Evaluation of the effects of nematode parasitism on grazing behaviour, herbage intake and growth in young grazing cattle

Parasitic nematodes, even in the absence of any clinical disease, can cause a reduction in voluntary feed intake in housed ruminants. This trial exami ned these effects on young cattle grazing pastures. Twenty dairy heifer cal ves, 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 g razed separately on two paddocks on predominately ryegrass swards. Liveweig hts were recorded every 28 days and faecal samples taken for worm egg count s 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 simila r award 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 w eeks after turnout, the untreated heifers spent on average 105 min less per day in grazing time and their daily herbage intake was 0.78 kg 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. Th ere 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 1 20 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 b ronchitis were observed in two of these animals in mid-July. Over the perio d from turnout until mid-July the untreated heifers showed a reduction in m ean daily liveweight gain of 150 g, compared to the treated animals. (C) 20 00 Elsevier Science B.V. All rights reserved.