The effect of TM-QTL and other QTLs on lean meat yield and meat quality in sheep and its evaluation using VISA

Lead Research Organisation: SRUC
Department Name: Research

Abstract

Lamb production is an important part of UK agriculture, contributing >10% of total livestock output. It is also important for maintaining employment and infrastructure in rural communities and helping manage and enhance landscape and biodiversity. However, for the UK sheep industry to continue as a major producer and exporter of lamb it is essential to improve carcass quality, since currently only ca. 55% of UK lambs meet core target specifications. Improving carcass quality by increasing lean meat yield (LMY) without a corresponding increase in fatness has been the focus of breeding programmes since the mid 1980's. This has been achieved utilising new technologies such as ultrasound and more recently CT scanning. While such breeding results in cumulative and permanent gains, the penetration to date has been modest, not least because of the belief that current payment systems do not reward producers for increased LMY. This project will help to change this situation. Firstly, there is one highly promising technology (video image scanning and analysis; VISA) currently being evaluated as the basis for a future value-based marketing system in the UK. Secondly, funding bodies, in the UK and elsewhere, in recent years have invested heavily in research to identify genes or chromosomal segments (QTL) responsible for a significant part of the genetic variation for important production traits. The challenge now is to successfully integrate these QTLs into breeding programmes. An earlier LINK project has identified a number of QTLs affecting muscle growth, the one on chromosome 18 in Texels (TM-QTL) being the most promising. Development of the best strategies for introducing and managing this QTL requires reliable information on the magnitude of its effects on LMY as well as possible indirect effects on other important traits. This comprehensive evaluation of new QTLs is important as genes have direct AND indirect effects, e.g. the callipyge mutation having negative effects on meat quality. There are suggestions that some of these mutations may also have negative impacts on fertility and animal welfare. It is therefore essential to understand both the direct and indirect effects of TM-QTL before exploitation. Recent business developments will soon make it possible for UK farmers to exploit other QTLs (e.g. LoinMaxTM and MyoMaxTM have recently been identified and validated in NZ, and are known to enhance muscle growth and decrease fatness in some NZ breeds. In recognition of the stratified crossbreeding structure inherent in the UK sheep industry in which terminal sire rams are mated to Mule ewes within the lowland sector to supply around 70% of slaughter lambs, we will here test the effects of these QTLs in crossbred lambs under UK conditions. This project will link together the comprehensive evaluation of the TM-QTL and other QTLs with the development and evaluation of a VISA system. It is essential that any improvements in LMY arising from exploitation of the QTLs can be differentiated by the VISA system that is likely to become the industry standard. The project will produce a range of carcasses that will be subjected to detailed evaluation through both CT scanning and dissection; providing ideal data sets for investigating relationships between CT and VISA traits and to provide the first estimates of genetic parameters for VISA traits so that these can be built into selection programmes. The project will also investigate the mode of inheritance of the TM-QTL, as well as it's interactions with other genes in the different genetic background of different breeds. For example, are the magnitude of the QTL effects measured in crossbred lambs out of Mule ewes the same as those in purebred Texels? This project will provide the essential information, which is required to avoid potentially inappropriate developments and recommendations, as well as to provide the industry with appropriate new technologies that have been fully evaluated.

Technical Summary

This project brings together 2 interlinked developments needing further investigation: i) evaluation of QTLs increasing LMY, and ii) the potential of VISA to estimate LMY. Recent research in the UK and NZ has identified 3 QTLs increasing LMY. However, comprehensive validation of both their direct and indirect effects is needed. VISA based carcass classification is about to be introduced into the UK. It may underpin a value-based marketing system with the potential to reward farmers for the use of 'new genetics', but requires detailed validation and refinement to predict LMY. This project has therefore 2 main aims: 1) provide a comprehensive validation of three QTLs for increasing LMY, and 2) test and calibrate the VISA-system, which will be delivered through following sub-objectives (O): O1.1 evaluate the direct and indirect effects of the TM-QTL, in purebred Texels using 210 lambs; O1.2 evaluate the direct and indirect effects (as in O1.1) above) in crossbred lambs produced out of Mule ewes, using 70 wildtype and 70 heterozygous lambs; Os 1.3 and 1.4 evaluate the direct and some indirect effects of LM-QTL & MM-QTL, as per O1.2. O1.5 to evaluate the direct effects of the TM-QTL & MM-QTL in crossbred lambs out of Welsh Mountain ewes; O1.6 introgress both the TM-QTL & MM-QTL into purebred Inverdale-Texel sheep, and to introgress the TM-QTL into other Terminal Sire breeds using marker assisted backcrossing, to produce some carrier rams for more widespread introduction of the QTL within these breeds; O2.1 investigate the associations of VISA-traits with carcass composition measured by CT and dissection; O2.2 predict the relationships between VISA-traits and meat quality based on 210 purebred Texel and 140 crossbred lambs, representing the different TM-QTL genotypes; O2.3 use the TM-QTL, LM-QTL & MM-QTL to estimate their effect on VISA characteristics; O2.4 estimate genetic parameters for VISA-traits as a pre-requisiste for their integration into breeding programmes.

Publications

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Description (i) Evaluation of direct and indirect effects of 3 muscling QTL/genes
a) TM-QTL (on CHR18, originally identified in purebred UK Texel sheep, had been reported to increase US muscle depth by around 4-7%. Muscling in other parts of the body had not been investigated.
The QTL increased loin muscling (LM) by 4-11% in heterozygous animals (crossbreds and purebred Texels) when they inherited from the ram.
Phenotypic effects on muscling were largely restricted to the LM, with only minor effects on other carcass parts.
TM-QTL appeared to show polar overdominance for muscling traits (only heterozygous animals inheriting the mutated allele from the ram showed increased muscling).
Homozygous lambs had increased body weights and carcass weights (by 1.4kg) with body growth showing an additive mode of inheritance.
There was a small but significant increase in shear force in meat ageing < 7d, which disappeared after 7d ageing.
TM-QTL carrier lambs had slightly higher incidence of birth difficulties in purebred Texels, with no effect in crossbred lambs. Additionally, if dams that carry TM-QTL need lambing assistance, it is more likely to need a higher grade of assistance. However, if TM-QTL was of interest to industry any selection for TM-QTL could be done in conjunction with selection for lambing ease for which sufficient genetic variance is available as the project showed.

b) LM-QTL (LoinMax): This QTL also on CHR18 was found Australian Poll Dorset sheep and had been reported to increase LM-area and weight by 11and 8%, respectively.
LM-QTL has similar effects on loin muscling to TM-QTL. It increased LM by 8-14% in carrier lambs
Phenotypic effects were restricted to the loin area, with little evidence for effects on live or carcass weights
There were no negative effects of LM-QTL on lamb or ewe health or welfare for Poll Dorset sired crossbred lambs out of Mule ewes. It remains unknown whether TM-QTL and LM-QTL are allelic or not and whether they might have additive effects on loin muscling
c) MM-QTL (MyoMax): This mutation is in the myostatin region; in several mammalian species associated with "double-muscling". However, the effects of this ovine mutation seem small to moderate. Phenotypic effects of this QTL were investigated in crossbred lambs out of Mule and Welsh Mountain ewes.
In contrast to TM/ LM-QTL, the phenotypic muscling effects of MM are not confined to the LM. In general, lean growth is increased and fat growth reduced. This mutation seems to be partial recessive. Carrying two MM-copies had significant positive effects on 8w weight, muscle growth, a negative effect on UFD and MLC fat class.
(ii) Evaluation of VIA and its use to measure the effects of the muscling genes/QTL
Work closely related to this project showed that VIA, under abattoir conditions and at line speed (800 lambs/h), improved the prediction of primal meat yields compared to the current MLC classification.
VIA has been calibrated in this study against dissection and VIA reflected accurately and precisely the value of sheep carcasses: the R2 values for the various primals were between 0.88 and 0.97.
Data of >7000 animals with pedigree information incl. 630 with repeated VIA allowed to estimate genetic parameters for predicted primal weights. The repeatability was high (> 0.9) and h2 values were low to moderate (0.08 to 0.26), suggesting that VIA information in breeding programs could help to improve carcass quality.
Using the same dataset it was shown that the h2 values for subjective characters (fat class, conformation) were low (h2 = 0.1) (likely a reflection of the subjective nature of this assessment.
VIA based dimensional carcass measurements showed moderate to high h2 values (0.20-0.53), suggesting their potential use in breeding for improved conformation in crossbred lambs.
The capability of VIA to measure phenotypic effects of the muscling QTLs/genes was also investigated an varied with the size of the phenotypic effect
Exploitation Route see final report submitted to BBSRC in 2011
Sectors Agriculture, Food and Drink

 
Description see final report provided to BBSRC in 2011
Sector Agriculture, Food and Drink
Impact Types Societal,Economic