EPIDEMIOLOGY OF NEMATODOSIS IN ROMNEY LAMBS SELECTIVELY BRED FOR RESISTANCE OR SUSCEPTIBILITY TO NEMATODE INFECTION

Field trials were undertaken to compare nematode population dynamics, lamb productivity and levels of breech soiling in experimental flocks of Romney lambs selectively bred for increased resistance or susceptib ility to nematode infection. In each year of the 2 year study, spring- born ewe lambs derived from Wallaceville Animal Research Centre's dive rgent nematode-resistant and nematode-susceptible breeding Lines were grazed as separate flocks on matched farmlets from weaning (at 3 month s old) until they were approximately 10-11 months old. Allocation of f armlets was reversed between Years 1 and 2 of the study to account for any possible paddock-related effects. Within each year both flocks we re subjected to identical management conditions, including anthelminti c treatment (which was administered only when the overall mean faecal worm egg count measured across both genotypes reached 1500 eggs g(-1)) . In both years, by mid-autumn (April) nematode larval infestation lev els on pasture were approximately 5-6-fold greater on the farmlet graz ed by susceptible (S) genotype lambs than on that grazed by their resi stant (R) counterparts (Year 1: 2506 cf. 544 larvae kg(-1) herbage; Ye ar 2: 431 cf. 74 larvae kg(-1) herbage). This led to 51-fold and 56-fo ld differences in faecal egg count between R and S lambs by late autum n (May) and winter (July) in Years I and 2, respectively. Although mea n growth rates were similar in the R and S lambs over summer (while pa sture infestation levels on the farmlets were still in the process of diverging), significantly higher growth rates occurred in the R than i n the S lambs over autumn-winter in both years of the study (P < 0.01) . In contrast, no significant differences in growth rate occurred in e ither year between male lambs derived from the nematode-resistant and nematode-susceptible breeding lines which were grazed together on anot her area of the Wallaceville farm from weaning until late autumn. Desp ite the substantially lower pasture infestation levels encountered by the R ewe lambs, they nevertheless temporarily suffered more breech so iling (dags) than their 8 counterparts(P < 0.01) in both years. Yearli ng fleece-weights of the R and S genotypes did not differ significantl y in either year. Although the results of our study confirmed that the re are potentially significant epidemiological benefits to be derived from breeding sheep for resistance to nematode infection, these benefi ts did not appear to be associated with large advantages in animal per formance. Further work is needed to establish how these results should be interpreted with respect to anthelmintic drench requirements of ge netically resistant animals. (C) 1997 Elsevier Science B.V.