Studies on the behavioural and genetic aspects of ewe rearing ability and lamb survival in South African sheep flocks

[摘要] English: The dissertation is based on a number of separate trials conducted since 1989. The centraltheme is ewe rearing ability and lamb survival, and the study includes 15 papers preparedover an 11-year period from 1992 to 2002. These papers are all linked by their emphasis onsheep production as influenced by lambing and neonatal behaviour and its interface withbreeding and management. The papers demonstrate the various phases the study wentthrough, and how it eventually transpired into an account of genetic change in lamb outputachievable through rigorous selection. The individual papers already contain abstracts. Thissummary is intended to provide readers with a broad overview of the most importantoutcomes of the study. The study was organised in three parts, as follows:Part 1:BackgroundInitially, two papers provided the background for the investigations into lamb survival andewe rearing ability. The first paper dealt with the average reproductive performance of sheepin the Southern Cape, and with levels of reproductive wastage in a number of flocks. Itdemonstrated that approximately one in four breeding ewes did not care for at least one lambat lamb marking. Losses were more or less evenly distributed between barrenness and rearingfailure. It was impossible to identify managerial practices conclusively associated with agood reproductive performance. Evidence was provided that an improvement in thereproduction of the current flock can be achieved by a simple method of selection that can bepracticed by commercial farmers with minimal record keeping.The second paper considered rearing ability in four experimental flocks of the Merino, SAMutton Merino and Dormer breeds. A minority of ewes was shown to contribute markedly tothe observed levels of rearing failure. Animals classified as being good or poor motherson their lamb rearing history were shown to differ in one or more pelvic dimensions in twoMerino flocks and the SA Mutton Merino flocks. Ewes classified as good had largerdimensions. Rearing ability was also demonstrated to be repeatable in the Tygerhoek Merinoflock, where sufficient data were available.Part 2: The dynamics of behaviour in lambing flocksThis part of the dissertation included four papers. The first paper detailed aspects ofparturition and separation from one or more lamb in Dormer and SA Mutton Merino ewes.Length of parturition was found to be dependent on breed, with Dormer ewes having shorterparturitions than SA Mutton Merinos. They also were less likely to be assisted and Dormerlambs were less likely to die during or shortly after parturition. Across years, length ofparturition was also repeatable in ewes. Early movement (within two hours) of ewes from thelambing site and high concentrations of lambed ewes in the same paddock were found toresult in higher levels of separation from one or more lambs, resulting in lamb mortality.The second paper detailed neonatal progress in lambs and related it to birth weight and lambsurvival. Survival was affected by neonatal progress as well as by live weight gain from birthto 3 days in lambs. No breed differences were found for the interval from birth to standing orfrom standing to apparently suckling in Dormer and SA Mutton Merino lambs. It was,however, found that the traits were moderately heritable, using paternal halfsib methods on arelatively small data set.The mechanisms of difficult births were subsequently investigated in Dormer and SA MuttonMerino ewes, since the two breeds were markedly different in this respect. The etiology ofdifficult births in the breeds was found to differ vastly. Dystocia and feto-pelvicdisproportions commonly led to prolonged parturitions and assistance in SA Mutton Merinos.Uterine inertia were more likely to be observed in Dormers. The ability of factors associatedwith birth difficulties (pelvic dimensions, litter size and weight, ewe live weight andconformation) to predict length of parturition within breeds was, however, limited.In the final paper of this part of the dissertation, it was possible to partition the geneticvariances for behaviour traits observed during lambing in Dormer and SA Mutton Merinoewes. Maternal additive variance ratios for length of parturition were moderate andsignificant in both breeds. Neonatal progress (time intervals from birth to standing and fromstanding to apparently suckling) was lowly heritable in both breeds. The interval fromstanding to suckling was also influenced by the maternal permanent environmental variancein SA Mutton Merinos. Direct and maternal breeding values for behavioural traits differedsignificantly between lambs that survived and those that died in most cases. This resultsuggested a genetic association of lamb mortality with lambing and neonatal behaviour.Part 3: Responses to selection for ewe multiple rearing abilityThis part of the dissertation was based on observed responses in two Merino lines that weredivergently selected for ewe multiple rearing ability since 1986. The line selected in anupward direction for multiple rearing ability is referred to as the High (H) line. The lineselected downwards is referred to as the Low (L) line. This part of the study consists of ninepapers.In the first paper, separation of ewes from one or more lambs was studied in the Merino linesreferred to above. Ewes in the H line were more likely to be separated from one or morelambs because of interference, and they tended to be more interested in other parturient ewes.Ewes in the L line tended to be more likely to desert their lamb(s). Ewes that left their birthsites within two hours of birth were more likely to be separated from one or more offspring.The ability of ewes and lambs from the Hand L lines to recognize each other wereinvestigated next. Ewes in the H line were able to find their lambs sooner after the lambbeing tethered 20 m away, compared to their L line contemporaries. They were also able toestablish contact with all litter members in multiples sooner after finding a tethered lamb.Lambs in the H line tended to be more likely to bleat and tug on the tethering rope whentethered. They also followed their dams more closely when chased away with their dams atthree days of age. The paper allowed the estimation of preliminary genetic parameters formutual recognition of one another by ewes and lambs, although it was not the primaryobjective. Significant maternal genetic variances were found for most of the traits.The following paper demonstrated a better lamb survival in H line lambs, despite a highermultiple birth rate. Line differences were reported, mostly involving behavioural adaptationsconducive to lamb survival in the H line. Ewes in the H line generally experienced shorterbirths, and H line lambs were less likely to succumb during or soon after parturition. Ewes inthe H line showed better cooperation with the first suckling attempts of their lambs, and Hline lambs apparently suckled sooner after standing than L line contemporaries. This linedifference remained after correction for the better cooperation of H line ewes. A markedadvantage in terms of weight of lamb weaned per breeding ewe in the H line wasdemonstrated for the five years of this study, when compared to the L line.It was found that a substantial proportion of births will not be supervised if the supervision ofMerino ewes in South Africa was confined to daylight hours in intensive systems. A largerthan expected frequency of very short intervals between subsequent onsets of parturition wasobserved. If the contention that births would trigger each other could be substantiated, itcould be of benefit to quantify the mechanism involved. Knowledge of this would facilitatethe synchronisation of natural births in pasture lambing flocks, where it is impractical to alterthe distribution of births by nutrition. Relatively few line differences were found in thispaper. The distribution of birth sites within paddocks could not be related to selection line.Ewes in the H line ewes were more likely to groom their lambs shortly after birth, while Lline ewes commenced grazing sooner after birth. These behavioural adaptations in the H linewould generally facilitate lamb survival. Behavioural data from the H and L lines were recorded over a lO-year period and subjected toa genetic analysis. The line differences obtained earlier could be confirmed in this study.Genetic parameters estimated from the data indicated that behaviour traits in lambs and eweswere lowly to moderately heritable. In lambs, a significant heritability estimate was derivedfor the interval from birth to standing. Maternal heritability estimates were significant forlength of parturition and for the interval from birth to standing. Maternal permanentenvironmental variances were significant for maternal cooperation and the interval fromstanding to apparently suckling. When behavioural traits were assessed in ewes, length ofparturition, maternal behaviour score and the period that ewes remained on or near the birthsite were found to be moderately heritable. Genetic divergence was found for the H linecompared to the L line. The derived genetic trends indicated changes conducive to lambsurvival in the H line.Direct additive, maternal additive and maternal permanent environmental variance ratios weresubsequently derived for lamb birth weight, birth coat score and lamb weaning weight. Birthcoat score was highly heritable and not influenced maternally. Genetic and phenotypic trends indicated divergence between the H and L lines for weaning weight over the period ofassessment. Genetic divergence was particularly strong in the case of direct breeding valuesfor weaning weight. Birth weight and birth coat score were independent of selection line.Genetic parameters were estimated for annual reproduction traits, ewe greasy fleece weightand ewe body weight at joining, using a repeatability model. Heritability estimates forreproduction traits were low, consistently below 10 % of the overall phenotypic variance.Ewe permanent environmental variances were higher, between 7 and 12 %. Geneticcorrelations of reproduction traits with ewe joining weight were favourable and very high inthe case of weight of lamb weaned per ewe. Corresponding genetic correlations with ewegreasy fleece weight were low and variable. Ewe permanent environmental correlations ofreproduction traits with ewe joining weight and greasy fleece weight were unfavourable andhigh in some instances. Phenotypic trends indicated divergence in the expected directionbetween the Hand L lines in the reproduction traits considered. These tendencies wereconfirmed by genetic trends based on averaged predicted breeding values within birth years.The results confirmed that genetic progress at a rate of 1-2 % per year was attainable inreproduction traits, despite low additive variance ratios.Genetic parameters were estimated for hogget wool and live weight traits, as well as for testismeasurements. Adequate genetic variation for exploitation in a well-structured breedingprogram was estimated in all traits. Genetic trends indicated change towards heavier andplainer sheep in the H line, while L line contemporaries became smaller and more developed.Genetic trends for testis circumference suggested divergence in the expected directionbetween lines. This result was, however, associated with the genetic change in live weight,since no line difference was found in testis circumference corrected for live weight. Genetictrends for clean fleece weight and fibre diameter indicated no divergence between lines.Age influences on reproduction, wool traits and live weight were assessed in H and L lineewes subjected to at least 10 years of divergent selection. Genetic parameters derived fromthis data set were generally consistent with previous estimates, despite the usage of a muchsmaller data set. The Hand L lines differed markedly for reproduction, but no differences inthe shape of age trends were found between lines. Compared to average L line performance, the superiority of H line ewes amounted to 56 % in the case of weight of lamb weaned perewe. Ewes in the H line were heavier than L line contemporaries at two-tooth age, but theline difference disappeared at later ages. Wool production of L line ewes were heavier thanthose of their H line contemporaries, particularly in the middle age groups (3 to 6 years),where the highest fleece weights are expected. Fibre diameter increased with ewe age, withno apparent line difference. It was contended that the stress of increased reproductionresulted in the different age trends for the respective selection lines for live weight and greasyfleece weight.In the final instance, this dissertation provides a framework for the genetic improvement oflamb output in pasture-fed sheep by selection for reproductivity. Unwanted correlated geneticchanges were minimal, although the stress associated with an increased reproduction impairedlive weight and wool traits in mature animals to an extent. The interface between behaviour,lamb survival and ewe rearing ability were clarified for a number of South African sheepbreeds, leading to a better understanding of the dynamics involved. These results contributedto the formulation of adapted husbandry and breeding practices, in the context of overall flockproductivity. Results pertaining to behaviour appeared to be fairly robust across studiesinvolving different resource populations, and should be appropriate for extension to thebroader industry.

[发布机构] University of the Free State