Resistance to gastrointestinal nematodes in dairy sheep: Genetic variability and relevance of artificial infection of nucleus rams to select for resistant ewes on farms.

Breeding sheep for enhanced resistance to gastrointestinal parasites is a promising strategy to limit the use of anthelmintics due to the now widespread resistance of parasites to these molecules. This paper reports the genetic parameters estimated for parasite resistance and resilience traits in the Blond-faced Manech dairy sheep breed and the putative impacts of the selection for resistance to gastrointestinal nematodes (GIN) on farms. Two datasets were used. First, the rams of the selection scheme were artificially infected twice with L3 Haemonchus contortus larvae. Faecal egg counts (FEC) and packed cell volume (PCV) loss were measured 30 days after each infection. Secondly, the FEC, PCV and body condition score (BCS) (1-6 measures per ewe) of naturally infected ewes on farms were measured in the spring, summer and autumn over a two-year period. Genetic parameters were estimated for each dataset independently but also globally based on the pedigree connections between the two datasets. For the experimentally infected sires, the FEC following the second infection was moderately heritable (heritability: 0.35) and strongly correlated with FEC after the first infection (genetic correlation: 0.92). For the naturally infected ewes, FEC was also heritable (0.18). Using the two datasets together, a genetic correlation of 0.56-0.71 was estimated between the FEC values of the experimentally infected rams and naturally infected ewes. Consequently, the genetic variability of parasite resistance is similar whatever the physiological status (males or milking/pregnant ewes) and the infection conditions (experimental infection with one parasite or natural infection with several parasites). In practice, when the sire population is divided into two groups based on their genetic value, the FEC of the ewes born to the 50% most resistant sires is half that of the ewes born to the 50% most susceptible sires. Our study shows the feasibility and efficiency of genetic selection for parasitism resistance based on the sires' FEC records to improve parasite resistance in naturally grazing ewes. For breed improvement, and to increase the selection pressure on parasite resistance, it seems more appropriate to measure FEC values on rams after experimental infection rather than on ewes in natural infection conditions because this limits the number and standardizes the conditions of FEC measurements.