Estimation of the numbers of trichostrongylid larvae on pastures.

Estimates of the density of larvae on herbage are used in epidemiologic studies to determine seasonal or monthly variations in contamination of pastures with third-stage larvae of the trichostrongyles, and to provide an index of the risk of exposure of grazing animals. Estimates of density are affected by many variables, including laboratory techniques, climate, forage types, and farm and animal management methods. Thus, a comparison of results given by different workers is not desirable. In obtaining and interpreting larval density estimates, it is important to standardize the technique over the period of observation. Because techniques for estimation of larvae are cumbersome and time-consuming, statistical design is often compromised. Herbage samples should be taken randomly and with sufficient replication to overcome the variability of the herbage sampling technique. Studies to determine an appropriate sampling design to obtain the best estimate for pastures of all sizes have not as yet been carried out.