How to work out ewe efficiency and what it can tell you

Calculating ewe efficiency can help inform fundamental flock management areas such as mature ewe weight, feeding management and weaning date.  

SAC Consulting sheep and grassland specialist Poppy Frater says the process of calculating ewe efficiency can be simplified to make it practical for a commercial flock. 

She says ewe efficiency records can be used to assess how well maternal genetics are performing and what you can do to improve feeding and body condition score management.

See also: Winter shearing improves ewe efficiency on Welsh farm

Eight-week weights can also be a baseline for lamb growth later in the year.   

Commercial ewes should rear 50% of their weight by eight weeks and lambs should be able to match the ewe’s weight by sale day. So a 75kg sheep rearing a 44kg lamb needs to rear 170%.

What you need to know

  • Mature ewe size
  • Lamb weights at eight weeks/li>
  • Scanning rate or number of lambs at tagging.

How to benchmark flock efficiency

  • Take a sample weight of mature ewes (three-crop and older) before they go to the tup. From this, work out a flock average. 
  • Estimate a birth date for lambs from the start of lambing plus 10 days. For example, if lambing starts on 25 April, add 10 days to get a birth date of 5 May. Eight weeks after that is 30 June.
  • Randomly sample an even batch of lambs. This could be done to coincide with Pasteurella and clostridial vaccinations or possibly a wormer if needed.  
  • Take an average of 50 lamb weights. Record 56 lambs, deducting the heaviest three and lightest three. If worming lambs, weigh every sixth lamb through the race to ensure a random selection. If a huge variation is being seen, try increasing the number to 86 lambs.  
  • Take the average lamb weight and multiply that by the number of lambs you’ve tagged that year.  
  • Divide this figure by the average ewe weight to give ewe efficiency as a percentage.

Example: If ewes weighed 65kg at tupping and lambs’ eight-week weights are 19.5kg and you’ve tagged 1.79 lambs, this is 19.5 x 1.79 = 34.9kg of lamb a ewe. To get a percentage from this, 34.9 / 65 x 100 = 53.7%.

Short-term benefits

The collected eight-week weights can be used a month later when weaning or deciding when to wean, explains Ms Frater.  

Weaning at 12 weeks is standard, but if ewe condition is suffering and grass supply is low then weaning at eight weeks can be an option.  

If lambs are growing at less than an average of 150g a day after eight weeks, it might be best to prioritise the best grass direct to the lamb.

“If lambs are growing less than 100g/day it’s definitely time to wean and investigate the problems,” says Ms Frater. 

Biologically, the younger lambs have better feed efficiency, so will make better use of grass or supplementary feed, she adds.

Long-term benefits

Ewe efficiency can be used to critique flock performance and influence system changes.   

Lamb weight, ewe weight and lamb volume can potentially flag up a variety of issues.

Troubleshooting using ewe efficiency numbers


What affects it


Possible problems

Lamb weight 

• Ewe milkiness and maternal ability

• Grass/creep quality – lambs’ main energy source is milk until six weeks of age

• Direct growth genetics

• 22kg for crossbred ewe progeny bred to terminal sires

• 320g/day lamb growth (4.5kg birthweight)

• Lambs weighing 30kg at 90 days

• Look at maternal genetics

• Is late gestation feeding sufficient?

• Colostrum quality

• Sire choice

• Lambing ease

Lamb volume 

• Scanning %

• Lambs born %

• Mortality

• Target depends on system and scale

• <13% losses from scanning to weaning could be a good rule of thumb

• Varies according to system and scale. Aim for abortions <2% and barren rate <5%. 

• Most low-input outdoor lambers target 1.5 lambs sold. Higher input indoor lambers target 1.7/1.8 lambs sold.

• Work out where losses are occurring. 

• Are you scanning? 

• Are you vaccinating for abortion? 

• Work with your vet to draw up a flock health plan. 

• Is grass quality good enough for dry sheep to flush them?

• Is slow lamb growth limiting the next year’s crop if sufficient quality grass isn’t available to flush ewes?

Ewe size 

• Selection policies if self-replacing

• Size and weight of dam and sire if using crossbred females

• Optimal ewe size depends on system, farm and labour 

• Hill ewes 50-55kg

• Lleyn 60kg

• Mule 75kg

• Continental-cross 80kg

• Run scenarios looking at stocking rates with different ewe sizes at current rearing percentage to see if smaller or bigger sheep would work better.

Case study: Weaning efficiency informs management and breeding  

A Kent sheep breeder is using weaning efficiency data to monitor maternal performance and help guide flock management and breeding decisions.   

Alan West has scrutinised eight-week and ewe weights in his flock since the Lleyn Sheep Society launched the Lleyn Gold scheme in 2015.  

Decisions to feed at lambing, paddock graze, wean early and creep feed have been informed by looking at ewe efficiency data along with body condition score and grass availability. 

And while 2016 was the best year, extreme weather conditions have made subsequent years harder on the sheep, leading Mr West to believe that, overall, progress is being made (see table).

Malling flock 

  • 50 pedigree Lleyn sheep 
  • 15ha (35 acres) of permanent pasture 
  • 190-200% scanning 
  • Lamb for 1.5 cycles (16-22 days) 
  • Ewes lambed outdoors and housed for 24-48 hours to mother up 
  • Signet recorded for three years

How Lleyn Gold works

The scheme aims to select for moderate-sized, prolific, milky, maternal ewes that rear twins. Ewes are ranked on the percentage of bodyweight they rear, with dams achieving 60% ranked as Lleyn Gold.  

Mr West marks ewe lambs out of Lleyn Gold sheep with a coloured spot on the back of their head. Marked lambs that are correct and look strong and balanced are retained for breeding.  

Individual lamb weights and ewe performance can be calculated because lambs are usually tagged and have parentage recorded within 24 hours of birth as part of Signet recording. 

“Lleyn Gold is starting to show through in the flock,” says Mr West. “One thing that does stand out is that mature ewes that achieve Lleyn Gold usually do so repeatedly.” 

Last year, 20% of the ewe tegs joining the flock from Lleyn Gold dams achieved Lleyn Gold, while this year, as two-shears, it was 60%.  

Mr West explains that simplicity is key to running his small flock on a commercial basis, with feed used only to supplement grass growth.

Clover, yarrow and plantain have been stitched in permanent pasture to improve persistence, palatability, mineral availability and drought hardiness.

Malling flock has maintained weaning efficiency through challenging years







Lleyn Gold rate 






Daily lamb liveweight gains 






Average ewe weight 






Twinning rate 







• Good grass growing year. Little feeding needed

• Weekly or fortnightly rotational grazing

• Dry summer. Twinning rate dropped and lamb weights dropped

• Drought followed Beast from the East

• Fed ewes post-lambing for two weeks longer and creep fed to take pressure off ewes

• Tupping in 2018 was good. Rain helped ewes recover condition 

• 2019 was dry but neighbours’ land was used to winter sheep to rest home grass

• Started three-day paddock grazing in 2019 and seeing more grass and better recovery. 

• Little creep offered for lambs in May and will wean slightly earlier in May.