period of food or water deprivation and the stresses associated with transport. The most commonly recognized consequences of poor pre- slaughter handling on lean meat quality are pale, soft, exudative (PSE) meat in pigs and dark, firm, dry (DFD) meat in pigs and cattle. The full economic consequences of lean meat quality defects are difficult to quantify accurately.
million birds. The mortality is higher in longer journeys (Fig. 7.2). As in pigs, higher ambient temperatures are associated with higher mortality. In both species adequate ventilation of the transport vehicles is therefore of paramount importance. In confined conditions birds lose heat (thermoregulate) at high temperatures largely by panting. At high humidities the effectiveness of panting is reduced or nullified.
Hot, humid conditions therefore limit the bird’s ability to lose body heat.
Fig. 7.1. The influence of temperature on transport deaths in pigs (based on data in Allen and Smith, 1974).
Fig. 7.2. Influence of journey time on the percentage of birds recorded as dead on arrival (DOA) at a poultry processing plant. The data relate to 3.2 million birds transported in 1113 journeys (based on information in Warriss et al., 1992).
Carcass damage
Carcass damage can take the form of bruising and haemorrhages, skin blemishes or, particularly in poultry, broken bones. Bruising can occur at any point in the marketing chain from handling on the farm, through transport, to the time immediately after stunning but before the animal is bled out. In a bruise, blood from damaged blood vessels accumu- lates. Bruised tissue therefore looks unsightly and is usually trimmed, reducing yield as well as frequently leading to downgrading. The cost of this downgrading may be greater than the value of the trimmed meat.
Surveys have shown that about 2% of pork hams are bruised in the USA and that over 30% of bulls and 80% of cows show some bruises.
In terms of quality, in red meat species bruising is an aesthetic rather than a hygiene problem. Bruised tissue probably has no greater initial microbiological load than normal tissue. However, because it tends to be handled more, for example at inspection and trimming, it may pick up a higher microbial load. For poultry the situation may be different, but for reasons that are unclear, and bruising can be associated with a higher susceptibility to spoilage. Although in the fresh state bruised tissue may be aesthetically undesirable, its use in processed products may be acceptable. The microbiological and organoleptic properties of bruised meat have been described (Gill and Harrison, 1982; Gill, 1994).
Bruising can occur in pigs by misuse of slap markers (used to tattoo an identification mark on the animal’s back), by rough handling, and internally. This occurs when injuries are caused by the pig’s back legs slipping apart on poor or wet surfaces. In cattle it can be caused by animals slipping and falling, or through the inappropriate use of sticks.
An important cause of bruising in cattle is the agonistic behaviour, such as butting and mounting, which occurs on mixing animals from different rearing pen groups. This is a particular problem with young bulls.
Bruising is often caused by trying to move animals too quickly, particularly over uneven or slippery floors. Too high or too low a stock- ing density during transport can cause bruising. Understocking allows animals to be thrown about when the vehicle is moving. This is especially important for adult cattle which, except on the longest journeys, may not lie down. In the close confinement of holding pens or vehicles, horned cattle can be a problem in causing damage to other stock. Cutting off the tips of the horns before transport, known as
‘tipping’ has not been found to reduce bruising. Other factors that have been shown to increase the level of the problem in cattle are long periods without food, and chronic stress. Marketing animals through live auctions increases the levels of bruising in both sheep and cattle (Table 7.2). This may be because it increases the number of journeys the animals make or because of the handling that occurs in the market.
The differences in overall levels of bruising in the two studies on sheep are because of the different recording methods used. Cockram and Lee (1991) recorded all bruises, Knowles et al. (1994b) recorded only bruises severe enough to be commercially important in that they led to economic loss. The studies on cattle also recorded only commercially significant bruising.
McNally and Warriss (1997) found that the prevalence of bruising varied with the particular market. It ranged from 2 to 18%. Distance travelled from market to slaughterplant could not explain the variation.
However, there was a correlation between bruising and the amount of stick-marking on the carcasses. This suggested that the greater bruising was caused through less careful handling by the drovers and stockmen, rather than poorer facilities, since greater stick-marking is caused by people hitting animals more.
Bruises can vary both in number and severity or size. Various methods of assessing bruising have been developed. An example is the Australian Bruise Scoring System (Anderson and Horder, 1979).
Attempts to estimate the age of bruises, in order better to identify the factors causing them, have not been very successful. The characteristic colour changes seen are caused by the breakdown of the red haemoglobin in the blood to bilirubin (yellowish) then to biliverdin (green) but they are relatively insensitive indices of bruise age. More sensitive methods to estimate bruise age have been based on measure- ment of the concentration of bilirubin (Hamdy et al., 1957) or histo- logical changes (McCausland and Dougherty, 1978).
Skin blemish in pigs
A related problem is the superficial skin damage caused by fighting in pigs, particularly between unfamiliar animals. This takes the form of Table 7.2. The effect of marketing through live auctions on the prevalence of bruising in sheep and cattle.
Method of marketing
Species Country Directly from farm Through live auction
Sheep UK 12 20a
Sheep UK 1.1 1.4b
Cattle UK 4.8 7.0c
Cattle Australia 2.8 3.5d
aCockram and Lee, 1991; bKnowles et al., 1994b; cMcNally and Warriss, 1996;
dEldridge et al., 1984. The figures represent the percentage of carcasses showing bruising in studies a, b and c, and the numbers of bruises per animal in study d.
unsightly lacerations (Fig. 7.3) and detracts from the appearance of rind-on products (those sold with the skin on). Pigs reared together develop stable social hierarchies. When animals are selected for slaughter, pigs from different rearing pens are frequently mixed together to make up batches of individuals with similar live weights. They may also get mixed in lairage. This disrupts the established hierarchies and individuals, particularly the more dominant ones, will often fight to establish new dominance orders. The fighting can be severe and leads to the lacerations on the skin, particularly on the shoulders and, usually to a lesser extent, along the loins and hams. In extreme cases it may lead to the carcass being downgraded. Surveys in the UK have shown about 5–7% downgraded carcasses (Warriss, 1984c; MLC, 1985). The problem is more common in boars than gilts or castrates, this reflecting the more aggressive nature of the uncastrated male pig (see Table 6.12).
Petherick and Blackshaw (1987) reviewed the factors that affect agonistic behaviour in pigs. Competition for food and, by implication,
Fig. 7.3. Lacerations on a pig carcass caused by fighting pre-slaughter.
factors that promote hunger, increase aggression. So also does com- petition for space. Having sufficient space to be able to easily retreat from aggressive congeners is important. It is known that pigs use pheromones to communicate, particularly in relation to potential aggression. Androstenone may be an important pheromone and there is evidence that spraying it on pigs can reduce agonistic behaviour.
Interestingly, Grandin and Bruning (1992) have found that the presence of a sexually mature boar in a group of slaughter-weight pigs reduced the incidence and intensity of fighting. It has been suggested that odour-masking substances, particularly those with a very strong smell, could help reduce aggression, but there is little or no evidence that they are effective. Tranquillizers such as azaperone will reduce fighting but their use in slaughter pigs is precluded by the need to ensure that carcasses and meat are free from residues.
Broken bones and bruising in poultry
Broken bones are a particular problem in poultry, and especially in culled hens. Gregory and Wilkins (1989b) found that 29% of battery hens in the UK had broken bones at slaughter. A corresponding survey of broilers indicated a level of 3%. The high level in hens is attribut- able to the weakness of the skeleton caused by demineralization of the bones and a restricted opportunity to exercise (Knowles and Broom, 1990). The situation is exacerbated by lack of sufficient care as the birds are removed from the battery cages when they are culled at the end of lay. Broken bones may cause bone splinters in the meat. These can be dangerous to the consumer if not detected after deboning.
Bruises in poultry may be caused by both ante-mortem handling or by stunning at slaughter. For example, red wingtips can be associated with severe flapping ante mortem as well as certain stunning proce- dures. The reported levels of bruising vary widely from about 2 to 20%.
Most bruises occur on the breast, followed by the legs and wings, then the backs and thighs. Strain of bird, season, degree of muscling, care during handling, particularly being picked up by one leg, and struggling on the shackles used to suspend the birds before slaughter, have all been suggested as factors influencing the level of the problem.
Modern strains of turkeys, which are bigger but younger at slaughter, are very prone to carcass damage (Barbut et al., 1990).
Reduction of live weight and carcass yield by inanition and transport It is inevitable that animals be deprived of food for some time before slaughter. Losses in live weight are due to loss of gut fill and excretory
losses. Losses in carcass weight are caused both by mobilization of tissues to provide energy for maintaining the vital functions of the body, and dehydration which often accompanies fasting and transport.
These losses in live or carcass weight are sometimes referred to as
‘shrinkage’. Shrinkage is loss of potential yield.
Loss of live weight in pigs begins almost directly after feed with- drawal; the rate of loss is about 0.2% h!1 (Warriss, 1985). The time when carcass loss begins is less clearly defined but is probably between about 9 and 18 h after the last meal. This reflects the rapid passage of food through the gut in pigs, food reaching the small intestine 4–8 h after ingestion, and most nutrient absorption occurring within 9 h. The rate of carcass loss is about 0.1% h!1. What this can mean in practice is that keeping pigs in lairage overnight without food may result in a reduction of 1.4% of potential carcass weight. This is nearly 1 kg in a 90 kg pig. On long journeys dehydration is probably an important factor. Although the influence of transport on yields is less well defined than that of fasting, a loss of 2% of carcass weight after a 6 h journey in hot weather has been recorded in pork pigs.
Because of their proportionally larger guts, ruminants are less susceptible than pigs to short periods of inanition. However, in sheep, economically significant losses in carcass yield can be detected after periods of food deprivation that occur during normal marketing. There is also a progressive loss of liver weight and, initially, a large part of this is attributable to loss of stored glycogen, which the animal uses as an energy source. With longer fasting, muscle glycogen stores are also mobilized.
In adult cattle the gut contents can account for 20% of the body weight and a large proportion of the loss in live weight over the first 24 h of food deprivation is attributable to loss of faeces. Gut fill is larger in animals on high roughage pasture than in those fed grain diets, and in animals that have recently drunk. Previous diet and access to water are therefore important influences on the patterns of live weight loss in cattle. Based on data collected from 26 publications, Shorthose and Wythes (1988) produced a relationship showing mean losses of live weight ranging from about 7% after 12 h to 11% after 72 h (Fig. 7.4). The effects of inanition on carcass yield are poorly defined. Various reports have recorded loss starting between 17 and 48 h after the beginning of a fast. Reported losses in carcass yield after 48 h range from 1 to 8%.
Much of the research on the effects of transport on shrinkage in cattle has been carried out in North America and Australia. Animals have lost between 6 and 12% of their live weight in journeys of 500–2000 km.
Losses in carcass weight range from 0 to 4% in journeys up to 2000 km.
Food and water deprivation in broiler chickens lead to live weight losses averaging about 0.2–0.3% h!1. Veerkamp (1986) pointed out that from estimates of a bird’s heat production (5 W kg!0.75) and the
energy value of animal tissue (7500 J g!1), one would expect a loss of 0.22% h!1 body weight. At high ambient temperatures the loss is greater because the bird’s heat production will increase and more moisture will be lost by evaporation from the respiratory tract. The size of carcass yield losses is poorly defined.
Other important quality implications of food and water deprivation As well as affecting carcass yield, the times for which animals are deprived of food and water before slaughter may have other implica- tions for quality. Pigs fed too soon before transport show a slightly increased mortality rate. Because of this, pigs should not have access to food within 4 h of loading on to the vehicle. Also, food that will not have had time to be digested will be wasted. To minimize carcass yield losses, however, the total time from last feed to actual slaughter should ideally be not more than about 12 h in pigs, and certainly not more than 18 h (Warriss, 1996a).
With longer food deprivation periods the stomach contents of ruminants become more watery, increasing the chances of head, tongue and carcass contamination, either through regurgitation or through accidental cutting of the gut wall during carcass dressing. Very full guts, however, make their handling and hygienic removal more difficult so a compromise must be reached. The disposal of large quantities of gut contents at the abattoir is also costly because they increase the dirtiness of the effluent water, incurring greater processing costs before discharge into the sewerage system.
Fig. 7.4. Liveweight loss in fasted cattle based on information derived from 26 studies by Shorthose and Wythes (1988) as summarized by Warriss (1995).
In sheep and cattle subjected to long periods without water, the removal of skin or hide is more difficult. This may result in greater tearing of the underlying tissues and a poorer appearance to the carcass surface. If the flaying is done by hand the greater effort required may increase chances of contamination of the carcass with dirt from the skin.
In red meat species there is some evidence that long food depriva- tion can lead to a build-up of pathogenic bacteria such as Salmonella in the gut. However, increased times between leaving the farm and slaughter also tend to increase the prevalence of Salmonella-infected cattle (Grau et al., 1968) and keeping pigs in lairage for 2–3 days prior to slaughter increased the incidence of Salmonellaeven when the pigs were fed (Hansen et al.,1964).
With poultry, the risk of contamination of the carcass with Salmonella and Campylobacter is perhaps greater. The transport of birds in stacked crates, often with perforated floors, gives great potential for faecal contamination of the outsides of the live birds.
Fasting periods of up to 10 h have been recommended for poultry but even prolonged food withdrawal will not completely prevent defaeca- tion. Paradoxically, longer feed withdrawal may increase the prevalence of Salmonellain the crop of laying hens (Humphrey et al., 1993) and has also been associated with a higher prevalence of birds testing positive for Campylobacter jejuniin cloacal swabs before slaughter and caecal swabs after (Willis et al., 1996).