Nematode genera diversity in cattle: similarity of between-sire progenies

Breeding cattle for resistance to nematode infection is mostly based on egg excretion. This, however, does not allow for generic identification of the nematodes involved. Unless we know whether the selected resistance is directed against one or several particular genera, a strong bias could be introduced in the selection programs. In order to estimate the likelihood of this potential bias we investigated nematode genera diversity in the progeny of four sires in 1992 and seven sires in 1994. Three of the four Aberdeen Angus sires used in 1992 were related while the seven sires in 1994 were unrelated. Diversity was assessed using at least ten individual faecal cultures for each progeny group during each of the two sampling periods (beginning and end of grazing period, April and September). It was estimated by the relative proportion of each genera (Ostertagia, Cooperia, Haemonchus, Trichostrongylus and Oesophagostomum) or by either the Shannon (genera diversity) or Pielou (genera evenness) index. The Shannon index was repeatable when measured at 2-week intervals within the same progeny group on ten random faecal samples. No significant difference was recorded between sire genera diversity over the two sampling periods. This indicated that hosts have a limited effect on the nematode genera diversity as assessed by faecal cultures, and that the selection of resistant hosts could probably be achieved using faecal egg counts.