Abstract:
Fluctuations in populations of infective larvae of cattle strongyles on a naturally grazed calf pasture were studied for 2 years. Susceptible calves were introduced to the pasture each month to determine how host infection was influenced by the degree of pasture contamination as measured by the number of larvae recovered per pound of pasture. The most numerous larvae recovered at all seasons belonged to the Cooperia spp. group. Larvae of Trichostronqylus axei were abundant throughout the winter to early summer, but scarce during midsummer and autumn. Larvae of Haemonchus placei were most abundant during the late summer and winter, and those of Oesophagostomum radiatum occurred in relatively small numbers at all times. Gross peaks of abundance occurred during the summer and midwinter, when the rainfall was higher than at other periods of the year. Although greater numbers of larvae per pound of pasture were recovered during the winter than during the summer, host infections were higher during the summer. This inverse relationship between the number of larvae and host infection is believed to be a result of a greater intake of pasture during the summer. An assessment of the degree to which a pasture may be dangerous therefore involves consideration not only of the number of larvae present, but also of the state of the pasture. Estimates of the daily intake of larvae by susceptible animals, allowed to graze for a restricted period and then examined at necropsy, ranged between 19 and 31 655 larvae.