Abstract

Gastrointestinal nematode parasites of first-season grazing cattle (FSGC) are ubiquitous and practically not feasible to eradicate. Heavy infections result in clinical parasitic gastroenteritis (PGE) characterized by diarrhoea, inappetence and weight loss. However, subclinical disease without obvious symptoms constitutes the majority of infections and may cause economic losses due to sub-optimal performance. This thesis is based on 4 separate publications that focus on epidemiology and control of gastrointestinal nematode infections in FSGC in Sweden. Two grazing experiments were carried out over consecutive years. One 2-year study was performed on semi-natural pastures and a 3-year study was conducted on improved pasturelands. In each study, groups of 10 FSGC were subjected to various parasite control methods in comparison with anthelmintic bolus treated animals (maximum control), and untreated, set-stocked cattle (minimum control). In companion ecological plot experiments, larval availability and overwintering survival on pasture were investigated. Results from the grazing trial on semi-natural pastures showed that nematode egg contamination of pastures by lightly infected animals during the first half of the season was sufficient to induce PGE the following spring. The rotation group suffered a weight gain penalty of the same magnitude as the untreated cattle (30 kg), compared with the bolus treated animals. The results were explained by the high degree of overwintering survival of the pre-parasitic stages from early season contamination, which was substantiated in the parallel plot study. The 3-year grazing experiment on improved pastures included evaluation of 1) the nematophagous fungus Duddingtonia flagrans, 2) pasture rotation using a turnout pasture grazed the previous season by adult cattle in combination with a mid-summer move to aftermath and 3) copper supplementation. Excellent results were recorded in the pasture rotation group, with growth rates of cattle equal to, or exceeding, those treated with the anthelmintic bolus. In the last year of the trial, the difference between the untreated and the anthelmintic treated cattle was 65 kg. The group fed D. flagrans had a weight gain advantage of 45 kg compared with the untreated group, whereas the copper had no control effect. Again, the accompanying plot study provided information to explain the outcome of the grazing trial.