ANNUAL VARIATION IN THE DISTRIBUTION OF BOVINE TRICHOSTRONGYLE INFECTIVE LARVAE ON PASTURE GRASS IN VALDIVIA, CHILE

The object of this study was to determine the section of grass which i nfective trichostrongyle bovine larvae (L3) prefer and whether larvae have a dairy vertical migration on the grass. Weekly grass samples fro m a 0.6 ha pasture with calves, from a farm near Valdivia, Chile (39 d egrees, 48'S.; 73 degrees 14'W.), were taken from September 1994 (spri ng) to September 1995 (end of winter). Parasite, genera found on the g rass samples were Nematodirus (74.9%) Ostertagia (7.2%), Cooperia (7.0 %) and Trichostrongylus (0.4%), without important seasonal variations. 10.5% of the larvae were not identified. No differences were observed between the weekly samplings of larvae per kg dry grass (L/kg dg) in the upper (230 L/kg dg) or lower (248 L/kg de) sections of grass (p > 0.05), during spring and summer. During autumn and winter, the mean am ount of larvae found, was significantly higher in the upper(1.322 L/kg dg) than in the lower (223 L/kg dg) section of the grass (p > 0.05). The highest number of Lj was found in autumn, with great fluctuations and a decreasing tendency during winter. In 92.8% of the samples, ther e was no difference in the amount of larvae on grass collected in the morning (at sunrise) and in the afternoon, (p < 0.05), indicating that there is no vertical daily migration of the larvae.