A synthetic criterion of resilience to improve genetic resistance to gastro-intestinal strongyles and growth in Creole goats under humid tropics

Breeding for resistance to gastro-intestinal strongyles is feasible in Creole goats during fattening on pasture. No genetic variation is observed on resilience (weight depression due to parasitism). Live weight and resistance criteria are slightly genetically related traits. A new criterion of resilience was proposed to increase breeding efficiency for growth traits and resistance to parasitism in Creole goats reared on pasture. Data consist of records collected between 1996 and 1998, from Creole kids in the experimental flock of INRA-Gardel. Kids were naturally infected with Haemonchus contortus, Trichostrongylus colubriformis and Oesophagostomum columbianum. They were drenched every 2 months, from weaning (at 82 days of age on average) to the end of fattening (10 months of age). Eggs outputs, packed cell volumes and live weights were recorded prior to drenching. When death due to parasitism occurred, date of exit was also collected. Individual productivity (IP) at fixed age (6 or 8 months) of alive kids was equal to their live weight. For dead kids, it was set to zero. Individual productivity of 759 kids sired from 43 bucks and 250 does were available for analyses. The VCE program was used to estimate genetic parameters by means of REML methodology. Fixed effects which significantly influenced the records were included in the models as age at weaning, parity of darn, combined effects of weaner group-sex and combined effects of birth-rearing rank Four mixed models were tested, including either only direct genetic effect, or several effects as direct genetic effect. maternal genetic and permanent effects and correlation between direct and maternal effects. The direct heritability of IP at 6 months and 8 months of age reached respectively 0.27 (±0.075) and 0.21 (±0.076), in the simplest model. This genetic variability was in the same range as variability already estimated for live weight traits. When maternal environmental and/or genetic effects were added, a significant part of phenotypic variation seems to be explained. However, the structure of the data set did not permit a good distinction between direct and maternal effects. Few dams were indeed recorded as progeny and parent. At 6 months of age, genetic correlation between IP and egg output was estimate from -0.23 to -0.41, depending of the model. For packed cell volume, the genetic correlation with IP was estimated from +0.26 to +0.33. So, a great level of genetic progress can be reached using Individual Productivity as resilience criterion. It may improve survival rate and live weight during grazing on infected pasture. If these results are confirmed on a larger data base, individual productivity criterion will be proposed to breeders. The same analyses will be conducted on data before weaning. The mortality due to strongyles is indeed much higher. (Texte intégral