Survival on Pasture of Larvae of Gastrointestinal Nematodes of Cattle. III. Fall Contamination

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Abstract
Development and survival of infective larvae of gastrointestinal nematodes of cattle following deposition of eggs on pastures in autumn at Beltsville, Maryland, were tested in four experiments. Indices of development and of plot-infec-tiousness, based on eggs deposited and worms recovered from initially worm-free calves that grazed the plots serially in time, were used to compare results within and between three of the experiments. Examinations of herbage and fecal samples from ungrazed plots yielded supplementary information. The cool, moist conditions prevailing early in the two experiments begun in October were favorable for development of Cooperia oncophora, Ostertagia ostertagi, and Trichostrongylus axei and detrimental for development of Haemonchus contortus and Oesoph-agostomum radiatum. Plot-infectiousness was maximal 2 to 4 months after contamination. In April it still averaged 39% of the average peak. C. oncophora and O. ostertagi larvae survived well until April. Observed infectiousness with Nematodirus helvetianus was low in one and moderate in the other of these experiments, but the peaks may not have been detected. The lower temperatures prevailing early in both experiments begun in November were detrimental for development of larvae of most species. That development of all species tested was almost completely prevented in the first one was evidenced by almost complete absence of eggs from the feces of a calf that test-grazed all the plots. In the other, the overall developmental index was much lower than in those started in October, though the index was appreciable for O. ostertagi and high for N. helvetianus. By spring, plot-infectiousness was only 8% of the peak. Overall development was higher and the rate of decline from peak plot-infectiousness was lower in the experiments begun in October than in experiments begun in spring and summer at Beltsville. Development was lowest in those begun in November.

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