The aim of this research was to assess the variability and genetic relationships among binary traits denoting the presence or absence of objectionable fibers, namely pigmented (BINPPF), medullated (BINPMED), and kemp fibers (BINPK), and of fleece traits (fiber diameter [FD] and clean fleece weight [CFW]) in Corriedale sheep. Additionally, the total response to selection against objectionable fibers and indirect responses when selecting for fleece traits were evaluated. Fiber records from 679 animals and fleece records from 795 animals obtained from 2 experimental flocks (from 2005 to 2007) were used; the pedigree file included a total of 3,792 animals. Heritability and genetic correlations among the traits were estimated with a multivariate animal model under a Bayesian setting. Heritability estimates (posterior SD) for BINPPF, BINPMED, and BINPK were 0.35 (0.08), 0.37 (0.10), and 0.63 (0.09), respectively; for CFW and FD, estimates were 0.42 (0.09) and 0.43 (0.08), respectively. The genetic correlations between CFW and the 3 types of objectionable fibers were very low (i.e., <0.2). Thus selection for CFW is not expected to affect the number of such fibers in any direction. The same occurred for the genetic correlation between BINPK and FD. Genetic correlations between FD and BINPMED and FD and BINPPF were positive and favorable (0.50 and 0.56, respectively). Selecting for lower FD would decrease the numbers of objectionable fibers. The expected correlated responses in BINPPF, BINPMED, and BINPK when selecting for CFW were -0.03 (0.11), 0.03 (0.11), and -0.05 (0.18), respectively; when selecting for FD, the correlated responses were -0.26 (0.11), -0.26 (0.11), and -0.14 (0.16). Overall, this study reports novel information on genetic parameters for the presence of objectionable fibers and their associations with fleece traits in sheep. Our findings suggest that it could be possible to improve FD while at the same time reducing the content of BINPMED and BINPPF in wool; however, improvements in CFW are likely to have no effect on the numbers of objectionable fibers.
The incidence of medullation in 549 Corriedale hoggets from two experimental flocks, its association with other characteristics of the fleece, and the sire effect were evaluated. The investigation was carried out during 2005–06 in hoggets from the Faculty of Agriculture and Faculty of Veterinary Science experimental flocks. Fleeces were weighed and samples were taken at shearing by picking 104 staples from fleeces to measure medullation. A mid-side wool sample was taken to determine wool characteristics (fleece weight, wool yield, mean fibre diameter, staple strength and length). A Dark Fibre Detector was used to identify total medullated fibres and fibre type: medullated fibres (med) and kemp, which were confirmed by light microscope. Mean total medullated fibres, med and kemp content per 10 g of clean wool were 4.2 ± 11.2; 2.0 ± 8.9 and 2.3 ± 4.9, and median values were 0.9, 0.0 and 0.7, respectively, in 549 fleeces. A high variability between samples and a higher proportion of hoggets with kemp fibres than those with med were observed. There were significant differences between sire families for total medullated fibre content/10 g, med fibre content/10 g and kemp fibre content/10 g, but there were no significant effects between flocks, years or interaction between them. Phenotypic correlations between medullated fibre content and wool characteristics were generally low (less than 0.19). In conclusion, medullated fibre content was highly variable between samples and was not strongly correlated with other fleece characteristics. Medullation varied between sire groups, suggesting genetic variation, which could be exploited to improve wool quality.
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