Investigating Poor Scanning Rates in Sheep Flocks

Lots of flocks will already have scanned for this year, although some are yet to do so. The barren rate within a flock should be <2%. Greater than that, (and certainly >4%) warrants investigation. Identifying a cause can be tricky and a diagnosis is not always reached due to the lack of samples available.

Below are some factors to consider following poor scanning results.

Reasons for a high barren rate include:

  • Poor conception rates due to females not cycling at the beginning of the mating period, or fertilisation failure. These cases may be identified as returns to oestrus (based on keel marks) or as empty at scanning.
  • Early embryonic death (EED) due to infectious causes or related to nutrition. These cases may be seen as late returns to oestrus, based on keel mark recording or may be identified by the scanner as resorbing embryos.


Poor conception rates – points to consider




Was mating time appropriate for the breed?


Moving the mating period too far from the natural timing for that breed will reduce conception rates

Were there any stressors around tupping time e.g. adverse weather?

Environmental stressors can impact upon mating behaviour, conception, and implantation of embryos

Were ewes in appropriate BCS at tupping?


BCS 2.5 to 3.5 depending on breed

Was their trace element status adequate/known?


Ideally sample and supplement ewes as required 6 weeks prior to tupping

Were hoggs/gimmers of appropriate size?


They should be at two thirds of their expected mature weight at tupping

Were they being fed brassicas?


Consider that ewes may have been on grass, while hoggs and gimmers were on rape or kale, the antioestrogenic effects of which could account for poorer scanning rates in the youngstock

Were they sound/not lame?


Lower conception rates may be seen with an increased incidence of lameness

Are GI parasites under control?

Ewes should be in top-notch condition for tupping. Consider worm egg counts

Is fluke under control?


Subacute fluke can have a major impact upon fertility – consider testing (fluke egg examination or coproantigen test)

If females were synchronised, was sponge removal and hormone treatment appropriate and timely?


Too high a dose of PMSG can result in multiple ovulations and potential loss of embryos

The timings on breeding programmes must be accurately adhered to, to ensure they work




Was tups’ BCS appropriate pre-tupping?

Ideally BCS 3.5 to 4

Were they free from disease/infection/lameness in the six weeks prior to, and throughout tupping?


This time-period is critical for healthy sperm production, and mating is an athletic pursuit

Did they undergo breeding soundness examinations?

Ideally performed a month pre-breeding

Was there sufficient ‘ram-power’?


The ratio of tups:ewes depends on tup maturity and whether the mating season has been advanced, or ewes have been synchronised



Early Embryonic Deaths

The main causes of EED are outlined below, along with their investigation options:


Border Disease

The outcome of infection with BDV depends on the stage of pregnancy when exposure occurred but early on, it results in EED. If identified, the farmer should remain vigilant for abortions and births of PI lambs.

Serology on empties/EEDs - if a significant seroprevalence is found amongst these animals, consider a PI hunt.

  • The period through which PI lambs are produced is longer than previously thought; PIs are not always classic ‘hairy-shakers’; PIs will/may have leukopaenia, as such they will be more predisposed to neonatal diseases.
  • Consider the BVD status of the farm if cattle are present.



Infection during early pregnancy can result in EED. Typically, the highest number of barren ewes is within the youngest age group.

Serology indicates past-infection – a single positive is not diagnostic for Toxoplasmosis; it only confirms that the animal has been exposed. Serology on younger stock may be more sensitive.


Subacute fluke

This can have profound effects on the ewe’s ability to maintain a pregnancy.

Faecal coproantigen testing can be useful to detect infection and potential drug-resistance should be investigated if animals were previously treated.



Infection within the first month of pregnancy can result in EED. This would require Cullicoides midge activity to coincide with tupping and early pregnancy. It is worth noting that SBV incidence is expected to be higher this year (the virus, due to waning herd immunity, is expected to peak approximately every four years).

Serology indicates previous exposure, although it could be carried out on barren hoggs who would not have been around during the previous midge season.


Selenium deficiency

This can cause early embryonic death 3 to 4 weeks after conception.

Biochemistry can be used to assess selenium levels (by measuring GSH-Px) and should drive the need for pre-breeding trace element assessment next season.


Iodine deficiency

This has been associated with reduced fertility.

Biochemistry can be used to assess plasma inorganic iodine levels.


Tick Borne Fever

Pyrexia associated with this could potentially cause EED.  Naïve animals e.g. recently introduced stock would be at most risk. It can also cause infertility in tups for a month post-infection.

PCR indicates if animals are infected.

In terms of prevention of poor conception and EED, health planning, maintaining excellent biosecurity, and investigating losses throughout this year’s pregnancies are important for future years.

*Serology and plasma inorganic iodine testing require a red-top tube. GSH-Px testing requires a green-top tube. Purple top tube (EDTA) is best for TBF PCR.

Posted by SRUC Veterinary Services on 15/02/2021

Tags: Sheep