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

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Cloete, S. W. P.
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University of the Free State
English: The dissertation is based on a number of separate trials conducted since 1989. The central theme is ewe rearing ability and lamb survival, and the study includes 15 papers prepared over an 11-year period from 1992 to 2002. These papers are all linked by their emphasis on sheep production as influenced by lambing and neonatal behaviour and its interface with breeding and management. The papers demonstrate the various phases the study went through, and how it eventually transpired into an account of genetic change in lamb output achievable through rigorous selection. The individual papers already contain abstracts. This summary is intended to provide readers with a broad overview of the most important outcomes of the study. The study was organised in three parts, as follows: Part 1:Background Initially, two papers provided the background for the investigations into lamb survival and ewe rearing ability. The first paper dealt with the average reproductive performance of sheep in the Southern Cape, and with levels of reproductive wastage in a number of flocks. It demonstrated that approximately one in four breeding ewes did not care for at least one lamb at lamb marking. Losses were more or less evenly distributed between barrenness and rearing failure. It was impossible to identify managerial practices conclusively associated with a good reproductive performance. Evidence was provided that an improvement in the reproduction of the current flock can be achieved by a simple method of selection that can be practiced by commercial farmers with minimal record keeping. The second paper considered rearing ability in four experimental flocks of the Merino, SA Mutton Merino and Dormer breeds. A minority of ewes was shown to contribute markedly to the observed levels of rearing failure. Animals classified as being "good" or "poor" mothers on their lamb rearing history were shown to differ in one or more pelvic dimensions in two Merino flocks and the SA Mutton Merino flocks. Ewes classified as "good" had larger dimensions. Rearing ability was also demonstrated to be repeatable in the Tygerhoek Merino flock, where sufficient data were available. Part 2: The dynamics of behaviour in lambing flocks This part of the dissertation included four papers. The first paper detailed aspects of parturition 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 shorter parturitions than SA Mutton Merinos. They also were less likely to be assisted and Dormer lambs were less likely to die during or shortly after parturition. Across years, length of parturition was also repeatable in ewes. Early movement (within two hours) of ewes from the lambing site and high concentrations of lambed ewes in the same paddock were found to result 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 lamb survival. Survival was affected by neonatal progress as well as by live weight gain from birth to 3 days in lambs. No breed differences were found for the interval from birth to standing or from 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 a relatively small data set. The mechanisms of difficult births were subsequently investigated in Dormer and SA Mutton Merino ewes, since the two breeds were markedly different in this respect. The etiology of difficult births in the breeds was found to differ vastly. Dystocia and feto-pelvic disproportions 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 associated with birth difficulties (pelvic dimensions, litter size and weight, ewe live weight and conformation) 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 genetic variances for behaviour traits observed during lambing in Dormer and SA Mutton Merino ewes. Maternal additive variance ratios for length of parturition were moderate and significant in both breeds. Neonatal progress (time intervals from birth to standing and from standing to apparently suckling) was lowly heritable in both breeds. The interval from standing to suckling was also influenced by the maternal permanent environmental variance in SA Mutton Merinos. Direct and maternal breeding values for behavioural traits differed significantly between lambs that survived and those that died in most cases. This result suggested a genetic association of lamb mortality with lambing and neonatal behaviour. Part 3: Responses to selection for ewe multiple rearing ability This part of the dissertation was based on observed responses in two Merino lines that were divergently selected for ewe multiple rearing ability since 1986. The line selected in an upward direction for multiple rearing ability is referred to as the High (H) line. The line selected downwards is referred to as the Low (L) line. This part of the study consists of nine papers. In the first paper, separation of ewes from one or more lambs was studied in the Merino lines referred to above. Ewes in the H line were more likely to be separated from one or more lambs 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 birth sites 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 were investigated next. Ewes in the H line were able to find their lambs sooner after the lamb being tethered 20 m away, compared to their L line contemporaries. They were also able to establish 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 when tethered. They also followed their dams more closely when chased away with their dams at three days of age. The paper allowed the estimation of preliminary genetic parameters for mutual recognition of one another by ewes and lambs, although it was not the primary objective. 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 higher multiple birth rate. Line differences were reported, mostly involving behavioural adaptations conducive to lamb survival in the H line. Ewes in the H line generally experienced shorter births, and H line lambs were less likely to succumb during or soon after parturition. Ewes in the H line showed better cooperation with the first suckling attempts of their lambs, and H line lambs apparently suckled sooner after standing than L line contemporaries. This line difference remained after correction for the better cooperation of H line ewes. A marked advantage in terms of weight of lamb weaned per breeding ewe in the H line was demonstrated 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 of Merino ewes in South Africa was confined to daylight hours in intensive systems. A larger than expected frequency of very short intervals between subsequent onsets of parturition was observed. If the contention that births would trigger each other could be substantiated, it could be of benefit to quantify the mechanism involved. Knowledge of this would facilitate the synchronisation of natural births in pasture lambing flocks, where it is impractical to alter the distribution of births by nutrition. Relatively few line differences were found in this paper. 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 L line ewes commenced grazing sooner after birth. These behavioural adaptations in the H line would generally facilitate lamb survival. Behavioural data from the H and L lines were recorded over a lO-year period and subjected to a 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 ewes were lowly to moderately heritable. In lambs, a significant heritability estimate was derived for the interval from birth to standing. Maternal heritability estimates were significant for length of parturition and for the interval from birth to standing. Maternal permanent environmental variances were significant for maternal cooperation and the interval from standing to apparently suckling. When behavioural traits were assessed in ewes, length of parturition, maternal behaviour score and the period that ewes remained on or near the birth site were found to be moderately heritable. Genetic divergence was found for the H line compared to the L line. The derived genetic trends indicated changes conducive to lamb survival in the H line. Direct additive, maternal additive and maternal permanent environmental variance ratios were subsequently derived for lamb birth weight, birth coat score and lamb weaning weight. Birth coat 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 of assessment. Genetic divergence was particularly strong in the case of direct breeding values for weaning weight. Birth weight and birth coat score were independent of selection line. Genetic parameters were estimated for annual reproduction traits, ewe greasy fleece weight and ewe body weight at joining, using a repeatability model. Heritability estimates for reproduction traits were low, consistently below 10 % of the overall phenotypic variance. Ewe permanent environmental variances were higher, between 7 and 12 %. Genetic correlations of reproduction traits with ewe joining weight were favourable and very high in the case of weight of lamb weaned per ewe. Corresponding genetic correlations with ewe greasy fleece weight were low and variable. Ewe permanent environmental correlations of reproduction traits with ewe joining weight and greasy fleece weight were unfavourable and high in some instances. Phenotypic trends indicated divergence in the expected direction between the Hand L lines in the reproduction traits considered. These tendencies were confirmed 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 in reproduction traits, despite low additive variance ratios. Genetic parameters were estimated for hogget wool and live weight traits, as well as for testis measurements. Adequate genetic variation for exploitation in a well-structured breeding program was estimated in all traits. Genetic trends indicated change towards heavier and plainer sheep in the H line, while L line contemporaries became smaller and more developed. Genetic trends for testis circumference suggested divergence in the expected direction between 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. Genetic trends 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 line ewes subjected to at least 10 years of divergent selection. Genetic parameters derived from this data set were generally consistent with previous estimates, despite the usage of a much smaller data set. The Hand L lines differed markedly for reproduction, but no differences in the 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 per ewe. Ewes in the H line were heavier than L line contemporaries at two-tooth age, but the line difference disappeared at later ages. Wool production of L line ewes were heavier than those 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, with no apparent line difference. It was contended that the stress of increased reproduction resulted in the different age trends for the respective selection lines for live weight and greasy fleece weight. In the final instance, this dissertation provides a framework for the genetic improvement of lamb output in pasture-fed sheep by selection for reproductivity. Unwanted correlated genetic changes were minimal, although the stress associated with an increased reproduction impaired live 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 sheep breeds, leading to a better understanding of the dynamics involved. These results contributed to the formulation of adapted husbandry and breeding practices, in the context of overall flock productivity. Results pertaining to behaviour appeared to be fairly robust across studies involving different resource populations, and should be appropriate for extension to the broader industry.
Afrikaans: Die proefskrif is gebaseer op 'n aantal afsonderlike studies wat sedert 1989 uitgevoer is. Die onderwerp wat ondersoek is, is die grootmaakvermoë van ooie en oorlewing van lammers. Die studie sluit 15 artikels wat oor 'n l l-jaartydperk van 1992 tot 2002 geskryf is, in. Die artikels handel almaloor skaapproduksie, die invloed van gedrag tydens lam en die neonatale fase daarop, sowel as die wisselwerking daarvan met aangepaste teelten bestuurspraktyke. Die artikels dui op die fases waardeur die studie gegaan het, en hoe daar gevorder is om genetiese vordering in lamopbrengs aan te toon. Die ingeslote artikels sluit reeds Engelse opsommings in. Hierdie opsomming verskaf dus slegs 'n breë oorsig aangaande die belangrikste bevindings van die studie. Die navorsing word in drie dele opgedeel, as volg: Deel 1: Agtergrond Twee artikels het aanvanklik agtergrond verskaf vir die ondersoek van lamoorlewing en ooigrootmaakvermoë. Die eerste artikel het oor die gemiddelde reproduksie en vlakke van produksieverliese van skaapkuddes in die Suid-Kaap gehandel. Tydens merktyd is gevind dat ongeveer een uit elke vier teeltooie nie een of meer lammers versorg het nie. Verliese is ongeveer gelyk verdeel tussen onvrugbaarheid en grootmaakmislukkings. Dit was onmoontlik om bestuursfaktore wat onomstootlik aan 'n goeie reproduksietempo verwant was te identifiseer. Die artikel het aangedui dat 'n verbetering in die reproduksie van die huidige kudde bewerkstellig kan word deur 'n eenvoudige stelsel van seleksie wat met minimale rekordhouding deur kommersiële produsente toegepas kan word. The tweede artikel het die grootmaakvermoë van ooie in vier eksperimentele kuddes (Merino, SA Vleismerino en Dormer) ondersoek. 'n Minderheid ooie het grootliks bygedra tot die totale grootmaakmislukking wat waargeneem is. Ooie wat op grond van hulle produksiegeskiedenis as "goed" of "swak" geklassifiseer is, het by die Merino of SA Vleismerinokuddes ten opsigte van een of meer pelvisafmetings verskil. Ooie wat as goed geklassifiseer is, het groter pelvisafmetings gehad. Grootmaakvermoë was herhaalbaar in die Tygerhoek Merinokudde, waar voldoende waarnemings VIr herhaalbaarheidsberamings beskikbaar was. Deel 2: Die dinamika van gedrag in lammerkuddes Hierdie gedeelte van die proefskrif het vier artikels beslaan. Die eerste hiervan het aspekte van die geboorteproses, sowel as die skeiding van ooie van een of meer lammers by SA Vleismerinos en Dormers ingesluit. Duur van geboorte was verwant aan ras, met Dormers wat gouer geboorte gegee het as SA Vleismerinos. Dormers is ook minder gehelp om te lam, en hulle lammers was minder geneig om gedurende of kort na geboorte te vrek as by SA Vleismerinos. Duur van geboorte by ooie was verder herhaalbaar oor Jare. Ooie wat vroeg van hulle lamplekke padgegee het, en hoë konsentrasies lammerooie in 'n kamp het skeiding van een of meer lammers in die hand gewerk. Neonatale gedrag in lammers is in 'n tweede artikelondersoek, en in verband gebring met geboortegewig en oorlewing. Oorlewing was fenotipies verwant aan neonatale vordering, sowel as aan gewigstoename van geboorte tot 3 dae. Dormers en SA Vleismerinos het nie verskil ten opsigte van die tydperke van geboorte tot staan, of vanaf staan tot oënskynlik suip nie. Die eienskappe was egter matig oorerflik, soos bepaal met 'n halfsibmodel op 'n betreklike klein datastel. Die prosesse betrokke by moeilike geboortes is vervolgens by Dormers en SA Vleismerinos ondersoek. Die oorsake van moeilike geboortes het aanmerklik tussen die rasse verskil. By SA Vleismerinos was distokia en feto-pelvis wanverhoudings meerendeels by verlengde geboortes en hulp tydens geboorte betrokke. Baarmoedertraagheid het meer algemeen by Dormers voorgekom. Alhoewel verskeie faktore met duur van geboorte in verband gebring is (o.a. pelvisafmetings, werpselgrootte en -gewig, ooi liggaamsgewig en -bouvorm), was dit onmoontlik om duur van geboorte met 'n groot mate van sekerheid te voorspel. In die laaste artikel van hierdie gedeelte, was dit moontlik om genetiese variansies vir lamgedrag (wat tydens intensiewe waarnemingsperiodes by Dormers en SA Vleismerinos bekom is) te beraam. Maternale genetiese variansie verhoudings vir duur van geboorte was matig en betekenisvol in beide rasse. Neonatale vordering (die tydperke vanaf geboorte tot staan en vanaf staan tot oënskynlik suip) was laag oorerflik in beide rasse. Die tydperk vanaf staan tot oënskynlik suip is ook deur 'n maternale permanente omgewingseffek beïnvloed in SA Vleismerinos. Direkte en maternale teeltwaardes vir gedragseienskappe het meestal betekenisvol verskil tussen lammers wat oorleef het, en die wat gevrek het. Hierdie resultaat het op 'n genetiese verwantskap van lamvrektes met lam- en neonatale gedrag gedui. Deel 3: Response op seleksie vir die vermoë van ooie om meerlinge groot te maak Die deel van die proefskrif is gebaseer op waargenome response in twee Merinolyne wat uiteenlopend vir maternale grootmaakvermoë geselekteer is. Daar word na die lyn wat vir die vermoë om meerlinge groot te maak geselekteer is, verwys as die Hoë (H) lyn. Die lyn wat afwaarts geselekteer is, staan ooreenstemmend bekend as die Lae (L) lyn. Die gedeelte van die pro efskri f het nege artikels beslaan. In die eerste artikel is skeiding van ooie van een of meer lammers in die lyne wat bo vermeld is, ondersoek. Ooie in die Il-lyn was meer geneig om weens inmenging van een of meer lammers geskei te word, en was meer geïnteresseerd in ooie wat in die proses van lam was. Ooie in die L-Iyn was meer geneig om hulle lammers te verlaat. Skeiding van ooie van een of meer lammers was verwant aan die tydperk wat ooie op die lamplek deurgebring het. Ooie wat hulle lamplekke binne twee ure verlaat het, was meer geneig om van een of meer lammers geskei te word as die wat langer gebly het. Die vermoë van OOIe en lammers om mekaar wedersyds te herken, is vervolgens ondersoek. Ooie in die H-lyn het hulle lammers gouer as L-Iyntydgenote gevind nadat hulle 20 m weg vasgemaak is. Hulle was ook in staat om by meerlinge gouer kontak met alle lammers te bewerkstellig nadat 'n vasgemaakte lam gekry is. Lammers in die If-lyn het geneig om meer te blêr en aan die tou te rem wanneer hulle vasgemaak is. Hulle het ook hulle moeders nader gevolg as hulle op drie dae aangejaag is. Die artikel het die beraming van voorlopige genetiese parameters vir die wedersydse herkenning van ooie en lammers toegelaat. Betekenisvolle maternale genetiese variansies is vir die meeste van die eienskappe verkry. Die volgende artikels het 'n beter lamoorlewing in die H-Iyn verkry, ten spyte van 'n hoër meerlinggeboortetempo. Lynverskille is gevind, in die meeste gevalle vir gedragseienskappe wat lamoorlewing in die hand sal werk ten gunste van die Il-lyn. Ooie in die H-Iyn het in die algemeen korter gevat om te lam, terwyl H-Iyn lammers minder geneig was om tydens of net na geboorte te vrek as L-Iyntydgenote. Ooie in die H-Iyn het beter samewerking met die eerste suippogings van hulle lammers getoon, en Hlynlammers het gouer na staan oënskynlik gesuip as L-Iyntydgenote. Die lynverskil is steeds waargeneem nadat die beter samewerking van H-Iynooie met hulle lammers se eerste suippogings in ag geneem is. 'n Aansienlike voordeel in terme van gewig lam gespeen per teeltooi is oor die vyf jaar van die studie aangetoon, in vergelyking met die prestasie in die L-Iyn. Indien toesig slegs tydens dagligure aan lammerooie verskaf word, sal 'n aansienlike proporsie van ooie in Suid-Afrikaanse intensiewe lamstelsels sonder toesig lam. 'n Hoër as verwagte frekwensie van baie kort intervalle tussen opeenvolgende geboortes is waargeneem. Indien die aanname dat geboortes mekaar 'n aanvang laat neem bevestig kan word, sou dit van waarde wees om die meganisme wat daaraan gekoppel is beter te verstaan. Die kennis kan ingespan word om natuurlike geboortes van weidende ooie te sinchroniseer, onder omstandighede waar dit onmoontlik is om die verspreiding van geboortes met voeding te manipuleer. Relatief min lynverskille is in die studie gevind. Die verspreiding van lamplekke in die kampe kon nie aan 'n lyn toegeskryf word nie. Ooie in die H-Iyn het meer geneig om hulle lammers skoon te maak kort na geboorte, terwyl hulle L-Iyn tydgenote korter na geboorte begin het om te wei. Die gedragsaanpassings in die H-Iyn salook lamoorlewing in die hand werk. Gedragsdata van die H- en L-Iyne is oor 'n periode van 10 jaar aangeteken, en aan 'n genetiese analise onderwerp. Die lynverskille wat vroeër verkry is, is deur die analise bevestig. Genetiese parameters wat beraam is, het aangetoon dat gedragseienskappe in lammers en ooie laag tot matig oorerflik was. In lammers is 'n betekenisvolle direkte oorerflikheidsberaming gevind vir die tydperk van geboorte tot staan. Maternale oorerflikhede was betekenisvol vir duur van geboorte, en die tydperk van geboorte tot staan. Maternale permanente omgewingsvariansies was betekenisvol vir maternale samewerking met die lam se eerste suippogings, en vir die tydperk vanaf staan tot oënskynlike suip. Met die analise van gedragseienskappe by ooie het dit geblyk dat duur van geboorte, maternale samewerkingspunt en die tydperk wat ooie op of nabyaan hulle lamplekke deurgebring het, matig oorerflik was. Genetiese tendense vir die H- en L-Iyne was uiteenlopend. Die verhaalde genetiese tendense in die Hvlyn was aanduidend van veranderinge wat lamoorlewing in die hand sal werk. Direkte en maternale genetiese, sowel as maternale permanente omgewingseffekte is vervolgens vir geboortegewig, geboortehaar en speengewig beraam. Geboortehaarpunt was hoog oorerflik en onafhanklik van maternale invloede. Genetiese en fenotipiese waardes het gedui op uiteenlopende tendense vir speengewig in die H- en L-Iyne. Uiteenlopende genetiese verskille was veralopmerklik by direkte teeltwaardes vir speengewig. Geboortegewig en geboortehaarpunt was grootliks onafhanklik van seleksielyn. Genetiese parameters is met 'n herhaalbaarheidsmodel beraam vir jaarlikse reproduksieeienskappe, rouvaggewig en liggaaamsgewig van ooie. Die oorerflikheid van reproduksie-eienskappe was laag, naamlik onder 10 % van die fenotipiese variansie. Permanente omgewingsvariansies van die ooie was hoër, tussen 7 en 13 %. Genetiese korrelasies van reproduksie-eienskappe met paargewig was gunstig en baie hoog in die geval van gewig lam gespeen per ooi. Ooreenstemmende korrelasies met rouvaggewig was laag en veranderlik. Permanente omgewingskorrelasies van reproduksie-eienskappe met paargewig en rouvaggewig was ongunstig en hoog in sommige gevalle. Uiteenlopende fenotipiese tendense in die verwagte rigting is vir reproduksie-eienskappe tussen die H- en L-Iyne verkry. Die neigings is bevestig deur verkreë genetiese tendense gebaseer op gemiddelde teeltwaardes binne geboortejare. Die resultate het bevestig dat genetiese vordering in die orde van 1-2 % per jaar haalbaar is in reproduksie, ten spyte van relatief lae oorerflikhede. Genetiese parameters is beraam vir tweetand wol- en liggaamsgewigeienskappe, sowel as vir testisafmetings. Voldoende genetiese variasie vir benutting in goedgestruktureerde teeltprogramme is vir al die eienskappe verkry. Genetiese tendense het op 'n verandering na groter en gladder skape in die H-Iyn gedui, terwyl L-Iyn tydgenote kleiner en meer ontwikkeld geword het. Uiteenlopende genetiese tendense tussen lyne is vir testisdeursnee verkry. Die tendense was in die verwagte rigting. Die resultaat het met die genetiese tendense in liggaamsgewig verband gehou, siende dat geen lynverskil vir testisgrootte wat vir liggaamsgewig gekorrigeer is, verkry is nie. Genetiese tendense vir skoonvaggewig en veseldikte het op geen noemenswaardige verskille tussen lyne gedui me. Die invloed van ouderdom op reproduksie, woleienskappe en paargewig is ondersoek in H- en L-Iynooie wat vir ten minste 10 jaar aan uiteenlopende seleksie onderwerp is. Genetiese parameters uit die ontleding was in algemene ooreenstemming met vorige beramings, al is 'n kleiner datastel gebruik. Die H- en L-Iyne het aanmerklik in reproduksie verskil, maar geen verskille het in die vorm van ouderdomstendense vir die lyne voorgekom nie. Uitgedruk relatief to prestasie in die L-Iyn, het die verskil tussen die lyne vir gewig lam gespeen per ooi tot 56 % beloop. Ooie in die Il-lyn was swaarder as L-Iyntydgenote by tweetandouderdom, maar die lynverskil het by ouer ouderdomsgroepe verdwyn. Ooie in die L-Iyne het hoër skoonvaggewigte gehandhaaf as H-Iyntydgenote, veral by die middelste ouderdomsgroepe (3 tot 6 jaar), die ouderdomsgroepe waar die hoogste wolopbrengs verwag word. Veseldikte het toegeneem met ouderdom, met geen opmerklike lynverskille. Dit is gestel dat die stres van dragtigheid en laktasie tot die lynverskille ten opsigte van paargewig en skoonvaggewig gelei het. Ten slotte het die proefskrif 'n raamwerk vir die genetiese verbetering van lamopbrengs by skape op weidings deur seleksie vir produktiwiteit verskaf. Ongunstige gekorreleerde response was gering, maar die stres van dragtigheid en laktasie het die liggaamsgewig en woleienskappe van volwasse teeltooie tot 'n mate benadeel. Die wisselwerking tussen gedrag, lamoorlewing en ooigrootmaakvermoë is duideliker gemaak vir 'n aantal Suid- Afrikaanse skaaprasse. Dit het gelei to 'n beter begrip vir die dinamiese prosesse betrokke. Die resultate het tot die formulering van aangepaste bestuurs- en teeltstelsels gelei, veral in die konteks van kuddeproduktiwiteit. Resultate uit die studie was redelik robuus wat verskillende hulpbronkuddes betref, en behoort in die breër bedryf toepaslik te wees.
Ewes -- Breeding, Lambs, Sheep -- Reproduction, Thesis (Ph.D. (Animal, Wildlife and Grassland Sciences))--University of the Free State, 2002