Gastrointestinal nematode control in sheep under three grazing management systems and factors influencing faecal worm egg count

Abstract

A six-year study on 50 ha farmlets on the northern tablelands of NSW compared control of gastrointestinal nematode (GIN) infection of sheep under typical (TYP), high input (HI) or intensive rotational grazing (IRG) management systems. The major sources of variation in faecal worm egg count (WEC) were also examined. The data set comprised 5644 faecal worm egg count (WEC) records and 322 larval differentiation tests. Worm infections in ewes, lambs, hoggets and wethers were, with some exceptions, adequately controlled through a combination of regular monitoring of WEC, anthelmintics and grazing management. The IRG farmlet had lower mean WEC (444 epg) and annual anthelmintic treatment frequency (3.1 treatments/yr) over the whole experimental period than TYP (1122 epg, 4.3 treatments/yr) or HI (1374 epg, 4.7 treatments/yr). The main factors influencing WEC were the time since the last anthelmintic treatment, and the anthelmintic used at that treatment. The magnitude of these effects dwarfed those climatic and management factors that might be expected to influence the epidemiology of GIN infections via environmental or host-mediated mechanisms. Nevertheless management factors associated with stocking rate and grazed proportion (proportion of each farmlet grazed at any one time), and climatic indicators of both temperature and moisture availability had significant effects on WEC. The results show that in a region with 'Haemonchus contortus' as the major GIN, improved host nutrition under the HI system did not provide more effective control of GIN than typical management, while IRG did provide considerably better control. Tactical worm control based on WEC monitoring provided adequate control of worms on all three farmlets for much of the experimental period but failed to prevent significant spikes in WEC on the TYP and HI farmlets to values associated with significant production loss on multiple occasions, and mortality on one occasion.