Soil moisture modulates the effects of the timing and amount of rainfall on faecal moisture and development of 'Haemonchus contortus' and 'Trichostrongylus colubriformis' to infective third stage larvae

Abstract

Recent experiments on the effects of rainfall and/or soil moisture (SM) on development of sheep gastro-intestinal nematodes to infective L3 stage have used soil of relatively low moisture content in small experimental samples that dry out faster than field soil. To determine whether higher and more sustained SM content modulates the effects of rainfall amount and timing on faecal moisture (FM) and development of 'H. contortus' and 'T. colubriformis' to infective third stage larvae (L3), a climate-controlled chamber experiment was conducted. It was designed to test the effects of rainfall amount (0, 12 and 24 mm), rainfall timing (days -1, 0 and 3 relative to faecal deposition) and soil moisture maintained at 10, 20 and 30% on these variables. Total recovery of L3 14 days after faecal deposition was significantly affected by SM, rainfall timing and their interaction ('P' < 0.01), but not by rainfall amount or species or other two-way interactions. Recovery of L3 was maximal (28%) with a SM treatment of 30% and simulated rainfall on day 3. Faecal moisture was significantly affected by collection day,SMtreatment, rainfall amount and rainfall timing with significant interaction between many of these effects (P' '< 0.05). A positive linear association between FM and total L3 recovery was strongest on day 4 after faecal deposition ('R'² = 0.64, 'P' < 0.001) for 'H. contortus' and day 6 ('R'² = 0.78, 'P' < 0.001) for 'T. colubriformis'. Overall the results show that SM is able to modulate the effects of rainfall timing and amount with increased SM acting to broaden the window of opportunity for the free-living stages to respond to post deposition rainfall to complete development to L3. If SM is maintained in the range 10-30%, the reported benefits of early rainfall (days -1 and 0) of up to 24mm appear to be negated with later rainfall (day 3) proving more beneficial. These results require field confirmation.