Genetic studies of resilience of Romney sheep to nematode challenge in New Zealand

C.A. MORRIS, S.A. BISSET, A. VLASSOFF, A.D. MACKAY, K.BETTERIDGE, M.J. ALDERTON, C.J. WEST AND B.P. DEVANTIER

AgResearch, Ruakura Agricultural Research Centre, PB 3123, Hamilton, New Zealand

ABSTRACT                                                                                                                                                                                                                    
“Resilience” in sheep can be defined as the ability to withstand nematode challenge, and to maintain acceptable healthand productivity, with minimal reliance on anthelmintic treatment. One heritable component trait is Total Drench Requirement (TDR), which is measured in lambs managed on a regime of “drench-on-demand” (where Trichostrongylus and Ostertagia are the predominant nematode genera in New Zealand sheep). A breeding experiment was established in 1994, selecting for increased resilience in Romney lambs. Currently, breeding ewes in the experiment are subdivided between Ballantrae, where four genetically equivalent sub-groups graze separate farmlets year-round, under Non-Chemical versus Conventional management (AgResearch’s Low-Chemical Farming Systems Programme), and Wallaceville, where an Elite Resilient line was re-established in 1999 alongside the Control Faecal Egg Count (FEC) line. Selection responses up to 1999/00 in the Elite Resilient-line lambs (relative to the Control line) included a 27% greater post-weaning weight gain (GAIN: Dec. to Apr.), a 0.48 unit reduction in dags, and a 45% reduction in TDR (Dec. to Apr.). A favourable genetic correlation (-0.54) was recorded between TDR and GAIN in lambs, whilst that between TDR and log FEC was -0.17 (not significant). Mean autumn weights in lambs at Ballantrae were 5.8 kg (18%) lower in the Non-Chemical than Conventional treatment groups (P < 0.01). Through intense genetic selection, 51% of Elite-line ram lambs are now resilient enough for low-chemical or organic production systems.

Keywords: sheep; nematodes; resilience; genetics.

Full Report click here  resilience1.pdf (32KB) 


Resilience to nematode parasite challenge in industry and AgResearch selection flocks

C.A. MORRIS, N.C. AMYES, S.A. BISSET and A.D. MACKAY
AgResearch, Ruakura Research Centre, PB 3123, Hamilton, New Zealand.

ABSTRACT

A large-scale industry project on the resilience of lambs to nematode parasite challenge was undertaken in 2001/02 and 2002/03, to evaluate the feasibility of recording standardised age at first drench (StAge) and postweaning weight gain under nematode challenge, and to estimate genetic differences among flocks. The project involved 15 Romney flocks from two ram breeding groups, with some flocks having been part of an earlier AgResearch study of the genetics of resilience traits. This provided the opportunity to assess any correlated changes in resilience over time (where direct selection over that time period in the industry flocks had been applied to productivity traits, but not to resilience), and to compare the industry performance with AgResearch Resilient-selected and Control lines. The overall estimates of heritability for StAge and weight gain (2 time periods: Gain1 and Gain2) were 0.14 ± 0.014, 0.30 ± 0.015 and 0.15 ± 0.007, respectively. Genetic correlations between StAge and either Gain1 or Gain2 were 0.30 ± 0.06 and 0.41 ± 0.06, respectively. Genetic differences for all traits between the two industry groups were small and not significant. The genetic differences of the current industry-group means (all 15 flocks) above the AgResearch Control line were small (0.8 days, 0.5 kg and 0.9 kg, respectively). In comparison, the genetic differences of the AgResearch Resilient-selected line over the Control line after eight years of selection were 19 days, 0.7 kg and 1.4 kg (11, 8.3 and 9.6% of the industry means). The results confirm that while selection for increased resilience is feasible (although very time-consuming) under commercial conditions, a ram breeder would need to consider an index approach which combined production and resilience traits, to make best use of his animal resources.

Keywords: sheep; nematodes; resilience; genetics; heritability

Full Report click here  resilience2.pdf (136KB)