PRGT: Developing increased understanding, awareness and potential mitigation strategies for perennial ryegrass toxicosis in sheep production systems

Summary

Ryegrass pastures infected with the Wild-type (WT) endophyte Neotyphodium lolii produces several classes of alkaloids which when ingested cause perennial ryegrass toxicosis (PRGT). The effects of PRGT can be devastating, with serious outbreaks causing considerable losses and animal welfare problems. Recent estimates place the economic cost to the Australian Sheep Industry resulting from clinical, sub-clinical and indirect effects on animal performance, at $100 million dollars annually.

Endophyte in ryegrass provides resistance to insect pests and confers agronomic advantages, and is therefore essential for enhancing the persistence and productivity of ryegrass. This project tested the novel endophytes AR1 and AR37 against wildtype (WT) endophyte and showed that there was no agronomic disadvantage from using them and that improved animal outcomes were possible.

With respect to animal safety, the endophytes ranked AR1>AR37 >WT, with WT having been shown to have a range of negative productivity and physiological effects on sheep. The potential for utilizing rumen detoxifying agents to provide short term mitigation and/or preventative strategies for managing PRGT was clearly demonstrated. Modelling clearly demonstrated that in high PRGT risk areas, renovating pastures to eliminate PRGT was economically viable even if no improvement in stocking rate was achieved.

Objectives

This project aimed to:
• Assess the pasture performance and physiological and productivity responses of sheep grazing AR1, AR37 and WT endophyte-infected perennial ryegrass
• Characterize the interaction between known endophyte alkaloid intake and a range of conditions (temperature, nutrition plane) on the productivity and physiology of sheep.
• Develop an animal model and experimental approach to investigate the efficacy of currently available rumen detoxifying agents on mitigating PRGT. Assess the production and economic impact of grazing weaner sheep on commercial, WT perennial ryegrass pastures and evaluate the efficacy of novel rumen detoxifying agent.

Key findings

Key findings from the grazing experiments were:
• Sheep grazing WT endophyte infected ryegrass experienced ryegrass staggers, had lower live weights, increased rectal temperatures, and increased respiration rates. These effects were dependent on a range of factors including seasonal conditions and feed availability.
• Sheep grazing AR1 endophyte infected ryegrass did not experience ryegrass staggers, elevated rectal temperatures, nor increased respiration rates.
• Sheep grazing AR37 did experience ryegrass staggers as equally severe as WT ryegrass however it was generally later and slower to develop.
• This project demonstrated that WT ryegrass can have significant impacts on sheep productivity and physiology. With respect to animal safety the endophytes ranked as AR1>AR37>WT – with WT considered to be unsafe.

Key findings from controlled, short term indoor experiments are as follows:
• Ewes fed ergovaline had reduced feed intake and sheep on switched diets lost weight at a quicker rate. Lamb growth rate was also lower in the ergovaline and ergovaline/nil treatments. Milk production and mammary gland size did not reduce due to ergovaline intake.
• Low levels of ergovaline, like in pastures during winter, increases heat load in sheep but are not likely to impact productivity.
• Feeding a low level of ergovaline and lolitrem B did not adversely affect animal productivity. However, rectal temperature and faecal moisture increased when ergovaline and lolitrem B were fed in combination.
• Crossbred and composite breed sheep have different responses to ergovaline. Crossbred ewes were unable to cope with the heat exposure as well as the composite breed ewes. This difference may be due to higher proportion of Merino genetics in Crossbreds.
• Feeding PRG alkaloids decreased feed intake in Merino ewes, this was countered by the consumption of a commercial mycotoxin eliminator, Elitox®, that may help increase production in sheep fed PRG alkaloids.

Benefits to industry

Perennial ryegrass toxicosis (PRGT) can cause considerable losses and animal welfare problems, estimating to cost the Australian sheep industry $100 million dollars annually in clinical, sub-clinical and indirect effects. Mitigating the effect of PRGT will reduce losses, improve animal welfare and reduce farmer stress.

MLA action

It was recommended that MLA develop:
• communication strategies to disseminate the key findings of the project and increase awareness of PRGT
• on-farm management practices for different classes/breeds of livestock
• a tool for objective assessment of the economic benefits of implementing different PRGT mitigation strategies on farm for different sheep production systems.

Future research

Further research is required to investigate; efficient delivery systems of alkaloid binders to livestock on farm, the efficiency of other alkaloid deactivators/binders in mitigating PRGT and PRGT like effects caused by alkaloids associated with alternative endophytes such as AR37, AR5/Endo 5 and NEA2.

Additionally, there is an opportunity to investigate the potential for early PRGT detection systems using a similar model as developed for the testing of alkaloid deactivators/binders and the study of low doses of alkaloids.