- Implications of selection against fleece rot in Merino sheep
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IMPLICATIONS OF SELECTION AGAINST FLEECE ROT IN MERINO SHEEP
G J Delport and G J Erasmus
S A Fleece Testing Centre
Private Bag X529
Middelburg Cape 5900
It is general practice to select against fleece rot in woolled sheep on account of its economic implications and relationship with blowfly strike (Bailie 1980). It has been confirmed that culling animals with fleece rot leads to within flock improvement (Atkins, McQuirk and Thornberry 1980). However the effectiveness of a direct selection programme will be influenced by the infrequent occurrence of causative environmental stresses, especially rainfall (Dunlop and Hayman 1958). According to the newsletter of the Australian Merino Society (A M S) in September 1981, the 14,8 percent higher fleece mass, achieved through selection for a number of years in that specific group breeding scheme, was associated with a higher incidence of fleece rot.
This study examines the economic and genetic consequences in other wool traits when selecting against fleece rot.
MATERIALS AND METHODS
The analysis was done on data of the 1980/81 season obtained from a group breeding scheme in the Eastern Cape.
The normal selection procedure of culling 43 percent rams and 15 percent ewes on functional cull faults as well as low body and fleece mass at 6 months of age was followed. After visual appraisal at 16 months of age, a routine fleece analysis was done by the S A Fleece Testing Centre. These data were analysed by normal statistical procedures. A selection index of 1 x body mass + 10 x fleece mass was used to rank the sheep for the calculation of selection differentials.
RESULTS AND DISCUSSION
1 Incidence of fleece rot
Although the sheep were run in a relatively high rainfall area (639 mm/year), the average incidence of 6,7 percent (Table 1) is low in comparison with Australian figures of 7,3 to 37,2 percent recorded between 1975 and 1977 (Atkins et al 1980). The existence of a close relationship between heritability and the incidence of a binomially distributed trait (Hill 1977) indicates the possibility of a lower selection response than in the Australian case (Atkins et al 1980). The sporadic incidence in this flock is clearly indicated in Table 1.
2 The relation between fleece rot and fleece characteristics
Mean values of wool traits were compared within sexes between animals with and those without fleece rot. The only statistically significant differences in both sexes were clean fleece mass and crimps per 25 mm (Table 2). The highly significant difference in fibre diameter of the rams may be the result of a bias due to the effect of preliminary culling, since this effect could not be demonstrated in the case of the ewes as well.
Estimates of phenotypic correlations between fleece rot and both clean fleece mass and n.d (crimps/ 25 mm x fibre diameter) were high (Table 3). It is interesting that the reasoning often heard that the fleeces of sheep with "undercrimped" wool, and therefore with a low n.d value, tend to fall open and be more exposed to conditions favourable for fleece rot, could not be indicated by differences between mean values (Table 2). On account of the generally accepted high negative correlation between n.d and clean fleece mass, it may be inferred that the abovementioned reasoning may be the result of an indirect effect through the correlation with fleece mass. The conclusion from Tables 2 and 3 is therefore that the elimination of animals with fleece rot will inevitably lead to lower average wool production figures in the current generation.
3 Economic implications
The following assumptions were made to calculate the economic loss in income from wool production by selecting against the incidence of fleece rot: a repeatability of 0.70 for clean fleece mass, two levels of selection intensity of 45 and 70 percent respectively and a price of R3.03 per kg for greasy wool for the 1981 season.
At both levels of selection intensity the loss amounted to R1.25 per sheep lifetime of five years in the flock. Unfortunately the amount of wool infested with fleece rot was not determined and the direct loss in income through the incidence of fleece rot could not be calculated.
4 Genetic implications
The effect of culling all sheep showing fleece rot on the genetic improvement of wool production was investigated. In the calculation the following assumptions were made: a heritability of 0,40 for clean fleece mass, a selection intensity of five percent for the ram progeny and that truncation selection for the selection index was used.
A loss of 24 percent in selection response per generation can therefore be expected (Table 4). Since the repeatability for the incidence of fleece rot varies from only 0,01 to 0,19 (Atkins et al 1980), high selection pressure to eliminate fleece rot, especially at times of an outbreak, is not a feasible practice. For the Australian situation Bailie (1980) reported a much higher incidence than in South Africa and stated that "Attempts to eliminate fleece rot completely may be a mistake. Sheep classers believe that high selection pressure to eliminate fleece rot will lead to a decline in fleece weight. To maintain reasonable fleece weights a certain amount of fleece rot may have to be tolerated". Examination of skin characteristics in search of indirect selection criteria (Thornberry, Kowal and Atkins 1980) might present an alternative solution to this problem in future.
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