Primary photosensitivity in lambs grazing forage brassica : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Forage brassicas are a diverse range of important, high-quality annual feed crops grown on farms throughout New Zealand (300,000 ha) and Australia (200,000 ha). An animal health concern occasionally associated with lambs consuming forage brassica crops grown during summer is photosensitivity; a sporadic, non-contagious disease where the epidermis and dermis of ear pinna, eyelids, bridge of the nose and midline of the back is uncharacteristically responsive to sunlight due to the presence of a phototoxin. The most prevalent clinical sign of this disease was a change in ear thickness, due to oedematous accumulation, affecting animal welfare, and potentially production. While the phototoxin in forage brassica is unidentified, compounds including perylenequinone, aglycone anthraquinone, furanocoumarin or a chlorophyll metabolite may be responsible. In order to identify mitigating factors, document gross pathology and provide practical management tools for this disease, three studies were conducted. In the first study, evidence of photosensitivity was compared between forage rape and raphanobrassica under low and high nitrogen (N) applications. There was little difference between forage species with both forages eliciting photosensitivity, peaking three days after introduction to brassica then gradually resolving. On Day 3 raphanobrassica fed lambs had a slightly higher increase in mean ear thickness at +1.77 mm, 95% CI [1.47, 2.08], compared with forage rape at +1.20 mm, 95% CI [1.01, 1.40], being no difference thereinafter (P > 0.1). There was no effect of N on photosensitivity (P = 0.531). A lack of alteration of liver enzymes, as demonstrated by the presence of gamma-glutamyl transferase (GGT) and glutamate dehydrogenase (GLDH), indicated this disease is a form of primary photosensitivity. A second study compared severity, incidence and onset of photosensitivity in lambs assigned to four brassica grazing treatments or a non-brassica control, chicory. The four brassica treatment strategies were designed to influence the intake of plant parts (i.e. leaf or petiole/stem). The strategy encouraging the highest leaf intake resulted in the greatest severity, highest incidence and earliest onset of the disease (P < 0.001). There was little evidence of photosensitivity in lambs that consumed predominantly petiole and stem, signifying the phototoxin may be present in leaf material. In both studies, clinical signs resolved while lambs continued to consume affecting feed, suggesting a developed ability to detoxify the causative agent(s). In a final study, the duration of this detoxification or tolerance ability was determined by measuring photosensitivity in four groups of lambs who had initially exhibited photosensitivity on brassica. Lambs continuously fed brassica for over eight weeks or those transferred from brassica to chicory for eight weeks showed no indication of photosensitivity. Lambs grazing brassica continuously, with a four-day break on chicory, before being returned to brassica for two weeks showed no signs either. While lambs initially on brassica removed to chicory for 8 weeks and returned to brassica for 14 days developed clinical signs (P < 0.001). This suggests their ability to detoxify the causative agent lasted at least four days but not eight weeks. As this disease was traditionally associated with forage rape (‘rape scald’), we propose the term ‘brassica-associated primary photosensitivity’ (BAPP) to encompass evidence of it occurring in other forage brassica species. Grazing strategies that reduce brassica leaf intake and retain lambs on forage brassica throughout the summer risk period may be effective tools to mitigate BAPP in lambs. A tentative diagnosis of BAPP may be given to lambs exhibiting an ear pinna thickness > 3.6 mm taken at the midpoint of the ear pinna and/or presence of non-pitting bilateral oedema and/or presence of erythema with a history of recent introduction to a forage brassica, i.e. within the last week, during summer.