Annual variation in the distribution of bovine trichostrongyle infective larvae on pasture grass in Valdivia, Chile

Abstract

The object of this study was to determine the section of grass which infective trichostrongyle bovine larvae (L3) prefer and whether larvae have a daily vertical migration on the grass. Weekly grass samples from a 0.6 ha pasture with calves, from a farm near Valdivia, Chile (39°, 48’S.; 73° 14’W.), were taken from September 1994 (spring) to September 1995 (end of winter). Parasite genera found on the grass 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 dg) sections of grass (p>0.05), during spring and summer. During autumn and winter, the mean amount 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 L3 was found in autumn, with great fluctuations and a decreasing tendency during winter. In 92.8% of the samples, there 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.