Animal Welfare Information Center Newsletter, Winter 1995/1996, Vol. 6 No. 2-4
Selection for Improved Efficiency of Lean Gain in Mice:
Population and Procedures
by
R. B. Holder and W. R. Lamberson,
Department of Animal Sciences, University of Missouri, Columbia, Missouri
Cost of feed for the slaughter animal accounts for about 45
percent of the total costs of producing lean tissue from the
swine herd. It represents the greatest single economic input in
swine production (9). Efficiency of feed utilization and
percentage lean in the carcass are traits over which there is
substantial genetic control (2,7). Improvement of feed
conversion to lean by 5 percent would be expected to decrease the
cost of producing a slaughter pig by nearly $2. Direct emphasis
on the trait in selection programs has been limited because of
difficulty in making individual measurements of feed intake and
lean percentage. In addition, selection on a trait defined as a
ratio (feed/lean gain) may not yield optimum response in the
components (3). In past studies in which feed:gain ratio was a
selection criterion, improvement in efficiency of feed
utilization resulted from decreased intake with little, if any,
improvement in lean gain. Decreased feed consumption limits
overall productivity of the animals and may also limit long-term
response to selection.
Laboratory animals have often been used as a model species for
genetic studies of swine because of their relatively low cost and
rapid generation turnover (5). Previous studies of selection for
lean efficiency in laboratory animals have relied on family
selection because of lack of an efficient method of estimating
body composition of the live animal (6). Advances in technology
for estimating body composition have yielded an accurate method
that can be applied to the live animal (10). Measurement of
total body electrical conductivity (TOBEC) allows prediction of
fat-free mass that is highly correlated to chemical composition.
Individually caging mice allows measurement of feed intake. The
objective of this study is to compare alternatives to selection
on the ratio of lean gain/feed intake on improvement of
efficiency of lean tissue deposition in mice.
Population
Experimental animals are outbred mice of the CF1 strain. Outbred
lines of mice are generally comparable to breeds of livestock.
The base population was produced by reciprocally mating CF1 males
and females obtained from two commercial sources. One generation
of random mating was practiced before selection was initiated.
Two replicates of five selection lines are included in the
experiment. Each line consists of 12 litters per generation
resulting from pair matings. Animals are mated at approximately
60 days of age. Four generations are expected to be produced per
year.
Figure 1. An anesthetized mouse is inserted into the
TOBEC instrument for measurement of electrical conductivity
subsequently used to predict fat-free mass.
Experimental animals are reared in litters standardized to four
male and four female pups. Number born, number weaned, litter
birth weight, and litter weaning weight are recorded. Litters
are weaned at 21 days of age and pups weighed. At 25 days of
age, mice are again weighed and placed in individual cages, and
recording of feed intake is initiated. Body weights and feed
intake are recorded at 31, 37, and 42 days of age. Intake is
defined as weight of feed placed in the cage minus that present
in the cage at the end of the recording period. Animals are fed
daily an amount expected to slightly exceed that consumed. At 45
days of age, mice are weighed and anesthetized by intraperitoneal
injection of 0.015 ml of 2.5 percent Avertin (4) per gram of body
weight. An EM-SCAN SA-2 instrument (EM-SCAN, Inc., Springfield,
IL; see sidebar) is used to obtain a measure of conductivity (E)
in triplicate (fig. 1). The mean of the three measures is
calculated and fat- free mass (FFM) estimated by using the
equation: FFM = -3.732 + 0.578 body weight + 2.967 E0.5. Previous
calibration of the instrument has yielded an R2 of 0.97 between
the TOBEC-estimated fat-free mass and chemical composition. The
relationship of predicted FFM and chemical composition of 79 male
and female mice is shown in figure 2.
Figure 2. Fat-Free mass predicted with the use of
TOBEC plotted against actual fat-free mass.
All mice have ad libitum access to a pelleted diet (23
percent crude protein and 4.5 percent fat; Lab Diet 5001, PMI
Feeds, Inc., St. Louis, Missouri) and distilled water. From day
17 of pregnancy until litters are weaned, females are housed in
polycarbonate cages measuring 28 x 17 x 12 cm. Litters remain in
these cages until 25 days of age. During the test period, mice
are individually housed in stainless steel hanging wire cages
measuring 24 x 10 x 13 cm. Animal rooms are maintained at 22° C
+/- 2° C with a relative humidity of 50 +/- 10 percent. The
light cycle is 12 hours light:12 hours dark.
Selection Criteria
Five lines are included in the selection experiment. A line in
which a male and female are selected at random from each litter
serves as a negative control. This line serves to measure
fluctuations and trends in the environment. A line in which the
selection criterion is weight of FFM gained divided by feed
disappearance (intake) between 25 and 42 days of age (gain/feed)
is the positive control line. This selection criterion represents
the standard criterion from past experiments. Three experimental
criteria represent alternatives to selection on the gain/feed
ratio. The first is intake deviation. Animals selected on this
criterion are those with the greatest negative deviation from the
regression line of intake on gain of FFM. This is equivalent to
selection on least intake after adjustment to a constant gain of
FFM. This criterion has been used for selection in the
commercial poultry industry. The second experimental criterion
is gain deviation. Animals selected on this criterion are those
with the greatest positive deviation from the regression line of
gain of FFM on intake. This is equivalent to selection on
greatest gain of FFM after adjustment to a constant intake. The
final criterion is denoted intrinsic efficiency (8). It is
similar to intake deviation except that adjustment is also made
for average FFM maintained.
Two replicates of each of the proposed criteria are included in
the experiment. Selection will be practiced for six generations.
Direct and correlated responses to selection will be measured as
regressions of line-generation means calculated as deviations
from controls on generation. Replication of lines allows tests
of significance to be performed using empirical standard errors.
Realized heritabilities and genetic correlations will be
estimated by regression of cumulative response on cumulative
selection differentials. Of particular interest is correlated
response in feed consumption.
Table 1. Correlations between experimental selection criteria and components of efficiency of lean gain.
| Variable | Intake | Gain/feed | Residual
gain | Residual
intake | Intrinsic
efficiency |
|---|
| Gain | 0.10 | 0.90 | 0.85 | 0.00 | 0.00 |
|---|
| Intake | | -0.35 | 0.00 | 0.90 | 0.90 |
|---|
| Gain/feed | | | 0.80 | -0.40 | -0.40 |
|---|
| Residual gain | | | | 0.00 | 0.00 |
|---|
| Residual intake | | | | | 0.96 |
|---|
Selection on any of the four positive selection criteria
described above would be expected to improve efficiency of feed
utilization by either increasing gain, decreasing intake, or a
combination of both. To determine the similarities among the
criteria, phenotypic correlations were calculated between
selection criteria and values rounded to the nearest 0.05
(table 1).
These correlations demonstrate the similarities and differences
among the selection criteria. In this population, gain is more
closely related to gain/feed than is intake. Residual intake and
intrinsic efficiency differ only by the adjustment for average
weight maintained in intrinsic efficiency, and the correlation
between these criteria is high. Since both use intake after
adjustment to a common gain as part of the criterion, the
correlation of each with gain is near zero and their correlation
with intake is high. Conversely, the correlation of residual
gain with intake is zero as it uses gain adjusted to a common
intake as the selection criterion. Its correlation with gain is
high.
Table 2. Descriptive statistics and standardized selection differentials by sex and line for generations one and two of selection.
| Generation 1 |
|---|
| | Mean | Standard deviation | Selection differential |
|---|
| Line | Female | Male | Female | Male | Female | Male |
|---|
| Gain/feed | 0.08 | 0.10 | 0.04 | 0.04 | 0.65 | 0.13 |
|---|
| Residual gain | 7.9 | 10.3 | 2.4 | 3.1 | 0.50 | 0.44 |
|---|
| Residual intake | 108.1 | 116.5 | 27.8 | 26.7 | 0.62 | 0.62 |
|---|
| Intrinsic efficiency | 112.4 | 118.3 | 21.5 | 23.1 | 0.51 | 0.54 |
|---|
| Generation 2 |
|---|
| Gain/feed | 0.09 | 0.12 | 0.04 | 0.03 | 1.03 | 0.96 |
|---|
| Residual gain | 9.2 | 12.7 | 3.0 | 2.2 | 1.30 | 1.39 |
|---|
| Residual intake | 98.0 | 103.0 | 13.0 | 14.8 | 1.25 | 1.14 |
|---|
| Intrinsic efficiency | 96.8 | 102.8 | 11.5 | 11.7 | 1.03 | 1.12 |
|---|
Descriptive statistics and standardized selection differentials
of the four selected lines for generations 1 and 2 are presented
in table 2. Standardized selection differentials tend to be
similar within generation and depend primarily on reproductive
rate. Low reproductive rate in generation 1 compared to that in
generation 2 resulted in lower selection differentials. The
expected selection differentials are expected to be about 1.25
standard deviations (1).
Efficiency of lean gain is a trait of great economic importance
to the livestock industry. Efforts to improve this trait by
genetic selection have been hampered by difficulty in measuring
its components --feed intake and lean gain--and have required
inefficient family selection procedures. Recent improvements in
technology for measuring body composition no longer require
sacrifice of the animal to determine lean content. Use of TOBEC
allows rapid determination of fat-free mass of an animal that can
later be used for breeding. Alternative methods of individual
selection for improving efficiency of lean gain are evaluated
using mice as models for the livestock industry.
- Becker, W. A. (1984). Manual of
Quantitative Genetics. Academic Enterprises: Pullman, WA.
- Bereskin, B. (1990) Genetic Parameters
for Feed Efficiency. In Genetics of Swine, L. D. Young,
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(1986). Manipulating the Mouse Embryo. Cold Spring
Harbor Laboratory: Cold Spring Harbor, New York.
- McCarthy, J. C. (1982). The
laboratory mouse as a model for animal breeding: A review of
selection for increased body weight and litter size. In
Proceedings of the 2nd World Congress on Genetics Applied to
Livestock Production, C. L. deCuenca, ed., Vol. V:66, Madrid.
- Notter., D. R., G. E. Dickerson, and J.
A. DeShazer (1976). Selection for rate and efficiency of lean
gain in the rat. Genetics 84(1):125-144.
- Stewart, T. S. and A. P. Schinckel
(1990). Genetic Parameters for Swine Growth and Carcass Traits.
In Genetics of Swine, L.D. Young, ed., U. S. Meat
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- Taylor, St. C. S. and G. B. Young
(1964). Variation in growth and efficiency in twin calves.
Journal of Agricultural Science 62:225-236.
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- Walsberg, G. E. (1988). Evaluation of a
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and mammals. Physiological Zoology 61(2):153-159.
This article appeared in the Animal Welfare Information Center
Newsletter, Volume 6, Number 2-4, Winter 1995/1996
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