Photosensitisation in sheep grazing biserrula

Page last updated: Tuesday, 15 February 2022 - 10:04am

Isolated cases of photosensitisation have occurred in sheep grazing both Casbah and Mauro biserrula throughout the wheatbelt. If photosensitisation is identified early and the animals immediately removed from the pasture to shady areas then they will usually recover well. This page provides guidelines to minimise the occurrence of photosensitisation in sheep.

Introduction

Biserrula (Biserrula pelecinus) is well adapted to a wide range of soil types with varying pH and can be grown with most other pasture species. It is a very productive species with a deep root system and provides green feed for up to a month longer than subclover at the end of the growing season.

Stock often preferentially graze the non-legume weed component which leads to a biserrula dominant pasture. This is highly beneficial for the management of herbicide resistant weeds such as annual ryegrass and wild radish. Well nodulated biserrula pastures can also supply valuable nitrogen for cereal crops grown in rotation.

There are two cultivars of biserrula: Casbah and Mauro, which differ in maturity and degree of hard-seededness. Casbah flowers 100-105 days after emergence whereas Mauro flowers about two weeks later.

Casbah has a high level of hard seed and is ideal for intensive cropping systems typical of the 325-500 millimetres (mm) rainfall region. It is recommended that Casbah pastures should be cropped in the year after establishment. Mauro has better regeneration in second year pastures and is suited to mixed farming and permanent pastures in the 450-700mm rainfall region.

Pods and flowers of biserrula
Pods and flowers of biserrula

Grazing tolerance and feed quality of biserrula

Biserrula has a prostrate growth habit which gives it a high tolerance of grazing compared with other aerial seeding pasture legumes. In the establishment year grazing should be limited, ensuring stock are removed at flowering and not replaced until after pod ripening. This allows for maximum seed production and the build-up of an adequate seed bank for future regeneration.

Biserrula can germinate rapidly on early autumn rain and the seedlings have good drought tolerance in comparison to other annual legumes. It can therefore provide an important source of early feed. Early grazing of regenerating stands of biserrula promotes prostrate growth. These stands are able to set seed even with moderate grazing pressure during flowering. Grazing can provide excellent control of wild radish and ryegrass and some control of doublegee.

During spring biserrula has the ability to stay green for up to four weeks longer than subclover, due to a deeper root system (up to 2m). This can be a valuable contribution in livestock systems such as weaning or finishing lambs.

Up to 45% of ingested seeds survive passage through the digestive tract of sheep (more in cattle) and there can be considerable spread of biserrula in manure. Consequently, summer grazing has little impact on persistence. Biserrula provides a high quality feed for stock over summer as it maintains a higher crude protein (CP), dry matter digestibility (DM) and metabolisable energy (ME) than cereal stubble.

Sheep grazing biserrula
Sheep grazing biserrula
Indicative nutritive value of senescing biserrula (Brookton 2007) in comparison to several cereal straws
Feed source Sample date ME (Megajoules per kilogram (MJ/kg) DM) CP (%) DM digestibility (%)
75% biserrula pasture 29/10/07 9.3 15.5 63.4
62% biserrula pasture 08/11/07 8.3 15.5 57.7
Wheat straw Shortly after harvest Nov-Dec 2006 6.9 4.3 49.7
Barley straw Shortly after harvest Nov-Dec 2006 7.5 6.1 52.9
Oat straw Shortly after harvest Nov-Dec 2006 8.0 6.9 55.9

Photosensitivity

Sporadic photosensitisation of sheep occurs in association with the grazing of a range of plant species, including various grasses, cereals and legumes, but the incidence is generally very low. Over the past 10 years, isolated cases of photosensitisation have occurred in sheep grazing both Casbah and Mauro biserrula throughout the wheatbelt.

Photosensitivity is a state of heightened sensitivity to sunlight caused by the presence of photodynamic chemicals in the skin, cornea and mucous membranes. Photosensitivity may result in photosensitisation, which is the dermatitis and conjunctivitis produced in a sensitised animal after exposure to sunlight.

The main types of photosensitisation are:

  • Primary; resulting when animals ingest particular plant compounds that are photodynamic.
  • Secondary; resulting when toxins in plants cause liver damage, which in turn results in the accumulation of the photodynamic compound, phylloerythrin, in the blood.

Biserrula and photosensitisation

There is currently no evidence that liver damage occurs in sheep grazing biserrula pastures. Therefore, the assumption is that biserrula contains photodynamic chemicals that when consumed in sufficient quantity will cause a primary photosensitisation. Photosensitisation in sheep grazing biserrula has only occurred when the plant has made up greater than 40% of the pasture on offer.

There is also some evidence indicating that lambs may be more susceptible to developing photosensitisation. It appears that biserrula pastures may only transiently contain high concentrations of the photodynamic compounds. Outbreaks of photosensitisation have occurred in late winter or early spring, when the green plant is growing rapidly and is about to flower. There is no evidence of photosensitisation having occurred when the dead plant is grazed in summer.

Sheep with photosensitisation; exposed raw tissue on the face and muzzle
Sheep with photosensitisation; exposed raw tissue on the face and muzzle

Clinical signs of photosensitisation

Photosensitivity can occur within a few days of sheep moving on to biserrula pastures though it may develop over several weeks. The first indication that animals may be photosensitised are:

  • restlessness
  • head shaking
  • rubbing
  • seeking out of shade.

Early signs of photosensitisation include swelling of the ears, eyelids, mulesed area (tail), backline and muzzles. These are all the most exposed parts of the body. If allowed to progress these areas will become reddened, inflamed, and the overlying skin will die and peel off, exposing raw tissue underneath. In severe cases lambs may lose the tips or all of their ears, a general break in the wool can occur and sheep may appear lame due to inflammation of the coronets.

Sheep with photosensitisation; loss of wool and exposed raw tissue on the back
Sheep with photosensitisation; loss of wool and exposed raw tissue on the back

Management

Farmers should adopt a cautious approach (regular monitoring) to the management of sheep grazing biserrula, especially when it dominates the pasture composition. If photosensitisation is identified early and the animals are immediately removed from the pasture to shady areas (or shedded if convenient), then the animals will normally recover well. Deaths due to biserrula photosensitisation are very rare.

The following practices will help to reduce the risk of photosensitisation when sheep are grazed on biserrula pastures:

  • Rotationally graze biserrula for short periods, especially during the high risk period of late winter and early spring.
  • Provide alternative grazing, preferably of grassbased pastures or standing oat crop rather than other legume dominant pastures.
  • Feed the sheep hay while they graze biserrula.
  • In particular, do not graze lambs or recently shorn sheep on biserrula for periods greater than two weeks. Many growers that have experienced incidents of photosensitivity have learnt to live with the problem and say that it is easy to manage.

Research

Current research is trying to identify the primary photosensitising compounds that are suspected to be present in biserrula. If they can be identified, breeding programs may be able to reduce or eliminate their presence in new cultivars. If any animals do develop photosensitisation while grazing biserrula, farmers should notify their local Department of Primary Industries and Regional Development office for advice and possibly to assist with continuing research.

Acknowledgments

We would like to thank the growers who took part in the biserrula case studies: Cameron Levett of Carnamah, Phil Bear of Dowerin, Andrew Chambers of Ravensthorpe, Jason Stokes of Chapman Valley, John and Gordon McDougall of Tincurrin Alf Niven of Carnamah and John Munckton of York. Also thanks to Noel Bubner from Balco Australia for supplying the straw feed test results.

Contact information

Author

Clinton Revell