Effect of simulated rainfall timing on faecal moisture and development of Haemonchus contortus and Trichostrongylus colubriformis eggs to infective larvae

Three climate-controlled chamber experiments were conducted to determine the effect of 32 mm simulated rainfall applied prior to (days −4 to −1) or after (days 0-7) faecal deposition and as a single (32 mm) or split (2 × 16 mm) application on faecal moisture (FM) and development of H. contortus and T. colubriformis to third stage infective larvae (L3). The timing of simulated rainfall regulated extra-pellet L3 recovery for H. contortus (P < 0.05) but not T. colubriformis. Recovery of L3 was highest (P < 0.05) when simulated rainfall was applied on the day of deposition followed by days −1, 1 and 2, which resulted in similar but lower development success rates. Recovery of intra-pellet T. colubriformis L3 was two-fold greater (P = 0.008) than for H. contortus and was higher (P = 0.007) following simulated rainfall on days 0 and 1 than on other days. There was a positive association between FM and total L3 recovery indicating the importance of FM in the period 48-72 h (H. contortus) and 72-96 h (T. colubriformis) after deposition. Simulated rainfall on the day prior to deposition was as effective in supporting total L3 recovery as application on days 1 or 2 and this effect could be predicted through FM. This highlights the importance of soil in transferring moisture to the faecal pellet. The importance of precedent rainfall and soil moisture in determining the development success of H. contortus and T. colubriformis, in addition to the general effects of the timing of simulated rainfall, need to be accommodated in grazing management programs to combat these species.