Last update: November 25, 2010 08:26:46 AM E-mail Print

 

CORRELATIONS OF SUBJECTIVELY ASSESSED FLEECE AND

CONFORMATION TRAITS WITH PRODUCTION AND REPRODUCTION

IN AFRINO SHEEP

 

M. A. Snyman, W. J. Olivier, P.J. Griessel & J.A.N. Cloete

Grootfontein Agricultural Development Institute, Private Bag X529, Middelburg (EC), 5900

 


The Afrino is a white woolled mutton sheep breed kept in the more extensive sheep grazing areas of the Republic of South Africa. The breeding objectives of the breed are to produce good quality slaughter lambs at a relatively early age from the veld, and to produce a reasonable amount of good quality fine wool. In practice, however, a lot of emphasis is placed on subjective conformation and fleece traits during the selection of breeding sires and dams. In many instances, animals are culled on the basis of these traits, but at the expense of the economically important traits such as reproduction. One such trait that is discriminated against in Afrino sheep, is creeping belly (the extent to which belly wool tends to creep up the side into the fleece). Other subjective conformation and fleece traits on which emphasis are placed, are general conformation of the head, softness of face covering, extent of pigmentation in the face and on the ears, crimp definition, density of fleece, evenness of fleece, straightness of the top line, hocks and pastern joints. Several subjective conformation and fleece traits of sheep in the Carnarvon Afrino flock were assessed on a linear scale. The heritabilities of and genetic and phenotypic correlations amongst these traits were estimated, as well as the genetic and phenotypic correlations of these traits with body weight, objective fleece traits and reproduction. This was done in an attempt to determine how selection emphasis on the subjectively assessed traits in the selection programme of Afrino, as well as other white woolled breeds, would influence genetic progress in the economically important production and reproduction traits. Data used for this study were collected on the Carnarvon Afrino flock from 1986 to 1998, and included data records on several subjectively assessed traits, body weight and fleece traits of 3291 animals, the progeny of 127 sires and 772 dams. Reproduction data of 686 ewes born from 1986 till 1997 were also available. Variance components and heritability estimates were obtained by fitting single trait animal models under the ASREML programme, while covariance components, genetic and phenotypic correlations among traits were obtained by fitting bi-variate animal models. Heritability estimates for the various subjective traits ranged from 0.06 " 0.02 for straightness of the top line to 0.51 " 0.04 for softness of fleece. Positive genetic correlations, ranging from 0.33 " 0.18 to 0.80 " 0.06 were estimated amongst the conformation traits, head, front quarters, top line and hocks. High genetic correlations were estimated amongst the subjective fleece traits and fibre diameter, where animals with lower fibre diameter had softer fleeces, better crimp definition, their fleeces were more even, less dense and had larger creeping bellies. Estimated genetic correlations between the subjective fleece traits and reproduction were variable in sign and magnitude. The most important being the unfavourable genetic correlation (-0.33 " 0.23) between creeping belly and reproduction. The conformation traits had moderate to high genetic correlations with body weight at all ages. Of the subjective fleece traits, creeping belly score had the largest genetic correlation with body weight, ranging from -0.26 " 0.10 for weaning weight to -0.38 " 0.07 for 18-month body weight. No noteworthy phenotypic correlations were discernable between the reproductive traits and any of the subjectively assessed traits. Of the objective fleece traits, only fibre diameter had some significant phenotypic correlations with the subjective fleece traits. These were similar in sign, but smaller in magnitude than the corresponding genetic correlations. It is concluded that, with the exception of two or three traits, the subjectively assessed traits would not be negatively influenced when selection is based on the economically important production traits.