Introduction

It is generally accepted that a period of feed withdrawal prior to slaughter reduces the risk of development of meat quality defects above the level determined by the genetic status of the animal. Feed withdrawal can enhance meat quality by reducing the amount of total carbohydrate available for postmortem conversion of glycogen to lactic acid (DeSmet et al., 1996). Muscle quality improvements from overnight feed withdrawal at the plant have been reported (Jones et al., 1985) as increased ultimate pH, loin color intensity, and water holding capacity. Eikelenboom et al. (1991) found that pigs held off feed (but not water) for a minimum of 12 h before slaughter resulted in fewer pale, soft, and exudative (PSE) carcasses. Murray et al. (1989) reported that a 24 or 48 h fast significantly reduced the frequency of PSE in stress susceptible pigs. However, the muscle quality advantages from feed withdrawal are not consistent in the scientific literature. DeSmet et al. (1996) found overnight feed withdrawal to have no effect on meat quality for all PSE related traits in normal, stress carrier and stress positive pigs. Becker et al (1989) found no relationship between feed withdrawal up to 72 hrs and ultimate pork quality but did report that meat tenderness was enhanced by transport and fasting.

Considering feed withdrawal from an animal welfare standpoint it may appear that the stress of transport combined with the stress of feed restriction could produce negative affects on the well being of pigs being transported to market. However, Becker et al. (1989) reported that the combination of feed restriction and transport stress did not present an additive demand on the pigs biological systems. In fact, Stephens and Perry (1990) found that a pig's aversion to transport was increased if they had eaten prior to shipment and Tarrant (1993) reported that death losses are greater in pigs fed on the day of transport.

Feed withdrawal has a more practical application even if one discounts the possible improvement of pork quality. Eilert (1997) suggests that a 16 to 24 h feed withdrawal period is consistently advantageous to the packer because of the greater ease of carcass evisceration, reduced waste handling at the plant, and reduced incidence of broken viscera on the slaughter floor. Feed withdrawal can facilitate the process of evisceration and minimize the chance for microbial contamination of the carcass from lacerations in the gastrointestinal tract. Eilert (1997) also suggested that producers consider approximate transport time and holding time at the plant when determining the length of on-farm feed withdrawal.

Given the recommendation (Eikelenboom 1991) to withdraw feed from hogs 12-24 hours prior to slaughter and the fact that producers are penalized for selling hogs outside a narrow weight range most feed withdrawal occurs during transportation and rest at the plant. Feed withdrawal can also be difficult to implement on farm when graded marketing occurs in all-in/all-out facilities over a 3-6 week period. In most cases, feed can practically be withdrawn only from the last load. Earlier loads are usually on full feed until they are shipped.

Withdrawing feed from hogs before they are slaughtered has the following potential benefits: decreased weight and contents of the gastrointestinal tract resulting in fewer lacerations and consequently decreased risk of carcass contamination, less feed wastage, increased yield, fewer carcasses with PSE, and decreased cost of manure treatment at the plant. The potential disadvantages include a possible reduction in live weight, possible reduction in tenderness and juiciness (Ellis 1996), a possible increase in the proportion of pigs excreting Salmonella spp. and consequently an increased risk of carcass contamination, and possibly increased prevalence of gastric ulcers. To better understand feed withdrawal as a standard production practice these benefits and disadvantages were evaluated as a system in a study conducted at North Carolina State University.

Study Objectives

To determine the effect of withdrawing feed from pigs at 0, 12, or 24 hours before they are shipped to slaughter and the effect of sorting by weight and withdrawing feed once, twice, or three times in a carcass-merit situation on:

• Meat quality as measured by ultimate pH, color, and water holding capacity
• Proportion of gastrointestinal tract lacerations
• Prevalence of Salmonella spp. in cecal contents at slaughter
• Prevalence and severity of gastric ulcers
• The economic impact of the treatments

Procedures

In March 1998, 1133 barrows from a nursery site were weighed, individually identified and assigned, blocked by weight, to 36 pens. The barn had 40 pens and the other 4 pens were used to hold the cull pigs and the extreme lightest and heaviest pigs that were excluded from the study. Each pen of 29-32 pigs had pigs of similar minimum and maximum weight with similar variation between pens. Maximum variation within a pen, rather than minimum, allowed the progressive selection of the heaviest third of pigs for slaughter from each pen and simulates the slaughter closeout of a barn where on three occasions the heaviest third of pigs are removed for slaughter. By design, however, this procedure confounds the effects of repeated feed withdrawal with pig weight because the lighter weight (presumably slower growing) pigs are exclude from the first marketing group. Pigs were presumed homozygous stress negative because they came from lines that had been DNA tested and found negative for the HAL 1843 gene. In June, the 6 pens that had the fewest pigs (attrition from death and culling) were deleted from the study because they exceeded our needs.

Experimental design: A 3 by 3 split-plot.

Treatments: Treatments included feed withdrawal of 0, 12, and 24 hours and marketing group (1, 2, and 3) selected on weight and having feed withdrawn once, twice, or three times prior to shipment. Treatments were allocated at random, blocked on pen prevalence of Salmonella spp. as determined from the fecal sampling in May and June. Feeders to the pens containing hogs for slaughter were shut off and any feed in the feeding troughs was returned to the pens' feeders.

Shipments: For the first and second marketing groups, the 10 heaviest pigs in each pen were visually identified and shipped (feed withdrawn once or twice). The third marketing group closed out the barn and consisted of all pigs remaining in all the test pens. In the second marketing group, an accident at the packing plant resulted in the loss of all data on all the pigs (60) for that day. Pigs were individually tattooed with a unique 4 digit identifying number coded to describe the day and treatment. The person accompanying the pigs recorded time in transport and lairage. In lairage, pigs had free access to water but not feed.

Composition and Ultimate Muscle quality: Data on 657 pigs were collected on 11 days (no Saturday data). Hot carcass weight was collected and fat and muscle depth were determined by the Fat-O-Meter optical probe (SFK Technology, Denmark) at 30 minutes post stunning. One chop was collected from each carcass at the tenth rib location at 24 h. post-mortem and after a minimum 20 minute bloom time was evaluated for color, water holding capacity, ultimate pH, and temperature (^oC). The loin was measured in triplicate (medial, middle, lateral) and mean values calculated for color lightness (L*), redness (a*) and yellowness (b*) using a Minolta Chromameter 200 (set to D65 illuminant, a 2 degree standard observer, using an 8 mm optical port with glass insert, and calibrated with Minolta white standard color plate). A visual color score was also determined on a scale from 1 to 6 (1 = pale, 6 = very dark) using plastic Japanese color standards. On the same sample ultimate pH was measured using an Engold electrode and an Omega pH-50 meter. Water-holding capacity was evaluated by using filter paper (4.5 cm circles; S&S Filter Paper; Keene, NH) absorption of excess fluids on the cut surface as determined by weight increase (Kauffman et al. 1986).

Statistical Analyses: For meat quality measures the statistical model included the fixed effects of feed withdrawal (0, 12, and 24), marketing group (1, 2, and 3), and withdrawal by market group interactions. The random cold temperature was fit as a covariate for ultimate pH, water holding capacity and Japanese color score. Contrasts among feed withdrawal treatments were also tested for linear and quadratic effects.

Economic analyses: Differences in revenues and costs were calculated for pigs in each of the 9 subgroups (0, 12, and 24 hour feed withdrawal combined with the three marketing groups that had feed withdrawal once, twice or three times prior to slaughter). Standard base values were assigned for price per pound of carcass weight ($0.60 per pound), feed cost per additional pound of gut weight ($0.025 per pound gut weight), non-feed cost of additional days on feed ($0.05 per day), and value of feed saved during feed withdrawal for animals that were returned to feed ($0.1312 per hog per 12 hour feed withdrawal and $0.2625 per hog per 24 hour feed withdrawal). Carcass merit value differences were calculated separately using the same carcass base price ($0.60 per pound) with the addition of a backfat adjustment (0.25% per mm. of backfat) and a muscle adjustment (0.20% per mm. of loin muscle depth). Carcass merit prices also included discounts of 30% for carcasses weighing less than 125 pounds, 20% for carcasses weighing 125 to 145 pounds, 3% for carcasses weighing 146 to 166 pounds, 2% for carcasses weighing 195 to 215, and 6% for carcasses weighing above 215 pounds. Values were calculated for each pig slaughtered and analyzed for effects of treatment and the marketing group.

Results and Discussion

Pigs were loaded and left the farm between 2-5am, traveled for 1hr 15 min (range: 1 hr 45 min to 48 min) and allowed to rest at the plant for 3 hr 50 min (range: 4 hr 47 min to 1 hr 58 min).

Meat Quality: Significant interactions between length of feed withdrawal and marketing groups were observed for hot carcass weight and Japanese color score. Feed withdrawal for 0, 12 and 24 hours did not influence hot carcass weight in the first marketing group. However, in subsequent marketing groups hot carcass weight was reduced over increasing feed withdrawal periods. This result may indicate that the hot carcass weight of presumably healthier, faster growing pigs is not as greatly influenced by withdrawal of feed as those pigs selected into the second and third marketing groups. In marketing groups one and two, feed withdrawal resulted in increasing Japanese Color Scores while in marketing group three, pigs on increasing feed withdrawal treatments did not differ in Japanese Color Score. The most likely explanation for this interaction is due to the subjectivity of the individual determining the score.

Withdrawal of feed prior to shipment for slaughter (Table 1) generally, resulted in a linear reduction (P < .1) in hot carcass weight. Feed withdrawal prior to shipment improved the color of the longisimus dorsi as indicated by linear increases (P <. 1) in Japanese Color Score and linear decreases (P < .01) in Minolta a* measures over increasing feed withdrawal periods. There was a numerical improvement in water holding capacity for pigs on the 24 hour feed withdrawal treatment. Small numerical improvements are also indicated for Ultimate pH and Minolta L* as feed withdrawal periods increase. However, pigs were not significantly (P > .3) different in carcass composition across feed withdrawal periods.

Results in Table 2 indicate pigs in the third marketing group that experienced three feed withdrawal periods had lighter carcasses, less fat depth and greater loin depth (P < .01). This result is expected as the smaller, slower growing animals naturally selected themselves into this group and would be expected to have lighter, leaner carcasses. Increased muscling may be due to increased age. Graded marketing combined with repeated feed withdrawal resulted in linearly decreasing pork quality over marketing groups. The heaviest one-third of pigs selected for the first marketing group had a more desirable ultimate pH, greater water holding capacity and more desirable color. This improved quality may be due to a combination of factors including less stress by not having repeated feed withdrawal, healthier, more productive, faster growing pigs, and less severe gastric ulcers (Eisemann et al., 1999).

These results suggest that on-farm withdrawal of feed for 24 h prior to slaughter enhance ultimate pork quality. However, in a graded marketing situation ultimate muscle quality was reduced over time. This reduction may be caused by any one, or the combination, of smaller, slower growing pigs that may have had increased health challenges, increased incidence of gastric ulcers, and repeated withdrawal of feed over time.

Economics: Excluding carcass merit or meat quality effects, the hogs that had feed withdrawn only once or were in the corresponding control group (the first marketing group) demonstrated no significant effect of feed withdrawal on the selected revenues and costs. Net revenue differences for the 12 hour feed withdrawal and 24 hour withdrawal were +$0.99 (P = .52 ) per hog and +$0.19 (P = .90), respectively versus no feed withdrawal. The hogs in the second marketing group (feed withdrawn twice) and their corresponding control group demonstrated a statistically insignificant effect from those withdrawn from feed for 12 hours (-$0.96, P =. 54) and significant effect from those off feed for 24 hours (-$5.51, P = .0004) versus the control group. The hogs in the third marketing group (feed withdrawn three times) also demonstrated a statistically significant effect from both those withdrawn from feed for 12 hours (-$3.71, P =. 0047) or 24 hours (-$6.29, P =. 0001) versus the control group. Effects of feed withdrawal on carcass price through backfat and loin muscle premiums and carcass weight discounts were not statistically significant (12 hour feed withdrawal: +$0.14 per cwt. (P = .80); 24 hours: -$0.34 per cwt. (P =. 54) versus the control group) for the first marketing group. Hogs in the second marketing group (feed withdrawn twice) also did not demonstrate significantly different carcass merit adjustments to carcass price (12 hours: -$0.07 per cwt. (P =. 92); 24 hours: -$1.12 per cwt. (P =. 0863) versus the control group). The hogs in the third marketing group (feed withdrawn 3 times) demonstrated statistically mixed effects on carcass merit adjustments to carcass price (12 hours: -$0.86 per cwt. (P =. 1331); 24 hours: -$2.20 per cwt. (P =. 0002) versus the control group).

A consistent economic result emerged from the analysis. Feed withdrawal had no statistically significant effects on net revenue from the hogs that had feed withdrawn only once excluding meat quality benefits. However, marketing groups did show statistically significant reductions in net income for animals that had feed withdrawn. The effect was most significant for the 24-hour withdrawal in the second and third marketing group. Reduced carcass weight in pigs that had feed withdrawn twice or three times as compared to their control groups appears to be a primary factor in net revenue effects. This "within marketing group" reduction in net revenue was in addition to the lower revenue earned by the third marketing group consisting of "tail-enders". While it is difficult to attach a dollar value to meat quality attributes, the economic results are consistent with the meat quality results: feed withdrawal may be a net benefit as long as feed is not withdrawn more than once.

Implications

Feed withdrawal prior to slaughter results in improved loin color. However, our results indicate that repeated feed withdrawal may be responsible, in part, for reduced muscle quality in subsequent marketing groups. The selection of healthier, faster growing pigs into distinct marketing groups for weight graded marketing may contribute to differences in muscle quality across groups. These findings need to be tested in other finishing sites to determine if our results can be generalized across farms. A remarkable finding for meat quality from this study is that those animals sent to slaughter in the first marketing group have a fifty- percent greater water holding capacity than later marketing groups. This needs to be tested in other finishing sites and the biological reasons investigated.

Suggested reading

Becker, B. A. , H. F. Mayes, G. L. Hahn, J. A. Nienaber, G. W. Jesse, M. E. Anderson, H. Heymann, and H. B. Hedrick. 1989. Effect of fasting and transport on various physiological parameters and meat quality of slaughter hogs. J. Anim. Sci. 67:334-341.

DeSmet, S. M., H. Pauwels, S. De Bie, D. I. Deymeyer, J. Callewier, and W. Eeckhout. 1996. Effect of halothane genotype, breed, feed withdrawal, and lairage on pork quality of Belgian slaughter pigs. J. Anim. Sci. 74:1854-1863.

Eilert, S. J. 1997. What quality controls are working in the plant? P. 59-63. In: Proc Pork Quality Summit. July 8-9. National Pork Producers council. Des Moines, IA.

Eikelenboom, G., Bolink, A.H., Sybesma, W. (1991). Effects of feed withdrawal before delivery on pork quality and carcass yield. Meat Science 29: (1) 25-30.

Jones, S. D. M., R. E. Rempala, and C. R. Haworth. 1985. Effects of water restriction on carcass shrink and pork quality. Can. J. Anim. Sci. 65:613-618.

Kauffman, R. G., G. Eikelenboom, P. G. van der Wal, G. Merkus. 1986. The use of filter paper to estimate drip loss of pork musculature. Meat Sci. 18:191-200.

Murray, A. C., S. D. M. Jones, and A. P. Sather. 1989. The effects of preslaughter feed restriction and genoty0pe for stress susceptibility on pork lean quality and composition. Can. J. Anim. Sci. 69:83-91.

Stephens, D. B. and G. C. Perry. 1990. The effects of restraint, handling, simulated and real transport in the pig (with reference to man and other species). Appl. Anim. Behav. Sci. 28:41-55.

Tarrant, P. V. 1993. An overview of production, slaughter and processing factors that effect pork quality - General review. P. 1-21. In: Pork Quality: Genetic and Metabolic Factors. Edit. E. Puolanne, D. I. Demeyer, M. Ruusunen, and S. Ellis. CAB International.