Breeding worm resistant sheep at Billandri.

Abstract

Proc. Assoc. Advmt. Anim. Breed. Genet. Vol12 BREEDING WORM RESISTANT SHEEP AT BILLANDRI W. Sandilands 'Billandri', Kendenup, WA 6323 SUMMARY Breeding sheep for increased resistance to worms can be incorporated into breeding objectives including other economically important production traits such as fibre diameter and clean fleece weight. This paper reports on the first year's results from Billandri Poll Merino stud comparing individual ram hoggets ranked using 4 different selection indices. Stud replacement sires were selected based on the Worm 50 Index. Keywords: Selection, wool sheep, resistance to parasites. INTRODUCTION The threat of resistance to the macrocyclic lactone group of drenches in addition to the high prevalence of resistance to the levimisole and benzimadizole anthehnintic drench groups is a significant problem for the wool industry. There is also increasing pressure from processors and consumers of wool to reduce the use of chemical pesticides. This has prompted serious consideration of breeding for worm resistance. Results from the Rylington Merino project indicated that selection for low faecal worm egg count (FWBC) was effective (Karlsson et al. 1995). Billandri has collaborated with Agriculture WA and the Nemesis project to demonstrate that resistance to worms can be incorporated into a commercial ram breeding program. The effect of including resistance to parasites in a selection index on the other production traits is examined. METHOD From the 1994 drop ram lambs 720 out of a total of 1288 born were retained entire and fleece measured in June 1995 at 12-13 months of age with 9 months wool growth. Greasy fleece weight was recorded and mid-side samples collected and tested for fibre diameter (FD), yield, coefficient of variation of fibre diameter (CVFD) and percentage of fibres greater than 30 microns (O/&30). Ram numbers were reduced to 464 based on a combination of production figures in an index and subjective features. The remaining rams were sampled for FWBC. All rams were drenched with ivermectin on 17' March 1995. The break of season rain was 10 mm on 12* May followed by 8.5 mm on 22'd May. One month after the opening rains the FWBC of young rams were monitored regularly by rounding up the ram mob against a fence for 5- 10 minutes and collecting 15-20 random samples of dung pellets. These were taken to Agriculture WA laboratory in Albany for FWBC and larvae differentiation analysis. From an average FWBC of 6 eggs/gram (epg) on the 13' of June the count progressively rose to 1.50epg by the 25 July and the number of zero counts fell from 15/17 to 2/20. At this point it was decided to sample the whole mob when heritability of FWBC is highest (Greeff et al. 1995). Sires retained need to be selected by early August to fit in with the annual selection and sale calendar. Faecal sampling was performed on a Harrington V- 338 Proc. Assoc. Advmt. Anim. Breed Genet. Voll2 belt machine. Faecal consistency was scored on a scale of 1 (firm) to 5 (sloppy). Dag score was scored on a scale of 0 (no dags) to 4 (very daggy). 455 rams were sampled. RESULTS AND DISCUSSION The average FWEC on the 3ti August when the whole mob was sampled was 209 (range O-1650), with Trichostrongylus 151 (range o-1050) and Nematodirus 57 (range o-500). The proportion of rams with zero counts was 68/455. Average FWEC was below the recommended minimum average of 300 (preferably 500) for WA. When considering selection of sires, production data was incorporated into several indices to assess the impact that adding resistance to worms to the selection programme would have on rates of response in other measured characteristics. Table 1. Predicted genetic change after ten years of selection for the various indices -1.0 Micron + SS worm 50 worm 70 worm 30 Index -0.9 Fibre Diameter (u) -1.0 -1.0 -0.7 Clean Fleece Wti (gms) +350 +270 +230 +300 maintained Body Weight (kg) maintained maintained maintained not included Staple Strength (N/Ktex) +2.0 not included not included The indices were calculated by Agriculture WA (Greeff, personal communication. 1995) The change in ranking of individual rams under the different indices is examined in Table 2. FWECs are given to show how they affect ranking under Worm indices. The effect on rams with above average FWEC is illustrated by ram 261 with the highest FWEC of 1650 which was ranked 1st under -1.0 micron plus staple strength index, 47' under Worm30 index and 337ti under Worm70. Ram 122 also with above average FWEC of 450 ranks 5* under -1.0 micron index, 17' under Worm30 and 32ndunder Worm 70. An example of a resistant ram is ram 144. Ram 144 with 0 FWEC sired by a ram whose progeny had a low average FWEC (average 75) ranked 22LLd in 1.Omicron plus staple strength index ranked higher relative to ram 44 also with 0 FWEC but sired by a ram whose progeny had above average FWEC (average 236) as more emphasis is placed on resistance. Ram 33 with FWEC 300 ranks lower when compared with ram 44 (same sire) with FWEC 0 when more emphasis is placed on resistance. 339 Proc. Assoc. Advmt. Anim. Breed. Genet. Vol12 Table 2. Ranking of the top 20 rams under various indices -1.0 Micron Index Rank TAGNO. 1 261 2 87 3 188 4 178 5 122 6 6 7 77 8 124 9 53 10 44 11 193 12 33 13 202 14 90 15 26 16 97 17 127 18 196 19 472 20 228 *Retained as sires. worm 30 FWEC 1650 100 250 200 450 100 200 300 150 0 200 300 250 0 0 150 100 0 50 400 TAG NO. 87 6 178 188 77 196 193 53 33 124 90 144 202 26 44 34 122 97 472 127 worm 50 TAG NO. 87* 6 178* 188 77* 196* 53 193* 90* 144 124 33 26 202* 44 34* 472* 127 97 83 worm 70 TAG NO. 87 6 196 178 77 188 53 90 144 193 26 34 44 202 472 127 124 33 83 126 The average performance of the top 20 ranking rams in each of five indices is given in Table 3. The top twenty ranking rams deserve attention because most of the sire replacements will come from this group. The tabIe also demonstrates the effect on other production characteristics when resistance to worms is included as a selection criterion. In other words the degree to which selection for other characteristics has to be relaxed to include varying degrees of reduction of FWECs. Table 3. Average of top 20 rams using different indices INDEX WoolplanOptl -1.0 micron+SS WORM30 WORM50 WORM70 FD 18.8 19.4 19.3 19.3 19.3 CFW 137% 142% 139% 137% 137% CV of FD 21.1% 20.5% 20.9% 21.0% 21.0% FAECAL FIBRE BODY FWEC WT AUG. 3 CONSISTENCY >30 1.5% 79.2 252.5 3.5 1.8% 79.2 242.5 3.4 1.9% 78.5 140.0 3.5 1.9% 77.7 122.5 3.5 1.9% 78.3 115.0 3.5 DAG SCORE 0.2 0.1 0.1 0.1 0.1 340 Proc. Assoc. Advmt. Anim. Breed. Genet. Vol12 More emphasis on FWEC led to a reduction of a Faecal Worm Egg Count from 242.5 micron finer plus improved staple strength index to a FWEC of 122.5 under the Worm a cost of 5% clean fleece weight, 0.5% increase in CV of FD and a slightly lower live kgs.) Other characteristics including fibre diameter, yield, percentage of fibres over wrinkle score, scrotal circumference, and dag score have not been affected. under the 1 50 index at weight (1.5 30 micron, The timing of testing was suitable in that after fleece measurement the number of rams to be tested had been reduced. The period of eight weeks after the break of season does not interfere with other major management procedures. An artificial worm challenge could be considered if a late break of season makes the sampling date less convenient although higher estimates for heritability have been obtained from natural challenge (Karlsson personal communication). The use of faecal antigens, a possible refinement to more accurately measure the worm burden, would not alter the procedure. Including a staple strength component based on the coefficient of variation of fibre diameter is desirable. Apart from the cost of egg counts the main limitations to the method used would be the sampling error involved in the egg count (half sib data is an important component to firm up estimates) and the possibility of low faecal worm egg counts being correlated with hyper-sensitivity and scouring in later life. In New Zealand there is some evidence to suggest there is a small genetic antagonism between FEC and wool production in NZ breeds which would slow genetic progress in these traits (McEwan et al. 1995). Genetic correlation estimates from the Rylington Merinos are more favourable and not antagonistic ( Greeff personal communication). This work in a Mediterranean environment is very important in assisting breeders to decide what degree of emphasis should be placed on resistance to worms. CONCLUSION Using Faecal Egg Counts to breed sheep resistant to worms and reduce pasture contamination can be incorporated into a ram breeding programme without great inconvenience and deserves consideration by at least ram breeders with clients in the over 500 mm rainfall areas. ACKNOWLEDGMENTS The assistance of John Karlsson Regional Veterinary Officer Agriculture WA with his experience from the Rylington Merino project, Jill Lyon Agriculture WA, Albany for the Faecal egg counts and Johan Greeff GSARI Agriculture WA in providing indices and EBVs is gratefully acknowledged. The NEMESIS network and newsletter have been valuable sources of information on selection for worm resistance. REFERENCES McEwan, J.C., Dodds, K.G., Watson, ., T.G., Greer, G.J., Hosking, B.C and Douch, P.G.C. (1995) Proc. Aust. Assoc. Anim. Breed Genet. 11:70. Greeff, J.C., Karlsson, L.J.E., and Harris, J.F. (1995) Proc. Aust. Assoc. Anim. Breed Genet. 11:117 Karlsson, L.J.E., Greeff, J.C., and.Harris, J.F (1995) Proc. Aust. Assoc. Anim. Breed. Genet. 11:122. 341