<p>Dark cutting beef is defined by Meat Standards Australia (MSA) as carcasses that have an ultimate pH (pH_u) > 5.70 when measured at grading. Dark cutting beef is associated with deteriorated meat quality, thus beef producers are generally penalised by abattoirs to compensate for reduced saleable quality product. Non-compliant Meat Standards Australia beef, based on pH, has been estimated to cost the Australian beef industry approximately A$55 million per year and the estimated cost to beef producers is approximately A$0.50 per kg carcass weight or A$7.09 for every carcass graded in the Meat Standards Australia system.</p> <p>Dark cutting (DC) is a complex multifactorial problem that is influenced by nutrition and preslaughter factors which induce stress and exercise and increase muscle glycogenolysis. The condition is generally attributed to low muscle glycogen stores at slaughter, which is either a function of insufficient glycogenesis on farm or too much glycogenolysis during the preslaughter period due to exercise or stress.</p> <p>The 2015 Meat Standards Australia Australian Beef Quality audit showed the incidence of dark cutting in grain fed beef was between 1.5% and 2.5% with a peak incidence of 2.5% in March. Australian feedlot producers have anecdotally suggested there is an increased incidence of dark cutting during the summer months. Cattle under heat stress have higher respiration rates and panting to induce evaporative cooling, and have a reduced feed intake. The lowered nutritional intake combined with increased levels of epinephrine (adrenaline) and catecholamine expression during heat stress decrease glycogenesis and stimulate glycogen mobilisation. However, the influence of environmental conditions and/or the duration of exposure to these hot conditions on the incidence of dark cutting had not been established. The overarching objective of this research was to establish whether there was an association between environmental conditions and the incidence of dark cutting in grain fed cattle. This thesis is comprised of four experimental chapters examining the seasonality of dark cutting, a retrospective analysis of historical MSA grading data, plus an in depth examination of feedlot and processor factors affecting the incidence of DC.</p> <p>The first experiment investigated if there was any inherent impact of seasonality on the incidence of dark cutting in Australian grain fed beef, using a Meat Standards Australia carcass data set. Carcasses were classified as dark cutting when pH measured at grading was > 5.70 in the rib eye muscle (<i>longissimus thoracis et lumborum</i>) of the chilled carcass at the quartering site approximately 12–24 hours post-mortem. Utilizing data from 2,005,925 Meat Standards Australia carcasses, which were supplied from 12 feedlots and sent to 17 different abattoirs over 5 years were analysed. Carcass data were categorised into five groups based on the location of feedlots across geographical regions. These data were then analysed using a seasonal and trend decomposition using locally estimated scatterplot smoothing (loess time series analysis) to establish if a seasonal trend in dark cutting exists in Australian grain fed carcasses. Results show that the majority of feedlots had seasonality scores below 0.3 suggesting that dark cutting in grain fed beef has either low or no seasonality. Three feedlots had slightly higher seasonality scores (0.3 to 0.4), however these are still considered to be low scores. These results highlight there is limited evidence of a seasonality effect on dark cutting in the carcasses evaluated in this study. However, the presence of seasonality scoring above 0.3 for three participating feedlots, highlighted the need for a deeper understanding of the impacts of climatic conditions and specific weather events at the feedlot on the incidence dark cutting.</p> <p>The second experiment evaluated the relationship between climatic conditions prior to feedlot departure on the incidence of dark cutting in grain fed beef by combining historical Meat Standards Australia carcass data with Bureau of Meteorology weather data. Data records for 2,795,754 carcasses from 17 commercial feedlots sent to 16 abattoirs over 6 years were evaluated in this study. Weather data recorded at 30 minute intervals were obtained from the weather station closest to each feedlot. Climatic data were amalgamated into daily observations and a series of predictors including ambient temperature, relative humidity, wind speed and direction, rainfall, solar radiation and temperature humidity index were included in the analysis. Lag interactions from 24 hours out to 28 days before leaving the feedlot were generated. The incidence of dark cutting was determined as percentage per cohort with an ultimate pH > 5.7. The data were analysed using three separate models. Model 1 included feedlot, abattoir, hormone growth promotants status and sex as fixed effects. Model 2 incorporated the fixed effects within Model 1 and minimum, maximum and standard deviation of ambient temperature and relative humidity, daily range in ambient temperature, average wind speed and rainfall as covariates. Model 3 incorporated minimum, maximum, range and standard deviation of temperature humidity index, average wind speed and rainfall as covariates in addition to the fixed effects of Model 1. The results showed that higher maximum ambient temperature, relative humidity and temperature humidity index in the 3 to 28 days before consignment were all associated with an increased incidence of dark cutting (<i>P</i> < 0.05), but not in the 48 hours before consignment (<i>P</i> > 0.05). Lower minimum ambient temperature and lower temperature humidity index values were also associated with an increase in the incidence of dark cutting across all lag periods (<i>P</i> < 0.05). Increased variation in the temperature humidity index and ambient temperature in the 48 hours before consignment increased dark cutting (<i>P</i> < 0.05) while increased standard deviation of temperature and temperature humidity index range also increased dark cutting in the 14 and 28 day before feedlot exit (<i>P</i> < 0.05). Smaller minimum ranges in ambient temperature in the 28 days before consignment also reduced dark cutting (<i>P</i> < 0.05). Even though many different climatic conditions impacted the incidence of dark cutting, they only accounted for a further 0.1% to 0.2% of variance in the incidence of dark cutting. In contrast, the feedlot and abattoir animal management factors included in the models accounted for 21% of the variation in dark cutting. This analysis showed the major causes of dark cutting in lot fed cattle were driven by factors within the feedlot and abattoir which needed to be quantified further.</p> <p>The supply chain experiment analysed the significance of feedlot and abattoir factors including carcass traits, lairage time, weather conditions during lairage and other abattoir factors on the incidence of dark cutting in 142,228 grain fed carcasses. This study analysed data from cattle that were supplied from seven feedlots and processed at three abattoirs over 1 year. The average incidence of dark cutting was 2.8%. Increased wind speed (P < 0.05) and rain (P < 0.001) during lairage at the abattoir was associated with an increased risk of dark cutting, whereas variation in ambient temperature, relative humidity or temperature humidity index in lairage did not influence dark cutting. Heavier carcasses with whiter fat, larger hump heights, more rib fat, higher marble scores and lower ossification had lower incidences of dark cutting (P < 0.001). The abattoir and the grader at the abattoir also had significant effects on the incidence of dark cutting (P < 0.001). Increased time in lairage and reduced time to grading markedly increased the incidence of dark cutting (P < 0.001), which suggests that reducing the time in lairage and increasing the time between slaughter and grading are the two major ways to reduce dark cutting in grain finished carcasses. These factors reduce the opportunity and time for the cattle to experience stressful situations before slaughter and maximise the opportunity for the meat to reach pH_u, reducing the incidence of false positive dark cutting caused by grading too early.</p> <p>The feedlot component of the supply chain experiment analysed the effect of climate, animal and feedlot factors on the incidence of pH non-compliance in Australian grain fed cattle. Data on the same 142,228 carcasses from seven feedlots processed at three abattoirs over 1 year were analysed. The production factors which were associated with increased pH noncompliance included feedlot, sex, hormone growth promotants, cattle morbidity and longer days on feed. pH non-compliance was also increased by reduced solar radiation, lower wind speeds, increased ambient temperature, more rain, higher average temperature humidity index and more hours spent above heat load index threshold of 86 in the week before consignment. Interestingly transport duration did not impact the incidence of dark cutting. </p> <p>The causes of dark cutting in grain fed cattle are extremely multifactorial with many small factors having an impact. This thesis identified the main drivers across the supply chain which increased the risk of dark cutting in Australian grain fed cattle. This will help lot feeders identify cattle that are at higher risk of dark cutting and alter their pre-slaughter management to ensure cattle have minimal time in lairage and maximal time post-mortem before grading. Climatic conditions do impact the incidence of dark cutting, however management of cattle before slaughter and of carcasses after slaughter has a larger effect on the incidence of dark cutting.</p>