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Abstract
Parasitic nematodes, even in the absence of any clinical disease, can cause a reduction
in voluntary feed intake in housed ruminants. This trial examined these effects on
young cattle grazing pastures. Twenty dairy heifer calves, born in the previous autumn,
were blocked according to liveweight and allocated to one of two groups: either untreated
or dosed with an IVOMEC((R)) (ivermectin) SR Bolus 10 days prior to turnout on 1 May
1998. The groups grazed separately on two paddocks on predominately ryegrass swards.
Liveweights were recorded every 28 days and faecal samples taken for worm egg counts
at the same time. In mid-May and mid-July, for two 14-day periods, animals were paired
within treatment groups and transferred to one of 10 replicate paddocks of similar
sward height and herbage mass. Grazing behaviour and herbage intake were measured
during these periods. In mid-May, 2 weeks after turnout, treated and untreated animals
showed no difference in grazing behaviour or daily intake of grass. By mid-July, 10
weeks after turnout, the untreated heifers spent on average 105min less per day in
grazing time and their daily herbage intake was 0.78kg dry matter (DM) per day lower
than that of the treated animals. Residual sward height, mass and composition in the
trial paddocks reflected these differences. There were no clinical signs of gastrointestinal
parasitism in the untreated group up to this time; in July, the mean worm egg count
in this group was 120 eggs per gram (e.p.g.) of faeces. Faecal samples from several
untreated animals were positive for lungworm larvae by July, mild clinical signs of
bronchitis were observed in two of these animals in mid-July. Over the period from
turnout until mid-July, the untreated heifers showed a reduction in mean daily liveweight
gain of 150g, compared to the treated animals.