282 K
. Kolstad Livestock Production Science 67 2001 281 –292
have resulted in a lower marbling score, and poorer lar fat Wood and Cameron, 1994. Also included
meat quality compared to less intensively selected was a crossbreed LLP between Norwegian Land-
breeds Gregor and Scholz, 1993; Cameron et al., race L and a selection line LP selected for high
1999. backfat and low growth rate Vangen, 1979. The LP
Differences between breeds of pig are found in selection line pigs have a backfat thickness of
amounts of internal fat Kolstad et al., 1996. approximately 31 mm at slaughter Vangen, 1980.
Internal fat depots do not influence carcass quality. Comparing amounts and distribution of fat in those
They will, however, influence the efficiency of meat genetic groups may help us to understand the
production. Whole body scanning is necessary to background of genetic variation in these traits, and
detect internal depots in live animals. indicate potentials for genetic changes.
Computer tomography CT can be used for detailed measurement of body components and fat
2.2. Experimental design, housing, feeding and distribution on live animals Afonso, 1992; Vangen
weight recording and Thompson, 1992; Kolstad and Vangen, 1996;
Kolstad et al., 1996. Earlier studies based on CT A total of 141 pigs were included in the experi-
observations on different breeds of pigs by Luiting et ment representing the three genetic groups Landrace
al. 1995, Kolstad and Vangen 1996, and Kolstad 63, Duroc 62 and LandraceLP 16. For the
et al. 1996 proved breed differences in mainte- experiment, two males and two females were select-
nance requirements, fat distribution and fat mobilisa- ed at random from each litter. The experiment
tion in pigs at about 60 kg live weight, i.e. when the included four batches, the first one starting in the
pigs grow most rapidly. The aim of the present study autumn 1996, and the last one ending in the spring
was to examine development of different fat depots 1998. All batches included all three genetic groups
in three genetic groups of pigs by repeated measure- and both sexes except the last batch where LLP was
ments of body composition by CT within animals not represented.
from weaning to slaughter, in order to increase the The animals entered the experiment at weaning 5
understanding of the background of genetic variation weeks of age. The animals stayed in mixed groups
in amounts and distribution of fat in pigs at different of about six during the whole experiment, with free
live weights, and indicate potentials for genetic access to water. The indoor temperature was kept at
changes. about 208C.
From weaning to 25 kg live weight, the animals were group fed. Piglets in the first two batches were
2. Material and methods fed ad libitum, or 80 of ad libitum during this
period. In the two last batches, all animals were fed 2.1. Animals
ad libitum, but with feed differing in lysine content 0.8 and 1.0. Genotypes and sexes were equally
Pigs of the two breeds Norwegian Landrace and represented in each of the feeding strategies. The
Duroc were included in the experiment, as they are effects of feeding level and lysine content will be
known to differ considerably in growth, body com- dealt with in another paper from the experiment.
position and fat distribution, partly because of differ- Those effects were, however, adjusted for in the
ent selection history. The Norwegian Landrace is a statistical analysis.
highly efficient breed which has been intensively From 25 to 105 kg live weight, the animals were
selected for leanness and rapid gain for 35–40 hand-fed ‘to appetite’ twice a day a standard feed
generations Vangen and Sehested, 1997; Norsvin, used for commercial slaughter pigs, with an energy
1999. The Duroc population in Norway is of content of 12.03 MJ ME per kg of feed and 12.3
Canadian origin, and its limited size has, until digestible protein. Excess feed was registered daily,
recently, not allowed for intensive selection. Duroc and individual net feed intake calculated. All animals
pigs are known for their high content of intramuscu- were weighed weekly during the whole experiment,
K . Kolstad Livestock Production Science 67 2001 281 –292
283
and slaughtered when they reached 105 kg live Tissue density 5 1.0062 1 mean tissue
weight. Hounsfield unit value 3 0.00601
2.3. Computer tomography The sum of all tissue weights between the first and
last CT image in each animal is referred to as Computer tomography CT was used for measur-
CT-weight in the following, while the sum of all fat ing changes in body composition during the experi-
depots between the first and last CT image is referred ment. The pigs were scanned at weaning | 10 kg,
to as total fat. at about 25, 50, 85 and 105 kg live weight, totaling
five times in each animal, except for the last batch, 2.4. Statistical analysis
where technical problems limited the number of scannings to the three first live weights. The pigs
The main effects on amounts and proportions of were fasted for more than 16 h before scanning. The
fat depots were analysed using an animal model anaesthetic Azaperon 4 mg kg live weight was
including a relationship matrix model 1. The administrated by intramuscular injection followed by
ASReml software developed by Gilmour et al. a vascular injection of Phentotal sodium 5 mg kg
1999, was used to estimate fixed effects genetic live weight. This immobilised them for about 30
group, sex, batch, the covariate age and one of the min and minimised artefacts in the CT images due to
two covariates CT-weight or total fat and random movements of the animals. A series of cross-section-
effects animal, error, and to test for significance al images was collected throughout the body of the
under a general mixed model. Interactions were animal. The first cross-sectional image was taken at a
included in the model whenever they were significant position proximate to the femur tibia articulation.
P ,0.05. Thereafter, images were taken with a constant dis-
tance between them throughout the body until the Y
5 m 1 b W
1 b AG 1 B 1 S
ijklmn 1
ijklmn 2
ijklmn i
j
first cervical vertebra was reached. At 10 and 25 kg live weight this constant distance was 40 mm, while
1 T 1 F 1 A 1 e
1
k kl
im ijklmn
at 50, 85 and 105 kg live weight, the distance was 50 mm. A total of 20 to 25 images were taken on each
where: Y is a fat depot at a live weight;
m is the
ijklmn
animal, depending upon the length of the body. The general mean for Y
; B is the effect of genetic
ijklmn i
PC based CT image analysis program
CATMAN
group i 51,2,3; S is the effect of sex j 51,2; T
j k
Thompson and Kinghorn, 1992 was used to quan- is the effect of batch k 51,2,3,4; F is the effect of
il
tify areas of fat, lean, non-fat visceral components feeding system from 10 to 25 kg live weight within
NFVC and bone in each image as well as depots batch l 51,2; A
is the random additive genetic
im
within tissues. These components can easily be effect of animal m; b
and b are the regression
1 2
recognised in the image and recorded with high coefficients; W
is the effect of the covariate
ijklmn 2
precision R 5 0.85–0.95 Vangen, 1988; Afonso, CT-weight or total fat weight; AG
is the effect
ijklmn
1992; Jopson et al., 1995. of the covariate age; e
is the random error.
ijklmn
The total weight of each depot and tissue com- When considering amounts of fat in each depot
ponent was estimated from total volume and mean relative to CT-weight, model 1 was used including
density. Total
volume was
determined using
the covariate CT-weight. When fat distribution was Cavalieri’s principle by multiplying the sum of areas
to be considered, total fat between the first and last in all images with the distance between each image
CT image was included as a covariate, while CT- assuming a random sampling of parallel sections
weight was excluded. Allometric growth coefficients separated by a known distance Gundersen et al.,
b were estimated within each animal according to
b
1988. Mean density was determined from a function the function y 5aX
to describe the changes in relating Hounsfield unit value to tissue density
proportion of each fat depot relative to CT-weight Fullerton, 1980:
and total fat over the five repeated scannings. The
284 K
. Kolstad Livestock Production Science 67 2001 281 –292
coefficient b was estimated by regressing the growth coefficients for each fat depot from 10 to 105
logarithm of component weight y on the logarithm kg live weight relative to CT-weight and total fat
of live weight or total fat X : log y 5log a 1b log X. respectively.
3.1. Genotype differences in amounts of depot fat
3. Results adjusted for CT-weight
