Low-stress livestock handling at the slaughter plant has the benefit of improving both animal welfare and meat quality. Resting pigs in the lairage for 2 h before stunning, and reduction of electric prods (goads), is especially important to preserve meat quality (Milligan et al., 1998; Warriss, 2003; Hambrecht et al., 2005a). Short-term handling stresses a few minutes before stunning can increase Pale, Soft, Exudative (PSE) pork in pigs (Hambrecht et al., 2005a; Hambrecht et al., 2005b) and tough meat in beef (Warner et al., 2007). High-stress handling methods will increase lactate in the blood (Benjamin et al., 2001; Hambrecht et al., 2005a; Edwards et al., 2010a; Edwards et al., 2010b; Rocha et al., 2016). This results in lower pH in the meat, and it may make beef tougher (Gruber et al., 2010). When glycogen is depleted, lactic acid can no longer be produced. This results in higher than normal ultimate pH.

Longer term stresses, such as temperature fluctuations 24 to 48 h before slaughter or long transport times, deplete the stores of muscle glycogen and may increase Dark, Firm and Dry (DFD) meat (Jones and Tong, 1989; Scanga et al., 1997; Gallo et al., 2003). A basic principle is that handling stresses that occur shortly before stunning increase lactate and lower pH. Animals subjected to long-term stress may have higher pH and DFD meat (dark cutter). There are exceptions to this rule, but the principle of a short-term stress versus a long-term stress is a concept that is easy to understand. It will assist abattoir managers in solving meat quality problems, because long-term stresses are more likely to occur outside the abattoir.

Both PSE in pigs and DFD beef in cattle are severe quality defects. PSE pork has poor water binding capacity and higher cooking losses (Van der Wal et al., 1988). DFD beef and pork have a shorter shelf life (Blixt and Borch, 2002). The shorter shelf life is due to higher pH favoring microbial growth. Meat that is DFD is darker and firmer. Other topics that will be covered in this review will be methods for reducing bruises and quality problems such as petechial hemorrhages (blood splash). Both of these problems may be related to handling methods. The main emphasis of this paper will be on improving handling practices in the abattoir. There will also be a short discussion of on-farm factors outside the abattoir that may increase handling problems in the slaughter plant. Stress in a slaughter plant is often due to the novelty of the new environment (Kilgour and deLangen, 1970; Grandin, 1997; Deiss et al., 2009; Bourquet et al., 2010). Animals with more reactive genetics will have a bigger reaction (Bourquet et al., 2010). These reactions may be affected by both genetics and how the animals were handled on the floor.

When the glycogen in the muscle is depleted, an animal is more likely to have DFD meat. In Ghana, a combination of both poor transport and abusive handling can result in high percentages of cattle with DFD meat (Frimpong et al., 2014). Bulls will often have a greater percentage of dark cutters compared to steers (Price and Tennessen, 1981). Bulls have a greater tendency to fight than steers (Tennessen et al., 1985). Mixing bulls from different pens on the farm will increase fighting (Tennessen et al., 1985). The physical exertion of fighting depletes glycogen. Bulls that are fed together with their penmates on the farm should be transported with their penmates and then penned in the abattoir with their penmates (Grandin, 2014). In European abattoirs, bulls are often unloaded directly into single-file races to prevent fighting. Each bull is held in the race between 2 sliding gates that form an individual pen. This system is used to prevent fighting. Fighting resulted in more DFD beef. It is the author’s opinion that for optimal animal welfare, bulls that are lairaged in single-file races should be delivered on a “just in time” basis. They should spend only a few hours in single-file races. From an animal welfare standpoint, the best way to manage fed bulls is to raise them on the farm and transport and lairage them in the same group of familiar penmates. When the groups are formed on the farm, the bulls will fight. When mixing is done early in the feeding period, the bull’s physiology will have sufficient time to recover from the stress of fighting. After the bulls have formed a stable dominance hierarchy, they will no longer fight.

Bruises on livestock cause huge losses to the livestock industry in many countries (Harris et al., 2017; Grandin, 2017). Training of employees in low-stress handling practices will reduce bruises (Grandin, 1981; de Costa et al., 2014). In cattle, bruises can occur after captive bolt stunning and before bleeding (Meischke and Horder, 1976). Strappini et al. (2013) reported that the stun box door hitting the backs of the cattle caused many bruises. Exact aging of bruises is difficult, but it is easy to determine a fresh bruise from a bruise that is several days old. Fresh bruises in cattle will be red (Hamdy et al., 1957), and older bruises in cattle or lambs over 48 h old may be darker or have yellowish mucous on them (McCausland and Dougherty, 1978). Strappini et al. (2009) and Mendonça et al. (2019) have good reviews of aging bruises in cattle. Betancourt-Garcia et al. (2019) found that either poor handling or poor loading facilities at the ranch of origin increased bruising. Additional handling procedures at auctions for disease testing may increase bruises on cows (Hoffman et al., 1998). In pigs, visual appraisal of skin lesions was not very accurate for determination of the age of the lesions (Vitali et al., 2017). A more accurate method for determining lesion age in pigs was a spectrophotometer (Vitali et al., 2017).

There are a number of different scoring tools available for quantifying bruises on cattle (Anderson, 1978; Strappini et al., 2013; Lee et al., 2017; Kline, 2018). When bruise prevalence is being compared between different abattoirs, it is strongly recommended that the same scoring tool be used. Some people working under commercial conditions do not know that using the same scoring tool is important. Visual assessment of bruises prior to trimming may miss internal deep damage. Kline (2018) found that some bruises have an “iceberg effect.” The damage visible on the surface may be very slight, but when the bruise is trimmed, extensive damage is hidden underneath the surface. There is also a type of bruise that can occur on sheep that does not occur in other species. Sheep can be severely bruised if they are grabbed or picked up by the wool (Richardson, 2002).

Both stunning and handling methods can have an effect on blood splash in meat of both pigs and sheep (Burson et al., 1983; Gregory, 2005). Poor handling methods and electric prod use can increase blood splash in pigs (Calkins et al., 1980). The use of a group handling system and elimination of electric prods reduced blood splash in pigs that were stunned with CO₂ (Franck et al., 2003). It is beyond the scope of this paper to discuss the extensive research that has been conducted on stunning methods and meat quality. The author has observed that electric prod use and excessive pressure from a restraint device will increase blood splash in kosher-fed steers and heifers. These animals were slaughtered without stunning. This review will be limited to handling methods in the abattoir.

There should be sufficient space in the lairage for all animals to lie down at the same time. This can be determined by observing pigs or cattle when they are all lying down. Kline et al. (2019) contains photos of an overstocked and a correctly stocked lairage after all the cattle laid down. Lairages should also be equipped with water troughs (Welfare Quality Network, 2009; NAMI, 2019). Resting pigs in the lairage for 1 to 4 h will improve pork quality (Milligan et al., 1998). There are many methods to make floors nonslip. Some examples of nonslip flooring in lairage pens are grooved concrete or epoxy grit finishes. In high-traffic areas, such as unloading ramps and other high-traffic areas, steel bars or rubber mats may be used. Assessment of slips and falls is strongly recommended (Welfare Quality Network, 2009; Grandin, 2010). Scoring slips and falls will enable managers to determine whether slips and falls are improving or becoming worse. Rocha et al. (2016) reported that drip loss in pork was correlated with slips during unloading. Sharp edges can cause bruises. A smooth rounded surface will not case a bruise. Edges on angle irons or I-beams can bruise. If livestock are hitting a sharp metal edge, the metal may be shiny or have tufts of hair on it.

Electric prod use can often be greatly reduced by stopping strong air movement that blows into an animal’s face when it enters a stun box or restrainer (Grandin, 1996; Grandin, 2014). Funnel-shaped crowd pens work effectively for cattle and sheep (Grandin, 2014; Grandin, 2015), but they work poorly for pigs (Hoenderken, 1976). A single offset step prevented pigs from jamming (Grandin, 1982). A single-file race must never be bent too sharply where it joins the crowd pen (Grandin, 2014) because bending it too sharply will create a dead-end effect, and animals may refuse to enter.

When new stainless steel equipment is ordered, the metal fabricator should be instructed to provide a dull, nonshiny surface. Moving lights or removing lights can sometimes eliminate reflections on shiny metal or a wet floor (Grandin, 1996; Klingimair et al., 2011). All species may refuse to enter a dark stun box or restrainer. Experimentation with portable lights to illuminate the entrance may facilitate entry for all species. The lamps should be positioned to provide indirect lighting. Illumination will facilitate animal movement from a darker to a brighter place (Van Putten and Elshof, 1978; Grandin, 1982; Grandin, 1996; Tanida et al., 1996; Grandin, 2001). Adding a light to a restrainer entrance reduced vocalizations from 8% of the cattle to 0% because electric prod use was reduced (Grandin, 2001). Addition of green lighting may reduce reflections (Eyes on Animals, 2015).

Addition of a solid panel prevented approaching animals from seeing people or moving conveyors, which often facilitated forward movement. Experimentation with large pieces of cardboard can be used to determine the best position for solid panels (Ecolano, 2018). Müller et al. (2008) reported that cattle remained calmer when a solid panel was installed to prevent them from seeing a person close to them. Animals will stop and refuse to move when they see small things that people do not notice. Some examples of small distractions that may need to be removed are paper towels hanging down, a coat on a fence, a hose on the floor, or a small piece of metal that moves (Grandin, 1996). Sudden intermittent noise is stressful to livestock (Stermer et al., 1981; Talling et al., 1998; Lanier et al., 2000). High sound levels may increase the risk for PSE pork (Vermeulen et al., 2016). Air exhausts should be muffled to prevent sudden loud hissing.

In pigs or cattle, vocalization is often caused by excessive pressure applied by a head holder or other mechanical restraint device (Grandin, 1998a; Grandin, 2001; Bourquet et al., 2012). Pressure-limiting devices may need to be installed if the animal vocalizes in direct response to application of a head holder or body restraint. Cattle, pigs, and sheep may respond by struggling. Further information on cattle restraint devices is in Ewbank et al. (1992) and Grandin (1992). Only flat or smooth rounded surfaces should come in contact with the animal. Animals will react or struggle if their skin is pinched by a restraining device. To determine the cause, watch closely and determine which part of the restraint apparatus moves when the animal reacts.

Lame animals of all species that have difficulty walking may be more difficult to handle in a low-stress manner (Grandin, 2015). Lameness scoring can be used to determine which producers or dealers have delivered high percentages of lame livestock. Edwards-Callaway et al. (2017) developed a simple, easy to use system for scoring animals when they unloaded. The scores are (1) normal, (2) lame but keeps up with the walking group, (3) lame and cannot keep up, and (4) almost a downer. A study by Davis-Unger et al. (2019) indicated that there are big differences in the percentage of lame feedlot cattle from different producers.

Livestock that are moved through auctions may have more bruises (Strappini et al., 2009). Changing the way that livestock transporters are paid may also help reduce bruises. They should be paid based on the condition of animals after they arrive; they should not be paid based on the weight being transported. Bruises on feedlot cattle were reduced by half when feedlots had them subtracted from their payment (Grandin, 1981). Other factors that will increase bruises are overloaded trucks (Tarrant et al., 1988). The way that a truck is driven may also have an effect on bruises (Tarrant et al., 1988). Larger cattle may hit their backs on low ceilings (Lee et al., 2017). In pigs, overloading of trucks increased both nonambulatory pigs and losses (Ritter et al., 2006; Pilcher et al., 2011).

When a meat company has control of the complete supply chain, it is often possible to make dramatic improvements. It is important to discuss on-farm factors because the author has observed that some severe welfare problems that have occurred in a slaughter plant were impossible to remedy at the plant. The author consulted with a large, new pork plant that had excellent new lairage and handling facilities. When this abattoir first opened, 5 or 6 full-time employees were required to handle several hundred nonambulatory downed pigs. The handlers at this slaughter plant were doing an excellent job, and no electric prods were used. Even though handling was excellent, they still had many downed pigs. In this case, the solution to the problem was changing conditions on the farm. After the improvements, the number of people required to handle downers was reduced to 1 part-time employee. The following 3 changes were made on the farms. •

Producers were trained to walk their pens to get the pigs accustomed to people walking through them. They walked quietly through the finishing pens every day to train the pigs to quietly get up and move away Research has clearly shown that a pig’s experience on the farm with handling will affect how it will react in the future (Abbott et al., 1997; Geverink et al., 1998; Lewis et al., 2008; Krebs and McGlone, 2009).

Implementing a comprehensive program of assessment of handling in the slaughter plant can help keep standards high (Grandin, 2010). When a problem is located, the first step is to determine whether it occurred before arrival at the slaughter plant or inside the abattoir. For example, there are differences in pig genetics and the tendency to have either PSE meat or a higher percentage of deads (Murray and Johnson, 1998). If bruises are elevated, the first step is to locate and correct problems that occurred inside the abattoir. If a particular supplier has elevated bruises, there will usually be a pattern of bruises that occurred on their livestock. When bruises are caused by conditions inside the plant, they will often occur on the same location on livestock from many different producers or transporters. Successful troubleshooting of either handling or meat quality problems will require both detective work and good record keeping. Records of bruises, deads, downers, lameness, and other conditions can be used to locate the sources of problems.

The use of calm, low-stress methods to handle and move livestock results in both improved animal welfare and improved meat quality. Short-term stresses a few minutes before stunning, such as electric prods or jamming in the race, may increase PSE in pigs or tougher meat in beef. Management should implement simple change in both handling methods and equipment. When a problem is detected, the next step should be to determine whether the cause is inside or outside of the abattoir.