The contribution of overwintering pasture larval infection and the spring rise as sources of Ostertagia circumcincta infection in lambs.

An investigation was made to study the acquisition of infection with Ostertagia circumcincta by lambs on land carrying residual infection, infection following the spring rise and both infections. Dry weather in June modified the usual pattern of herbage larval infection by delaying the peak of larvae derived from residual infection and cutting out the first half of the diphasic peak normally associated with the spring rise. It is suggested that prediction of the time when damaging infections develop on the herbage may be possible by a study of meteorological observations.