Predation by feral pigs on Merino lambs at Nyngan, New South Wales

Transcription

1 570 Aust. 3. Ex#. Agric. Anim. Husb., 1987, 21 : Predation by feral pigs on Merino lambs at Nyngan, New South Wales P. M, Pavlov, J. Hone, R. J. Kilgour and H. Pedersen Summary-The effects of feral pigs on the lambing performance of Merinos was studied at Nyngan, New South Wales. Ewes lambed in two paddocks; one with feral pigs and one with no pigs in autumn and spring in 1978 and Ewes were pregnancy tested before lambing, and after lambing, classified as dry, lactating or lambed-and-lost. Lambs were mustered weekly in three lambings. Pig movement in three lambings was controlled by electric fences. There were effects (P < 0.05) of feral pigs, years, seasons, and pigs x years on the percentage of ewes lactating, of those lambing. Feral pigs lowered the number of lambs per 100 ewes by 37.9 and 31.9 in autumn and spring 1978, respectively. In 1979 pig predation was an insignificant cause of lamb loss. Feral pigs have been known to kill lambs (Moule 1954; Rowley 1970; Giles 1976). Excluding pigs from lambing ewes increased the lamb marking percentage to 117% compared with 80% in an adjacent paddock with feral pigs (plant et al. 1977, 1978). This experiment was designed to estimate the effect on lamb losses of excluding pigs from lambing paddocks. The study was conducted to determine if the findings of Plant et al. would be applicable in an area of lower rainfall. Information was also gathered on the age of lambs killed by pigs. The behaviour of feral pigs in this experiment is reported by Pavlov and Hone (in press). Materials and methods Site The study site was on a commercial sheep and cattle property at Nyngan (147".52'E, 30" 12's) in central New South Wales. The area has mild winters and hot summers, with an average rainfall of 425 mm. Total rainfall was 640 mm in 1978 and 354 rnm in 1979 (figure 1). There was abundant green pasture on the study site at the start of each lambing. The only periods of low pasture growth The Authms-Mr I? M. Pavlov, and Mr R. J. Kilgour, Agricultural Research Station, Trangie, New South Wahs 2823; and Mr J. Hone, and Mr H. Pedersen, Veterinaly Rtsearch Station, GlenfiM New South Wales Mr Pavlovk present address is c/o Post O@e Girilambone, New South Wales 2841, and Mr Pedersenk present address is Willandra National Park, via Hillston, New South Wales were during the second half of lambing in autumn 1978 and spring The vegetation associations of the area were described as Eucalyptus bicolor (Beadle 1948) and in a more detailed survey as Stipa aristoglumis (Chinnick and Key 1971). Figure 2 shows the habitats in the lambing paddocks. Design The experiment was conducted over four lambings - in the autumn and spring of 1978 and On each occasion ewes lambed in two adjacent paddocks. Feral pigs were excluded from one paddock (pig free) but not from the other (pigs present). The latter paddock was 120 ha and the former slighter larger (figure 2). The same paddocks were used for the pig free and pigs present groups at each lambing. Experimental procedure The number of pigs in the pigs present paddock in autumn 1978 was not known, although some pigs were observed. At the end of lambing a helicopter survey was conducted over a surrounding area of 108 km2. Numbers were counted on a strip of 80 m from an altitude of 20 m at a speed of 40 knots. The proportion of the area sampled in this manner was.5.7%; observed feral pig density was 5/km2. After the spring 1978 lambing 4 adult males and 11 adult female pigs were in the pigs present paddock. Before the 1979 lambings all pigs in the paddock were killed and replaced so that the population was the same as in spring

2 Pavlov et al.: Predation by fmalpigs on lambs 57 1 Total rainfall (mm) Ave known records (Nyngan) Moonagee Station Lambing period n I Adult feral pigs averaged 20 to 30 months of age and from 41.3 to 59.5 kg weight in each lambing. These weights were not significantly different, as shown by analysis of variance, from those of pigs in the surrounding area. Four pigs were shot in the pig free paddock throughout the study. Existing fences around both paddocks were of ringlock (sheep and lamb mesh), supported by steel and wooden posts. Before the autumn 1978 lambing, the fences around the pig free paddock were electrified with a single live wire, held approximately 10 cm off the fence and cm from the ground to prevent pig entry. After the first lambing, fences around the pigs present paddock were also electrified to prevent movement of pigs in or out. J J Month Figure I -Rainfall at Nyngan (average of known records) and at the study site on Moonagee Station, and the lambing periods in Low scrub (Acacia pendula) Dead timber (Medicago spp., Hordeum sp.) Swamp depression (Medicago spp., Cyperus spp.) Dense canegrass (Eragrostis sp., Leptochloa sp.) Low open ridge (Medicago spp., Chloris spp.) Canegrass regrowth (Eragrostis sp., Leptochloa sp.) Open canegrass (Eragrostis sp., Leptochloa sp.) Woodland (Eucalyptus bicolorl 0 Plain (Medicago spp., Bassia spp., Hordeum sp.) Sheep For each of the first three larnbings about 500 adult Merino ewes were joined, as one flock, with rams at the percentage of 2.5. In the fourth lambing the ewes were mostly maidens. Joinings for the four lambings were for 10,9, 6 and 6 weeks; for the last three lambings rams were fitted with mating harnesses. The ewes for the autumn 1978 lambing were randomly allocated into two flocks just before lambing. For subsequent lambings, the ewes were divided into two groups and randomized on the pattern of mating determined from raddle marks. The ewes were pregnancy tested by udder palpation (Dun 1963) immediately before each of the four lambings. During the last three lambings all lambs were colour coded according to the week of birth and weekly records kept of all lambs that survived. The difference in the number of lambs between paddocks was assumed to be due to feral pigs. Other causes of mortality were assumed to be similar between paddocks. When lambing was completed, ewes were classified as dry, lactating or lambedand-lost ( ~un 1963). The method of Rowley (1970) was used to determine the cause of death of lambs. Figure 2-Habitats in the lambing paddocks. Analysis The number of ewes lambing in the pigs present and pig free paddocks was recorded at each lambing. The effect of excluding pigs from the lambing paddock was estimated as the difference in the numbers of lambs surviving in the pigs present and pig free groups, on the assumption that the original number of lambs born per lambing ewe was. the same in each group. The pigs present and pig free groups were compared for the number of ewes lambing expressed as a percentage of those alive at lambing, and for the number of ewes lactat-

3 572 Australian Journal of Experimental Agm'culture and Animal Husbandry, Volume 21 December 1981 ing (i.e. with a lamb) expressed as a percentage of the number of ewes lambing. Percentages were used because the total number of ewes and the number of ewes lambing were not equal between lambings. The former was due to limits on the number of sheep available for the experiment, the latter, to variable joining successes. The comparisons were made in unweighted least squares analyses of variance after all percentages had been transformed to angles. The effects of the presence of pigs, years and seasons and their interactions were estimated against an error of 821 divided by the mean of the number of ewes per cell (~nedecor and Cochran 1967). Results The percentage of ewes lambing was higher (P < 0.01) in 1979 (87.0%) than in 1978 (73.3%) (table 1). There was also a season x year interaction (P < 0.05) as in 1978 the percentage of ewes lambing was highest in spring but in 1979 it was highest in autumn (table 1). Because of these results further analyses were made on the percentages of ewes lactating, of those that had lambed.?'here were differences (P < 0.05) in the percentages of ewes lactating (table 1) between years, seasons and pigs. There was also a significant interaction (P < 0.01) between pigs and years (1978, pigs present, 22.8%; pig free, 59.0%; 1979 pigs present, 66.3'0; pig free, 68.4%). Averaged over the four lambings the percentage of ewes lactating, of those alive, in the pig kee paddock was 51.7% compared with 34.2% in the pigs present paddock. Ewes in the pig free paddock had on average lambs per 100 ewes alive at the end of lambing, compared with 39.2 lambs per 100 ewes in the pigs present paddock. The distribution of new lambs at mustering in the pigs present paddock in spring 1978 is given in table 2. The number of new lambs was highest in the first two fortnights and then declined. While the number of lambs killed by pigs increased as the number of new lambs increased, the percentages killed were lowest in the first two fortnights when lamb numbers were highest. Of all TABLE I Reproductive data on ewes and lambs in ead season and year. Attribute Autumn Spring Autumn Spring Total no. of ewes at lambing No. of ewcs lambingt No. of ewes lactating $ No. of ewes lambed-and-lost $ No. of lambs alive at the end of lambingt Difference in lamb marking due to pigs ("/;I) t Figures in parentheses are the percentagr of the total number of ewes alive at lambing $ Figures in parentheses are the percentage of the number of ewes lambing.

4 Pavlov et al.: Predation by feral pigs on lambs 573 'TABLE 2 Distribution of new lambs mustered and death attributed to pigs in the pigs present jock in spring I Fortnight of No. new lambs Estimated no. Percentage lambing mustered in pig lambs killed killed? free paddock in pigs present paddock 'I Calculated as a percentage of the new lambs mustered lambs whose deaths were attributed to pigs in spring 1978, 87.5% were up to one week old. The total number of lambs mustered (149 - table 2) is higher than the number alive at the end of lambing (122 - table 1) as some lambs died after mustering and before lambing ended. For the same reasons the estimated number of lambs killed (86 - table 2) is not the same as the difference between the number of lambs in the paddocks at the end of lambing ( = 78 table 1). Of lamb carcases found in the pigs present paddock, 21 out of 48 (44%) had been killed by feral pigs. Twenty-nine lamb carcases were found in the pig free paddock, three of which had been killed by feral pigs. Few lamb carcases were found in either paddock because of the large areas and height of pasture. Discussion The results of this experiment show that excluding feral pigs from lambing ewes can in some years lead to a significant increase in lamb survival. The results from 1978 were similar to the earlier observations of Plant et al. (1977, 1978). Hone and Waithman's (1979) survey shows that a number of other sheep raising areas have high feral pig densities similar to those in our study. This suggests that lamb predation of the magnitude reported here may occur widely in New South Wales. Several aspects of the results need discussion. The presence of twin lambs in the paddocks may have biased the estimated effect of feral pigs. If pigs kill one of a set of twin lambs, the percentage of ewes lactating will not change. Analysis of those data will underestimate the effect of pigs on the number of lambs. The susceptibility of single and twin lambs to predation by feral pigs needs further investigation. The different habitats in each paddock did not appear to produce a paddock effect. The pigs present paddock had dense canegrass (Eragrostis and Leptochloa spp.) but as the ewes and lambs spent less than.5'% of their time in that habitat (Pavlov and Hone, in press) it is unlikely that significant extra mismothering occurred. Similarly, it is considered that the electric fences may not have altered the behaviour of the feral pigs. The pig predation of lambs was similar in autumn and spring 1978 despite the pigs being unconfined and confined in the respective lambings. The benefits of excluding pigs from the lambing paddock were not uniform in all lambings. In 1979 there was no benefit at all. This variation may be explained in several ways. Only a small number of feral pigs appears to kill lambs, as suggested by Pullar (1950). In spring 1978 one of the 15 pigs in the pigs present paddock was responsible for most of the observed lamb kills (~avlov and Hone, in press). A similar phenomenon has been reported in other species (Curio 1976). The pigs released into the lambing paddock in 1979 may not have included any killer or rogue pigs. An alternative, but not mutually exclusive, explanation for the variation in results between years is the length of lambing. Lambing continued for six weeks in 1979 compared with nine and ten weeks in It has been suggested (Estes 1976) that wildebeest have a short calving season to reduce the effect of calf predation. The higher survival rate of lambs in our study during the first six weeks of spring 1978 supports Estes' idea. During a short lambing, feral pigs will have less opportunity to learn to kill, develop a taste for, or a search image (Curio 1976) of lambs. Exactly which does occur is unknown and is an area for future research. While a short lambing may reduce predation, killing may not be eliminated, hence the first explanation is probably more important. The results in this paper have shown that feral pigs can cause significant lamb loss. Sheep breeders should therefore consider one or more of the following: (i) lamb away from pig populations if possible, (ii) exclude pigs from specially protected paddocks, (iii) draft off ewes due to lamb at the same time so lambing is not protracted, (iv) control pigs before lambing,

5 574 Australian Journal of Experimental Agriculture and Animal Husbandy, Volume 27 December 7987 (v) identify and remove killer or rogue pigs. The choice between these strategies will be determined largely by the economic situation of each breeder. Acknowledgments The co-operation of Killen Brothers for the use of their animals and paddocks at Moonagee Station through the Manager, J. Campbell, and Assistant Manager, H. March, is gratefully acknowledged. Technical assistance from members of the Agricultural Research Station, Trangie, and the Noxious and Feral Animals Research Centre, Glenfield, is appreciated. The useful comments of a referee are acknowledged. The project was supported in part by the Australian Meat Research Committee and a supplementary grant from the Swine Compensation Fund. REFERENCES Beadle, N. C. W. (1948)-The vegetation and pastures of western New South Wales with special reference to soil erosion. Government Printer, Sydney. Chinnick, L. J. and Key, K. H. L. (1971)-Soils of the Bogan-Macquarie outbreak area of the Australian plague locust. CSIRO, Division of Entomology Technical Paper no. 12. Curio, E. (1976)-The Ethology of Predation'. Vol. 7 in series, Zoophysiology and Ecology. (Springer-Verlag: Berlin.) Dun, R. B. (1963)-Recording the lambing performance of ewes under field conditions, Australian Journal of Experimental Agriculture and Animal Husbandry 3 : Estes, R. D. (1976)-The significance of breeding synchrony in the wildebeest. East Afncan Wildlife Journal 14 : Giles, J. R. (1976)-Feral pigs and agriculture. Proceedzngs of a Symposium: Agriculture, Forestry and Wildlife - ConJlict or Co-existence?, University of New England, p. 125, Hone, J. and Waithman, J. (1979)-Feral pigs are ~preadin~. Agricultural Gazette of JVX W 90(2) : Moule, G. R. (1954)-Observations on mortality amongst lambs in Queensland. Australian Veterinary Journal 30 : Pavlov, P. M. and Hone, J. (in press) -The behaviour of feral pigs (Sus soof.) in lambing flocks. Australian Wildlife Research. Plant, J. W., Rees, D., Marchant, R. S. and Mitchell, 1'. D. (1977)- Feral - Predators of Lambs. Agricultural Gazette of NS. W. 88(5) : Plant, J. W., Marchant, R., Mitchell, T. D. and Giles, J. R. (1978)- Neonatal lamb losses due to feral pig predation. Atlstmlian Veterinary Journal 54 : Pullar, E. M. (1950)-The wild (feral) pigs of Australia and their role in the spread of infectious diseases. Australian Veterinary Journal 26 : Rowley, I. (1970)-Lamb predation in Australia: Incidence, predisposing conditions and the identification of wounds. CSIRO Wildlife Research 15(1) : Snedecor, G. W. and Cochran, W. G. (1967)-'Statistical Methods'. Sixth Edition. (Iowa State University Press: Iowa.) Paper received December 7 6, 7980; accepted June 7 6, 798 1