Comparison between tracer calves and herbage samplings for the assessment of pasture infectivity in trichostrongylosis of cattle.

Two trials were designed in "Haute Normandie" to assess the value of the number of third stage larvae in herbage samples for monitoring the infection of cattle. Two techniques of sampling were used and were compared to the actual infection of tracer calves. An index of infectivity of the pasture was defined: the number of third stage larvae in 1 kg of dry matter of herbage multiplied by the quantity of grass (dry matter) ingested by calves. There was a significant correlation between this index and the actual worm burdens of tracer calves when parasites were numbered by genera, and for herbage samplings done according to "classical" techniques. The correlation is improved when data are combined from samples taken "close to fecal pats" and "far from fecal pats".