Sources of Variation and Repeatability of Litter Size in Pedigree Galway Sheep Flocks

Sources of Variation and Repeatability of Litter Size in Pedigree Galway Sheep FlocksAuthor(s): J. P. HanrahanSource: Irish Journal of Agricultural Research, Vol. 16, No. 3 (Dec., 1977), pp. 285-291Published by: TEAGASC-Agriculture and Food Development AuthorityStable URL: http://www.jstor.org/stable/25555879 .

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Ir. L agric. Res. U: 285-291, '1977

SOURCES OF VARIATION AND REPEATABILITY OF LITTER SIZE IN PEDIGREE GALWAY SHEEP FLOCKS

J. P. Hanrahan

An Foras Taluntais, Belclare, Tuam, Co. Galway

ABSTRACT

The litter size at birth of Galway ewes in more than 30 flocks was recorded during 1970-73. Litter size of 2-, 3- and 4-year-old ewes was analysed to determine the importance of flock and year effects as sources of variation in litter size. The repeatability of litter size was also examined. Average litter sizes were 1.38, 1.41 and 1.47 for 2-, 3- and 4-year-old ewes, respectively. Approximately 90%

of all variation in litter size was within flock X year subgroups. The repeatability of 2-year-old litter size was very low. being 0.08 at 3 years of age and 0.01 at 4 years. The repeatability of 3-year-old litter size was higher with an estimate of 0.17 at 4 years of age. Present culling practices in pedigree flocks leave very little opportunity for genetic improvement.

INTRODUCTION

In the case of lowland sheep enterprises the fecundity of the ewe flock is a very im

portant determinant of economic efficiency. The Galway breed, which is the most

important lowland ewe breed in Ireland, has been reported to have a very low lamb

ing percentage (1) and this was due mainly to a low litter size at birth. The econ

omic gains from increased litter size are expected to be greatest in flocks with a low

litter size (2). Litter size may be increased by either improved management practices,

including hormonal intervention (3), or by genetic means. The genetic approaches fall into two main categories: a) within breed selection and b) crossbreeding, with or

without breed development. Both of these approaches have been initiated with the

Galway breed (4, 5). Genetic improvement through within-breed selection requires an

efficient recording and selection programme in the pedigree population. In order to

develop the most appropriate selection scheme information is necessary on the im

portance of various sources of variation in on-the-farm performance.

In 1969 a recording programme was initiated for pedigree Galway flocks for the

purpose of achieving genetic improvement in litter size. This was a co-operative ven

ture among the Galway Sheep Breeders' Society, the Department of Agriculture and

An Foras Taluntais. This programme has been described by Timon (5) and involves

recording the litter size at birth together with lamb growth rate. The present paper



286 IRISH JOURNAL OF AGRICULTURAL RESEARCH, VOL. 16, NO. 3, 1977

reports an analysis of sources of variation in litter size at birth and estimates of the

repeatability of litter size. The implications of the results for the breed improvement

programme are discussed.

EXPERIMENTAL

All ewes involved in this study were ofthe Galway breed and were recorded on farms

in Counties Galway, Mayo, Roscommon and Clare. The flocks included those of all

registered breeders of pedigree Galway sheep. The data relate to the lambings of 1970

through 1973. Flocks were inspected at mating and regularly during the lambing

season to record litter size (lambs bora alive plus dead) and lamb weights. All lambs

were weighed again at weaning. Table 1 summarises the number of ewes with litter

size records at 2, 3 and 4 years of age.

Analysis ofthe sources of variation in litter size was confined to flock x year x

age groups with at least five ewes lambing. The estimation of repeatability was per

formed by correlating litter size at 2 or 3 years of age with litter size at 3 or 4 years.

All flock x age x year groups with at least three pairs of values were used. Sums of

squares and cross-products were then pooled.

The set of flock x year averages for a given age of ewe may be described by the

model Xij ? ju + ft + ys + du where Xy as the observed mean litter size for the

n,j ewes of a given age in the ith flock at the jth year, |x is the overall mean litter size

TABLE 1: Number of flock x year groups with litter size records for 2-, 3-, or

4-year-old ewes and average number of ewes lambing per group

Flock x year Ewes per group Ewe age (years) groups (no.) Ewes (no.) (no.)

T ~ls5 "

2893 lij 3 163 2421 14.9

4 139 . 1189 8.5

TABLE 2: Expected mean squares for the analysis of unweighted means

Source df Expected mean squares

Flock a?1 kaw2 + afy2 + bdr2 Year b-1 k<yw2 + crfy2 + acr^

Flock x year (a-l)(b-l) kcrw2 + Ory2 Within flock a^

k = inverse of the harmonic mean number of records per flock X year average



HANRAHAN: LITTER SIZE IN SHEEP 287

and fj and yj represent flock and year effects with d^ being the residual effect. The

expected mean squares for this analysis are given in Table 2 under the assumption

that the flock and year effects are random (6). An estimate of within flock x year variance was obtained from the repeatability analysis and hence estimates of all the

sources of variation were obtained. These sources of variation were flocks (of2),

years (ay2), flock x year interaction (crfy2) and within flock x year (ow2). Because litter size is not normally distributed and because the analysis of variance

was confined to flocks with at least five ewes per mean each year, the importance of

flock effects was also investigated using rank order correlation coefficients. Signifi cance tests were according to Otten (7) or using the f-test when the number of pairs was large (8).

RESULTS

Flock averages

The total number of flock x year averages based on at least five ewes is shown

in Table 3 for 2-, 3- and 4-year-old ewes. The total number of ewes and the correspond

ing weighted mean litter size is also given. Average litter size increases with age of ewe.

Sources of variation

There were 13 flocks with 2-year-old averages based on at least five ewes for each

year. The corresponding values for 3- and 4-year-old ewes were 15 and 12, respec

tively. The weighted mean litter sizes for these flocks were 1.36, 1.36 and 1.43 for

2-, 3- and 4-year-old ewes, respectively. These values are all less than the correspond

ing values in Table 3. No explanation can be offered for this observation and it must be pointed out that the flocks involved included most of the longest established

pedigree flocks (9). The results of the analysis of variance are shown in Table 4

together with estimates of the appropriate error variance. Flock and year effects were

significant for litter size at 2 and 3 years of age while the flock x year interaction was

only significant for 4-year-old litter size. Estimates of the variance components are

shown in Table 5 from which it is clear that variation among flocks and years accounts for approximately 10% of the total variation in litter size.

TABLE 3: Number of flock x year groups with at least five ewes lambing and the corresponding average litter size

Flock x year groups Ewe age (years) (no.) Ewes (no.) Litter size

2 134 2835 1.38 3 141 2364 1.41

4 96 1085 1.47




TABLE 4: Analysis of variance of flock x year averages for litter size at 2, 3 and 4 years of age

Ewe age (years)

2 3 4

Source df MS df MS df MS

Flock 14 0.094** 12 0.103** 11 0.053 Year 3 0.082** 3 0.063** 3 0.038

Flock X year 42 0.014 36 0.015 33 0.047** Error 1053 0.011 1053 0,016 525 0.027

Significance levels in this and subsequent tables:

**p<0.01; *p<0.05; fp<0.10

TABLE 5: Estimates of components of variance for litter size at 2, 3 and 4 years of age

Ewe age (years)

Component 2 3 4

Flock 0.017 (7.8)a 0.012(4.6) 0.006 (1.9) Year 0.005 (2.5) 0.007 (2.5) -0.009 (-)

Flock x year 0.002 (LI) -0.006 (-) 0.020 (6,6) Within flock x year 0.200 (88.6) 0.244 (93.0) 0.279 (91.4)

a Percentage of total variance

TABLE 6: Rank order correlation coefficients among flocks for average litter size

Year

Year (years) 1971 1972 1973

1970 "

2 0.38* 0.22 0.82** 3 0.48* 0.47* 0.52*

4 0.29 -0.19 -0.02 1971 2 0.38* -0.03

3 0.47** 0.43* 4 0.53* 0,36f

1972 2 0.27 3 0.60** 4 0.36f

Repeatability of flock rank The repeatability of flock rank based on average litter size was examined by com

puting the rank correlation between years for all combinations of ewe age groups* The results are presented in Table 6 and Fig. 1. The correlations within age classes were




generally positive and averaged 0.34, 0.50 and 0.22 for 2-, 3- and 4-year-old ewes,

respectively. The results for rank correlations between different age classes, which are summarised in Fig. 1, show that most correlations were positive. In fact only 3 out of 48 correlations were negative (-0.01, -0.04 and -0.40) which indicates that flock effects were consistent over years and age classes.

Repeatability of litter size The analysis of repeatability of litter size is summarised in Table 7. The repeat

ability of 2-year-old litter size was extremely low, especially at 4 years of age. The correlation between 3- and 4-year-old litter size was 0.17.

DISCUSSION

As expected, litter size increased with age of ewe but the increases were relatively small. The mean litter size is in general agreement with the observations of More

O'Ferrall and Vial (10) and Hanrahan and More O'Ferrall (11) but is considerably higher than has been reported in two other studies (1,12) of Galway sheep. In a 5-year study of various ewe breeds McGloughlin and Curran (13) found a value of 1.72 for the litter size of Galway ewes and explained this high value as being due to the very

good husbandry conditions which achieved an average ewe body weight of 77 kg

postpartum. This explanation does not agree with the results of a recent study of ewe

bodyweight in a sample of pedigree flocks which found average ewe weight in October

to be 71 kg and a regression of 0.012 (lambs/kg) for litter size on bodyweight (14).

** *......--# 2-_ind 3-year-old ranks

j^_.-^ 2- and 4-year-old ranks n

50- D??d* 3-and 4-year-old ranks

/ \

tr li *?\ A

i in-'' %

;,^ .J/ \i N%...s

0 ** ?* i? t? j ?"-n-?i?"?r ?

T ?"Q 0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

Rank correlation

Fig.1: Relative frequency distribution of' rank order correlation coefficients




TABLE 7; Estimates of the repeatability of litter size together with the number

of flock x year groups and number of ewes contributing to each estimate

Flock X year Records groups (no.) Ewes (no.) Correlation

2-year-old with 3-year-old 93 1146 0.08*

2-year-old with 4-year-old 62 505 0.01

3-year-old with 4-year-old 68 593 0.17**

The observation that 90% of the variation in litter size was within flocks means

that, when ewes are being ranked on the basis of a single record, the benefits of

evaluating them relative to their age-contemporary average are negligible. However*

as the number of records per ewe increases the relative importance of variation among

flocks also increases. Thus, for ewes with three litter size records the within-flock variance is reduced by a factor of 0.4 (approximately) and variation among flocks will then account for about 20 % of the total variance. Even then the correlation between actual performance and performance adjusted (without error) for flock effects will be in the region of 0.9. When this level of correlation is considered together with the discrete nature of the variation in average litter size over three lambings (minimum difference among ewes = 0.33 = 3of approx.) there is little justification for ranking ewes on the basis of within-flock deviations.

The repeatability of litter size at 2 years of age was much lower than the values

reported by Hanrahan (15) for a flock of Galway ewes (0.08 and 0.01 compared with 0.15 and 0.24). The increase in the repeatability of litter size with age at the first record observed in the present study agrees with the observations of Purser (16). The values for repeatability found in the present study, while they agree with the results of McGloughlin and Curran (13), are lower than most values in the literature for other breeds (17).

The low repeatability of litter size in the flocks under study, especially 2-year-old litter size, means that ewes should be allowed to have three records before being culled on performance. From the flock structure evident in Table 1 it is clear that pedigree Galway breeders are not evaluating ewes very accurately since only 45% of ewes survive to have three records. Another advantage of having more records per ewe is that it allows a higher selection intensity among dams of rams as well as

making possible the matching of selection/rejection boundaries with discontinuities in the actual level of performance. This follows from the effect of repeated measures on the distribution of average litter size (18).

The results of the present study indicate that, if selection is based solely on litter size, there is little gain in accuracy to be obtained by ranking ewes according to deviations from their contemporary flock average. The low level of repeatability found for litter size, particularly 2-year-old litter size, means that considerable gains in




accuracy may be obtained by ensuring that all ewes have three lambing records before culling on performance. Finally, the low level of repeatability indicates that breed improvement, through selection within flocks for higher litter size, will be extremely slow and consequently breed development using genetic material from prolific breeds should be a priority.

ACKNOWLEDGMENTS

The co-operation of the Department of Agriculture in operating the flock recording scheme is gratefully acknowledged. The co-operation ofthe Galway Sheep Breeders' Society made the recording programme possible. The contribution of Drs V. M. Timon and G, J. More O'Ferrall is acknowledged and the technical assistance of Miss K. Walsh with data analysis is also acknowledged.

REFERENCES

1. Daly, P. J. ''Sheep husbandry in Kilmaine, Co. Galway." An Foras Taluntais, Dublin, 44p., 1966.

2. Blaxter, K. L. Relative efficiencies of farm animals in using crops and byproducts in production of foods. Proc. 2nd Wld Conf Anim. Prod., Maryland, p. 31, 1968.

3. Gordon, I. Controlled breeding in sheep?its potential application under Irish farming con ditions. Ir. vet. J. 24: 227, 1970.

4. Hanrahan, J. P. Breeding more productive sheep. Fm Fd Res. 3; 135, 1972. 5. Timon, V. M. Breeding a more prolific lowland sheep. Fm Fd Res. 1: 14, 1970. 6. Anderson, R. L. and Bancroft, T. A. "Statistical theory in research." McGraw-Hill Book -Co.,

New York, 399p., 1952. 7. Otten, A. Note on the Spearman rank correlation coefficient. /. Am. statist. Ass. 68: 545, 1973. 8. Zar, J. H. Significance testing of the Spearman rank correlation coefficient. J. Am. statist. Ass.

67:578, 1972. 9. Martin, I. The genetic history and breed structure of the Galway sheep breed. Unpublished

M.Agr.Sc. thesis, University College, Dublin, 1971. 10. More O'Ferrall, G. J. and Vial, V. E. Correction factors for use in sheep flock recording, Ir. J.

agric. Res. 2: 13, 1963. 11. Hanrahan, J. P. and More O'Ferrall, G. J. Effects of crossing Finnish landrace and Galway

sheep. Proc. Br. Soc. Anim. Prod, 3: 81, 1973. 12. Gordon, I. Aspects of reproduction and neonatal mortality in ewe-lambs and adult sheep.

X Dep. Agric. Fish. Repub. Ire. 64: 16, 1967. 13. McGloughlin, P. and Curran, C. A comparison of four breeds of sheep as dams for fat lamb

production. 1. Reproductive performance and fleece weight. Ir. J. agric. Res, 8: 67,1969. 14. Hanrahan, J. P. Ewe bodyweight, litter size and lamb growth in pedigree Galway flocks. Res.

Rep. Anim. Prod., An Foras Taluntais, Dublin, p. 165, 1975.

15. Hanrahan, J. P. Repeatability of ovulation rate and its relationship with litter size in four sheep breeds. Proc. E.A.A.P. meeting, Zurich, 1976.

' ;.

16. Purser, A. F. Repeatability and heritability of fertility in hill sheep. Anim. Prod. 7: 15, 1965,

17. Turner, H. N. Genetic improvement of reproduction rate in sheep. Anim. Breed. Abstr. 37:

545,1969. 18. Hanrahan, J. P. Ovulation rate as a selection criterion for fecundity in sheep. Proc. 1st Wld

Congr. Genet. Appl Anim. Prod., Madrid 3: 1033,1974.

Received February 5,0}r6



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Sources of Variation and Repeatability of Litter Size in Pedigree Galway Sheep Flocks