The effect of fertilisers on gastrointestinal L3 larvae survival : A dissertation submitted in partial fulfilment of the requirements for the Degree of Agricultural Science with Honours at Lincoln University

Abstract

Traditional gastrointestinal nematode (GIN) control relies heavily on anthelmintic administration, targeting the parasitic nematode stage within the host. With increasing levels of anthelmintic resistance within GIN, targeting the free-living population outside the host may provide a feasible complimentary approach to conventional parasitic nematode control. Nitrogenous pasture fertiliser application has been shown to reduce egg hatch and larvae development in Trichostrongylus colubriformis, however this suffers from practical limitations as little research has been conducted beyond the egg stage during the free-living larvae L3 phase. Therefore, this series of research experiments investigated the effects of various fertilisers - ultimately nitrogen based - on L3 larvae migration, including the potential influence of exposure time and temperature on effectiveness. Preliminary trials explored the ability of various widely-used fertilisers at a range of concentration levels (1:1 to 1:32 fertiliser to water ratio) to inhibit larvae migration over a four hour period. A range including urea, Epsom salt, superphosphate, potassium nitrate, diatomaceous, sulphate of ammonia, sulphate of potash and sulphur. The most significant effects, when compared with the water (control) larvae migration percentage of 97%, were observed in salt and nitrogenous fertilisers at full concentrations, with migration limited to 33%, 0%, 70% and 71% for urea, salt, potassium nitrate and sulphate of ammonia respectively. No significant effect was observed under the remaining fertiliser treatments. Further larval migration assays were then completed, utilising urea, salt and potassium nitrate in a reduced range of concentration levels (1:1 to 1:4), and with the introduction of three exposure time variables. At full concentration all fertiliser types used showed a reduced larvae migration to <30% when compared with the control (>95% migration), with both urea and salt performing best at 8% or lower. There were variations in effectiveness between fertiliser types at different concentrations (P <0.001), but no apparent interaction effect, or significant effect of varied soak time (P >0.05). Urea proved the most restrictive overall, limiting migration across all results (P <0.05 for all) with <62% migration at a 1:4 concentration. At up to 50% concentration salt was the more effective, though its effect dropped significantly at dilution rates beyond that. Exposure time trials of larvae under urea, salt and potassium nitrate treatments provided varied results between both treatment types and soak times (p <0.001), with a strong interaction (P <0.001) indicating lengthened exposure time increased migration inhibition. Regardless of fertiliser type, exposure of larvae to solution for two minutes resulted in no effect on migration (P >0.05 for all). Urea was significantly different from all other treatments in its performance, proving to be most effective, reducing migration to 34% and 0.1% after two and four hour soaks respectively (P <0.001). Concentrations of 75%, 62% and 56% urea at one, two and four hours exposure times respectively, reduced migration by 90%. The percentage of larvae migration was temperature dependant. Migration was significantly reduced outside of the temperature range 1-30°C, while at 15-20°C, migration appeared to be largely unaffected, with similar migration rates of >95% under both water and urea treatments. Temperatures below 4°C, at 25°C and above 34°C showed significant differences to water treatments, which were largely unaffected by temperature. It appears there is an interaction between temperature and urea, although it is unclear whether this is due to increased larval susceptibility at temperature extremes, or temperature-altering physical properties of urea. Strategic application of nitrogenous fertilisers has the potential to break the parasitic lifecycle over time through targeted treatment at the free-living stage, and presents a viable complimentary option in establishing a diversified GIN reduction regime, as an alternative to solely anthelmintic control. Further research is required regarding optimum urea concentrations and exposure time, and the suitability and feasibility of inputting nitrogenous fertiliser management plans in place in a farming environment.