Date Published: November 7, 2007
Publisher: BioMed Central
Author(s): Fintan J McEvoy, Anders B Strathe, Mads T Madsen, Eiliv Svalastoga.
The thickness of the subcutaneous fat layer is an important parameter at all stages of pig production. It is used to inform decisions on dietary requirements to optimize growth, in gilts to promote longevity and finally to assist in the calculation of payments to producers that allow for general adiposity. Currently for reasons of tradition and ease, total adipose thickness measurements are made at one or multiple sites although it has been long recognized that up to three well defined layers (outer (L1), middle (L2), and inner (L3)) may be present to make up the total. Various features and properties of these layers have been described. This paper examines the contribution of each layer to total adipose thickness at three time points and describes the change in thickness of each layer per unit change in body weight in normal growing pigs.
A group of nine pigs was examined using 14 MHz linear array transducer on three separate occasions. The average weight was 51, 94 and 124 kg for each successive scan. The time between scanning was approximately 4 weeks. The proportion of each layer to total thickness was modeled statistically with scan session as a variable and the change in absolute thickness of each layer per unit change in body weight was modeled in a random regression model.
There was a significant change in ratios between scans for the middle and inner layers (P < 0.001). The significant changes were seen between the first and second, and between the first and final, scan sessions. The change in thickness per unit change in body weight was greatest for L2, followed by L1 and L3. These results demonstrate that subcutaneous adipose layers grow at different rates relative to each other and to change in body weight and indicate that ultrasound can be used to track these differences.
Measurements of subcutaneous adipose tissue are used in decision making during pig production for optimal growth, for longevity in gilts and for quality control and carcass classification post mortem [1-4]. Typically these measurements are made using ultrasound. Transducer frequencies of 3.5 to 7 MHz are reported for this application with data displayed as an image for B-mode (brightness mode) and as a number or numbers indicating either the total adipose thickness or the thickness of individual layers for A-mode (amplitude mode). The scan site and the use of a depth measurement that includes all fat layers are historically based as these sites and parameters were measured either by palpation or by sharp dissection prior to the advent of the use of ultrasound.
Satisfactory ultrasonograms were obtained from all pigs in the study. A typical image is shown in Figure 1. The image shows that despite the excellent imaging capabilities of the machine used, differentiation between the edge of the outer aspect of L1 and skin is diffcult for the eye to identify. Differentiation here is based on the tissue structure, which for skin, is more uniform than for L1. The middle layer (L2) is composed of uniform hypoechoic tissue, producing few internal reflections. It has a sharp boundary with the overlying L1 and the underlying L3. Being hypoechoic (dark on the image) it contrasts well with the hyperechoic tissue of L1 and L3. This contrast with adjacent tissue and its sharp margins render L2 as well defined and easily recognized. L3 is readily identifiable. This layer contains a series of internal hyperechoic linear structures together with hypoechoic tissue. Being hyperechoic it contrasts well with L2 and also with the hypoechoic muscle fibers beneath. Its linear striations result in sharp edges, L3 is thus readily differentiated from adjacent tissues.
Meat quality is a function of the interplay between multiple variables and is of ongoing concern to pig producers, meat processors retailers and consumers alike . Indicators of meat quality include pH, tenderness, intramuscular fat percentage and color. However the production of animals with high overall fat content is inefficient and is financially penalized as farmers are generally paid by weight after adjustments for the total body fat present are made. This has resulted in steps by the industry to optimize efficiency which include selecting for decreased backfat thickness. This in turn has lead to the production of meat with reduced palatability due to decreased fat content within the muscle .
This study indicates that during growth, the middle and inner subcutaneous adipose layers change in the relative contribution they make to total back fat thickness and the middle layer shows the greatest increase in thickness per unit body weight. Ultrasound monitoring strategies would be better devoted to measurement of these individual layers than to the measurement of total back fat thickness.
The author(s) declare that they have no competing interests.
FM conceived and participated in the design of the study, carried out the ultrasound examinations and drafted the manuscript. AS performed the statistical analysis. MM participated in the design of the study and provided feeding and management protocols for the animals used. ES participated in the design of the study. All authors read and approved the final manuscript.