Seed contamination in sheep carcasses by barley grass: An analysis of prevalence, management and economic impact

This project investigates the prevalence, causal factors and key weeds associated with weed seed contamination in sheep across southern Australia, in an effort to identify optimal IWM strategies for effective long term management of seed contamination in sheep production systems. An analysis of Australian abattoir, weather and weed datasets were undertaken during 2016, while replicated trials from 2016 to 2018 were undertaken at Wagga Wagga, NSW to investigate individual and IWM strategies against Hordeum spp. in lucerne pasture. Trials provided key data for the development of a barley grass submodel and a bioeconomic model for simulating the impacts of barley grass seed contamination and associated weed control programs on lamb profitability in lucerne pastures.
Abattoir analysis results revealed significant variability in carcass contamination incidence with time and region, and was associated with Hordeum and Bromus spp. distribution. Contamination rates were highest in adult male sheep, varied between abattoirs, fluctuated between states in response to mean monthly temperature and increased with mean monthly rainfall. Incidence also decreased with elevation, varying between years.
Field trial results in 2016 demonstrated propaquizafop to be 99% effective against barley grass. Paraquat was less so (58%), attributed to suboptimal timing of applications, but resulted in increased lucerne and other weed biomass. Repeated mowing applied at barley grass boot stage reduced seedling emergence 12 months after application. Integrating mowing with herbicide application significantly influenced lucerne and other weed biomass after 24 months, highlighting the crucial influence of control timing and frequency on botanical composition in infested legume pastures. Further investigations in 2017 revealed haloxyfop-R + simazine, or paraquat, applied at the early tillering stage, to be over 95% effective against barley grass. Repeat defoliations conducted at post-inflorescence emergence at all population densities, reduced barley grass survival and fecundity by 35% and 75% respectively under heaviest competition.
A dynamic and stochastic bioeconomic model was constructed using these results to simulate the efficacy of individual and IWM strategies against barley grass on lamb production at various weed density thresholds within an infested lucerne pasture over ten years. Simulations demonstrated the superiority of integrated management to individual applications against barley grass seedbanks. Livestock enterprise profitability was maximised at $67.98/ha/yr when IWM strategies were implemented at low barley grass densities, representing an economic benefit of $169.98/ha/yr from utilising IWM in comparison to no control (-$102/ha/yr).
Findings demonstrate the value of proactive and accurately timed IWM strategies for influencing legume pasture botanical composition, reducing barley grass populations in heavily infested regions across Australia, and mitigating losses associated with barley grass seed contamination in grazing sheep across southern Australia. Further research is required to 1) more accurately match contamination rates with causal weed distribution patterns to predict future contamination prevalence, 2) better elucidate relationships between weed density and carcass contamination, improving model prediction and capacity, 3) investigate barley grass phenology and IWM impacts under variable climatic conditions, and 4) develop and validate the bioeconomic model under various field conditions to inform development of subsequent models simulating weed impacts on livestock production systems.