The effects of parasitism on ewes for prime lamb production in western Victoria

B Kirk,J W A Larsen,N Anderson,M A Stevenson

doi:10.1071/an20414

Abstract

Context Internal parasites are estimated to cost the Australian sheep industry AUD436 million per annum (p.a.). Aims To assess the effects of parasitism in flocks producing prime lambs in the 500–700 mm p.a rainfall. area of Victoria. Methods Ewes on two farms that followed ‘best practice’ gastrointestinal parasite control programs (BP) and two farms that did not (regionally typical, TYP) were compared. Separate cohorts of ewes were monitored from pregnancy scanning to their subsequent joining each year for three consecutive seasons. Observations included worm egg count (WEC), bodyweight, condition score and presence of breech soiling (dag). These were compared between groups that were treated to suppress parasitism (SUP) and those treated according to the usual program used on that farm (NSUP). Data from individual ewes were analysed using a multivariable, mixed-effects regression model. Key results After adjusting for known confounders, SUP ewes were 1.2 (95% CI 0.80–1.6) kg heavier than NSUP ewes. Mature SUP ewes were significantly heavier than NSUP ewes at their next joining on 6 of 18 occasions, mostly following winters when ewes experienced nutritional stress. Ewe hoggets and Merino ewes were generally more susceptible to parasitism than mature non-Merino ewes; single-bearing ewes were less susceptible than those bearing twins. The effects of parasitism were reduced when peri-parturient ewes had an optimal condition score and grazed adequate pastures. Conclusions Ewes were more vulnerable to parasitism when immature, twin-bearing, or under nutritional stress. Some of the greatest differences between SUP and NSUP ewes occurred following periods of low feed availability and/or ewe condition score. The difference between the mean bodyweight of SUP and NSUP Merinos was not always greater than that of the non-Merinos. WECs are not a sole reliable indicator of the effects of parasitism in this class of sheep. Implications Immature or twin-bearing ewes, and those in suboptimal body condition, should be managed considering their increased vulnerability to parasitism, and WEC interpreted alongside other factors. Controlled release capsules were not cost effective in reducing production loss from gastrointestinal nematodes in most years but may be effective in reducing the effects of clinical parasitism in some cases.

Highlights

Prime lamb production has increased in Australia in recent years; the gross value of lamb production was AUD4 billion in 2017–2018 (ABARES 2019), having risen from AUD 0.5 billion in 1988 (CPI adjusted figure AUD1.1 billion) (ABS 2013)
Immature or twin-bearing ewes, and those in suboptimal body condition, should be managed considering their increased vulnerability to parasitism, and worm egg count (WEC) interpreted alongside other factors
This program included: (1) one or two strategic anthelmintic treatments given in summer, with additional treatments based on season and worm egg counts (WEC); (2) regular use of WECs for decisions on treatment; (3) knowledge of resistance to anthelmintics in the nematode population, via worm egg count reduction test (WECRTs); and (4) the use of rams with Australian Sheep Breeding Values for WEC (Anon 2014a)

Summary

Introduction

Prime lamb production has increased in Australia in recent years; the gross value of lamb production was AUD4 billion in 2017–2018 (ABARES 2019), having risen from AUD 0.5 billion in 1988 (CPI adjusted figure AUD1.1 billion) (ABS 2013). The southern part of western Victoria has a pasture growing season of 7–8 months, with an annual rainfall of 500–700 mm, most of which occurs in winter. This provides ideal conditions for prime lamb production. Lambing in western Victoria typically occurs from June to August, which coincides with the peak availability on pastures of the infective stage of nematode parasites (Anderson 1972; Niven et al 2002). The effects of parasitism acquired during winter and early spring on wool producing sheep have been reported; Anderson (1972) identified a 7% decrease in bodyweight of Merino ewes not receiving any anthelmintic treatment, compared with those treated fortnightly. Internal parasites have been estimated to cost the Australian sheep industry AUD436 million per annum (Lane et al 2015) and the total annual cost of gastrointestinal parasitism in Australian prime lamb flocks has been estimated to be over AUD90 000 (Sackett et al 2006)

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