Genetic selection for health and welfare traits in lambs
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Lamb mortality remains a significant welfare and economic issue for sheep production. Two significant causes of mortality are dystocia and low lamb vigour; both requiring high levels of human care to ensure lamb survival. Genetic solutions to reduce lamb mortality and its main causes (dystocia and low vigour lambs) are desirable, with at least two possible solutions available: (i) use of suitable breeds or strains and (ii) intrabreed selection. Approach (i) requires the existence of breed/strain differences in the desired traits and approach (ii) needs sufficient intrabreed genetic variance. Reproductive and behavioural traits are, however, difficult to quantify and measure on farm. On dedicated research farms, lamb vigour has been measured using latencies to perform specific behaviours (e.g. standing and sucking) but this methodology is difficult to transfer to a commercial setting - timed behavioural traits are not as easy to measure on farm when compared with categorical indicator traits. Therefore, proxy methods (categorical scoring systems) are needed to measure behaviour traits in a manner that allows for collection of sufficient data to enable genetic analysis. The main purpose of this thesis was to develop such proxy methods, to estimate the heritability of lamb traits, and thus to investigate whether it is possible to improve the welfare of lambs through selection of parents with superior vigour and lambing ease characteristics. Scoring systems were developed for quantifying neonatal lamb fitness and behaviour traits. Detailed historical behaviour data were analysed to develop criteria for three scores: birth assistance (BA), lamb vigour (LV) and sucking assistance (SA). These scoring systems were then validated in a separate flock by simultaneously recording scores and the latency to perform certain landmark behaviours. The results obtained indicated that the scoring systems developed were a practical, reliable and sensitive indicator of lamb fitness traits. To determine whether neonatal lamb vigour traits were heritable, scores from the scoring systems previously developed and validated were recorded in an experimental flock of pure-bred Texel sheep for the purpose of estimating genetics parameters for each trait. Results indicated that heritabilities for all traits range from low-moderate, BA 0.43 (s.e. 0.063), LV 0.15 (s.e. 0.059), SA (0.27 (s.e. 0.045), suggesting there is sufficient variation present within this Texel sub-population to allow for selection for improved neonatal fitness traits. Thus far, we have determined that neonatal traits are heritable and can be measured using proxy scoring systems. The next step was to establish whether the proxy scores developed were feasible, in a commercial setting, for the mass data collection needed for estimation of genetic parameters and to determine the relationship between neonatal traits and later production traits, with the aim of integrating this data into breeding programmes. A total of 11,092 lambs with complete neonatal records, from 290 flocks belonging to the Industrial Partner, the Suffolk Sheep Society (UK), were analysed to report the genetic variance present within the UK population of registered pure-bred Suffolk sheep. The results from this analysis show that heritabilities were moderate for BA, 0.26 (s.e. 0.03), LV, 0.40 (s.e. 0.04) and SA, 0.32 (s.e. 0.03) with genetic correlations between neonatal traits all moderate to high and positive. This demonstrates that neonatal fitness traits can have heritabilities comparable to those of production traits. The analysis also shows that neonatal survival traits of birth assistance and sucking assistance are moderately heritable, when treated as a lamb trait rather than a sire or ewe trait, indicating the selection should target the lambs in order to successfully, and efficiently, improve survival. A possible alternative method for improving dystocia and lamb vigour would be to introgress genes for improved lambing ease and lamb vigour from the New Zealand strain of Suffolk sheep into the British Suffolk strains. However, there has been no published record of how much ‘NZ genetics’ would improve (or compare to) British Suffolk’s under standard UK management practises. Therefore, the objectives of this study were to examine possible differences in neonatal behavioural traits (birth assistance, lamb vigour and sucking assistance) between NZ and UK Suffolks when used as terminal sires on commercial cross-bred ewes. Thus, neonatal scores from cross-bred lambs sired by rams from one of the three main Suffolk strains currently used in the UK were compared. The analysis indicated there was no significant effect of sire strain on any of the neonatal traits, and that individual sire variation was greater than the variation between the strains. In conclusion, the work contained within this thesis shows that neonatal lamb behaviour traits can be measured accurately and easily using well-realised and biologically relevant scoring systems. Furthermore, these scoring systems are a feasible and practical method of measuring neonatal lamb vigour which may be used to evaluate management systems and to improve selection criteria for neonatal traits.