Sex differences in the heritabilities of economic traits in South African Merino sheep

No Abstrac

Genetic parameters for reproduction rate in the Tygerhoek Merino flock. 2. Genetic correlations with wool and live mass traits

Data of the Tygerhoek Merino flock were used to obtain estimates of genetic correlations of 18 months wool and pre-mating live mass traits with average reproduction over two to five lambing opportunities, as well as between pre-mating live mass and subsequent reproduction in age groups of 2 - 6 years by paternal halfsib procedures. The wool traits considered were clean fleece mass (CFW), greasy fleece mass (GFW) and fibre diameter (FD). Reproduction traits were lambs born/ewe conceived (Lb!Ec), lambs born/ewe mated (Lb/Em), and lambs weaned/ewe mated (Lw/Em). The genetic correlations of 18 months wool traits with reproduction traits were not very accurate and estimates larger than twice their standard errors were obtained only on a few occasions. The genetic correlation between CFW and Lb/Ec tended to be positive. There was, however, evidence of a negative genetic correlation between CFW and the composite trait of Lw/Em both in literature cited and in the present investigation. The genetic correlations of 18 months FD with reproduction traits were variable. The genetic correlation of live mass with reproduction traits was positive, both within age groups and when 18 months pre-mating live mass was correlated with average reproduction over a number of lambing opportunities. Several estimates involving Lb/Ec, Lb/Em, and Lw/Em were larger than twice their obtained standard errors. The obtained genetic correlations were in correspondence with literature cited. It was concluded that selection for live mass is unlikely to cause negative correlated responses in reproduction, unless live mass at a very young age is used as a selection criterion.Data van die Tygerhoek-Merinokudde isgebruik om beramings van genetiese korrelasiesvan 18-maande-wol- en liggaamsmassa-eienskappe met gemiddelde reproduksie oortwee totvyf lamgeleenthede en van paarmassa met daaropvolgende reproduksie binne ouderdomsgroepe volgens halfsibmetodes te bereken. Die woleienskappe wat ondersoek is, was skoonvagmassa (CFW), rouvagmassa en veseldikte (FD). Reproduksie-eienskappe was lammers gebore/ooi beset (Lb/Ec), lammers gebore!ooi gepaar (Lb/Em) en lammers gespeen/ooi gepaar . (Lw/Em). Die genetiese korrelasies van 18-maande-woleienskappe met reproduksiewas nie baie akkuraat nie en beramingsgroter as twee maal hul standaardfoute is slegs by 'n paar geleenthede verkry. Die genetiese korrelasie tussen CFW en Lb/Ec was oorwegend positief. Daarteenoor was daar aanduidings van 'n negatiewe genetiese korrelasie tussen CFWen die saamgestelde eienskap Lw/Em, beide in aangehaalde Iiteratuur en in die huidige ondersoek. Die genetiese korrelasie van FD met reproduksie was veranderlik. Die genetiese korrelasie van Iiggaamsmassa met reproduksie-eienskappe was positief, beide binne ooi-ouderdomsgroepe en tussen 18-maande-paarmassa en gemiddelde reproduksie cor 'n aantal lamgeleenthede. Etlike beramings op Lb/Ec, Lb/Em en Lw/Em was meer as twee maal groter as hulle verkree standaardfoute. Die beraamde genetiese korrelasies was in ooreenstemming met aangehaalde literatuur. Dit is onwaarskynlik dat seleksie vir Iiggaamsmassa nadelige gekorreleerde reaksies in reproduksie sal veroorsaak, tensy Iiggaamsmassa op 'n baie jong ouderdom as seleksiekriterium gebruik wor

Die evaluasie van 'n prestasietoetstegniek met Afrikanerbulle

No Abstrac

Phenotypic and genetic aspects of production in the Dohne Merino. III The influence of age of the ewe on reproductive performance

No Abstrac

The effect of sampling frequency on the accuracy of estimates of milk-fat yields of dairy cows

Estimation of 300-day lactation yields of milk, fat, protein and lactose from combined morning and afternoon milk samples collected at five-weekly intervals is time-consuming, and proportional bulking of morning and evening samples is not always done correctly by farmers. This study was conducted to determine whether samples could be collected once daily without compromising the accuracy of yield estimates. The results of this study support the five-weekly sampling procedure currently used by the South African National Dairy Cattle Performance Testing Scheme. However, replacement of proportional bulking of individual morning and evening samples with a single evening milk sample would not compromise accuracy provided that the relevant corrections are made to prevent over-estimation of milk-fat yields.Articl

Maintaining genetic stability in a control flock of South African Merino sheep

A genetic control flock of Merino sheep consisting of 160 ewes and 16 rams was established in 1969 at the Tygerhoek Experimental Farm near Riviersonderend in the South Western Districts of South Africa. In 1976 it was extended to 200 ewes and 20 rams. Ewes are replaced by their second ewe lamb when it reaches joining age and rams are replaced yearly, by a randomly chosen son. Four body-mass and six wool traits of the off-spring are measured every year, Random discrepancies between the means of whole progeny and replacement groups for the measured characters varied from 0 to 6,2 % over a period of 11 years, but was on average, roughly zero for individual traits over all years. Regression equations for trait means on time showed the following total changes from 1971 to 1981: Birth mass -23,4 %, 42-day body mass -8.9 %, 120-day body mass + 4,7 %, 18-rnonth body mass -1,7 %, greasy fleece mass -11,1 %, percentage clean yield + 1,5 %, clean fleece mass -9,8 %, staple length + 1,0 %, crimp frequency -1,5 % and fibre diameter -0,7 %. Only the regression for birth mass (-0.096±0,01 kg/year) was significant. That the changes with regard to 42-day body mass and greasy and clean fleece mass were not caused by genetic drift, is indicated by the low increase in inbreeding of only 1,2 % over the 11 years covered in this report. It is concluded that genetic stability can be maintained in a Merino control flock of this size under practical conditions

Environmental and genetic trends in clean fleece mass, live mass and fiber diameter in selection and control flocks involving a selection experiment for increased clean fleece mass in South African Merino sheep

This study was undertaken to investigate the genetic stability of an unselected Conlrol Group of South African Merino sheep and genetic change in a flock selected for increased clean fleece mass (Selection Group) under the same environmental conditions. Data regarding 14-17 months clean fleece mass (CFM) and fibre diameter (FD) of 5867 progeny from these groups (3186 and 2681 individuals in the Selection and Conlrol Groups, respectively) were analysed to investigate genetic change over the period 1969-1989. A smaller data set, involving 5273 progeny (2782 and 2491 individuals in the respective groups) born in 1971-1989, was used to investigate change in 16-17 month live mass (LM). An animal model was used to obtain predicted breeding values (PBVs) for all individuals by mixed model analysis (MMA). Average PBVs of Selection and Conlrol group progeny within birth years were taken as a measure of genetic change, rendering genetic trends independent from environmental bias. Genetic change in the Selection Group was also obtained by deviating average PBVs from those obtained in the Control Group. This approach is analogous to the method of expressing genetic change as least squares deviations of Selection Group progeny from Control Group contemporaries, and will be affected by genetic change in the Control Group. Prior heritability estimates, derived for the MMA by paternal halfsib procedures, were within ranges reported in the literature. Year-to-year variation, as derived from environmental trends from the MMA, appeared to be less for FD when compared to LM and CFM. Average PBVs for Control Group progeny increased (