The use of bioactive forages towards organic/sustainable control of gastrointestinal parasites in sheep

The aim of this thesis was to investigate the potential use of bioactive plants as alternative grazing forages to control gastrointestinal parasites of growing lambs. For this purpose four different grazing experiments were conducted using the following bioactive forages: Lotus pedunculatus (lotus), Onobrychis viciifolia (sainfoin), Hedysarum coronarium (sulla), and Cichorium intybus (chicory); the forages were tested against two parasitic ovine nematode species, Teladorsagia circumcincta and Trichostrongy’lus colubriformis. The first two experiments tested the effects of short-term grazing on bioactive forages against the viability and fecundity of adult, and the establishment and development of larval nematodes of T. colubriformis (Chapter Two) and T. circumcincta (Chapter Three) species. In the first experiment, grazing on bioactive forages showed no evidence of forage effects upon incoming T. colubriformis larvae or against adult T. colubriformis worms during a two-week period. In contrast, the results form the second experiment showed significant reductions in adult T. circumcincta worm burdens of lambs grazing chicory for a two-week period compared to lambs grazing grass/clover {folium perenne/Trifolium repens). Immature worm burdens were, affected by physiologically and/or immunologically mediated mechanisms, which reduced larval establishment in all treatments. The following experiment (Chapter Four) aimed to investigate the effects of sequential grazing on bioactive forages, firstly on chicory followed by sulla, towards an established adult T. circumcincta worm population of growing lambs. The results showed that grazing on chicory reduced the faecal egg counts (FEC) of infected lambs, whilst the following grazing on sulla provided little evidence of additional benefits. During this grazing period, evidence of emergence of immunity, declining FEC and low wonn establishment rate, were observed across all treatments. The last experiment (Chapter Five) aimed to separate the direct (anthelmintic) and indirect (immunologicaFphysiological) mediated effects of bioactive forages and investigate their effects on acquired immunity against the abomasal nematode T. circumcincta. The results showed elevated immune responses, in terms of nematode larval development and mucosal cell counts, of growing lambs grazing on either sulla or chicory compared to those grazing on grass/clover. It is therefore suggested that the effects of these two bioactive forages are due to a combination of their direct and indirect properties. The potential application of bioactive forages as an alternative to control gastrointestinal parasitic infections in organic/sustainable sheep production systems is considered in the General Discussion (Chapter Six)