Gastrointestinal nematodes (GINs) are a common type of parasite found in cattle worldwide. It is important to evaluate the intensity of infection and identify the species present in order to reduce their frequency and choose effective anthelmintic treatments. However, assessment of GIN species is usually based on necropsies, which are costly and time-consuming. Faecal cultures are non-destructive, enabling species identification (Van Wyk et al., 2004) and monitoring at various times of year. However, it is unclear whether proportions based on larval cultures accurately reflect actual GIN infection, as few studies have attempted to relate GIN infection based on necropsy to infection established through larval cultures in faeces. In this study, we investigated the relationship between the proportions of species estimated from necropsies (adults) and faecal cultures (L3) in 29 grazing meat steers. The steers were 12–14 months old and originated from six commercial farms in the Pampas region of Argentina, around the town of General Pico. The animals had not received any anthelmintic treatments for at least the last three months. The animals were ready for commercialisation, and organs and faeces were collected at the slaughterhouse. The GIN species identified were as follows: Haemonchus placei, Ostertagia ostertagi, Cooperia oncophora, Cooperia punctata, Trichostrongylus axei, and Oesophagostomum radiatum. Linear regressions with the Napierian logarithm (Ln) were forced to zero (i.e. 0 larvae corresponded to 0 adults). These were the following:

H. placei: Ln adults = 0.75 Ln (L3 + 1) (R² = 0.84, P < 0.001)

O. ostertagi: Ln adults = 1.24 Ln (L3 + 1) (R² = 0.94, P < 0.001).

C.oncophora: Ln adults = 0.87 Ln (L3 + 1) (R² = 0.87, P < 0.001).

C.punctata: Ln adults = 0.68 Ln (L3 + 1) (R² = 0.74, P < 0.001).

T. axei: Ln adults = 1.42 Ln (L3 + 1) (R² = 0.78, P < 0.001).

O. radiatum: Ln adults = 0.38 Ln (L3 + 1) (R² = 0.16, P < 0.05).

All the regressions were significant. The proportions of species identified in faecal cultures can be used as an indicator of actual infection in cattle, except for O. radiatum.