Two groups of 28 Finnish Landrace (FL) lambs were given (group Mg) or denied (group O) access to a magnesium (Mg)-rich mineral mixture to establish its role in the hitherto frequent development of diarrhoea in the flock. Diarrhoea developed in most lambs but became marginally worse in group Mg (P < 0·05) by weaning. At weaning, 28% of the lambs had salivary sodium: potassium (Na: K) ratios < 4 indicating Na deficiency but pasture was not low in Na (60·9 mmol/kg dry matter (DM)) and providing the mineral mixture (which also contained Na) did not alleviate the deficiency. Raised plasma pepsinogen (PP) concentrations were indicative ofabomasal parasitism and all lambs were drenched with levamisole. Groups were subdivided at weaning, either continuing on their previous treatments (groups Mg/Mg and O/O) or being offered pure salt (NaCl) (groups Mg/Na and O/Na). Mineral consumption was measured from weaning and varied widely between groups and with time for the first 2 weeks. Early consumption was maximal at 19·6 g per head per day in group Mg/Mg and zero in group Mg/Na and faecal DM was significantly lower in the former group (165 v. 223 (s.d. 16·9) g/kg DM). All groups showed an improvement in Na status after weaning followed by a decline and NaCl consumption showed opposite changes about a mean of 6·3 g per head per day: faecal DM became uniformly low (ca. 200 g/kg fresh weight) in all groups. A further rise in PP prompted a second treatment with levamisole 6 weeks after weaning. When salivary Na: K was low (1 and 7 weeks after weaning), a significant relationship (P = 0·002) was found between salivary K (mmol/l), faecal DM (g/kg) and plasma pepsinogen (PP U per l;):
K = 22·9 + 4·5 PP – 0·047 DM (d.f. 82: r = 0·34).
Nematodiasis may have caused an ion imbalance and an associated diarrhoea, inducing a craving for NaCl which was ill-met by a mixture rich in laxative Mg.