Purdue University 1996 Swine Day Report

Effects of Genotype, Sex, Slaughter Weight, and Dietary Fat on Pig Growth, Carcass Composition, and Pork Quality

J.M. Eggert, E.B. Sheiss, A.P. Schinckel, J.C. Forrest, A.L. Grant, S.E. Mills, and B.A. Watkins
Departments of Animal Sciences and Food Sciences

Consumers and carcass-value marketing systems demand the efficient production of lean pork. Unfortunately, swine genotypes which provide a higher percentage of carcass lean also tend to produce carcasses which are poor in quality. The effects of dietary fat on pig growth and carcass composition are not well understood, especially for high lean gain genotypes. A study of fat distribution in pork carcasses from pigs of two genotypes which have different rates of lean growth and fat deposition was conducted in the fall of 1995. While the effects of sex and slaughter weight are well documented, this study further examined the effects of supplemented dietary fat in two divergent genotypes on growth, carcass traits, and pork quality.

Materials and Methods

Two genotypes which have different rates of lean growth and fat deposition were identified. Line 1 is a European Large White Landrace which represents the upper 5th percentile for percent lean. Line 2 is a commercial terminal cross which represents average U.S. pigs for percent lean. Pigs were reared using segregated early weaning procedures. From 30 to 132 lbs. liveweight, the pigs were fed a conventional corn-soybean meal diet which contained .85% lysine and no added fat. At 132 lbs. liveweight, forty-eight pigs of each genotype (24 barrows and 24 gilts) were selected and half of these were fed a diet with the same formulation as above except that it contained .90% lysine and 5% added beef tallow. The diets were calibrated to have equivalent lysine:calorie ratios (Table 1) and offered on an ad-libitum basis. At liveweights of 231, 264, and 297 lbs., four pigs of each sex x diet x genotype combination were slaughtered.

Results and Discussion

Growth Performance. Line 2 pigs had higher daily feed intakes and lower gain:feed ratios than Line 1 pigs (Table 2). There was no effect of genotype on average daily gain. Barrows had higher daily feed intakes and average daily gains than did gilts (Table 2). Gain:feed ratios decreased as slaughter weight increased (Table 3). Slaughter weight did not affect daily feed intake or average daily gain. Pigs fed 5% added beef tallow demonstrated higher gain:feed ratios and lower daily feed intakes than those with no added fat in their diets (Table 4). A sex x diet interaction was observed for daily feed intake, as the addition of dietary fat decreased the feed intake of barrows more than gilts. Average daily gain was also affected by a sex x diet interaction, as the diet which included 5% added fat increased the average daily gain of the gilts but had no effect on the average daily gain of the barrows (Table 4). No genotype x sex, genotype x slaughter weight, genotype x diet, sex x slaughter weight, or slaughter weight x diet interactions were observed.

Pork Quality and Carcass Composition. Gilts and Line 1 pigs had larger loin eye areas, higher percent fat-free lean, less 10th rib fat depth, and optical probe measurements of higher predicted percent lean, higher lean depth, and lower fat depth than barrows and Line 2 pigs, respectively. Line 1 pigs also had lower marbling scores and had less last rib backfat than Line 2 pigs (Table 2). There were no effects of genotype on color or firmness and no effects of sex on color, firmness, marbling or last rib backfat. As slaughter weight increased, last rib backfat, 10th rib fat depth, loin eye area, and fat depth (optical probe) increased while percent fat-free lean and percent lean (optical probe) decreased (Table 3). Slaughter weight did not affect color, firmness, marbling, or lean depth (optical probe). A genotype x sex interaction was observed for color scores, as barrows received higher color scores than the gilts in Line 1, and gilts had higher scores than barrows in Line 2. Loin eye area was also affected by a genotype x sex interaction, as the gilts of Line 1 had larger loin eye areas than the gilts of Line 2, whereas the loin eye areas of the barrows were not different between the two lines (Table 2). Supplemental dietary fat had no effect on carcass traits, nor were there any significant interactions of diet x genotype, sex, or slaughter weight.

Summary

The addition of 5% beef tallow has minimal influence on pork carcass composition and quality when equivalent dietary lysine:calorie ratios are maintained. However, growth performance was affected as the added fat diet decreased daily feed intake and increased the gain:feed ratio. Effects of supplemental fat on growth may be dependent on genotype. Further analysis of carcass composition is being conducted to determine changes in fat distribution and fatty acid profiles of different muscles and fat depots.

Table 1. Composition of Dietary Treatments.

^a Dry Polyanox®, contains ethoxyquint and BHT.
^b Trans 18 is a fatty acid added to trace the amount of fat directly synthesized from dietary fat.
^c Micro Aid contains 2 oz/ton active ingredient

Table 2. Least squares means of growth and carcass traits for genotype and sex.

^a These effects were significant at P<.05. G = Genotype, S = Sex
^b 1-5 NPPC Scoring System, evaluated on the loin at the 10th rib.
^c Estimated using NPPC (1991 equation) and adjusted to lean containing 0% fat.

Table 3. Least squares means of growth and carcass traits for slaughter weight.

^a This effect was significant at P<.05. WT = Slaughter weight
^b 1-5 NPPC Scoring System, evaluated on the loin at the 10th rib.
^c Estimated using NPPC (1991 equation) and adjusted to lean containing 0% fat.

Table 4. Least squares means of growth traits for sex and diet.

^a These effects were significant at P<.05. S = Sex, D = Diet.

Index of 1996 Purdue Swine Day Articles

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