A Review of Genetic and Nutritional Factors
Affecting Fat Quality and Belly Firmness

A. P. Schinckel*, S. E. Mills*, T. E. Weber*, and J. M. Eggert†
*Purdue University, Department of Animal Sciences
†Monsanto Choice Genetics, St. Louis, MO

Introduction

Genetic selection has been successful at increasing lean growth rate and carcass lean percentage. Pigs have been selected to have decreased daily feed intakes and to more efficiently convert feed to carcass muscle tissue growth (Schinckel, 1999, 2001). Fat tissue growth has been substantially reduced as a result of selection for improved lean efficiency. Consequently, genetic selection has altered the underlying biological and physiological properties of the fat tissue (Allen et al., 1976).

Numerous studies have examined subcutaneous fat tissue quality, and have identified problems with soft fat and separation of the fat from the lean in extremely lean carcasses (Lea and Swoboda, 1970; Wood et al., 1986, 1989; Warris et al., 1990). The most notable fat quality concern is belly firmness as soft bellies can result in bacon slicing difficulties. Bellies not suitable for processing into bacon go into trim which significantly reduces the carcass value for the pork processor.

The emphasis placed on reducing carcass fat has led to the fat quality concerns; however, the potential for continued genetic improvement in carcass leanness exists. Can carcass fat percentage be further reduced without undermining fat tissue quality? An understanding of the genetic and nutritional factors which impact fat quality may lead to useful approaches.
Factors influencing fat quality

The quality of fat associated with meat products can be described in physical, chemical and sensory terms. Figure 1 lists three major properties of fat tissue that influence quality and some of the important factors that affect these three properties. Physical properties of fat tissue including firmness, cohesiveness and color describe the textural role of fat in determining meat quality. The chemical composition of the fat refers to the protein, lipid, and moisture content of the fat tissue. The fatty acid profile refers to the fatty acid composition of storage triglyceride, and to a lesser extent, the composition of membrane phospholipids. Interest in chemical composition reflects consumer demand for products with reduced lipids and saturated fatty acids. Sensory properties of fat tissue refer to palatability characteristics such as flavor that fat imparts to meats.

Adipose tissue growth and development

Adipose cells develop from loose connective tissue, often likened to a collapsed sponge, which has numerous cavities present and is initially filled with a watery matrix material. Fat cells may be irregularly dispersed or densely packed, as seen in adipose tissue. Furthermore, it is the extent to which this loose connective tissue is occupied by fat cells which will determine the perceived firmness of the tissue.

Adipocyte size varies among the different fat depots of meat animals. For a given fat depot site, the increased in adipose tissue mass can be caused by different relative amounts of hypertrophy and hyperplasia. Generally, cellular development at a specific adipose tissue depot progresses from periods of primarily hyperplasia to primarily hypertrophy. The larger adipocytes have a greater lipid percentage and decreased water percentage.

During pig growth protein accretion eventually plateaus and fat tissue growth and lipid accretion exceeds lean tissue growth. Pigs of different genetic populations grow to different live weights to achieve a similar carcass composition. Some insight was achieved from the evaluation of middle layer subcutaneous backfat and the lipid-free carcass weight in three substantially different genetic populations of pigs (Allen et al., 1976). When pigs of different genetic populations were studied at a similar carcass composition (30.6 to 31.7% carcass lipid) at 37, 54, or 109 kg live weight, the genetic populations with a heavier lipid-free carcass mass had a greater number of adipocytes, but the ratio of adipocytes to lipid-free carcass mass was similar for all three genetic populations. The rate of increase in adipocyte volume was slowest for the leanest genetic populations. Thus, adipocyte volume and fat tissue composition are indicators of the stage of physiological maturity and potential to produce a heavy lean carcass.
In market pigs, nearly 85-90% of empty body fat is carcass fat, and most of this is subcutaneous (SQ) fat tissue. Intermuscular, intramuscular, and abdominal fat depots constitute a relatively small proportion of the total fat tissue in a market weight pig (Wood, 1984). The origin of lipids deposited in adipose tissue is illustrated in Figure 2. Fatty acids used for the formation of triglyceride of phospholipids are synthesized de novo from dietary carbohydrate or are contributed directly from dietary fat. The relative contribution of each of these two sources depends on the amount of fat in the diet. Typical corn-soybean meal rations are low in fat (3-4%) and it has been estimated that 80% of deposited triglyceride is derived from dietary carbohydrate (Anderson and

Kauffman, 1973) in older genetic populations of pigs with over 1.0 inch of backfat. The type and amounts of fatty acids synthesized from carbohydrate is reflected in the composition of adipose triglyceride. Linoleic acid (18:2) is not synthesized in the adipose cell and its presence reflects the contribution from the dietary lipid. Increasing the amount of dietary lipid will increase the proportion of tissue triglyceride contributed by dietary fatty acids by diluting the contribution from carbohydrate, and by inhibiting the de novo synthesis of fatty acids from carbohydrates. Dietary polyunsaturated fatty acids are the most effective inhibitors of de novo fatty acid synthesis and, therefore, vegetable oils may be expected to have a greater impact on fatty acid profiles than would dietary fats of animal origin (Clarke et al., 1990; Bee et al., 1999, 2002).

Development of the backfat layers

Subcutaneous fat is the major fat depot in pigs. The three backfat layers have distinct patterns of growth and should be considered as three separate tissues (Lee and Kauffman, 1974a, 1974b; Moody and Zobrisky, 1966; Fortin, 1986; Eggert and Schinckel, 1998). The outside layer is predominant at the lighter weights; while at heavier weights, the middle layer is predominant. The relative growth rate, the rate of growth divided by the current thickness, is greater for the middle and inner layers than the outer layer. The growth of the individual backfat layers is affected by sire breed and sex of the pig (Eggert and Schinckel, 1998). A comparison of individual backfat layer growth for Duroc sired barrows (fattest) and Large White-sired gilts (leanest) is presented in Figure 3. These curves demonstrate differences in the pattern of individual fat layer deposition both within and across genetic populations. Differences in the relative contribution of each layer to total backfat depth are also apparent.

Mersmann and Leymaster (1984) nutritionally manipulated growing pigs to study depletion, depletion followed by repletion, or deposition of the individual backfat layers. Obese and lean pigs were used to gain insight into the possible role of genetic selection for backfat depth on each backfat layer. Genetically obese pigs increased backfat depth by accretion of middle layer to a greater extent than outer. Genetically lean pigs had more equal accretion of the two layers with a preference toward the outer layer in some cases. Selection for total backfat depth appears to have selected middle layer in preference to outer. Outer layer changed to a small extent compared to middle layer during depletion, deposition, or genetic selection possibly indicating a greater metabolic role for the middle layer in vivo (Mersmann and Leymaster, 1984; Leymaster and Mersmann, 1991).

The relative thickness of the three layers of backfat may provide an indirect measure of the maturity of the fat tissue and firmness of the fat tissue. In a small trial, gilts of three genetic populations were reared using segregated early weaning procedures (Eggert et al., 1997). Two genetic populations, obtained from different seedstock sources, (1) Large White sires ? (Large White-Landrace) dam and (2) Pietrain sires ? (Large White-Landrace) dam, had comparable backfat thickness that represented the top 10% leanness for U.S. pigs. The third genetic population, synthetic terminal sires ? (Yorkshire-Landrace) dams, was a commercial terminal cross that represents average leanness for U.S. pigs.

Although genetic populations one and two were comparable in total 10th rib fat depth, their backfat was distributed between the individual layers disproportionately. Genetic population one (G1) had more outer layer backfat and less inner layer backfat when compared to genetic population two (G2; Table 1). The lesser amount of inner layer backfat seen in G1 is associated with softer middle layer backfat and a greater incidence of separation between the middle and inner backfat layers (Table 1). In attempting to attain leanness, G1 may have been selected for slower maturing backfat layers, which resulted in softer carcass fat.

Gilts of G2 had slightly higher marbling scores than did G1 gilts (Table 1), which was associated with having more inner layer backfat. Bellies of G1 gilts were thinner and less firm (Table 1). Thin, soft bellies can result in processing and slicing difficulties, which could make this lean line less desirable to pork processors.

Fatty acid profile of tissue lipids

The firmness or hardness of adipose tissue triglycerides is related to their average melting point, which is a function of several factors including fatty acid chain length, the number of double bonds and the positional and geometric configuration of the double bonds within the fatty acids (Deuel, 1951). With conventional feeding practices, the most important factor is the number of double bonds, which dramatically affects the physical characteristics of the lipids contained within the fat tissue. It is well recognized that dietary fatty acids are directly deposited into fat tissue triglycerides, and therefore, the final fatty acid profile reflects the relative contribution of each source (Figure 2).
The extent to which the fatty acid profile changes with age depends on diet as well as the pigs genetic propensity for fat accretion (i.e. endogenous synthesis). Therefore, it is reasonable to expect that pigs with greater or lesser rates of endogenous synthesis might have a greater or lesser contribution of dietary fat, and that fatty acid profiles will reflect these differences. Adipose tissue from pigs with less backfat, and presumably lower propensity for endogenous synthesis, contains a lower proportion of saturated fatty acids and a larger proportion of polyunsaturated fatty acids resulting in a higher calculated iodine value (IV). Within the same population of pigs, the leaner pigs have increasingly higher linoleic acid concentrations in the fat tissues (Figure 4).

Similar trends have been shown between genetically lean and obese pigs (Scott et al., 1983) and for pigs limit-fed vs. full fed (Wood, 1984).

The impact of dietary ingredients on IV of a specific tissue can be predicted by calculation of an iodine value product (IVP; Madsen, 1992; Boyd et al., 1997). The IVP is calculated as: (IV of the dietary lipids) ? (percentage dietary lipid) ? .10. In a recent trial, Boyd et al. (1997) fed five isocaloric diets ranging in IV and IVP values to modern U.S. terminal cross pigs (Table 2). A direct relationship was observed between increasing dietary IVP and increasing IV of the backfat layers and bacon fat increased (Table 3). The mean body fat IV values were regressed on the iodine value product on each diet. The resulting equation was backfat IV = 52.4 ? 0.315 (diet IVP). The equation allows for the prediction of the resulting backfat IV from commonly made diets composed of normal corn, high oil corn, and alternative types of added fat (Figure 6). The equation has good predictive ability within the range of dietary IVP’s used in the original trial. Diets containing low fat feedstuffs such as wheat and barley, will have lower IVP’s and result in firmer fat tissue with lower IV values. Lean pigs reared on barley-based diets could have fewer fat quality problems. Yet, the same genetic population reared on high energy, high IVP diets could have serious fat quality problems.


One alternative strategy is to feed less-saturated, low-cost dietary fats such as poultry fat or animal-vegetable blends in the grower phase and switch to low IVP diets during the finisher phase. The rate in which the fat quality improves is depot-specific and measured as the rate in which the fatty acid concentrations or IV values change per day or unit BW gain (Wiseman and Agunbiade, 1998). Fat depots with high relative growth rates such as the middle and inner backfat layers or belly fat will change more rapidly than the outer layer. In the same experiment Boyd et al. (1997) had one treatment (number 6) in which pigs were fed diets containing 3.7% 18:2 (diet 5) to 90 kg BW followed by a diet containing 1.9% 18:2 (diet 6) to 118 kg BW. The inner backfat layer and belly fat were affected by the dietary change to a much greater extent than the outer backfat layer (Table 3).

The impact of feeding a low IVP diet in the finishing stage will not have the same impact on pigs with different backfat thickness (Eggert et al., 1998a and 1998b). A research trial compared gilts of two genetic populations (.56 vs .85 in backfat at 250 lbs BW) fed diets containing 5% add poultry fat from 88 to 176 lbs and either 5% poultry fat or 5% beef tallow from 176 to 253 lbs live weight. A second trial looked at the impact of feeding beef tallow versus no added fat when a high IVP diet (10% added soybean oil) had been previously fed (Eggert et al., 1998b). A number of genotype ? diet interactions were found for the fatty acid composition of the different fat depots and belly firmness scores. Long term selection for leanness has altered the biology of adipose tissue in the pig. Differences in the effects of dietary treatments between genetic populations, and across depots, may be due to differences in gene expression or to differences in the timing and rates of depot growth. Lean pigs must be fed a more saturated fat from six to eight weeks after an unsaturated fat to produce a desired fatty acid profile (Gatlin et al., 2002).
Depending on the pork processor, the acceptable (tolerable) level of carcass fat IV differs. The acceptable IV threshold is 70 for the Danish (Barton & Gade, 1987; Madsen et al., 1992), while an IV value of 75 has been used as the threshold for U.S. pork producers (Boyd et al., 1997). The Swiss pork industry has taken the most proactive approach to the fat quality (Scheeder and Wenk, 1998; Scheeder et al., 1998, 1999; Glaser et al., 2001). In Swiss pork processing plants, a semi-automated method is used to estimate the number of double bonds in the outer layer of backfat. A fat score is established that has a high linear relationship with the IV value (IV value = 1.267 (fat score) – 12.26). Fat tissue is taken from each carcass and combined for the entire group. Fat scores of 60 to 62 are acceptable (approximate IV of 64). Fat scores of 62 to 64 (IV values of 66.3 to 68.8, respectively) are discounted .10 SFr per kg carcass weight. Carcasses with fat scores of 64 to 66 (IV values of 68.8 to 71.4) are discounted .40 SFr per kg and fat scores above 66 (IV values above 71.4) are discounted 1.0 SFr per kg carcass weight when the average price is about 4.2 SFr per kg. These discounts have led to careful evaluation of the diets fed to pigs and rapidly improved fat quality. The percent unacceptable declined from approximately 50% during 1987 through 1989 to less than 10% in 1998.

Composition of fat tissue

Because adipose tissue is a specialized connective tissue it is reasonable to expect that adipose tissue with low triglyceride content will have high water content. Wood (1984) has shown the period of rapid lipid accumulation in the pig during suckling is accompanied by dramatic reduction in SQ water concentration from 90% to 20%. Increased accretion of lipid post-weaning further alters the composition of adipose tissue to nearly 85% lipid and less than 10% moisture. It is also reasonable to expect that adipose tissue with greater fat and less water would appear more firm and cohesive than adipose tissue with less fat and more water. Data presented in Figures 6 and 7 illustrate that pigs with less total backfat have adipose tissue which contains less total lipid and more water. The significance of this is that the low fat quality scores associated with lean pigs may result from both increased concentrations of linoleic acid and tissue water.


Fat tissue grows via a combination of hyperplasia and hypertrophy. As pigs mature, cellular development progresses from periods of larger percentage increases in cell number (hyperplasia) to larger increases in cell volume (hypertrophy). As adipocyte cells fill with lipid, approaching 200 µm in diameter, a signal is given to recruit a new population of small adipocytes. This creates a biphasic adipocyte diameter distribution. There is a strong relationship between mean fat cell size and fat tissue lipid percentage (r = -.81, Allen et al., 1976).

The percentage of lipid contained within the fat tissue increases and then tends to plateau (Stant et al., 1968; Hiner, 1971; Metz, 1980). A review of the literature indicates that over the last 35 years, the lipid percentage of the fat tissue has decreased and the live weight in which lipid percentage plateaus may have decreased. In 1968, pigs were evaluated that had .87, 1.22, and 1.65 inches of backfat thickness at 100, 150, and 200 lbs live weight with lipid percentages averaging 79.6, 82.1, and 84.1%. The pigs evaluated by Hiner et al. (1971) had 83.7, 85.1, and 85.1% lipid in their dissected carcass fat tissue at 175, 225, and 275 lbs live weight. In pigs averaging 1.2 inches backfat and 43.9% fat-free lean, fat tissue moisture decreased from 40% at 55 lbs live weight to 23.3, 21.2, and 18.6% at 220, 252, and 284 lbs live weight (Schinckel et al., 2001). Lipid percentage of the dissected carcass fat tissue increased from 51% at 55 lbs to 72.0, 74.5, and 77.4% at 220, 252, and 284 lbs live weight. In a recent trial, barrows averaging .73 inches backfat and 45.7% fat-free lean at 249 lbs live weight had 63.0% lipid within their dissected carcass fat tissue (Schinckel et al., 2002). These trends may indicate that genetic selection for decreased backfat thickness has indirectly resulted in slower maturing backfat tissue.

Maturity of connective tissue

Collagen is the major protein forming the fibers in connective tissue and its role in imparting structural integrity and strength to muscle is well characterized (Bailey and Light, 1989). The role of collagen and connective tissue in determining the structural integrity of adipose tissue, and the connections made between the different layers of adipose tissue and between the adipose and muscle tissues is less well understood. A compromise in the integrity of any of these connections may translate into tissue that is soft, floppy or less cohesive.

One example, which illustrates the role of connective tissue in tissue integrity, is the differences seen in the various layers of subcutaneous fat. When subcutaneous fat is mechanically stretched, it is readily apparent that the integrity of the outer layer of fat (nearest the skin) is maintained while the layers adjacent to and attached to the muscle tissue are much more loosely organized. These differences in integrity between layers may be associated with connective tissue differences. The degree of saturation of adipose depots in a pig increases from exterior to interior, such that the outer later of backfat is the least saturated, and has the highest iodine value (IV; reflects the concentration of unsaturated fatty acids) and lowest melting point of all depots. Based on fatty acid composition, the outer layer should be the softest layer. The outer layer, however, has a greater collagen content than the middle layer (Table 4). Therefore, it seems likely that the amount, and perhaps degree, of crosslinking of the collagen contributes to the resilient character of the outer layer.

Genetics

There are few genetic parameters for the fatty acid concentrations in combination with meat quality traits. Cameron and Enser (1991) determined pork quality on 176 lb Duroc and Landrace boars and gilts. The heritability of the fatty concentrations were high ranging from .53 to .71 (Table 5). As found previously, the highest phenotypic correlations (Table 6) were those of 18:2 concentration with fat firmness (-.50), fat moisture (.41), subcutaneous fat mass (-.52), and P2 fat depth (-.52). The highest genetic correlations were those at 18:2 with fat firmness (-.85), fat moisture (.50), subcutaneous fat mass (-.74), and P2 fat depth (-.71). Selection for reduced fat depth and increased carcass lean percentage would result in increased 18:2 and moisture concentrations and decreased fat tissue firmness (Table 7).

Swiss researchers noticed that leaner pigs had higher concentrations of 18:2, IV, and a lower lipid percentage within the fat tissue (Schwörer et al., 1995). Heritability estimates for the fatty concentrations ranged from .39 to .98 (Schwörer et al., 1988, 1990). The heritability values for 18:2 concentration ranged from .40 to .64 (Table 8). The genetic correlations of 18:2 concentration of four fat depots (outer layer, inner layer, belly fat, and lean fat) with the mass of superficial fat ranging from –.54 to –.84 and with carcass premium cut percentage ranging from .41 to .84. The genetic correlations of 18:2 percentage of backfat with carcass fat percentage (-.90), and backfat weight (-.81) were similar in magnitude (Table 9). The results indicated that selection for increased premium cut percentage would result in increased 18:2 concentrations in all fat depots.

Reducing equivalents in the form of reduced nicotinamide-adenine-dinucleotide-phosphate (NADPH) are required for the de novo synthesis of fatty acids (Langdon, 1957). Müller (1985) analyzed the effects of selection for lipogenic enzymes in backfat of pigs on carcass traits. The selection criterion used was low or high activity of NADPH-generating enzymes (sum of G-6-PDH, 6-PGDH, NADP-ICDH, and NADP-MDH) in backfat. A third line was selected for low backfat thickness based on ultrasonic measurement. There also was a control line. After eight generations of selection, for high or low enzyme activity, the difference between the selection lines was found to be about 2.5 phenotypic standard deviations for the selection criterion (enzyme activity) and about 3.6 phenotypic standard deviations for backfat thickness with the higher level in the high-enzyme activity line. In all measured traits related to adipose tissue deposition, the levels of control pigs were between those of the low- and high-enzyme activity lines. In the low backfat thickness line, nearly all biochemical parameters were similar to those of the low-enzyme activity line. These data indicate that selection for decreasing backfat thickness indirectly results in reduced activity of the NADPH-generating enzymes required for synthesis of fatty acids.

Cliplef and McKay (1993) selected for reduced backfat thickness and increased growth rate in Yorkshire (eight generations) and Hampshire (seven generations) pigs. Selection resulted in less backfat, longer carcasses, and higher percentages of dissectible lean in the fresh hams. Carcasses from selected pigs also had a higher incidence of severe loin lean-fat separation, subjectively perceived softness in the subcutaneous fat, and thinner bellies. Adipose depots of pigs in the selection groups contained less saturated fat than their respective controls.

Selection for improved feed efficiency favors pigs which have a lower lipid percentage in their body fat tissues. The fat-free lean muscle growth and fat growth would be expected to have similar live weight growth, backfat thickness and loin eye size. A 10% reduction in the lipid content of the fat tissue of current high lean gain U.S. terminal cross pigs, carcass fat-free lean growth rates of .83 lb/d, and total carcass fat tissue growth of .59 lbs/d will reduce lipid accretion from 50 to 250 lbs by seven lbs and feed intake by 24 lbs. Since the percent moisture of the fat tissue is a moderately heritable trait (.27; Table 5), it is likely that selection for improved feed efficiency will tend to reduce the percent lipid within the fat tissue.

Conjugated Linoleic Acid

Conjugated linoleic acid (CLA) is a collective term to describe a group of positional and geometric isomers of linoleic acid (18:2). Dietary CLA increases the level of saturated fatty acids and firmness of the fat tissue while slightly decreasing the overall carcass fatness (Eggert et al., 2001). Dietary CLA increases the concentration of saturated fatty acids (14:0, 16:0 and 18:0) and reduces the concentration of 18:1 and 18:2 fatty acids (Eggert et al., 2001; Ramsey et al., 2001; Smith et al., 2002). Typically, CLA oil containing 60% active CLA isomers is fed at 1.0 to 2.0% of the diet.

Schinckel et al. (2000) fed CLA to two genetic populations of pigs. CLA reduced backfat thickness to a greater extent in the genetic population with greater backfat thickness (Table 10). The first genetic population (G1) was produced by mating Large White terminal sires to Large White-Landrace females. The second population (G2) was produced by mating a synthetic terminal sire line to (Large White-Duroc ? Large White-Landrace) crossbred females. The diets consisted of a conventional corn-soybean meal diet supplemented with either 1.0% CLA oil (containing 60% CLA isomers) or 1.0 % sunflower oil.

Bellies were removed from the carcasses and subjectively graded for firmness. A score of three was assigned to the firmest bellies that maintained a flop angle of greater than 120o. A firmness score of one was designated for the softest bellies, which yielded a flop angle of less than 60o. Any intermediate bellies were recorded as a firmness score of 2.
Supplemental CLA reduced midline last rib backfat thickness (1.01 vs. 1.13 in; P < .01) and 10th rib backfat depth (.64 vs .73 in; P < .01). Dietary CLA improved color (2.28 vs 2.02; P < .01), firmness (2.22 vs 1.98; P < .01), and marbling scores (1.96 vs 1.63; P < .01). Dietary CLA substantially increased belly firmness scores (2.63 vs 1.93; P = .02) and predicted fat-free lean (54.85 vs 53.26; P = .02).
Carroll et al. (1999) randomly assigned high-lean gilts (n = 224) in a 4 ? 3 factorial arrangement of treatments based on corn type and duration of CLA supplementation. CLA treatments included no CLA (1% sunflower oil), finishing with 1.0% CLA oil from 195-256 lb (CLA1), and finishing with 1.0% CLA oil from 144-256 lb (CLA2). CLA oil contained 60% conjugated linoleic acid.

Pork quality scores tended to increase as CLA duration increased (Table 11). Loin muscle area and percent fat-free lean tended to increase and 10th rib fat depth tended to decrease as the duration of CLA supplementation increased, but the results were not statistically significant
(P > .10). CLA supplementation demonstrated a drastic improvement in belly firmness (P < .01). CLA decreased the thickness of the outer fat layer (P < .05), but did not affect the inner or middle layers or the overall 10th rib fat depth.

Weber et al. (2001) investigated the combined effects of CLA, ractopamine, and added dietary animal fat on fatty acid profiles and belly firmness (Table 12). Lean terminal cross gilts (n = 180) were assigned to a 2 ? 2 ? 3 factorial consisting of dietary CLA, ractopamine, and supplemental fat. Conjugated linoleic acid and dietary fat treatments lasted eight weeks and the ractopamine treatments (0 or 9 g/ton) were assigned four weeks prior to the end of the trial. The dietary fats were either 0% added animal fat or 5% choice white grease or beef tallow. The fat tissues from 72 pigs were analyzed for fatty acid composition, six per CLA ? dietary fat ? ractopamine treatment at the end of the trial.

Subjective belly firmness scores were assigned by placing bellies over a horizontal bar
(1 = soft, 5 = firm). Objective scores were assigned to the bellies by measuring the distance between the anterior and posterior ends of the belly when suspended over a horizontal bar.
Dietary CLA caused the lipids contained in the carcass fat tissue to become more saturated. After eight weeks, gilts fed CLA had higher concentrations of saturated fatty acids (14:00, 16:00, and 18:00), decreased monounsaturated fatty acids (18:1n-9 and 18:1n7) and polyunsaturated (18:2n6) fatty acids. Supplementation with CLA lowered IV by 5.3, 5.1, and 6.9 points on the outer backfat layer, inner backfat layer, and belly fat respectively (Table 12).
The addition of 5% animal fat, either choice white grease or beef tallow had relatively smaller effects on fatty acid profiles than CLA. The addition of 5% animal fat resulted in slightly lower concentrations of saturated fatty acids (16:0 and 18:0) and slightly increased concentrations of monounsaturated fatty acids (18:1n-9, 18:1n7). The IV values were increased by approximately three points in the backfat belly fat after eight weeks of fat supplementation (Table 12). For the inner layer of backfat, significant CLA by dietary fat interactions existed for total saturated fatty acids, total mono unsaturated fatty acids, total unsaturated fatty acids and IV value.
Dietary ractopamine, fed for four weeks, had very little impact to decrease the concentration of saturated fatty acids and increase the concentration of polyunsaturated fatty acids. Ractopamine increased the IV values of the carcass fat tissues by approximately 2 to 2.5 points (Table 12).

Summary


Continued selection for increased carcass leanness is expected to result in an increased incidence of fat quality problems. Selection for increased leanness will result in fat tissues with greater moisture content, lower lipid content, and increased concentration of unsaturated fatty acids, especially linoleic acid. Maturation of the fatty tissue has likely been delayed as a result of genetic selection for carcass leanness. The role of connective tissue maturity on fat firmness and cohesiveness has not been evaluated.
The moderate to high genetic correlations of 18:2 concentration with backfat thickness
(rg = -.50 to –.80) indicate that concurrent selection for increased fatty acid saturation and carcass leanness would result in substantially reduced rates of genetic improvement for the individual traits.
Genetically lean pigs are more sensitive to the type and amount of dietary fat. High fat diets and ractopamine will result in fat tissues with slightly higher IV values. Dietary CLA is a potential nutritional means to improve fat quality in genetically lean pigs.

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