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The results of five experiments are reported as tests of a theory of the growth and feed intake of sheep. An attempt is first made to describe potential growth in the sheep, that is, growth under non-limiting conditions, and to generalise this description across breeds. The two ways of providing non-limiting conditions are then considered. This approach necessarily confronts the problem of predicting the desired intake of feed resources required to fulfill the potential. Whilst ad libitum access to a single,nutrient-rich feed is found an acceptable method, data collected in three experiments, using a total of 64 Suffolk X Greyface lambs, lead to theconclusion that sheep can also attain their potential for growth when given free and continual access to more than one feed, a possible combination of which is non-limiting to growth. It is suggested that the technique may be applied as an independent test of other estimates of resource requirements for growth and to the prediction of feeding behaviour in sheep. The responses of sheep to limiting conditions for growth are studied on the assumption that sheep have rules for partitioning the feed's first limiting resource. The limits to this view are investigated by experiment using 73 Scottish Blackface wethers offered amounts of feed which provide similar crude protein allowances but different yields of energy. It is concluded that the body composition of sheep can be manipulated by nutrition, since the animals will partition scarce energyresources to protein growth before fattening, but that this effect is complicatedin ruminants because protein supply is not independent of feeding level. The theory is then applied to the problem of predicting the recovery of sheep from the effects of growth restraint. An experiment involving 30 Scottish Blackface wethers provides data which support the idea that sheep, following a period of underfeeding, will resume their known potential for growth and, only when necessary, restore their body composition to the state dictated by their inheritance. No compensatory growth is observed. Finally, the work as a whole is discussed in relation to animal science and the sheepmeat industry. Possibilities for future research are also suggested. The experimental data are published in full in the appendices.

Thesis (MScAgric)--University of Stellenbosch, 2001. / ENGLISH ABSTRACT: The objectives of this study were to investigate genetic parameters of early growth traits, lifetime ewe productivity (total number born, number weaned and weight weaned per ewe lifetime production) and testis measurements in the Merino flock maintained at the Tygerhoek Experimental Farm. Non-genetic fixed factors influencing these traits were evaluated by using appropriate models. The influence of non-genetic factors on preweaning lamb survival rate and the effect of scrotal circumference on ewe fertility was also studied. Fixed effect models were fitted to the data using either Procedure GLM or CATMOD in SAS (1996) depending on the parameters being estimated. Variance components were estimated by Restricted Maximum Likelihood (REML) procedures, fitting an animal model. Records of 8310 lambs born from 2538 ewes and sired by 681 rams covering the period 1970 to 1998 were used in the analysis of birth weight (BW), weaning weight (WW) and preweaning average daily gain (ADG). All three traits were affected (p < 0.001) by type of birth of lambs (single, multiple), sex, year of birth of lambs, group of animals (selection, control) and age of dam at lambing (2- to 6-yr old). Male lambs and singles were heavier both at birth and weaning and grew faster (p < 0.001) than females and multiples, respectively. Non-selected animals were lighter than selected animals at birth and weaning with an inferior growth rate. BW increased with increasing dam age at lambing until a maximum of 3.7 kg was reached at 6-yr of age. However, WW and ADG reached a maximum at 4.5- and 4.2-yr of age, respectively. Variance components for BW, WW and ADG were estimated using Restricted Maximum Likelihood (REML) procedures. Four different animal models were fitted, differentiated by including or excluding maternal effects. The direct heritability estimates (h") ranged from 0.19 to 0.38, 0.25 to 0.40 and 0.26 to 0.40 for BW, WW and ADG, respectively. Estimates of direct heritability (h") were substantially higher when maternal effects were ignored. The direct heritability (h") estimate for BW decreased from 0.38 to 0.19 when both or either of the maternal effects, genetic or environmental, were fitted in the model. The h2 estimate for WW was 0.36 when both maternal effects were ignored from the model and decreased to 0.29 and 0.25 when maternal effects were fitted. A similar trend was observed for ADG. The maternal heritability (rn-) for BW ranged from 0.38 to 0.41 when only maternal additive genetic effects were fitted in the model, and decreased to 0.25 and 0.27 when the maternal permanent environmental effect (c") was fitted. The m" for WW and ADG ranged from 0.02 to 0.11 and from 0.01 to 0.10, respectively. The respective c2 estimates ranged from 0.05 to 0.10 and from 0.02 to 0.09. Moderate negative genetic correlations (ram) between direct and maternal additive genetic effects were observed in BW, while close to zero estimates were obtained for WW and ADG. The direct additive genetic correlation estimates between BW and WW and BW and ADG were 0.16 and 0.04, respectively. The corresponding maternal additive genetic correlation estimates were 0.93 and 0.60. The direct and maternal additive genetic correlation estimates between WW and ADG were 0.99 and 0.85, respectively. Data of 3272 lambing records collected on 818 ewes born from 689 dams and sired by 371 rams were used in the evaluation of ewe lifetime productivity (total number born, number weaned and total weight weaned). These records were collected from ewes born from 1969 to 1994 and gave birth from 1971 to 1999. Year of birth of ewe and ewe two-tooth liveweight affected (p < 0.001) the ewe reproduction traits investigated. Multiple born ewes were superior both in total number of lambs born and weaned than singles. Heritability estimates of 0.23, 0.17 and 0.20 were obtained for total number of lambs born (TLB), total number of lambs weaned (TLW) and total weight of lambs weaned per ewe lambing over four lambing opportunities (TWW), respectively. For total weight weaned per ewe lambing over the first lambing opportunity (TWWl) a heritability estimate of 0.02 was computed. Genetic correlation estimates of -0.10, 0.57 and 1.00 were obtained between TWW1 and TLB, TWW1 and TLW and TWWl and TWW, respectively. High and positive genetic correlations ranging from 0.61 to 0.92 were estimated between ewe lifetime reproduction traits. Data from 1380 rams born from 1986 to 1998 were used to investigate genetic parameters for scrotal circumference (SC), testis diameter (TD) and two-tooth liveweight (LW). The other objective of this study was to estimate the magnitude of the effect of SC on ewe fertility. Year of birth, selection group and LW were significant (p < 0.001) sources of variation both for SC and TD. Heritability estimates ranged from 0.29 to 0.40, 0.25 to 0.38 and 0.49 to 0.52 for SC, TD and LW, respectively. Adjustment for LW decreased heritability estimates of SC and TD by 11.0 and 8.0 %, respectively. The genetic correlation between SC and TD was unity. Rams with larger SC had a significant effect on ewe fertility (ewes lambing per ewe mated). Average survival rate from birth to 100 days of age was 0.79 and was influenced by lamb birth weight, year of birth, age of dam at lambing, type of birth (single, multiple) and sex of lambs. Lambs with a liveweight between 4.0 to 4.9 kg at birth showed the highest survival rate while lambs with a liveweight of:s 2.0 kg showed the lowest survival rate. / AFRIKAANSE OPSOMMING: '0 Genetiese studie van vroeë groei-eienskappe en ooiproduktiwiteit by Merinoskape: Die doel van die studie was om genetiese parameters VIr vroeë groei-eienskappe, ooileeftydsproduksie (totale aantal gebore, aantal gespeen en totale gewig gespeen) en testismaatstawwe in die Merinokudde van die Tygerhoek Navorsingstasie te beraam. Niegenetiese vaste effekte wat hierdie eienskappe beïnvloed, is met behulp van gepaste modelle geëvalueer. Die invloed van nie-genetiese faktore op voorspeense lamoorlewing en die invloed van skrotumomvang op ooivrugbaarheid is ook ondersoek. Vaste-effek modelle is, afhangende van die parameters wat beraam is, deur middel van of die GLM of CATMOD prosedures van SAS (1996) op die data gepas. Variansiekomponente is met behulp van die Beperkte Maksimum Aanneemlikheidsprosedure (REML), deur die passing van 'n dieremodel, beraam. Rekords van 8310 lammers wat tussen 1970 en 1998 van 2538 ooie gebore is en die nageslag van 681 ramme was, is vir die ontleding van geboortegewig (BW), speengewig (WW) en voorspeense daaglikse toename (ADG) gebruik. Al drie eienskappe is deur geboortestatus (enkeling, meerling), geslag, jaar van geboorte, groep (seleksie, kontrole) en ouderdom van die moeder met lamming (2- tot 6-jr oud) beïnvloed (p<0.001). Ramlammers en enkelinge was met beide geboorte en speen swaarder en het vinniger (p<O.OOI) as onderskeidelik ooilammers en meerlinge gegroei. Nie-geselekteerde lammers was ligter as dié van die seleksiegroep met beide geboorte en speen en het ook stadiger gegroei. BW het met toename in moederouderdom tot 'n maksimum van 3.7 kg by 6-jr oud ooie gestyg. Maksimum WW en ADG is egter by 3- en 5 jr oud ooie onderskeidelik bereik. Variansiekomponente vir BW, WW en ADG is met behulp van Beperkte Maksimum Aanneemlikheidsprosedures (REML) beraam. Vier verskillende modelle is gepas, waarin mateme effekte óf in- óf uitgelaat is. Die direkte oorerflikhede (h2 ) het van 0.19 tot 0.38, 0.25 tot 0.40 en 0.26 tot 0.40 vir BW, WW en ADG onderskeidelik gevarieer. Beramings van die direkte oorerflikheid (h2 ) was aansienlik hoër waar die mateme effekte geïgnoreer is. Die direkte oorerflikheidsberaming (h2 ) vir BW het vanaf 0.38 tot 0.19 verlaag toe beide of een van die mateme effekte, geneties of omgewings, in die model ingesluit is. Die h2 beraming vir WW was 0.36 toe beide mateme effekte geïgnoreer is en het tot 0.29 en 0.25 gedaal toe beide in die model gepas is. 'n Soortegelyke tendens is vir ADG waargeneem. Die mateme oorerflikheid (rrr') van BW het vanaf 0.38 tot 0.41 gevarieer waar slegs die direkte mateme effek gepas is maar tot 0.25 en 0.27 gedaal toe die mateme permanente omgewingseffek (c2 ) additioneel gepas is. Die m2 vir WW en ADG het onderskeidelik van 0.02 tot 0.11 en van 0.0 I tot 0.10 gewissel. Die ooreenstemmende c2 beramings het vanaf 0.05 tot 0.10 en vanaf 0.02 tot 0.09 gewissel. Matige negatiewe genetiese korrelasies (ram) tussen die direkte en mateme genetiese effekte is vir BW gevind, terwyl bykans zero beramings vir beide WW en ADG gevind is. Die direkte genetiese korrelasies tussen BW en WW en tussen BW en ADG was onderskeidelik 0.16 en 0.04. Die ooreenstemmende mateme genetiese korrelasies was 0.93 en 0.60. Die genetiese korrelasies tussen die direkte en mateme genetiese effekte tussen WW en ADG was onderskeidelik 0.99 en 0.85. Data van 3272 lamrekords van 818 ooie vanaf689 moeders en 371 vaders is vir die evaluasie van ooileeftydsproduktiwiteit (totale aantal gebore, aantal gespeen en totale speengewig) gebruik. Hierdie rekords is van ooie wat tussen 1971 en 1999 gelam het, versamel. Jaar van geboorte van die ooi en ooi-tweetandgewig het die reproduksie-eienskappe ondersoek, beïnvloed (p<O.OOI). Meerlinggebore ooie was beter as enkelgebore ooie ten opsigte van aantal lammers gebore en gespeen. Oorerflikheidsberamings van onderskeidelik 0.23, 0.17 en 0.20 is vir aantal lammers gebore (TLB), totale aantal lammers gespeen (TLW) en totale gewig oor vier lamgeleenthede gespeen (TWW), gevind. Vir totale gewig per ooi met die eerste lamgeleentheid gespeen (TWWJ), is 'n oorerflikheidsberaming van 0.02 bereken. Genetiese korrelasies van -0.10, 0.57 en 1.00 is onderskeidelik tussen TWW, en TLB, TWW, en TLW en TWW, en TWW beraam. Hoë en positiewe genetiese korrelasies wat tussen 0.61 en 0.92 gevarieer het, is tussen ooileeftydsreproduksie-eienskappe beraam. Data van 1380 ramme wat tussen 1986 en 1998 gebore is, is gebruik om genetiese parameters van skrotumomvang (SC), testisdeursnee (TD) en tweetandgewig (LW) te beraam. 'n Ander doel van die ondersoek was om die effek van SC op ooivrugbaarheid te ondersoek. Jaar van geboorte, seleksiegroep en LW was betekenisvolle (p<0.001) bronne van variasie vir beide SC en TD. Oorerflikheidsberamings het van 0.29 tot 0.40, 0.25 tot 0.38 en 0.49 tot 0.52 vir onderskeidelik SC, TD en LW gevarieer. Korrigering vir LW het die oorerflikhede van SC en TD met onderskeidelik 11.0 en 8.0 % verlaag. Die genetiese korrelasie tussen SC en TD was een. Ramme met hoër SC het 'n betekenisvolle invloed op ooivrugbaarheid (ooie gelam per ooie gepaar) gehad. Die gemiddelde oorlewingstempo vanaf geboorte tot 100 dae was 0.79 en is deur geboortegewig van die lam, jaar van geboorte, ouderdom van die ooi, geboortestatus (enkeling, meerling) en geslag van die lam beïnvloed. Lammers met 'n geboortegewig van tussen 4.0 en 4.9 kg het die hoogste oorlewingstempo gehad, terwyl lammers met 'n geboortegewig van ~2.0 kg die laagste oorlewingstempo gehad het.

Made available in DSpace on 2014-06-11T19:26:06Z (GMT). No. of bitstreams: 0 Previous issue date: 2011-07-29Bitstream added on 2014-06-13T20:14:36Z : No. of bitstreams: 1 ono_rk_me_jabo.pdf: 515778 bytes, checksum: 1290bcd92cebca595d379194178c40d4 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Objetivo do presente estudo foi estimar componentes de (co)variância e parâmetros genéticos para as características de crescimento: Peso ao nascer (PN), Peso ao desmame (PD), Ganho de peso pré-desmama (GDPRÉ) e Ganho de peso pós-desmama (GDPÓS) e de carcaça: Área de olho de lombo (AOL) e Espessura de gordura subcutânea (EGS) para uma raça materna composta. Os dados utilizados foram oriundos de uma fazenda pertencente ao programa de melhoramento genético de ovino (OviGol®), coletados entre os anos de 2008 e 2011. Os efeitos ambientais considerados nos modelos foram determinados a partir de análise de variância, pela metodologia dos quadrados mínimos, considerando apenas efeitos significativos (p<0,01). Os componentes de (co)variâncias e os parâmetros genéticos para as características foram estimados pelo método da máxima verossimilhança restrita (REML), sob modelo animal, utilizando o programa REMLF90. Avaliaram-se dois modelos em análises unicaracterísticas pelo teste da razão de verossimilhança (LRT), sendo escolhido o modelo mais completo, o qual incluía como efeitos aleatórios o ambiente permanente materno e o genético aditivo. As herdabilidades diretas estimadas em análise multi-característica, sob modelo animal, foram: 0,26; 0,20; 0,23; 0,15; 0,18 e 0,10, para PN, GDPRÉ, PD, GDPÓS, AOL e EGS respectivamente. As correlações genéticas variaram de baixa a alta magnitude, sendo a mais alta entre GDPRÉ e PD (0,99) e a mais baixa entre PN e GDPÓS (0,02). Os resultados indicam que há variação genética para PN, PD e GDPRÉ, com possibilidade de seleção dentro da população / The aim of this study was to estimate the (co)variance components and genetic parameters for growth: Birth Weight (BW), Weaning Weight (WW), average daily gain pre-weaning (ADGpre) and average daily gain post-weaning (ADGpos) and carcass: Eye Muscle Area (EMA) and Fat Depth (FD) for a composite maternal breed. The data used were from a farm belonging to the breeding program for sheep (OviGol®), collected between 2008 and 2011. The environmental effects considered in models were determined from analysis of variance by least squares method, considering only significant effects (p<0.01). The components of (co)variance and genetic parameters were estimated by restricted maximum likelihood (REML), under the animal model, obtained by the REMLF90 program. Two models were tested, differentiated only by random effects in single-trait analysis of the likelihood ratio test (LRT), whichever is the more complete model, which included as random maternal permanent environmental and additive genetic. The direct heritability, analysis in multi-trait and in an animal model, was 0.26, 0.20, 0.23, 0.15, 0.18 and 0.10, for BW, ADGpre, WW, ADGpos, EMA and FT respectively. Genetic correlations ranged from low to high values, being the highest among ADGpre and WW (0.99) and lowest between BW and ADGpos (0.02). The study indicates that there is genetic variation for PN, GDPRÉ and PD, with scope for selection within population

Orientador: Ricardo da Fonseca / Banca: Aníbal Eugênio Vercesi Filho / Banca: Henrique Nunes de Oliveira / Resumo: Objetivo do presente estudo foi estimar componentes de (co)variância e parâmetros genéticos para as características de crescimento: Peso ao nascer (PN), Peso ao desmame (PD), Ganho de peso pré-desmama (GDPRÉ) e Ganho de peso pós-desmama (GDPÓS) e de carcaça: Área de olho de lombo (AOL) e Espessura de gordura subcutânea (EGS) para uma raça materna composta. Os dados utilizados foram oriundos de uma fazenda pertencente ao programa de melhoramento genético de ovino (OviGol®), coletados entre os anos de 2008 e 2011. Os efeitos ambientais considerados nos modelos foram determinados a partir de análise de variância, pela metodologia dos quadrados mínimos, considerando apenas efeitos significativos (p<0,01). Os componentes de (co)variâncias e os parâmetros genéticos para as características foram estimados pelo método da máxima verossimilhança restrita (REML), sob modelo animal, utilizando o programa REMLF90. Avaliaram-se dois modelos em análises unicaracterísticas pelo teste da razão de verossimilhança (LRT), sendo escolhido o modelo mais completo, o qual incluía como efeitos aleatórios o ambiente permanente materno e o genético aditivo. As herdabilidades diretas estimadas em análise multi-característica, sob modelo animal, foram: 0,26; 0,20; 0,23; 0,15; 0,18 e 0,10, para PN, GDPRÉ, PD, GDPÓS, AOL e EGS respectivamente. As correlações genéticas variaram de baixa a alta magnitude, sendo a mais alta entre GDPRÉ e PD (0,99) e a mais baixa entre PN e GDPÓS (0,02). Os resultados indicam que há variação genética para PN, PD e GDPRÉ, com possibilidade de seleção dentro da população / Abstract: The aim of this study was to estimate the (co)variance components and genetic parameters for growth: Birth Weight (BW), Weaning Weight (WW), average daily gain pre-weaning (ADGpre) and average daily gain post-weaning (ADGpos) and carcass: Eye Muscle Area (EMA) and Fat Depth (FD) for a composite maternal breed. The data used were from a farm belonging to the breeding program for sheep (OviGol®), collected between 2008 and 2011. The environmental effects considered in models were determined from analysis of variance by least squares method, considering only significant effects (p<0.01). The components of (co)variance and genetic parameters were estimated by restricted maximum likelihood (REML), under the animal model, obtained by the REMLF90 program. Two models were tested, differentiated only by random effects in single-trait analysis of the likelihood ratio test (LRT), whichever is the more complete model, which included as random maternal permanent environmental and additive genetic. The direct heritability, analysis in multi-trait and in an animal model, was 0.26, 0.20, 0.23, 0.15, 0.18 and 0.10, for BW, ADGpre, WW, ADGpos, EMA and FT respectively. Genetic correlations ranged from low to high values, being the highest among ADGpre and WW (0.99) and lowest between BW and ADGpos (0.02). The study indicates that there is genetic variation for PN, GDPRÉ and PD, with scope for selection within population / Mestre

Prepartum, multiparous ewes were randomly assigned to experimental groups and sacrificed at 50(n=5), SO(n=4),115(n=5), and 140(n=4) days of gestation. Serum harvested the week prior to slaughter was assayed for progesterone (PG), prolactin (PRL) and growth hormone (GH) concentrations. Mammary tissue obtained at slaughter was assayed for receptor concentrations of progesterone (PGr), prolactin (PRLr) and insulin (Ir). Quantitative biochemical, histological and autoradiographical analyses were used to measure mammary gland growth and indicated no significant glandular growth occurs prior to 8O days of gestation. However, a major phase of parenchymal growth occurred between 8O and 115 days which coincided with significant increases in PG, PGr and PRLr concentrations. Parenchymal growth continued further into late pregnancy. GH and Ir concentrations did not change significantly during pregnancy and were not strongly correlated to growth measurements. These results suggest that mammary gland growth is receptor-mediated and direct or indirect regulation of PGr and PRLr is primarily responsible for the observed growth phenomenon. / Master of Science

"May 2002" Includes bibliographical references (leaves 148-164) Describes a series of novel experiments designed to enhance our understanding of nutrient utilisation for growth of wool and the whole body.

"May 2002" / Includes bibliographical references (leaves 148-164) / xi, 164 leaves : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Describes a series of novel experiments designed to enhance our understanding of nutrient utilisation for growth of wool and the whole body. / Thesis (Ph.D.)--University of Adelaide, Dept. of Animal Science, 2002

Made available in DSpace on 2014-06-11T19:26:06Z (GMT). No. of bitstreams: 0 Previous issue date: 2011-07-29Bitstream added on 2014-06-13T19:12:58Z : No. of bitstreams: 1 pires_mp_me_jabo.pdf: 1499100 bytes, checksum: 36601d48bc1009bc104fa6338a8ec4ee (MD5) / Os objetivos desse estudo foram planejar e implementar um programa de melhoramento genético de ovinos no oeste paulista e estimar parâmetros genéticos de características de crescimento e de carcaça em ovinos Suffolk, afim de traçar estratégias de seleção para programas de melhoramento genético. Os dados analisados para a realização do primeiro objetivo, foram coletados da prova de ganho de peso realizada em 2009, sendo os animais oriundos de 11 propriedades próximas a região de Dracena. As características avaliadas foram peso ajustado aos 150 dias de idade (P150) e ganho de peso médio diário (GPD). Para o auxílio da classificação dos cordeiros um índice da prova de ganho de peso (Ipgp) foi elaborado e 5 reprodutores foram indicados para a seleção. Entretanto, por falta de interesse dos produtores, nenhum dos animais indicados para a reprodução foram utilizados, sendo estes, vendidos para o abate. Desta forma, para o início de um programa de melhoramento genético de ovinos na região, será necessário a formação de um rebanho com mérito genético superior, adequado ao sistema de manejo adotado na região e ao mercado consumidor, através de produtores interessados em selecionar seus animais ou então pela faculdade, se responsabilizando pelo fornecimento de material genético para a região. Para a realização do segundo objetivo, os dados avaliados foram coletados entre os anos de 2007 e 2009, oriundos de uma propriedade localizada no Estado de São Paulo, participante do programa de melhoramento Ovigol, desenvolvido pela empresa Aries Reprodução e Melhoramento Genético Ovino – Ltda em parceria com a empresa AbacusBio Limited da Nova Zelândia. As estimativas dos componentes de (co)variâncias e dos parâmetros genéticos foram obtidos pelo software REMLF90, que usou... / The objectives of this study were to design and implement a program of genetic improvement of sheep in western São Paulo state and estimate genetic parameters for growth traits and carcass in Suffolk sheep in order to trace selection strategies for genetic improvement programs. The data analyzed to achieve the first objective, we collected evidence of weight gain took place in 2009, and the animals from 11 properties near the Dracena.These characteristics were adjusted weight at 150 days of age (P150) and average daily weight gain (ADG). To aid the classification of lambs an index of evidence of weight gain (IPGP) was developed and five players were nominated for selection. However, due to lack of interest of producers, none of the animals listed were used for reproduction, the latter being sold for slaughter.Thus, for the beginning of a breeding program for sheep in the region will require the formation of a herd with superior genetic merit, appropriate to the management system adopted in the region and to the consumer market by producers interested in selecting their animals or for college, taking responsibility for providing genetic material for the region. To achieve the second objective, the data evaluated were collected between 2007 and 2009, coming from a property located in the State of São Paulo, participant Ovigol improvement program, developed by Aries Reproduction and Breeding Sheep - Limited in AbacusBio partnership with New Zealand Limited. Estimates of (co) variances and genetic parameters were obtained by REMLF90 software, which used the restricted maximum likelihood method under multi-trait animal model. The predicted heritability for BW, GPPré, PD, GPPós, AOL and EGS were 0.06, 0.48, 0.45, 0.16, 0.10 and 0.07, respectively. Genetic correlations between weight traits ranged from -0.67 to 0.98... (Complete abstract click electronic access below)

Thesis (PhD(Agric) (Animal Sciences))--University of Stellenbosch, 2006. / This project consisted of five studies. The objectives were to determine the essential amino acid (AA) requirements of growing woolled lambs (Merino and Dohne Merino) and the essential AA profile of duodenal digesta pre-dominantly derived from microbial protein. The limiting essential AA`s in high rumen degradable protein (RDP) diets to growing lambs, where microbial protein is the primary source of AA`s, were also identified. The first study determined the essential AA profile of duodenal protein on a high rumen degradable diet and evaluated the impact of dietary RDP concentration and source [true RDP vs. non-protein nitrogen (NPN)] on the AA composition of supplied in the duodenum. The first trial in this study evaluated the effects of increasing true RDP levels on the essential AA composition of duodenal protein primarily derived from rumen microbes. The lambs had free access to wheat straw and fresh water. The daily RDP supplements were administered in two equal doses into the rumens through rumen cannulas at 07:00 and 19:00. Duodenal digesta was extracted with 6h intervals through T-type cannulas, inserted proximally to the common bile duct. Sampling time was advanced 2h every day to obtain duodenal samples on every even hour of a 24h period after three days. As expected, deficient RDP limited the supply of essential AA`s in the iv duodenum. When the true RDP supplements increased, the duodenal flow of essential AA`s also increased concomitantly, but appeared to level off at the higher RDP levels. Despite the positive quantitative effects of true RDP supplementation on AA supply to the duodenum, the AA profile in the duodenum was unaltered. Consequently, the essential AA profile of duodenal protein of sheep receiving high RDP diets, where microbial protein is the primary source of AA`s in the duodenum, is relatively constant and insensitive to dietary RDP concentration. In the second trial the effects of RDP source (true RDP vs. NPN) on the essential AA profile of duodenal protein on high RDP diets were evaluated by substituting increasing amounts of urea for true RDP in isonitrogenous teatments. Higher NPN increments reduced the daily supply of essential AA`s in the duodenum. In corroboration of the first trial, the AA profile of the duodenal protein was very constant, irrespective of the RDP source. Since microbial protein is the major source of duodenal AA`s on high RDP diets, this study supports the view that microbial protein has a relatively constant AA profile, but microbial protein yield varies according to several rate limiting factors in the rumen. A constant microbial AA profile allows accurate estimates of microbial essential AA supply in the small intestine if microbial protein production and fluid and particulate outflow rates from the rumen can be accurately predicted. This allows the development of more accurate undegradable protein (UDP) supplementation strategies, based on the essential AA requirements of animals. In the second study growing male Merino and Dohne Merino lambs were slaughtered at different weights and body condition scores. The digesta was removed from the stomachs and intestines and every organ or body part were weighed to determine the whole empty body (WEB) composition. The WEB was partitioned into the carcass, internall offal (stomachs, intestines, organs and blood) and external offal (head, feet, skin and wool). No differences were apparent in the proportional weight distribution of similar body components of the same breed at different ages. In a comparison between breeds, the proportional weight contributions of the carcasses from both breeds to the WEB weight were remarkably similar at both slaughtering stages. The Dohne Merino lambs had proportionally larger internal offals and smaller external offals than the Merino lambs at both slaughters. Unless the essential amino acid compositions of the internal and external offals were identical to the carcass, the dissimilarities in weight and protein allocation to these two components within the WEB`s of Merino and Dohne Merino lambs imply a distinct WEB essential AA composition for each breed. The apparent digestibilities of dry matter (DM), crude protein (CP), energy, acid detergent fibre (ADF), neutral detergent fibre (NDF), fat and ash did not differ between Merino and Dohne Merino lambs. Energy retention was also similar for the two breeds, but the Merino lambs retained considerably more N than the Dohne Merino lambs. This may also impact on the respective amino acid requirements of the lambs. Since the Merino lambs utilise N more efficiently, they may have potentially lower essential amino acid requirements to achieve a similar growth rate. The WEB essential AA compositions of growing Merino and Dohne Merino lambs were determined in the third study. Based on the ideal protein concept, the WEB essential AA profile was accepted as representative of the AA requirements for growth. The use of a single body part as a representation of the WEB AA profile was also evaluated. Differences in the proportional weight and protein contribution of the three body components (carcass, internal offal and external offal) of the two breeds strongly suggested that the WEB AA composition of the breeds would differ, because of likely differences in the AA profiles of these components. The essential AA profiles of the carcasses from the two breeds were surprisingly similar. However, the essential AA compositions of the internal offal and external offal differed substantially from each other, as well as from the carcass. In addition, the internal offal and external offals of each breed had characteristic essential AA profiles. Inevitably, the WEB essential AA profiles of Merino and Dohne Merino lambs differed considerably. Only the leucine and phenylalanine concentrations in the WEB`s of Merino and Dohne Merino lambs did not differ. Significant differences in the concentrations of eight essential AA`s implied that the two breeds have different AA requirements for growth. The different AA compositions of the internal and external offal within each breed also illustrated that the use of a single body component, like the carcass, as a predictor of WEB essential AA composition contains considerable inaccuracies. The essential AA index indicated that the duodenal protein, primarily derived from rumen microbes, provided approximately 81 % of the qualitative AA requirements of growing lambs. During periods of sufficient availability of very low-quality forage, as the diet in this study simulated, microbial protein is not able to support maximum growth. The first two limiting AA`s (histidine and methionine) could not even support daily growth rates of 100 g/d. This is very low and stresses the need for effective undegradable AA supplementation under these conditions. Chemical scores identified histidine as the first limiting AA in high RDP diets (predominantly microbial protein), followed by methionine, leucine, arginine and phenylalanine. However, the requirements for histidine and arginine are frequently over estimated and these AA`s should actually be considered semi-essential, which could render methionine, leucine and phenylalanine the first three limiting AA`s to growing lambs receiving high RDP diets. Because of the limitations of static measurement systems for the determination of AA requirements, a more comprehensive evaluation method was introduced for determination of the limiting AA`s in duodenal protein of lambs on high RDP diets, in the fourth study. The fourth study focused on the identification of limiting AA`s to growing lambs being limit-fed a high RDP diet. The diet consisted primarily of soybean hulls, for its’ low rumen UDP content. Microbial protein production was calculated as 13 % of total digestible nutrient intake and complementary AA supplements prepared to simulate the WEB AA profile, determined in the previous study, in the small intestine. To eliminate the influence of the rumen on the AA supplements, the latter were infused into the abomasums via flexible tubing. Each essential AA was in turn removed from the control treatment (simulating the WEB composition) and the effect on N retention measured. When methionine or the branched-chain amino acids (BCAA`s) were removed from the infusate, N retention of the lambs was reduced. Consequently, methionine and at least one of the BCAA`s limited growth performance of young lambs when microbial protein was the predominant source of AA`s. The concomitant increased plasma concentrations of total AA`s when methionine or the BCAA`s were removed from the infusate corroborates the effects on N retention, since it indicates that AA utilisation was reduced when these AA imbalances were introduced. Amino acid imbalances had no effect on apparent DM, organic matter (OM) or NDF digestion, but N digestibility was reduced. The final study verified whether the BCAA’s were co-limiting the growth of lambs, or if any single BCAA was responsible for the limitation. Again the WEB AA profile of growing lambs was simulated in the small intestine via abomasal infusions to lambs receiving a soybean hull-based diet. Leucine, isoleucine and valine were individually or simultaneously removed from the infusate and the impact on N retention measured. On an individual basis valine had the largest negative impact on the efficiency of N utilisation. However, the simultaneous removal of the BCAA`s resulted in the lowest N retention, suggesting that valine might be limiting, but the three BCAA`s are more likely to be co-limiting in diets to growing lambs where microbial protein is the primary source of AA`s. Once again, neither DM, OM or NDF digestibility were affected by the AA imbalances. Nitrogen digestibility was, however, negatively affected by AA imbalances. This project succeeded in establishing the essential AA profile of duodenal protein in sheep receiving high RDP diets. The WEB essential AA compositions of growing lambs from two prominent sheep breeds in South Afica were then determined and the duodenal essential AA profile evaluated against the calculated AA requirements. Finally, the AA`s that limit growth in diets where microbial protein is the predominant source thereof were identified. These results contribute to the current knowledge of AA requirements in growing lambs, and highlight areas for future research, as discussed in the General Conclusion.