Pork Safety and Quality: Feed Withdrawal Prior to Slaughter
Table of Contents
The pork industry continues to look for ways to improve pork safety and quality, production performance, animal welfare and the environment. One of the good production practices recognized by the pork industry is withdrawing feed from market hogs prior to slaughter. Pork producers are encouraged to work with hog transporters and packers to implement an effective on-farm feed withdrawal program.
Pre-slaughter feed withdrawal (also referred to as withholding feed, removal of feed, holding feed off or fasting prior to slaughter) is the period of time when pigs are denied access to feed prior to slaughter. To calculate the number of hours of feed withdrawal, the overall time from feed removal to slaughter must be considered.
Time without feed on farm
For example, if feed is withdrawn from pigs at 7:00 pm, and they are slaughtered at 10:00 am the following day, the total time of pre-slaughter feed withdrawal is 15 hr.
The timing of feed withdrawal is critical to the success of a feed withdrawal program. Numerous studies have shown that the optimal pre-slaughter feed withdrawal time is 12-18 hr. To properly assess the timing of feed withdrawal, monitor the feeding cycles in the barn so you know when the majority of pigs have last eaten. Researchers in Illinois studied the eating behaviour of market hogs and found that un-crowded pigs ate little feed overnight between 6:00 pm and 6:00 am. However, where pigs were crowded or housed in hot barns, feeding was more likely to continue throughout the night. The pigs' housing situation can affect how long they may have gone without feed by morning.
Improving Pork Safety
Pigs with full stomachs at slaughter are prone to intestinal rupture
or laceration, gut spill and fecal leaking from the rectum, which all
cause fecal contamination of the carcass - one of the major food safety
risks and concerns at the packing plant. Withdrawing feed from pigs for
12-18 hr prior to slaughter can reduce gut fill, reduce the incidence
of carcass contamination and improve pork safety.
Improving Pork Quality
Fresh pork quality, as measured by colour, water-holding capacity and meat appearance, is closely related to meat pH. A rapid drop in muscle pH after slaughter and low ultimate muscle pH cause denaturation of muscle cells and lead to the formation of soft, pale and exudative (PSE) pork. One of the key factors causing meat pH drop is the presence of muscle glycogen (a form of sugar). Since muscle glycogen is converted to lactic acid after slaughter, the more glycogen the muscle contains at slaughter, the more lactic acid will be produced, resulting in lower meat pH. Pre-slaughter feed withdrawal reduces the amount of muscle glycogen and, therefore, prevents rapid muscle pH drop. Studies show that a 12-18 hr pre-slaughter feed withdrawal reduces the incidence of PSE and drip loss and improves meat colour (redness) without compromising other meat quality attributes. A slightly darker red meat colour is preferred by consumers, especially those in the Asian markets. Feed withdrawal has also been shown to improve the meat quality of hogs carrying the halothane or porcine stress syndrome (PSS) gene. However, it has no effect on the meat quality of pigs that carry the Rendement Napole (RN) gene.
Reducing In-Transit Loss and Improving Animal Well-Being
In-transit loss is a term for animal deaths that occur during transportation. This loss varies among hog producers, transporters and packing plants. There are numerous factors associated with in-transit loss, including genetics, handling practices and weather (humidity and temperature). Pork producers, hog transporters and handlers along the pork chain can all take measures to minimize or even eliminate such losses. Studies show that during shipping and transportation, pigs with full stomachs are more likely to be subject to transport death, motion sickness and vomiting than pigs subjected to pre-slaughter feed withdrawal. Furthermore, there are also reports indicating that pigs that have been subjected to feed withdrawal are easier to move and handle during loading, transporting and unloading than animals that have not. Pre-slaughter feed withdrawal, therefore, could reduce in-transit losses and improve animal well-being.
Reducing Feed Cost
Feed consumed within 9-10 hr prior to slaughter is not converted into carcass weight gain. Feeding pigs before slaughter increases live animal weights but not carcass weight. Since it is carcass weights that are used for hog settlement, feed consumed by pigs just hours before slaughter is actually wasted. A recent study in the U.S. found that withholding feed for 24 hr pre-slaughter decreased feed consumption by approximately 2 kg per pig (on an as-fed basis). A study at the Prairie Swine Centre found that pre-slaughter feed withdrawal for 12 hr reduced feed consumption by 3 kg - $0.48 of feed cost per pig. Based on this study, the total saving for the Ontario pork industry from pre-slaughter feed withdrawal can translate to over $2 million.
Reducing the Amount of Gut Fill and the Cost of Its Disposal
Pigs denied access to feed for 12-18 hr prior to slaughter generate less waste in the form of gut contents at the packing plant. Consequently, the amount of waste and the cost associated with removal and disposal of the waste at the packing plant are reduced. Less waste is also better for the environment.
Losing Carcass Weight
The most significant effect of pre-slaughter feed withdrawal is the loss in live animal weight. The main source of live weight loss in the first 24 hr is gut contents loss, accounting for 80% of the total body weight loss. It has been reported that during the first 24 hr of feed withdrawal, pigs lose about 5% of their live body weight, at a rate of approximately 0.2% per hour or 50 g/kg of body weight. However, carcass weight loss only represents approximately 20% of the live weight loss during this 24-hr period and does not occur until 9-12 hr of feed withdrawal. Therefore, feed withdrawal 12-18 hr prior to slaughter could result in a slight loss in carcass weight. One study reported that overnight feed withdrawal resulted in a loss of 0.8%-0.9% in carcass yield.
Increasing Skin Damage and Stomach Ulcerations
Overnight feed withdrawal may cause increased carcass skin damage due to mixing and increased fighting between pigs. Research reports suggest that the problem is more pronounced in pigs fasted on-farm than those at the abattoir.
Another potential disadvantage of feed withdrawal is the formation of stomach ulcers, although research has shown that this problem only occurs with an extended period of feed withdrawal (over 24 hr). Withdrawal of feed for 12-18 hr prior to slaughter does not seem to cause stomach ulcerations.
Increasing Incidences of Dark, Firm and Dry Pork
Studies have shown that pigs denied feed for an extended period of time (over 24 hr) prior to slaughter can produce dark, firm and dry (DFD) pork. DFD pork occurs when the pH of the pork is higher than normal (ultimate meat pH greater than 6.2) as a result of depleted muscle and liver glycogen levels. Withdrawing feed for 12-18 hr does not seem to cause formation of DFD pork.
Based on the study results currently available, it is safe to say that 12-18 hr of pre-slaughter feed withdrawal is beneficial to the pork industry. Packers benefit from the decreased cost for disposal of gut contents, decreased incidence of nicking the gastrointestinal tract, PSE pork, in-transit and lairage mortality, and increased pork safety and quality. Producers may also benefit from reduced in-transit losses, improved pork safety and quality, and feed savings. Feed withdrawal may result in a slight loss in carcass weight (in the neighbourhood of 0.8%-0.9% of carcass yield after overnight feed withdrawal). There may be other costs associated with feed withdrawal to producers, such as the labour required for emptying feeders in some production systems.
Withdrawing feed from market hogs for 12-18 hr prior to slaughter enhances pork safety, pork quality and animal well-being. It also reduces the incidence of PSE pork and in-transit loss without compromising other quality and economic parameters, and reduces the amount of waste generated and the cost associated with waste disposal at the packing plant.
The length of pre-slaughter feed withdrawal time is important to the success of this production practice. To determine the appropriate feed withdrawal time for a particular situation, all the time between the pigs' last meal consumption and their slaughter should be counted. For example, if it takes 0.5 hr to load pigs on farm, 3 hr on the road to the packing plant, 0.5 hr to unload pigs and 2 hr of rest time at the packing plant prior to slaughter, feed should be removed from the pigs 6-12 hr prior to loading them on-farm. If these animals are going to be slaughtered at 9:00 am, they should be off feed between 3:00 pm and 9:00 pm the day before. Feed withdrawal can be easily done with automatic sorting systems. Without automatic sorters, producers have to remove feed manually. In practice, producers usually empty feeders the night before shipping hogs.
It must be pointed out that drinking water should be available to pigs whenever possible. Having access to drinking water is especially important during the summer months when the temperature and humidity are high. The optimal feed withdrawal time is 12-18 hr prior to slaughter; this should become a standard production practice in the pork industry. Hog producers, truckers and packers should work together to implement a pre-slaughter feed withdrawal program on-farm for the benefit of the whole pork industry.
Murray A, Robertson W, Nattress F, Fortin A. Effect of pre-slaughter overnight feed withdrawal on pig carcass and muscle quality. Can. J. Anim. Sci. 81: 89-97.
Bidner B, McKeith F. Feed withdrawal prior to slaughter: Effects on pork quality and safety. University of Illinois. Facts. National Pork Board. 1998.
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