The effect of repeated moves to clean pasture on the build up of gastrointestinal nematode infections in calves.

The build up of gastrointestinal nematode infections was followed in two grazing experiments. Both experiments included four groups of six calves, a permanently housed non-infected control group and three groups which were grazed from May to October. One of these was moved to aftermath in the beginning of July, the second in the beginning of July and August and the third in the beginning of July, August and September. The build up of gastrointestinal nematode infections was followed by performing faecal egg counts, differentiation of faecal larval cultures, pasture larval counts, serum pepsinogen values, serum antibodies against Cooperia oncophora, weight gain and worm counts. In the second experiment four of the principal trial animals of each group were treated with oxfendazole and subsequently challenged with 100,000 larvae of Ostertagia ostertagi to examine development of immunity against O. ostertagi. The faecal egg counts and the worm counts of the sentinels necropsied in July indicated low initial infections in both experiments. Infection levels in experiment 1 remained low in each group until the beginning of September. However, during the last month, moderate to high infections were acquired by the groups which were moved once or twice. In contrast, low to moderate infections were maintained in the group moved three times. In the second experiment moderate C. oncophora burdens were already observed in the sentinels grazed until the beginning of August. Tracers grazing in August-September with the group moved once acquired high O. ostertagi and C. oncophora infections, whereas those grazed with both other groups acquired moderate infections. In October high infections with both species occurred in the groups moved once and twice, whereas low to moderate infections were observed in the group moved three times. The challenge infection demonstrated a reduction of establishment of O. ostertagi of approximately 70% in all three groups on pasture. The results demonstrate that moving calves at monthly intervals to clean pasture can be an effective method for the control of parasitic gastroenteritis. In addition, the data indicate that it is essential that the last move does not occur more than 1 month before the end of the grazing season.