Nematode parasitism affects lying time and overall activity patterns in lambs following pasture exposure around weaning

•Effects of nematodes on lambs was evaluated using commercial accelerometers.•Subclinical infections affects lying time and overall activity around weaning.•There is a potential use of automated activity detection as a diagnostic tool. We investigated the effects of gastrointestinal nematode (GIN) challenge on activity in first season grazing lambs naturally exposed to two different levels of multispecies GIN infections. Ewes and their twin-born lambs were turned-out to graze in two permanent pasture enclosures naturally contaminated with GIN the previous year, thereby exposing them to overwintering strongyle larvae. Animals in the low parasite exposure group (LP) were dewormed monthly with 0.2 mg ivermectin (Ivomec® vet, oral suspension) per kg body weight, whereas those in high parasite exposure group (HP) were left untreated. At weaning, lambs were allocated to one out of four groups based on weight and sex (HPE, n = 15; HPR, n = 15; LPE, n = 14; LPR, n = 14), in four nearby non-contaminated ley enclosures of similar size. Activity patterns were monitored from day −7, i.e. 7 days pre-weaning, until day 49, i.e. 49 days post-weaning, by fitting all lambs with IceQube sensors (IceRobotics). Body weight was monitored weekly from day −21, whereas faecal samples were investigated at days −21, 7, 35 and 49 for nematode faecal egg counts (EPG) using McMaster-technology and a validated Droplet Digital PCR protocol to determine nematode composition. All statistical analyses were performed in R studio, using mixed models with repeated measures. In the data analyses, weekly recordings was treated as a period, generating a total of eight periods. Average daily lying time had a significant interaction between parasite exposure and period (P = 0.0013), with animals in HP having a 101 ± 31 min shorter daily lying time compared to LP. Motion Index (MI; absolute value of the 3-D acceleration) had a significant interaction between parasite exposure and period (P = 0.0001) with lambs in group HP having a lower average daily MI compared with LP. Both body weight gain and EPG levels were significantly different (P