Growth and Characterization of Individual Backfat Layers in Swine:
A New Method for the Prediction of Intramuscular Fat in the Live Animal or Carcass


J.M. Eggert and A.P. Schinckel
Department of Animal Sciences
Purdue University
West Lafayette, IN


Over the last three years, we have observed that pig genotypes differ not only in the total amount of backfat, but also in the relative amounts of the three individual backfat layers - outer, middle and inner. While it is apparent that these layers differ in chemical, structural, and textural properties, their relative contributions to carcass quality remain unknown. Furthermore, we have observed the presence of a well-developed innermost backfat layer to be associated with higher levels of intramuscular fat within the longissimus dorsi. This suggests that marbling is deposited concurrently with the innermost layer. Thus, the presence of a growing innermost backfat layer may be indicative of a developmental and energy state in which a pig deposits intramuscular fat. Monitoring fat depot development may provide insight into the relationships between individual fat layers and carcass quality. In this trial we describe the growth of individual backfat layers for three genotypes of pigs and use individual backfat layer thickness to predict intramuscular fat.

Materials and Methods

Three genotypes that have different rates of fat and lean growth were identified. 120 barrows and gilts were randomly assigned (4 pigs/pen basis) to a 3 x 2 factorial arrangement with sire line and sex as main effects. The three genotypes consisted of different purebred sire lines - Duroc (D), Pietrain (P) and Large White (W) - each crossed with the same dam line (Large White x Large White - Landrace). Pigs were reared using segregated early weaning procedures and fed conventional corn-soybean meal diets on an ad-libitum basis. Individual live weights and ultrasonic measurements of backfat and loin-eye area were collected biweekly from 50 lbs. live weight until slaughter. Data on growth traits was collected for the last 100 lbs. of finishing growth (164 - 264 lbs.). At 264 lbs. live weight, eight pigs from each sire line x sex combination were transported to the Purdue Meat Laboratory for slaughter, tissue collection and carcass evaluation. At 24 hours postmortem, standard carcass measurements such as backfat depths, loin-eye area, and subjective loin-eye quality (color, firmness/wetness and marbling) were taken. Standardized loin slices were obtained for evaluation of drip loss and intramuscular fat.

Prediction of intramuscular fat

In an effort to predict % intramuscular fat using data that is obtainable from either live animals or carcasses, stepwise regressions were conducted. Regressions were conducted for all pigs and for each sire line. Dependent variables included: slaughter weight (W), loin-eye area (L), total 10th rib backfat (T), outer layer 10th rib backfat (O), middle layer 10th rib backfat (M), inner layer 10th rib backfat (I), O/T, M/T, I/T, M/O, M/I, and the quadratic form of each of these terms (W2, L2, etc.). In addition, allometric functions were determined by regressing each variable onto % intramuscular fat (Wa, La, etc.). All regressions were limited to four terms.

Results and Discussion

Growth and carcass traits

From 165 to 264 lbs. live weight, Duroc-sired pigs demonstrated greater average daily gain and greater feed intake than did Pietrain- and Large White-sired pigs (Table 1). Duroc-sired pigs had lower loin weights, higher % intramuscular fat, received higher marbling scores of the loin muscle at the 10th rib, and had more middle layer, inner layer and total fat depth at the 10th rib than did Pietrain- and Large White-sired pigs (Table 1). Pietrain-sired pigs had larger loin-eye areas and less outer layer backfat than did Duroc- or Large White-sired pigs (Table 1). Sire line did not affect feed efficiency, 24-h postmortem pH (longissimus dorsi), drip loss, loin color or loin firmness evaluations.

Growth curves

The growth curves for total 10th rib backfat and outer, middle, and inner backfat layers are presented in Figures 1-4. As expected, these graphs demonstrate differences between the genotype x sex combinations of pigs for the amount of total backfat and individual backfat layers. However, these graphs further demonstrate different patterns of fat growth for different types of pigs. The outer layer appears to grow linearly for most pigs, while the middle layer grows at an increasing rate. Most notable is that inner layer fat growth is approaching a plateau for Pietrain-sired barrows and Large White-sired gilts while all other pigs are depositing inner layer backfat at an accelerating rate.

A comparison of individual backfat layer growth for Duroc-sired barrows (fattest) and Large White-sired gilts (leanest) are presented in Figure 5-6. These curves demonstrate differences in the pattern of individual fat layer deposition both within and across genotypes. Differences in the relative contribution of each layer to total backfat depth are also apparent.

Table 1. Effects of sire line on finishing growth (160 - 260 lbs.) and carcass composition
Trait
Duroc-sired
Pietrain-sired
L. White-sired
SE
Sig.*
Average daily gain (lbs.)
1.93
1.68
1.69
0.05
P < .01
Average daily feed intake (lbs.)
6.32
5.58
5.56
0.11
P < .01
Feed efficiency
3.28
3.36
3.29
0.07
Not sig.
Adjusted days to 269 lbs. (NSIF)
167.6
174.6
176.7
1.63
P < .01
10th rib fat depth, total (in.)
1.12
0.78
0.96
0.05
P < .01
10th rib fat depth, outer layer (in.)
0.37
0.31
0.36
0.01
P < .01
10th rib fat depth, middle layer (in.)
0.49
0.31
0.41
0.03
P < .01
10th rib fat depth, inner layer (in.)
0.27
0.16
0.17
0.02
P < .01
Loin eye area (sq. in.)
6.84
7.59
6.78
0.21
P < .05
Loin color **
2.78
2.72
2.69
0.08
Not sig.
Loin firmness **
2.81
2.59
2.59
0.15
Not sig.
Loin marbling **
2.41
1.25
1.41
0.14
P < .01
Loin ultimate pH (24-hr.)
5.45
5.45
5.48
0.02
Not sig.
Loin drip loss (%)
2.94
3.42
3.59
0.43
Not sig.
Loin weight (lbs.)
6.03
6.88
6.58
0.14
P < .01
Loin intramuscular fat (%)
3.05
1.77
1.63
0.18
P < .01
* Not sig. = not significant, p >0.05. ** NPPC Scoring System.
Color: 1 = pale, pinkish gray, 5 = dark, purplish red.
Firmness: 1 = very soft and very watery, 5 = very firm and dry.
Marbling: 1 = devoid to practically devoid, 5 = moderately abundant or greater.

Prediction of intramuscular fat

A summary of significant terms, r2 and residual standard deviations (RSD) for stepwise regressions are reported in Table 2. The RSD from the genotype-specific equations were smaller than the RSD from the overall equation. Table 2 also demonstrates that different terms are significant for each genotype. Backfat layers growing more rapidly at the time of slaughter appear to contribute more to the prediction of % intramuscular fat.

Table 2. Residual standard deviations (RSD) for the prediction of % intramuscular fat
Group
RSD
r2
4 most significant terms
All pigs (n=48)
.67 %
.60
I
Wa
(O/T)2
La
Duroc-sired (n=16)
.53 %
.82
Ta
Ia
(M/I)2
Ma
Pietrain-sired (n=16)
.36 %
.83
L
O/T
Ta
(O/T)a
Large White-sired (n=16)
.38 %
.50
(M/O)a
Ia
I
I2

Future Objectives

This trial demonstrates that by considering individual backfat layer thickness, % intramuscular fat can be accurately predicted for live animals and carcasses. However, the results of this trial (n=48) need to be repeated on a larger scale. This trial evaluated the phenotypic relationships between the thickness of individual backfat layers at slaughter and % intramuscular fat. Because phenotypic relationships may be due to both genetic and environmental correlations, future research must focus on determining the genetic relationship between individual backfat layers and % intramuscular fat.

As hypothesized, individual backfat layers appear to grow at different rates and to be affected by both genotype and sex. These findings will contribute to the growing body of data that suggest each backfat layer should be considered as a separate tissue. Should further studies determine that a certain layer is more closely related to pork quality or production traits, the potential for differential selection of each backfat layer exists. Thus, it may be possible to improve traits that are negatively affected by leanness without increasing total backfat.