The rôle of body reserves stored during early pregnancy on the periparturient breakdown of resistance to nematode parasites was investigated. From the 35th day of pregnancy, 46 ewes were distributed in four groups and given access to a lucerne pelleted diet. Two groups were given food at maintenance (M and C) and the other two were offered food either ad libitum (H) or at 0·70 their maintenance requirements (L) in order to achieve three well differentiated levels of body reserves by the 90th day of pregnancy when backfat thickness reached a mean value of 10·6, 7·5, 7·4 and 5·0 mm for H, M, C, and L treatments respectively. After the 90th day of pregnancy all animals were given food at 50 g/kg live weight per day. All ewes in groups H, M and L received 5000 infective larvae per week (40% Teladorsagia circumcincta, 40% Trichostrongylus colubriformis, 20% Haemonchus contortus) from 5 weeks before to 3 weeks after lambing. The animals in group C remained as uninfected controls.

Host resistance, as measured by faecal egg counts, was significantly affected by the nutritional treatment established during early pregnancy, showing a mean excretion of 546, 766 and 1007 eggs per gram (e.p.g. ) for H, M, and L treatments, respectively, until 3 weeks post lambing. This response was accompanied by a significant effect on circulating eosinophils and better fed ewes showed a higher concentration with a maximum mean value of 10·1, 6·9, 5·5 and 4·0 cells per mm3 for H, M, L and C treatments, respectively, 2 weeks post infection. The effect of the worm infection per se was evident in that the lambs of the M treatment ewes grew at 0·90 of the rate of those in the control treatment (189 v. 213 g/day). Ad libitum feeding in early pregnancy appeared to sufficiently compensate for this depressive effect, with the H ewes producing lambs that grew at a rate similar to those of the C ewes (213 v. 213 g/day).

The results support the view that fat mass stored by ewes in early pregnancy is involved in the expression of immunity against gastro-intestinal nematode infection around parturition. Furthermore, the depressive effect of ewes’ infection on lambs’ growth rate can be overcome by increasing the plane of nutrition of their dams. These results have clear implications for grazing animals, particularly their nutritional management in early pregnancy and provide a simple management strategy in sustainable management systems.