- Efficiency of selection against kemp and medullated fibres in mohair
|Last update: August 15, 2011 08:53:49 AM|
Efficiency of selection against kemp and medullated fibres in mohair
Margaretha A Snyman* & P R King
Grootfontein ADI, Middelburg (CP), 5900, South Africa
Two projects, involving the establishment of a genetically fine mohair flock and the development of a hardy genotype derived from Angora x Boer goat crosses, are currently being conducted at the Jansenville Experimental Station. As a result of these projects, the presence of kemp and medullated fibres in the fleece have again become an important issue. The level of kemp in South African mohair is exceptionally low. This was achieved mainly through subjective selection against kemp for many decades. As most medullated fibres can not be identified visually, a limited amount of these fibres is still present in the South African mohair clip. Accordingly, a study was undertaken to determine the efficiency of selection against medullation based on objective measurement. Subjective assessment of kemp and medullated fibres was done on the second fleece of 1164 ewe kids from 8 different studs. Objective measurements of kemp and medullated fibres were done on samples taken from the back (anterior to the tail) of 250 of these kids. Subjective scores and objective measurements were also done on the second fleece of 470 kids from the Jansenville experimental flocks. Objective measurements were done by means of a projection microscope. The fibres were classified according to the diameter of the medulla as (a) kemp fibres, when the diameter of the medulla was 60%, or more, of the diameter of the fibre, and (b) medullated fibres, when the diameter of the medulla was less than 60% of the diameter of the fibre. Kemp (0.000 %) and medullated fibre (0.194 ± 0.022 %) content of the stud animals were exceptionally low. Only nine of the 250 animals tested, had 1.0% or more medullated fibres, while none of the animals showed any kemp fibres. A phenotypic correlation of 0.45 was found between the subjective kemp score and percentage medullated fibres, while the heritability of the subjective kemp score was 0.027 ± 0.055 in the stud animals. The average percentage kemp (0.078 ± 0.026) and medullated fibres (0.919 ± 0.111) in the Jansenville kids were higher than those of the stud animals. Variation in the Jansenville animals were also larger than that of the stud animals, and percentage medullated fibres varied from 0.0% to 10.0% for the Jansenville kids. Phenotypic correlations between the subjective kemp score and objective kemp percentage, percentage medullated fibres and percentage total medullated fibres were 0.47, 0.38 and 0.44 respectively. These relatively low correlations stress the importance of objective measurement of kemp and medullated fibres, especially when the occurrence of these fibres are low. The preliminary heritabilities of percentage medullated fibres and percentage total medullated fibres were 0.590 ± 0.219 and 0.816 ± 0.255. These estimates indicate that selection against medullation based on objective measurement should be effective in these flocks.
Proceedings 33rd SASAS congress, Warmbad, 28-31 March