Genetic Variation influencing reproduction and growth of White Rock chickens was studied. The populations used were lines high (HH) and low (LL) selected for 56-day body weight, reciprocal F₁ crosses (HL and LH), F₂ crosses of the F₁ and dwarf populations (HD and LD) originating from lines HH and LL. Inheritance of egg production and egg composition was examined (Experiment 1). Although reciprocal effects were generally unimportant, nonadditive genetic variation was evident for reproductive traits and egg component measures of shell and yolk. Comparisons in Experiment 2 involved dwarf and normal chickens from lines HH and LL. Yolk weight increased while the ratio of albumen to yolk decreased with age. Line and genotype differences were evident for egg shell and yolk traits while line by genotype interactions showed that genetic background influenced expression of dw. Embryo growth of dwarfs was similar to that of nondwarfs at all ages (Experiment 3). Reduced egg size of dwarf pullets did not reduce embryonic weights. Embryo length and weight and yolk sac weight were greater for line HH than LL. In Experiment 4, allometric growth and cellular content of brains were measured in normal and dwarf chickens from lines HH and LL and reciprocal F₁ crosses from hatch to maturity. Allometric slopes were similar for lines HH and LL with significant heterosis. Within an age, DNA, RNA and protein content (mg/g) were similar for all populations. Brain weight differences were due to equal filling of a greater number cells for line HH than LL. Final brain size was influenced more by embryonic hyperplasia than postembryonic hypertrophy. The final experiment measured growth and cellular content of pectoralis and gastrocnemius muscles in populations HH, LL and F₁ from hatch to day 273. HH chicks had a smaller DNA unit size and a greater unit number than LL chicks at hatch. During rapid muscle growth, cell size was larger for HH than LL chickens. Pectoralis muscle grew at a faster rate than gastrocnemius muscle. Also cellular filling as measured by the DNA unit size was higher for pectoralis than gastrocnemius muscle through day 4. By day 10 the pattern reversed with values greater for gastrocnemius than pectoralis muscle. / Ph. D.

Thesis (MScFoodSc)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: Modern consumers are health conscious and are shifting towards more naturally produced products such as free range chicken. Commercial broiler strains are not suitable for free range rearing and an alternative genotype is needed that will serve the South African market with the acceptable meat quality as a broiler. The objective of this study was to investigate the effect of production system (free range and intensive reared) and genotype (Broiler (COBB™), Ross 308 X Potchefstroom Koekoek hybrid and Potchefstroom Koekoek) on chicken meat quality. This was quantified on the morphological, physical (pH, colour, drip and cooking loss, water holding capacity and tenderness), chemical composition (moisture, protein, fat, ash contents and fatty acid profile), sensory quality and consumer preference of various chicken meat portions. The results of this study indicate that genotype had a more pronounced effect than production system on the morphological and growth properties of chicken meat, as well as on the sensory characteristics and consumer preference. The broilers had the best (P ≤ 0.05) feed conversion ratio (FCR), highest average daily gain (ADG) and European production efficiency factor (EPEF), followed by the Hybrid and then the Potchefstroom Koekoek. For each genotype, the free range chickens produced heavier (P ≤ 0.05) live weights than intensively reared chickens. Despite the poorer growth performance and efficiency of the medium growing Hybrid birds, they had less mortality and fewer leg disorders than the broiler. Additional to these factors, the Hybrid Free Range had higher thigh, drumstick and wing yields (P ≤ 0.05) than the broiler. When investigating the correlation between the chemical and sensory data, it was observed that the Hybrid scored significantly higher (P ≤ 0.05) in both flavour and aroma than the Broiler and Koekoek genotypes for both production systems. For colour, pH and polyunsaturated to saturated fatty acid ratio (PUFA:SFA), the effect of production system was more pronounced than the effect of genotype. Rearing chickens in a free range environment increased the PUFA:SFA ratio (P ≤ 0.05), making it beneficial to human health. Free range rearing resulted in lower muscle pHu (P ≤ 0.05), darker (L* value) (P ≤ 0.05), less red and yellow (a* and b* value) (P ≤ 0.05) chicken meat. It also influenced the chemical composition in different carcass portions; for example, a lower fat content in the thigh and higher protein in the breast of the Broiler. Correlation with the sensory results indicated that juiciness, tenderness, chicken aroma and chicken flavour are the main drivers of liking for consumer’s preference towards chicken meat. The consumers predominantly preferred the Hybrid (P ≤ 0.05) in a blind tasting session, but when information was given on the production system of a chicken product, the consumers lean more towards a free range reared product than an intensive reared product. This indicates that consumer perception plays an immense role in consumer decision making. Cluster analysis was also performed to ascertain whether the consumers differed in their degree of liking of the intrinsic character of the respective chicken samples. Three different clusters of consumers were identified: 1) Consumers that prefer free range reared chicken meat, 2) Consumers that prefer intensively reared chicken meat, 3) Consumers that prefer both free range and intensive reared chicken meat. In conclusion, the Hybrid seems to be a viable option for free range production systems in South Africa, without negatively affecting the overall quality of the meat or consumer acceptance. / AFRIKAANSE OPSOMMING: Moderne verbruikers is baie meer gesonheidsbewus en verkies meer natuurlik geproduseerde produkte soos vrylopende (free range) hoenders. Die kommersiële braaikuiken is nie geskik vir vrylopende produksie nie en `n ander genotipe word benodig wat die Suid-Afrikaanse mark sal kan voorsien met aanvaarbare vleiskwaliteit vergelykbaar met dié van die braaikuiken. Die doel van hierdie navorsing was om die effek van produksiestelsel (vrylopend en intensief) en genotipe (braaikuiken (COBB™), Potchefstroom Koekoek en Ross 308 X Potchefstroom Koekoek kruising) op die morfologiese, fisiese (pH, kleur, drip- en kookverlies, waterhouvermoë en taaiheid), chemiese samestelling (vog-, proteïen-, vet-, asinhoud en vetsuurprofiel), sensoriese kwaliteit en verbruikersaanvaarbaarheid van verskeie hoender vleis porsies te bepaal. Hierdie navorsing het getoon dat genotipe `n groter invloed gehad het as produksiestelsel op die groei en morfologiese eienskappe van die hoenders, asook op die sensoriese eienskappe en verbruikersaanvaarbaarheid. Die braaikuiken, gevolg deur die Ross X Koekoek kruising en dan die Koekoek, het die beste (P ≤ 0.05) voeromsetverhouding (FCR), gemiddelde daaglikse toename (GDT) en Europese produksie effektiwiteitsfaktor (EPEF) getoon. Vir elke genotipe het die vrylopende hoenders swaarder (P ≤ 0.05) lewende massa by slag getoon. Ten spyte daarvan dat die Ross X Koekoek kruising swakker groei en effektiwiteitsresultate getoon het, het hulle laer mortaliteite en minder been breuke en beserings as die braaikuiken gehad. Die Ross X Koekoek kruising wat vrylopend groot gemaak is, het ook swaarder dy, boud en vlerkie massa (P ≤ 0.05) as die braaikuiken getoon. Die navorsing het ook getoon dat kleur, pH en die poli-onversadigde tot versadigde vetsuur verhouding (PUFA:SFA) meer beïnvloed is deur die effek van produksiestelsel as genotipe. Die hoenders wat in ʼn vrylopende omgewing grootgemaak is se PUFA:SFA verhouding is hoër as dié van intensiewe boerdery, wat dit voordelig maak vir menslike gesondheid. Vrylopende hoenders se vleis is donkerder (L*) (P ≤ 0.05) en het ook laer rooi, geel (a* en b*) en pH (P ≤ 0.05) waardes getoon. Produksiestelsel effek het ook variërende chemiese waardes in verskillende karkas porsies tot gevolg gehad: ʼn laer vetinhoud is gevind in die dy en ʼn hoër proteïeninhoud in die borsies van die braaikuikens wat vrylopend grootgemaak is. Korrelasies met die sensoriese data het ook getoon dat sappigheid, taaiheid en hoendervleis geur die grootste dryfvere is in verbruikersaanvaarbaarheid. Tydens die verbruikerstoetse waar die verbruikers die gaar hoendervleis blind geproe het, het die verbruikers oor die algemeen meer gehou van die Ross X Koekoek kruising in vergelyking met die ander hoender genotipes (P ≤ 0.05), maar sodra inligting oor die verskillende produksiestelsels gegee is, het die verbruikers aangedui dat hulle hoenders wat vrylopend groot gemaak is, verkies. Dit dui daarop dat persepsies ʼn baie belangrike rol speel in die verbruiker se finale besluitnemingsproses. Statistiese segmentasietegnieke is ook op die data uitgevoer ten einde te bepaal of verbruikers in groepe verdeel kan word wat betref hul voorkeur van die sensoriese of intrinsieke eienskappe van die hoenderprodukte. Drie verskillende groepe is geïdentifiseer, nl. verbruikers wat 1) vrylopende hoender vleis verkies; 2) intensiewe hoender vleis verkies; 3) beide vrylopende en intensiewe hoender vleis verkies. In die lig van bogenoemde resultate wil dit voorkom of kruisteling tussen die gewone braaikuiken en die Potchefstroom Koekoek ʼn moontlike opsie is vir die Suid-Afrikaanse vryloop hoenderbedryf. Hierdeur word daar van vrylopende produksie stelsels gebruik gemaak sonder om die vleiskwaliteit of gebruikers aanvaarbaarheid negatief te beïnvloed.

Poultry provide affordable animal protein products compared to other animal products in agricultural industry. The demand of organic food by world health organisation and call for discard of conventional laying cage production method led to this research study. The aim of the study was to determine how (4 x 4) full diallel crossbreeding of the Potchefstroom Koekoek (PK), Naked neck (NN), Lohmann Brown (LB) and White Leghorn (WL) had an effect on production performance, egg parameters, genetic and phenotypic characteristics of F1 crossbreed offspring. The study was conducted at the Agricultural Research Council (ARC), Livestock Production Improvement at the Irene Campus, which is situated about 25 km south of Pretoria. The (4 x 4) full diallel crossbreeding design used on four chicken breeds to produce four pure breeds, six crossbreeds and six reciprocal crosses. The total number of 352 chickens with16 treatments (2 cocks and 20 hens) used in phase 1 and 384 chickens 16 F1-treatments (3 cocks + 21 hens) used in phase 2. Data was analysed by full factorial analysis of variance (ANOVA), General Linear Model procedures and Scheffe post-hoc for multiple comparison of the means of different variable data. The outcome had shown that crossbreeding had an effect on the production performance, genetic and phenotypic characteristics. The performed F1 crossbreeds emerge from crossbreeding between the local dual-purpose PK and commercial LB chicken breeds. PKLB dominated on growth and production performance traits compared to other crossbreeds. All set null hypothesis differ significantly at (p < 0.05), the outcome of all five hypothesis of this study were rejected. In conclusion PKLB was the best performing F1 crossbreed, based on its best performance on growth, FCR, cost of rearing, productive, high quality safe eggshell, economic efficiency and consumer preference (brown eggshell and yolk colour). / Dikgogo di neelana ka dikumo tsa poroteine ya diphologolo go tshwantshanngwa le dikumo tsa diphologolo tse dingwe mo intasetering ya temo. Tlhokego ya dijo tse di bolang mo mekgatlhong ya boitekanelo ya lefatshe le pitso ya go latlha mekgwa ya kumo ya dikgetshe tsa go beela tsa tlwaelo di ne tsa isa kwa thutong ya patlisiso eno. Maikaelelo a thuto eno ke go tlhomamisa gore tsadiso ya kgabaganyo ya dilo tse pedi kgotsa go feta go tshwantshanya kgolagano ya mofuta wa dijene le tikologo tse di tletseng tsa (4 x 4) tsa Potchefstroom Koekoek (PK), Naked Neck (NN), Lohmann Brown (LB) le White Leghorn (WL) di na le ponalo mo tiragatsong ya kumo, diparametera tsa mae, le dijene le diponagalo tsa kgolagano ya mofuta wa dijene le tikologo tsa ditsadiso tsa kgabaganyo tsa ngwana wa F1. Thuto e ne ya diragadiwa kwa Agricultural Research Council (ARC) le Tokafatso ya Kumo ya Diruiwa kwa khempaseng ya Irene, e e agilweng bokana ka 25 km jwa borwa jwa Pretoria. Ditsadiso tsa kgabaganyo tsa dilo tse pedi kgotsa go feta go tshwantshanya kgolagano ya mofuta wa dijene le tikologo tse di tletseng tsa (4 x 4) di ne tsa dirisiwa mo mefuteng ya ditsadiso tsa dikgogo go ntsha mefuta ya ditsadiso e e tletseng e mene, ditsadiso tsa kgabaganyo tse thataro le dikgabaganyo tse di tshwanang tse thataro. Palo e e tletseng ya dikgogo tse di 352 ka ditiragatso di le 16 (mekoko e le 2 le dithole di le 20) di ne tsa dirisiwa mo letlhakoreng la 1 le dikgogo di le 384 ka ditiragatso tsa F1 di le 16 (mekoko e le 3 + dithole di le 21) di ne tsa dirisiwa mo letlhakoreng la 2. Data e ne ya tshetshereganngwa ka tshetshereganyo ya dintlha tse di tletseng tsa pharologantsho (ANOVA), dikgato tsa General Linear Model le tshwantshanyo ya bontsintsi ya morago (ANOVA), dikgato tsa General Linear Model le tshwantshanyo ya bontsintsi ya morago ga tiragalo ya Scheffe ka mekgwa ya data ya pharologantsho e e farologaneng. Ditlamorago di ne tsa bontsha gore ditsadiso tsa kgabaganyo di na le ponalo mo tiragatsong ya kumo, ga mmogo le diponagalo tsa dijene le setlhopha sa kgolagano ya mofuta wa dijene le tikologo. Go ne ga diriswa mefuta ya ditsadiso tsa kgabaganyo ya F1 tse di tlhagelelang go tswa mo ditsadisong tsa kgabaganyo magareng ga mefuta ya ditsadiso tsa dikgogo tsa PK tsa lebaka la gabedi la selegae le LB ya kgwebo. PKLB e ne ya fekeetsa metlhala ya tiragatso ya kgolo le kumo go tshwantshanngwa le mefuta ya ditsadiso tsa kgabaganyo tse dingwe. Setlhopha sotlhe sa dikakanyo tsa lefela se x farologana mo go bonagalang ka (p < 0.05) le ditlamorago tsa dikakanyo tse tlhano tse tsotlhe tsa thuto eno di ne tsa kganediwa. Kwa bokhutlong, PKLB e ne ya nna mofuta wa ditsadiso tsa F1 o o diragatsang go gaisa, go ikaegilwe ka tiragatso mabapi le kgolo, FCR, tshenyegelo ya go tsadisa, kumo, boleng jo bo kwa godimo jwa dikgapetla tsa mae tse di babalesegileng, bokgoni jwa ikonomi le boikgethelo jwa modirisi (dikgapetla tsa mae tse di tshetlha le mmala wa tlhae). / Agriculture and  Animal Health / Ph. D. (Agriculture)