Illini PorkNet Papers
An area of considerable current interest in the swine industry is that of pre-slaughter handling of pigs and it effect on pig meat quality. One important issue is the optimum time of feed withdrawal prior to slaughter. Time off-feed before slaughter can potentially affect a number of aspects that are of interest to producers and/or packers, including live and carcass weights, dressing percentage, the incidence of punctured stomachs during evisceration, and meat quality. The evidence from the scientific literature relating to the effect of time off-feed on these parameters is reviewed in this article.
Live and carcass weights and dressing percentage
One of the obvious consequences of holding animals off-feed is that their live weight will decline and the longer the period of feed withdrawal, the greater the reduction in live weight will be. For example, a number of studies have suggested that pigs will lose approximately 3% of their live weight if kept off feed for 24 hours prior to slaughter.
This reduction in live weight results from losses in gut fill and visceral weights, particularly from the liver, and a reduction in the weight of the carcass. The relative losses from these various sources appears to vary considerably between studies, but reductions in carcass weight represent a substantial proportion of the loss in live weight. Observed reductions in carcass weight have generally ranged from under l% up to 3% per 24 hour period without feed. In fact, there is evidence that feeding pigs prior to loading may produce a small increase in carcass weights. Therefore, fasting pigs prior to loading may produce a small increase in carcass weights. Therefore, fasting pigs prior to despatch for slaughter is likely to reduce carcass weights and, therefore, result in lower returns to producers who sell on a dead-weight basis and the extent of this loss will increase with increasing time off-feed.
The impact of time off-feed on dressing percentage will vary depending on how this parameter is measured. Dressing percentage is the ratio between carcass weight and live weight, and if this is calculated using the live weight of the pig immediately before slaughter, then dressing percentage will generally increase with increasing time off-feed because the reduction in live weight due to feed withdrawal will normally be proportionately greater than the reduction in carcass weight. However, if the live weight before feed withdrawal is used, then dressing percentage is likely to decrease with increasing time off-feed. There is a belief in some quarters that improving dressing percentage by withdrawing feed prior to slaughter is beneficial; however, this is achieved with an accompanying reduction in carcass weight and, therefore, makes little economic sense.
An interesting sideline to this issue has recently emerged from research carried out at the University of Illinois investigating eating behavior in finishing pigs. This work showed that in uncrowded situations, pigs consumed relatively little feed during the night time between 6.00 pm and 6.00 am. This suggests that if pigs are loaded early in the morning then the majority will have been off-feed for approximately 12 hours. If, however, the pigs are in crowded pens or are in hot conditions then this may not be the case.
Incidence of punctured stomachs
A major concern of the packer is the potential to puncture full stomachs during evisceration, which results in carcasses being contaminated with stomach contents. With the increasing importance of food safety, this is an area of great concern. Withdrawing feed before slaughter will reduce gut fill and should reduce the incidence of punctured stomachs, although there is little objective data to support this concept. There is a need for further research to quantify the relationship between time off-feed and the incidence of punctured stomachs under practical conditions.
It has been proposed that withdrawing feed from pigs pre-slaughter will reduce the incidence of pale, soft exudative (PSE) pig meat. The PSE problems is caused, in part, by a rapid breakdown of the energy in the muscle immediately after slaughter. Withdrawal of feed will ultimately lead to a reduction in the energy content of the muscle and should reduce the incidence of PSE. A number of research studies have demonstrated this effect and have also shown that the benefit is greatest in situations where the incidence of PSE is high, such as with pigs that are stress susceptible. However, fasting periods of 24 to 48 hours have generally been necessary to produce a significant improvement in meat quality. At these times off-feed, the reduction in carcass weight is likely to be substantial.
Fasting pigs before despatch for slaughter has the advantage of saving of feed costs and the potential to reduce the incidence of punctured stomachs and improve meat quality. However, the reduction in carcass weights that will occur will reduce the financial return to the producer. The optimum approach to use in practice, will obviously, depend on the economic balance between these advantages and disadvantages.