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M.A. Snyman*, J.J. Olivier*, G.J. Erasmus & J.B. van Wyk

*Grootfontein Agricultural Development Institute, Private Bag X529, Middelburg Cape, 5900, South Africa

Department of Animal Science, University of the Orange Free State, Box 339, Bloemfontein, 9300, South Africa



1. Introduction

Knowledge of genetic parameters for economically important traits is essential for deriving a selection strategy for any breed. In breeds such as the Afrino - a white woolled breed developed under harsh conditions for wool and slaughter lamb production - the primary selection objective should be to increase the weight of lamb produced per ewe over her lifetime 8. Lambing percentages in excess of 150% under harsh, extensive conditions and in wool producing sheep breeds lead to the production of a high quantity of lambs, but the quality of these lambs are in many instances not acceptable. Selection for increased reproductive performance in such flocks should be aimed at increasing the quality and monetary value of the product in terms of weight and carcass quality. Selection for litter size, without taking the weaning weight of the individual lambs into consideration, would be short sighted.

As approximately 20 % of the income from Afrino sheep is generated through wool production, fleece traits such as clean fleece weight and fibre diameter should also receive attention during selection. Numerous genetic and phenotypic correlations between production and reproduction traits for different breeds have been published 2. No correlation estimates between these economically important traits are, however, available for Afrino sheep.

The objective of this study was to estimate genetic and phenotypic correlations among weaning weight (WW), nine- (W9) and 18-month body weight (W18), clean fleece weight (CFW), mean fibre diameter (MFD), total weight of lamb weaned over three parities (TWW), as well as number of lambs born (NLB) and weaned (NLW) over three parities in the Carnarvon Afrino flock.


2. Material and methods

Data collected from 1972 to 1994 on the Carnarvon Afrino flock - kept on natural pasture at the Departmental Experimental Station near Carnarvon (30E 59'S, 22E 9'E) in the North-western Karoo region of the Republic of South Africa - were used for this study. A detailed description of the Afrino breed, management and selection procedures followed in this specific flock is given by Snyman 6.

Covariance components were estimated using the DFREML programme of Meyer 4,5. Bivariate animal models, including only direct genetic effects, were fitted throughout. The number of animals with records for bivariate analyses was 3748 for the production traits and 618 for the reproduction traits.


3. Results and discussion

Genetic and phenotypic correlations estimated between body weight, clean fleece weight, mean fibre diameter and the reproduction traits are presented in Table 1. Heritability estimates for body weight and fleece traits 7 and for TWW 8 are also presented. With some exceptions, most of the correlations estimated in this study fall within the ranges reported in the literature 2. The estimated genetic correlations which should have the most important influence on the formulation of a viable breeding plan for Afrino sheep are the high positive genetic correlations between TWW and body weight at all ages, as well as the negative correlations between CFW and the reproduction traits.

Genetic correlations of 0.83 and 0.84 between lifetime total weight of lamb weaned and lifetime number of lambs born and weaned respectively, were estimated. These estimates indicate that selection for litter size would increase total weight of lamb weaned, which could be expected, as litter size is the major determinant of total weight of lamb weaned. However, genetic correlation estimates of weaning weight with lifetime total weight of lamb weaned (0.75), lifetime number of lambs born (-0.01) and lifetime number of lambs weaned (0.11), indicate that selection for litter size would not increase the individual weaning weight of each lamb, which is just as important as the number of lambs weaned. Selection for total weight of lamb weaned would, however, result in a correlated genetic increase in weaning weight of the individual lambs as well.

Table 1. Heritabilities (on diagonal), genetic (above diagonal) and phenotypic (below diagonal) correlations between body weight, clean fleece weight, fibre diameter and reproductive traits


WW 0.41 0.96 0.90 0.04 -0.10 0.75 -0.01 0.11
W9 0.78 0.63 0.96 -0.01 -0.04 0.75 0.23 0.29
W18 0.64 0.80 0.60 -0.09 -0.04 0.88 0.31 0.40
CFW 0.15 0.14 0.10 0.62  0.18 -0.52 -0.33 -0.39
MFD -0.02 0.02 0.04 0.16 0.73 -0.11 -0.17 -0.09
TWW 0.15 0.24 0.27 -0.06 -0.03 0.17 0.83 0.84
NLB 0.04 0.12 0.16 -0.02 -0.03 0.79   0.99
NLW 0.03  0.10 0.15  -0.02  -0.01 0.92 0.89  

The objective of the Afrino is that it should be able to produce and reproduce under extensive conditions. As 80% of its income is generated through reproduction and growth (mutton production), and bearing in mind the negative genetic correlation estimated between CFW and TWW, selection for an increase in fleece weight under extensive conditions, given present price structures, would probably not be advisable. However, due to this negative genetic correlation it is advised that fleece weight should at least be monitored should selection be aimed at only increasing TWW and direct growth. MFD has low negative genetic correlations with all traits, except CFW. Negative selection pressure on MFD would therefore not adversely affect other economically important traits.

Selection for TWW would result in an increase in body weight at all ages, as well as in NLB and NLW. This would also be accompanied by a favourable decrease in MFD, but a substantial unwanted decrease in clean fleece weight. However, TWW is sex limited as well as a labourious and time consuming measurement. The results of this study imply that TWW can be improved by indirect selection for body weight at any age. Selection for W18 would be practical under certain circumstances, but not possible where selection takes place at an earlier age. In such instances, WW or W9 could be used as selection criteria. Preferably, W9 would be a better choice as it also includes a measure of post weaning growth, as opposed to WW which is largely influenced by maternal effects 7. Furthermore, W9 also has a higher heritability than WW.

Indirect selection will be more effective than direct selection if rAhY is greater than hX, where rAhY is the correlation between breeding values of the desired trait X and phenotypic values of the selected trait Y, while h is the accuracy of direct selection 1. As rAhY (0.432) is greater than hX (0.412), indirect selection for WW to improve TWW should be more effective than direct selection. In the case of indirect selection on W9, rAhY (0.578) is also greater than hX (0.412). A higher selection intensity with indirect selection for body weight is also possible because males can be selected, which should make it even more effective. In practise it is, however, difficult to quantify selection intensity as other traits are also considered.


3.1 Ram selection

All the information needed for the selection of potential sires are available at an early age. Selection can therefore be based on a selection index incorporating the relevant traits. The results of this study were used to construct four possible selection indices for Afrino ram selection 3. These, as well as the genetic response obtained per generation in the selection objectives with each of these recommended indices,3 are summarized in Table 2.


3.2 Ewe selection

The results of this study indicate that selection for WW or W9 will lead to a correlated genetic increase in TWW. However, the low phenotypic correlations estimated between TWW and WW (0.15) and between TWW and W9 (0.24) would not guarantee that the highest producers be selected for the current flock. Snyman (1996, Unpublished) indicated that total weight of lamb weaned is the most accurate predictor of current lifetime reproductive performance.

Ewe selection is aimed at increasing lifetime reproductive and productive efficiency in the current flock, as well as the genetic merit of future generations. The high genetic and phenotypic correlations estimated between total weight of lamb weaned at the first parity (TWW1) and future performance indicate that selection based on TWW1 will ensure that the highest producers be selected and therefore that gains in the current flock would be increased 6. The genetic variance exploited in this way, should also increase the genetic merit of these ewes' daughters in terms of lifetime reproductive efficiency.

Not all the information required for the accurate identification of superior ewes is available at selection age. It is therefore recommended that ewe selection should take place in two phases. In the first phase, ewes could be assessed subjectively for breed standards and conformation or wool faults. Preliminary selection on the basis of WW or W9 could then be done. These ewes should then be mated and final selection (second phase) could be done after their first parity. With the second phase of ewe selection, selection should be based solely on reproduction performance and young ewes that fail to wean a lamb or produce below average TWW1 could be culled.

Table 2. Selection indices for Afrino sheep and the genetic response obtained per generation in the selection objectives with each of these recommended indices

Index 1 +4   +1 -3
Index 2   +3 +2 -2
Index 3 +1     -1
Index 4   +1   -1


Genetic response in :

  (kg) (kg) (µm) (kg)
Index 1 2.19 0 -0.42 5.68
Index 2 2.68 0 -0.30 7.42
Index 3 2.10 -0.01 -0.56 5.70
Index 4 2.63 -0.02 -0.42 7.62


4. Conclusion

The most important result of this study is, in all probability, that indirect selection for reproductive performance, defined as total weight of lamb weaned (TWW), based on early recorded traits, which are not sex limited, could be more effective than direct selection. The holistic approach of this study, where production and reproduction traits were analysed simultaneously, provided useful information for aggregate genetic improvement and could be followed for other breeds and with other data sets.


5. References

1 FALCONER, D.S. & MACKAY, T.F.C., 1996. Introduction to quantitative genetics, Longman, Essex

2 FOGARTY, N.M., 1995. Genetic parameters for live weight, fat and muscle measurements, wool production and reproduction in sheep : a review. Anim. Breed. Abstr. 63(3) : 101-143

3 CUNNINGHAM, E.P. & MAHON, G.A.T., 1977. SELIND - A fortran computer program for genetic selection indexes, User’s guide

4 MEYER, K., 1991. Estimating variances and covariances for multivariate animal models by restricted maximum likelihood.

Genet. Sel. Evol. 23 : 67-83

5 MEYER, K., 1993. DFREML : Programs to estimate variance components by restricted maximum likelihood using a derivative-free algorithm. User notes, Ver. 2.1

6 SNYMAN, M.A., ERASMUS, G.J. & VAN WYK, J.B., 1995. Non-genetic factors influencing growth and fleece traits in Afrino sheep. S. Afr. J. Anim. Sci. 25(3) : 70-74

7 SNYMAN, M.A., ERASMUS, G.J., VAN WYK, J.B., & OLIVIER, J.J., 1995. Direct and maternal (co)variance components and heritability estimates for body weight at different ages and fleece traits in Afrino sheep. Livest. Prod. Sci. 44 : 229-235

8 SNYMAN, M.A., ERASMUS, G.J., VAN WYK, J.B., & OLIVIER, J.J., 1997. Genetic parameter estimates for total weight of lamb weaned in Afrino and Merino sheep. Livest. Prod. Sci. (In press)