The free-living stages of gastro-intestinal nematodes on pasture (i.e. eggs, developing and infective larvae) are often overlooked and not considered by producers. The factors that influence the development, survival, distribution, or migratory behavior of the free-living larvae seen on pasture are primarily weather related. The deposition of helminth eggs in feces is the starting point for the contamination of the pasture and the subsequent development of the parasites. The cumulative numbers of eggs can be considerable by the end of the grazing season. Environmental factors will influence both development and survival of the larvae on pasture, as well as their distribution onto the herbage. Eggs deposited on the pasture will hatch and develop to the infective L3 stage. Under optimal conditions of moisture and temperature, larvae of Ostertagia ostertagi will reach the infective stage in approximately 5-6 days. Developmental time varies from one geographic region to another depending on prevailing weather. Eggs hatch and develop more slowly at lower temperatures. The rate of development increases to a maximum at higher temperatures after which development will be adversely affected and death of the larvae occurs. It is difficult to evaluate the effects of temperature without considering moisture. Moisture must be present to prevent desiccation and death of the developing larvae. Moisture has an effect on tile movement and motility of the larvae. As the fecal pat dries out the larvae may desiccate and die. If the environment is dry, movement onto surrounding herbage would probably not be possible, thus forcing movement or migration into the soil beneath the fecal pat. Too much moisture (rain) may disrupt the pat, moving the larvae onto the surrounding herbage or carrying them away in the runoff. When moisture is adequate the movement of larvae on pasture may be affected by temperature. Temperature in the optimal range allows for larval activity and thus motility. Moisture provides a medium in which the larvae can move (there is probably little movement on dry surfaces). The effect of rainfall on dispersal of the larvae is important, a drop may transport larvae as far as 90 cm from the dung pat. However, optimal larval recovery is only approximately 5 cm (2-3 inch) from the edge of the fecal pat, with decreasing numbers out to approximately 25 cm. Several authors report larval migration into the soil as deep as 15 cm and as far as 40 cm from the center of the fecal pat. Soil type may have a major effect of the ability of larvae to migrate. The predilication of larvae to remain relatively close to the fecal pat may have substantial impact on transmission, as cattle do not graze close to fecal pats until forage is very limited. Therefore, intensive rotational grazing may enhance transmission by forcing animals to graze the highly contaminated areas. At the same time, intensive rotational grazing causes increased disruption of fecal pats, thus enhancing desiccation. Many investigators have utilized tracer calves grazing a pasture for a prescribed period of time to provide an indication of both species and numbers of worms present. Some investigators have used pasture larval counts, reporting number of larvae per kg of dry herbage and the genera present.