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Ovine Observer

Using scanned fetal numbers to inform reproduction trait performance in the MERINOSELECT evaluation

Dr Forbes Brien, University of Adelaide, SA; Dr Kim Bunter, Animal Genetics and Breeding Unit (AGBU), Armidale NSW; and Peta Bradley, Sheep Genetics, Armidale NSW

Author correspondence: forbes.brien@adelaide.edu.au

Introduction

Reproduction rates in wool sheep are an important contributor to profit, particularly in recent years where high returns have been received for lamb and sheep meat. Interest in improving sheep reproductive rates is widespread, not only through improved nutrition and innovations in general management, but also from genetic improvement programs.

To achieve effective genetic gain in reproductive rates requires accurate recording of reproductive performance for individual ewes. This normally includes records on lambs born alive and dead, collected at lambing time, pedigree information, and records of failure of individual ewes to conceive. However, the majority of Merino breeders have traditionally not collected much or any of this information. While other methods of collecting pedigree information are used, such as mothering up lambs in yards after lambing, using DNA testing or electronic systems to associate lambs with their dams, these generally are not able to capture any information on lamb losses when used in isolation, which invariably occur either at birth or within the first 3 days after birth. Further, a ewe’s failure to conceive must be actively recorded.

Pregnancy scanning data can help

In addition to data on lambs, Sheep Genetics use pregnancy scanning data submitted by breeders to inform conception and litter size traits in the MERINOSELECT and LAMBPLAN analyses where birth data is not available. Pregnancy scanning identifies which ewes did or didn’t conceive, and the fetal count at scanning. The fetal count represents the litter size. This data is used in the calculation of Australian Sheep Breeding Values (ASBVs) for conception (CON) and litter size (LS), whilst data on the proportion of the litter weaned informs ewe rearing ability (ERA). Together these component traits of reproduction (CON, LS and ERA) make up the weaning rate (WR) ASBV.

How does it work?

Pregnancy scanning records detailing the number of fetuses present per ewe are submitted by the breeder or their service provider to Sheep Genetics, alongside a mating record for individual ewes. This means scanning data can be used to evaluate conception outcomes and litter size. Recording performance for ewe rearing ability (the proportion of lambs born which are reared) is not possible using pregnancy scan data alone. However, breeders can use pregnancy scanning in combination with pedigree methods such as DNA parentage data to inform the Sheep Genetics database of lamb losses. With this information, Sheep Genetics generates ASBVs for the component reproduction traits (CON, LS and ERA) as well as the combined weaning rate (WR) ASBV.

Accuracy of pregnancy scanning for genetic evaluation

The agreement between litter size (lambs born per ewe) assessed by pregnancy scanning and litter size reported by mothering up at birth was investigated in a study of data submitted by ram breeder flocks (approximately 66 000 adult ewe records) to MERINOSELECT, the national genetic evaluation scheme for wool sheep. This study was part of the MLA/AWI-funded project L.LSM.0021 ‘Increasing lambing percentages through better use of pregnancy scanning technology’. Results are shown in Figure 1.

Figure 1. Mean litter size observed at birth or afterwards vs. mean scanned litter size
Figure 1. Mean litter size observed at birth or afterwards vs. mean scanned litter size

Many breeders do not mother up at birth, and therefore lambs observed are typically lower than lambs scanned due to lamb losses after birth. When the difference in means for scan count versus lamb count is >0.1, we know from other research activities that this exceeds expected lamb losses post birth. The blue dots in Figure 1 represent situations where this is not the case, and there is very good agreement in means for scans vs lambs. In contrast, the orange dots demonstrate that for every lamb scanned, on average 0.8 lambs are observed at weaning, which is typical of overall mortality rates.

This study demonstrates that scanned litter size data is a very good indicator of the reproductive rate of ewes, when account is taken of lamb losses, and can make a valuable contribution to the genetic evaluation of sheep for reproductive traits.

Further information

For more information on recording reproduction or MERINOSELECT visit the Sheep Genetics website https://www.sheepgenetics.org.au/ or phone +61(0)2 8055 1818.

Funding for the L.LSM.0021 Project ‘Increasing lambing percentages through better use of pregnancy scanning technology was provided by Meat and Livestock Australia and Australian Wool Innovation Limited.