Last update: January 17, 2011 03:24:31 PM E-mail Print

 

DIRECT AND CORRELATED SELECTION RESPONSE IN MERINO SHEEP WITH SELECTION

FOR TOTAL WEIGHT OF LAMB WEANED

 

 W.J. Olivier1, M.A. Snyman1,J.J. Olivier2, J.B. van Wyk3, & G.J. Erasmus3

1Grootfontein ADI, Private Bag X529, Middelburg E.C., 5900, South Africa

2ARC-AII, Private Bag X529, Middelburg E.C., 5900, South Africa

3Department of Animal Science, University of the OFS, Box 339, Bloemfontein, 9300, South Africa

 

 


 

INTRODUCTION

The improvement of the reproduction performance of a flock is of paramount importance for the economic survival of small stock farmers and the best measure of the reproduction performance is probably total weight of lamb weaned per ewe over her lifetime (Snyman et al., 1997). The main objective of this study was to calculate the direct and correlated selection response per generation in total weight of lambed weaned per ewe mated (TWW), number of lambs born (NLB) and number of lambs weaned (NLW) and individual weaning weight (WW) of each lamb for selection based on any of these four traits. For this purpose, genetic and phenotypic correlations among these traits were estimated.

 

MATERIAL AND METHODS

Data collected on the Grootfontein Merino stud (from 1968 to 1996) and the Carnarvon Merino flock (from 1964 to 1983) were used for this study. Traits analysed in this study were number of lambs born to a ewe mated over three parities (NLB), number of lambs weaned to a ewe mated over three parities (NLW), total weight of lamb weaned per ewe mated over three parities (TWW) and weaning weight of individual lambs (WW). TWW was calculated as described by Snyman et al. (1997). Only data of ewes with three consecutive lambing opportunities were used in the analysis.

(Co)variance components were estimated using the DFREML programme of Meyer (1993). The standard errors for the genetic correlations were calculated as described by Falconer & Mackay (1996). The direct selection response for each trait was calculated as R = ih2δP, while the correlated selection response was calculated as CRY = ihXhYrAδPY.

 

RESULTS AND DISCUSSION

Heritability estimates for WW, TWW, NLB and NLW, as well as the genetic and phenotypic correlations among these traits are summarized in Table 1 for both flocks.  The highest heritability estimate for GMS was obtained for NLB, while in CMF, WW had the highest estimate. The lowest heritability estimate in both flocks was obtained for NLW.

High estimates of genetic correlations among the reproduction traits were obtained in both GMS and CMF, while the genetic correlations of WW with TWW, NLB and NLW were moderate. The phenotypic correlations between WW and the reproduction traits were low, while the phenotypic correlations among the reproduction traits were moderate to high. Although the estimates found in the literature vary substantially the estimates of this study were within the reported range of correlations.

The selection response per generation in WW and TWW were measured as the average gain in kg and for NLB and NLW as the average gain in number of lambs born or weaned. In GMS the expected direct selection response in WW, TWW, NLB and NLW were 1.16 kg, 9.03 kg, 0.41 lambs and 0.31 lambs respectively, while in CMF, the respective response was 1.36 kg, 6.37 kg, 0.27 lambs and 0.24 lambs. The correlated selection response per generation to selection based on TWW in the GMS were 0.70 kg, 0.33 lambs and 0.33 lambs in WW, NLB and NLW respectively and in CMF the correlated response was 0.89 kg, 0.25 lambs and 0.26 lambs respectively.

As the best measurement of a ewe=s lifetime reproduction is the composite trait total weight of lambed weaned per ewe mated, the deviation of the selection responses of selection based on WW, NLB and NLW were therefore expressed as a percentage of the selection response obtained for selection based on TWW in each of the traits and these deviations are given in Table 2.

It is evident from Table 2 that selection based on TWW and NLB in the GMS and TWW and WW in CMF would have the highest selection responses in TWW. The low phenotypic correlations between TWW and WW in both these flocks indicate that the highest producers for the current flock would not necessarily be selected if selection is based on WW. In large areas of South Africa sheep is run under harsh conditions and an unnecessary increase in the number of lambs born would be undesirable. The results of this study support claims that TWW is the best indication of lifetime reproduction in the current flock, as well as in future generations.

 

Table 1. Heritability (SE) estimates (on diagonal) for and genetic (above diagonal) and phenotypic (below diagonal) correlations among the traits in the Grootfontein Merino stud and the Carnarvon Merino flock


                                 WW                                       TWW                                     NLB                                        NLW


Grootfontein Merino Stud

WW                       0.21 (0.04)                              0.65 (0.15)                              0.32 (0.16)                              0.34 (0.16)

TWW                     0.08 (0.03)                              0.19 (0.05)                              0.91(0.03)                               0.97 (0.01)

NLB                       0.06 (0.03)                              0.71 (0.01)                              0.23 (0.05)                              0.97 (0.01)

NLW                      0.04 (0.03)                              0.95 (0.01)                              0.77 (0.01)                              0.17 (0.05)

Carnarvon Merino Flock

WW                       0.30 (0.05)                              0.78 (0.08)                              0.45 (0.12)                              0.57 (0.12)

TWW                     0.19 (0.02)                              0.21 (0.04)                              0.89 (0.04)                              0.98 (0.01)

NLB                       0.14 (0.02)                              0.82 (0.01)                              0.19 (0.04)                              0.93 (0.02)

NLW                      0.14 (0.02)                              0.96 (0.01)                              0.84 (0.01)                              0.16 (0.04)


 

Table 2. The deviation of the selection responses of selection based on WW, NLB and NLW expressed as a percentage of the selection response obtained for selection based on TWW

 


Trait under selection                WW (%)                   TWW (%)               NLB (%)                     NLW (%)


Grootfontein Merino Stud

TWW                                     100.00                     100.00                     100.00                     100.00

WW                                       165.71                       66.56                      36.37                       36.37

NLB                                         54.29                     101.88                     124.25                     109.10

NLW                                        51.43                       94.13                     103.03                      96.97

Carnarvon Merino Flock

TWW                                     100.00                     100.00                     100.00                     100.00

WW                                       152.81                       92.94                       28.00                       15.38

NLB                                         23.6                         84.46                    108.00                       92.31

NLW                                        15.73                       85.87                      92.00                       92.31


 

CONCLUSION

The selection responses estimated in this study indicate that direct selection for TWW would be the most suitable selection criteria to improve reproductive performance, without an unnecessary increase in the number of lambs born or weaned. In the case of a flock with a low reproduction rate (lambing percentage < 100 %), such as the Carnarvon flock, the selection for improved reproduction should be based on fertility, i.e. cull all the ewes that failed to lamb. This would increase the reproduction rate, but not  necessarily the TWW of a ewe. Selection for WW could be combined with fertility in the selection objectives to improve TWW. In the case of a flock with a high reproduction rate, selection should be aimed at improving TWW. An important fact to note is that the environment under which a flock is kept would dictate the acceptable reproduction rate for that specific environment. In areas where the lambing percentages  are already at an acceptable rate, an increase in the number of lambs born would proved undesirable. It would therefore be more sensible to improve the quality of the lambs of a ewe, rather than the quantity of lambs per ewe. In the past WW, especially in the Merino sheep, was not regarded as an important selection criteria and was therefore not recorded in most flocks. From the results, of this study it is evident that WW should be recorded, as it forms an important part in the calculation of an ewes total weight of lambed weaned.

 

REFERENCES

Falconer, D.S. & Mackay, T.F.C, 1996. Introduction to quantitative genetics, Longman, New York

Meyer, K., 1993. DFREML: User notes, Ver. 2.1

Snyman, M.A., Olivier, J.J., Erasmus, G.J., & Van Wyk, J.B., 1997. Livest. Prod. Sci. 48 (2), 111