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dscn2148The use of ultrasound scanning to measure muscle and fat depth in Signet recorded flocks has been one of the great success stories of the last 30 years. The scanning services provides proxy measures for body composition in sheep breeding programmes designed to enhance carcase yield and conformation, whilst optimising fatness.

Over the last 10 years alone the Signet team have scanned over 400,000 lambs from over 30 different breeds.

When should lambs be ultrasound scanned?

Traditionally breeders have been asked to scan lambs when they reach 21 weeks of age and weighing at least 45kg. This protocol ensured that variation could be identified between individuals – particularly in terms of fat depth, optimising the accuracy of data used to produce Estimated Breeding Values (EBVs).

A number of factors mean this protocol has been reviewed

  • Timeliness: In many flocks scanning lambs at 21 weeks of age a proportion of the lamb crop have either been sold for slaughter or are sold after scanning when they are too heavy and possibly fat to achieve the optimal commercial carcase value. This discourages breeders from scanning all their lambs and using EBVs to inform their decisions.
  • Commercial slaughter criteria: Prime lambs are marketed on the basis of live weight (to a maximum carcase weight of 21kg) and fat cover, not age. It could be argued that the future sires and maternal grandsires of prime lambs should be evaluated on criteria that reflect this commercial payment system as closely as possible.
  • Accuracy of measurement: The technology used in ultrasound scanning machines has advanced. We cannot assume that recommended minimum levels of fat for accurate measurement still apply.
  • Changing selection objectives: When ultrasound scanning was first introduced in the 1980’s reduction of fat in lamb carcases was a major priority. Since then considerable genetic improvement has been achieved. Increasing the lean meat content of the carcase through increased muscle growth now takes greater priority over reduction of fat depth per se. Maximising the variation expressed in fat depth measurements is now less important than it was.

Several small studies have been completed looking at scanning sheep at younger ages.

  • Signet commissioned work in two forage fed flocks of Lleyn and Suffolk sheep which showed a high repeatability in the ranking of lambs scanned between 15 and 19 weeks of age.
  • Repeatability trials undertaken by the Suffolk Sheep Society
  • Analysis of 2,756 commercial lambs scanned as part of RamCompare showed acceptable levels of variation being expressed for muscle and fat depth on fast finishing commercial systems where lambs were scanned between 10-16 weeks of age

AHDB also commissioned a more detailed piece of work undertaken by independent geneticist, Dr Janet Roden using Lleyn and Meatlinc datasets to see whether lambs could be scanned at younger ages. These breeds were chosen as they provided information on lambs reared under relatively commercial conditions.

Janet’s work concluded that:

  • Reducing age at scanning from 21 weeks to 17 weeks for Lleyn and Meatlinc wouldn’t significantly reduce genetic progress in live weight and ultrasonic muscle depth. It might result in slightly less progress in changes to fat depth.
  • The optimum live weight for scanning is in the range 35-45 kg for the Meatlinc and 30-45 kg in the Lleyn, which is consistent with the target live weight for the slaughter of commercial prime lambs.

 

What are the implications for Signet clients?

The key message is that breeders now have more flexibility in when they can present lambs for scanning – but they do need to use their judgement to ensure lambs are heavy enough and expressing variation in fat cover/finish.

Flocks that wish to can now scan lambs around 17 weeks of age, assuming:

  • a relatively narrow spread in lambing dates – to avoid scanning large numbers of very young lambs
  • lambs average at least 35kg and the lightest weigh over 30kg
  • lambs are expressing variation in fat cover /finish

This protocol will apply to terminal sire and maternal breeds. Signet’s protocol of hill breeds is unchanged as these lambs tend to be lighter and less well developed at scanning time.

What are Signet flocks doing now?

In recent years there has been a move to scan lambs at a younger age, with most of these flocks achieving satisfactory weights at this age. Some flocks are scanning lambs at lighter weights than recommended – however this doesn’t tend to be because they are young. When looking at differences between flocks, age has little bearing on scan weight – indicating the massive variation in performance that is seen between farms.

Studies looking at variation in fat depth and muscle depth also confirm that flocks with low levels of variation are more likely to be scanning lambs that are too small, rather than too young. So the message to breeders is clear, make sure lambs are well grown and heavy enough to scan rather focussing on getting them scanned at the right age.

How young can you go?

Janet’s study doesn’t make recommendations for lambs scanned younger than 17 weeks, as these are not widely represented within the datasets used – but it is logical that younger, well grown lambs can be scanned with confidence.

The main concern about scanning very young lambs is to what degree the variation in measurements expressed might be attributable to the maternal performance of a lamb’s mother as opposed to the genes of the lamb. This influence is already observed when analysing 8 week weights – where the analysis teases out direct and maternal genetic influences on lamb performance.chart-png

Chart by Emma Steele, Signet Breeding Services

 

Future research – Breeding better muscled sheep at commercial slaughter weights

Signet recorded flocks scan lambs at a wide variety of ages and weights. The genetic evaluations currently produced for Signet by SRUC/Egenes adjust muscle and fat depth measures for age at scanning, so that all animals are effectively compared at an equal age.

This effectively means sheep can achieve a high muscle depth EBV in two ways – being genetically very big or having a very deep muscle depth relative to their size.

Janet’s report also looked at whether genetic analyses ranked animals in the same way if data was adjusted for weight, rather than age – with a view to producing analyses that identify differences between sheep at the same weight – say 40kg live weight – rather than age.

Her provisional study indicates that when adjusting for weight, rather than age both traits are still highly heritable – but that animals do re-rank as a result of this approach. The new approach would favour animals that are very muscly for their size, rather than those that are just big. Further work at SRUC/EGENES will look at the implications of making this type of change to sheep evaluations – but with this work in mind, it may be advisable for flocks scanning lambs at very heavy weights to consider bringing their scanning age forward.

Click the link below to see the new Signet manual explaining the changes to Signet ultrasound scanning recommendations.

New Guidelines for Ultrasound Scanning of Sheep