Cumulative incidence and associated risk factors of dark, firm and dry (DFD) meat in South African beef carcasses

Abstract

The present study investigated the cumulative incidence and associated risk factors of dark, firm and dry (DFD) meat, also described as dark-cutting beef (DCB) or dark-cutters, in South African beef carcasses. The aim was to analyse possible causative risk factors associated with DFD beef in South African conditions, through a comprehensive retrospective review of production data from feeding systems, transportation, lairage, slaughter and cooling to retail. The studied material consisted of 29,787 cattle from 52 suppliers, distributed over Eastern Cape, Free State, Gauteng, KwaZulu-Natal, Limpopo, Mpumalanga, North West and Western Cape provinces in South Africa and different production systems (Angus = 2,116; Commercial = 10,580; Free-range = 12,976) for the period from January 2020 to December 2020 from a typical high throughput abattoir. Climatological data were obtained from the closest meteorological stations to the loading location of cattle at the farms and the unloading location at the slaughter plant, which were combined with the large data set from the abattoir that included the following information per individual animal: farm production system, estimated standing time, breed type, age class, gender, cattle conformation, live weight, carcass weight, fat content and post-mortem carcass pH-temperature measurements. Because of the large variation in pH values at which DFD beef is identified between geographical locations and from previous studies, the 24-hour pH values post-mortem were divided into risk categories namely: Normal (pHu < 5.8), Intermediate (pHu 5.8 ≤ X < 6.2) and High (pHu ≥ 6.2). The effects of pre-slaughter stress factors on the risk of DFD beef were analysed by means of the General linear mixed model procedure in SPSS IBM Statistics version 27 (2089, 2020). The overall estimated incidence of DFD beef at a South African abattoir was 0.40% high-risk (pHu > 6.2), 43.2% intermediate risk (pHu 5.8 ≤ X < 6.2) and 56.4 % (pHu < 5.8) low-risk DFD carcasses. The proportion estimate effect of the extrinsic and intrinsic pre-slaughter stress factors on the pHu was significant but small. However, combining the estimated effects of all factors on individual cattle significantly increased the risk of DFD beef by c.a 17%. Because the pre-slaughter stress factors are confounded by one another by minimising the effect of one factor, there may be a reduced effect of other related factors. In summary, suppliers had a significant effect (0.044%) on the incidence of DFD beef along with production systems (Angus = -0.508%; Commercial feedlots = 0.054%; Free-range = 0%) and climatic factors (Tx_FARM - Tx_ABATTOIR = 0.017914; Tn_FARM - Tn_ABATTOIR = 0.016504; RHx_FARM - RHx_ABATTOIR = 0.001758). Although climatic conditions had a small but meaningful effect on increasing the risk of DFD meat, it cannot be compared with the variation in pHu caused by the other factors as it cannot be controlled. Transportation distances did not affect the pHu post-mortem, but loading density (0.002%) significantly increased the risk of DFD beef. Furthermore, the study suggests that cattle should be rested after transportation (1 day = 2.651 %; 2 days = 2.147 %; 3 days = 0 %) but more research is required to quantify a general estimated resting time according to the pre-slaughter stress factors cattle experience before slaughter. Cattle breed did not have a significant effect on DFD beef, however, gender (Cow = -0.027; Heifer = -0.474; Steers = 0 %) and age (A = -0.811 %; AB = -0.794 % ; B = -0.858 % ; C = 0 %) significantly affected the pHu post-mortem. Furthermore, the outcome of this study indicates that the seasonal variation in pHu (Autumn = 1.176%; Spring = -0.969%; Summer = -1.528%; Winter = 0%) was mainly determined by the variation in cattle body conformation (live weight, warm carcass mass and fat content) throughout the year. Based on the findings of this study, a DFD beef window was defined under South African conditions, in which cattle have a lower body conformation (live weight (< 458 kg) and low warm carcass mass (< 281 kg)) compared to the rest of the year and therefore responded more severely to the extrinsic and intrinsic pre-slaughter stress factors, increasing the risk of DFD. Thus, body conformation can be used as the main determining factor to identify individual cattle at risk to produce DFD beef prior to slaughter, especially in the DFD window period. This study indicates that it is possible to decrease the risk of DFD by focussing on the factors that can be controlled, such as nutritional management, low-stress management techniques especially during transportation, avoiding the mixing of unfamiliar cattle, reducing the loading density and standing period and increasing carcass weight. One of the biggest concerns of the effect of pre-slaughter stress factors on an individual animal is the lack of knowledge by suppliers and abattoirs to use basic skilful management techniques to minimise the risk of DFD beef. Therefore, it is important that more awareness campaigns and training of suppliers, advisers and policymakers should occur to explain the key impact each section in the supply chain has on meat quality attributes. Then only can dark-cutting beef be minimised to a manageable extent by skilful manipulation of cattle management techniques in beef production.