Proc. Aust. Soc. Anim. Prod. (1972) 9: 65
CULLING FOR FERTILITY IN SHEEP FLOCKS IN WESTERN AUSTRALIA
J. F. SMITH and D. R. LINDSAY
Summary
Two methods of culling for fertility were studied in Western Australian flocks.
Individual lambing records of 6,178 ewes in 12 flocks were studied over two successive years to determine whether the proportion of ewes which lambed in the second year could be enhanced by culling ewes which were dry in the first year.
Only two of the 12 flocks were significantly improved by this method of culling.
Mating records on 3,718 ewes in seven flocks were used to determine the effect on lambing performance of culling those ewes which returned to service. A significant improvement was seen in two of the flocks.
The practicability of either method is questionable because, in most cases, very large proportions of the flocks have to be culled to attain significant improve- ment.
I. INTRODUCTION
The lambing performance of flocks in Western Australia is low and in many flocks this is due to a high proportion of dry ewes (Chopping and Lindsay, 1970;
Lindsay and Smith unpublished data). As there is no readily apparent economic method of inducing dry ewes to lamb, one method of attack is to predict those ewes that will be dry and cull them either before mating or before lambing.
Dun (1961) and Mdule (1966) indicated that lambing performance is repeat- able to some extent and that ewes dry in one year are more likely to be dry in the next. However, Barrett and May (1958) have shown that in general the chance of a ewe lambing in any year is not affected by its success or failure in preceding years, and concluded that no marked increase in the percentage of ewes lambing would be achieved by culling ewes which failed to lamb. In a review of this subject Turner (1969) concluded that selection against ewes failing to lamb was “not likely to lead to any spectacular increase in the current flock”.
This paper presents the results of an investigation into two possible methods of culling for fertility. First, the effect of culling ewes that were dry in the previous year and, second, the effect of culling those ewes which return to service during the mating period.
Department of Animal Science and Production, University of Western Australia, Nedlands, Western Australia, 6009.
II. MATERIALS AND METHODS
As part of a survey of reproductive wastage in Western Australia the lambing performances of individual ewes in 12 flocks over two successive years were recorded. Flocks were examined in 1969 and 19’70, or in 1970 and 1971.
1969 was drier than average but the other two seasons were typical of the mediterranean climate of the region.
In seven flocks the lambing performances of ewes served once, twice or three times in a 6 week mating period were compared. Ewes were classified into two categories when lambs were tailed, according to the method of Dun (1963) - wet, when ewes were still suckling a lamb, and dry, when the ewe either lambed and lost the lamb or had not lambed. In the “dry” category the two classes of ewes were pooled for convenience of presentation as they performed similarly throughout the investigations.
In all flocks joining lasted for 6 weeks and rams were fitted with “sire sine”
harnesses (Radford, Watson and Wood, 1960). Raddle colours were changed each 14 days and returns to service recorded.
Flocks were joined during November to March and individual flocks were mated on approximately the same date each year. No ewes were culled from these flocks but missing and dead ewes were eliminated from the records of both years.
The size of mating flocks and the areas of mating paddocks were those normally used on each individual property.
The gain in reproductive performance the previous year or ewes which returned calculated by the formula:
gain = pd
due to culling either dry ewes from to service in the current year was where p = proportion culled
and d = difference in subsequent performance between selected and culled (Turner, 1969).
III. RESULTS
(a) Reproductive performance
of
ewes in successive yearsTable 1 shows the numbers of wet and dry ewes over two successive years.
A total of 6,178 ewes were examined. The average lambing percentage was 81 per cent, with a range of 71 per cent (Flock No. 8) to 96 per cent (Flock No. 4) in Year 1, and 77 per cent, with a range of 55 per cent (Flock No. 2) to 86 per cent (Flock No. 6) in Year 2.
Ewes dry in Year 1 were theoretically culled and the performances of the modified flocks in Year 2 were recalculated. (The gain due to culling these ewes is shown in the last column in Table 1).
When considered over all ewes in all flocks this advantage was 4.3 per cent and highly significant (P < 0.001 ), but when considered within individual flocks there was a significant advantage in only two cases (No. 2, 10.4 per cent, P <
0.001; No. 17, 4.9 per cent, P < 0.01).
The reproductive performance in Year 2 of the ewes which were dry in
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TABLE 1
TABLE 2
Year 1 varied greatly among flocks and ranged from 25.2 per cent (Flock No. 2) to 81.1 per cent (Flock No. 14).
(b) Reproductive performance of ewes in relation to their mating performance Table 2 shows the reproductive performance of ewes which mated once, twice, or three times during the 6 week joining period.
Few ewes did not mate during the 6 week period. Of 3,7 18 ewes joined only 173 (4.7 per cent) failed to mate, although in Flock Nos. 9 and 10 9.7 per cent and 12.8 per cent of ewes failed to mate.
In all flocks, ewes which did not return to service had a higher lambing percentage (77.7 per cent) than those ewes which did (55.2 per cent). By culling ewes which returned to service it would have been possible to increase the lambing percentage of all flocks (last column Table 2). Overall this advantage was 6.8 per cent which was statistically significant (P < O.OOl), but when con- sidered within flocks, only 2 flocks showed a significant advantage (No. 2, 12.7 per cent, P < 0.001; No. 13, 7.0 per cent, P < 0.01).
IV. DISCUSSION
The most serious category of reproductive loss in most flocks was ewes which mated but did not rear a lamb to marking. Failure of ewes to mate was important in only two of the flocks examined, but to a lesser extent. This agrees with the results of Chopping and Lindsay (1970).
When the results for all the flocks were pooled, theoretically, culling ewes dry in the first year increased the lambing performance of the modified flock in the subsequent year by 4.3 per cent. The biological and economic significance of this are questionable, because only two individual flocks were significantly improved by such culling. In these, approximately 25 per cent and 29 per cent of the flock had to be culled to achieve improvements of 10 per cent and 5 per cent respectively.
Ewes which were dry in two successive years represented only 7.8 per cent of the 6,178 ewes examined. The lambing percentage of these ewes in the third year is not available, but even if all failed to lamb the maximum increase possible by culling would be 7.8 per cent. In general, this cannot be justified in terms of costs and effort in identifying these animals over two years.
Ewes which returned to did not, but this fact does not on flock statistics two options service is small, little gain is is large as much as 40 per improvement is achieved.
cent
service proved to be less fertile than those which seem promising as a basis for culling. Depending are open. In cases where the number of returns to achieved. In cases where the number of returns of the flock might have to be culled before sign&ant Thus if a flock owner has a problem of low fertility and is in a position to reduce the size of his flock then one or other of these two methods might be used as a basis for culling. If the desire is to maintain flock numbers then, in most cases, culling on the criteria suggested appears to offer little promise.
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V. ACKNOWLEDGMENTS
We are grateful to the participating farmers for their co-operation and to Messrs. T. Johnson, C. Oldham and M. Sanders for their assistance in the conduct of the work. This project was supported by a grant from the Australian Meat Research Committee. One author (J.F.S.) was holder of the E. H. B. Lefroy Research Fellowship in Agriculture.
VI. REFERENCES
Barrett, J. F., and May, P. F. (1958). Proceedings of the Australian Society of Animal Production. 2: 13 1.‘
Chopping, M. H., and Lindsay, D. R. (1970). Proceedings of the Australian Society o f
Animal Production. 8: 312.
Dun, R. B. ( 1961). Wool Technology and Sheep Breeding. 8: 9.
Dun, R. B. ( 1963). Australian Journal of Experimental Agriculture and Animal Husbandry.
3: 228.
Moule, G. R. ( 1966). Australian Veterinary Journal. 42: 13.
Radford, H. M., Watson, R. H., and Wood, G. F. ( 1960). Australian Veterinary Journal. 36:
57.
Turner, H. N. (1969). Animal Breeding Abstracts. 37: 545.
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