P., Kenyon, P., Ramilan, T., Cranston, L., 2019. The effect of ewe wastage in New Zealand sheep and beef farms on flock productivity and farm profitability. Agricultural Systems 174, 125-132. https://doi.org/10.1016/j.agsy.2019.04.013


The research is funded by The New Zealand Merino Company and Callaghan Innovation with the model also used to investigate use of terminal sires and use of Merino sires to breed a Romney to a ¾Merino-Romney crossbred flock producing more valuable, mid-micron wool.

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The two editors, Isabel Vialoux and Rhiannon Handcock are PhD students/employees at Massey University.

This blog represents the views and opinions of Isabel and Rhiannon, not Massey University.

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