Understanding photosensitisation in sheep grazing the pasture legume Biserrula pelecinus L.

Summary

​Biserrula pelecinus L. is an annual legume native to the southern Mediterranean (Howieson et al., 1995). It was first introduced to Australia in 1991 as a potentially valuable rotational pasture species for livestock production. It produces large quantities of biomass, exhibits drought tolerance and is effective for weed suppression in pasture rotations. However, despite proving to be a valuable addition to the pasture toolbox, producers in NSW and WA have reported a limiting factor to uptake: incidence of severe photosensitisation when grazing sheep on biserrula pastures. Biserrula photosensitivity, anecdotally, appeared to be associated with a particular phase of crop maturity and showed an increased severity of clinical signs in young animals, however, the pathogenesis of this photosensitisation and the compound responsible were unknown.
This project aimed to determine the nature of the photosensitisation (PS) observed with animals grazing biserrula,  provide mitigation strategies to prevent livestock photosensitivity when grazing this useful pasture species in the future, and to define the critical growth stage of phototoxicity. It examined the aetiology and pathogenesis of biserrula PS, defining the disorder as a primary photosensitivity, and surveyed producers known to have implemented this pasture species in New South Wales and Western Australia, the major zones of utilisation. Both meat sheep and wool producers were identified in the cohort, and a single cattle producer. In all bar one case the cultivar sown was biserulla 'Casbah' as part of a crop rotation. Results showed that 55% of producers grazing livestock on biserrula pasture had experienced outbreaks of photosensitisation. All ages and sexes of animals were found to be affected. A higher incidence of outbreaks of PS was observed in WA compared to NSW. In most cases the outbreaks were identified to be either mild (47%) or moderate (40%). Only a small proportion of producers contacted a veterinarian in the advent of these outbreaks, the vast majority managing them themselves without veterinary involvement.
As part of this project, plant samples were collected from a number of sites in the Riverina, for future analysis, as well as from the grazing trials identified below. These samples ranged in nature from early vegetative, through to senescent. Outbreaks of PS were associated with samples from vegetative through late flowering stages of plant growth, but not with senescent material.

A clinical case study identified a mob of 120 meat lambs grazing biserulla 'Casbah'. This study showed histopathological features of clinical PS in ears and facial tissues of affected lambs without underlying hepathopathy. This was the first reported case study identifying primary PS in lambs grazing Biserrula pelecinus cv. 'Cashbah' and is the first reported case study identifying primary photosensitisation in animals grazing this pasture species.

Further supporting evidence that PS subsequent to ingestion of commercially-available biserrula cultivars exerted no hepatic damage was confirmed in a subsequent series of grazing trials which examined prevalence and severity of PS in sheep of various ages and breed characteristics. A PS scoring system was established to quantitate clinical signs of PS in affected animals and this was applied to animals entering controlled grazing trials on pastures of various compositions during 2013 and 2014.