[Characterization of areas at parasitic risk for sheep reared under conditions of uncontrolled breeding on the ranges of the Causses. 1. Study methodology and its application to gastrointestinal strongyles].

A flock of 50 Lacaune ewes grazed three paddocks (approximately 17ha each) in rotation. Thirteen areas were defined in each of the two selected paddocks on the account of vegetation and spatial utilization by the ewes. These areas represented one tenth of the total area of the paddock. For each area the following measures were estimated: species of grasses, bushes, grass disponibility, amount of faeces deposited, infective larval population of strongyles, duration of the utilization by the ewes for resting or grazing. The measures were achieved during two rotational cycles from May to July 1981. Among the several methods utilized for the assessment of the various parameters influencing the probable infection of animals, one was unsatisfying (estimation of grass intake) and another was of particular good value (estimation of the amount of faeces deposited by an index method). The areas at risk were characterized by a vegetation with Bromus erectus associated either with Festuca duriuscula or Brachypodium pinnatum. The infectivity of each area (number of larvae X duration of grazing) was correlated to the number of strongyle eggs deposited, the amount of faeces accumulated and to an index of pasture quality (pastoral value X biomass of available grass). This work demonstrates that in heterogeneous pasture, even if the level of parasitism (Ostertagia sp. and Nematodirus sp.) is low, area at risk could be determined.