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Factors impacting non-compliance of southern Australian grass fed cattle

Loudon, Kate Margaret Windeyer (2019) Factors impacting non-compliance of southern Australian grass fed cattle. PhD thesis, Murdoch University.

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Producing a product that delivers a consistently high-quality eating experience is paramount to the Australian beef industry to ensure consumer satisfaction and return protein purchasing. Dark cutting meat, a carcass with a high ultimate pH (>5.7), is one of the biggest economic impacts affecting meat quality worldwide. The importance of adequate muscle glycogen levels at slaughter and the impact of pre-slaughter stress is widely understood however there is variability in incidence of dark cutting rates in pasture raised cattle at certain points of the year that is currently unexplained. Furthermore it has recently been demonstrated that pre-slaughter stress independent of ultimate pH is having an impact on beef eating quality.

The first experiment evaluated the effect of animal, management, pasture and environmental factors on the impact of dark cutting incidence in pasture raised cattle (n=3145) on King Island, Tasmania. Cattle were consigned to slaughter in 66 lots, on 13 kill days, with 3 to 6 groups harvested per kill day. All cattle were trucked from farm gate to King Island port which ranged in distance from 15 to 90 km. The transport from King Island port to Tasmania mainland abattoir was the same for all cattle, to minimise any compounding variables during pre-slaughter transport. Cattle were shipped overnight for 8 to 10 hours then trucked for 100 km to the same abattoir. There was a 26% increased relative risk of dark cutting in cattle grazing pasture with magnesium concentration ≤ 0.24%. In pastures that contained ergot alkaloids there was a 45% increased relative risk of dark cutting and a 37% increased relative risk if FumoninsinB1 was present in pastures. There was a 50% increased risk in dark cutting in cattle drinking from dam water compared to trough. There was a 25% decreased risk in cattle fed supplements (hay/silage) prior to slaughter and a decrease in heifer groups compared to steer groups.

The second experiment investigated if supplementing magnesium prior to slaughter to pasture fed cattle would improve muscle glycogen and incidence of dark cutting. For certain producers there was an increase in slaughter muscle glycogen concentration with magnesium supplementation (p<0.01) suggesting that this may be a viable option to improve MSA compliance rates.

The third experiment evaluated if blood metabolites, enzymes and electrolytes could distinguish between various pathways of commercial pre-slaughter stress and if a rest period prior to slaughter would be beneficial to muscle glycogen repletion. The design aimed to test groups consisting of steer only, heifer only, and mixed sex cattle under a direct kill versus rested (14 days in abattoir holding paddocks) scenario. Cattle (n=488) were sourced from 8 farms and a comparison was made between a shipping or saleyard pre-slaughter pathway. Plasma creatine kinase (CK) was the metabolite most correlated with muscle glycogen and ultimate pH (p<0.01). A rest period prior to slaughter was effective at lowering this enzyme and improving muscle glycogen concentration (p<0.01). However, the plasma biomarkers were not sufficient at distinguishing between the various types of pre-slaughter stress.

The final experiment evaluated the impact of pre-slaughter stress pathways and plasma metabolites on consumer eating quality. Approximately 24 hours after the animals in experiment three were slaughtered, 5 muscles were collected from the left side of each carcass. Each cut was aged for 7 or 21 days under vacuum packaging after which samples were evaluated in 187 consumer sensory sessions, each utilising 60 untrained Australian consumers (n=11,220 consumers). Mixing of unfamiliar cattle (p<0.001) and saleyard pre-slaughter pathways (p<0.01) were associated with eating quality erosion in certain cuts. Plasma metabolites of acute stress and muscle damage had a small but negative impact on consumer eating quality. A two week rest period prior to slaughter was effective at improving consumer sensory score (p<0.01). Plasma CK was the metabolite that was most correlated with consumer eating quality, as this was the metabolite with the most correlation to muscle glycogen further research and economic modelling is warranted to determine if CK could be used on a commercial scale to distinguish if individual animals would benefit from a two week rest period prior to slaughter.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Veterinary and Life Sciences
Supervisor(s): Pethick, David and Gardner, Graham
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