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The genetics of lean tissue growth in the pig |
 
Who saw this article? New!
Author: Dr. Mike Varley, SCA Nutrition Ltd, North Yorkshire, UK
The following article is a special collaboration
from AFMA (Animal Feed Manufacturers
Association) www.afma.co.za
We thank their kind support.
Introduction
Over the course of the last 30 years, the commercial hybrid companies worldwide
have generated selection responses in pig populations that have produced novel
breeding lines that grow faster, convert their food much more efficiently and
also have superior carcases compared to the pigs of the past. Pig improvement
has been exceptionally fruitful for the pig industry and significant economies
of production have accrued. In addition, the reduction in carcase fat levels and
the increased lean tissue accretion has gone a long way to providing consumers
with meat products that are both healthy products and also what was required by
consumers. Through the 1970's and 1980's genetic change moved further and further
into private hands from the public sector as the large scale commercial genetics
companies successfully entered this arena. From the basic original European White
breed lines (Yorkshire, Large White, Landrace) that were the foundation of most
breeding programmes, inevitably the specialised sire and dam lines have changed.
This has also led to a different level of nutritional requirements for each of
these different lines and hence nutritionists and geneticists have had to work
closer together. The purpose of this paper is to focus on these changes and to
review knowledge in this area.
Genetic Change
A major factor determining the profitability of a pig enterprise is lean tissue
growth and this is genetically correlated to lean tissue food conversion efficiency.
In practice the selection criteria that are principally used are daily liveweight
gain, food conversion ratios, ultrasound backfat and probably various slaughter
pig traits such as killing out (dressing) percentage, lean depth at the eye muscle,
length, and perhaps some meat quality parameters. All of these traits are easily
and cheaply measured and also carry medium to high heritabilities. The pig also
has a high rate of reproduction (litter size = 10-12, 2.4 litters per sow per
year) and hence the deployment of very high selection differentials is possible.
Together with low generation intervals (12 months) this means that the rate of
genetic progress has been very high indeed. The Meat & Livestock Commission
(MLC) in the United Kingdom have estimated in the past from control herd population
data that the annual genetic progress in selected populations was about 30 points
change from a moving average base of 100 points. As a consequence, modern hybrid
pigs are totally different to the pigs of 30 years ago and the phenotypic differences
reflect this. Figures 1 and 2 illustrate from MLC Yearbook data, how pigs have
changed over the years 1970 to 1999.
Figure 1 - Phenotypic Changes in Hybrid
Pigs 1970 -1999
Figure 2 - Carcase Changes in Hybrid Pigs
Clearly the production efficiency and carcase traits continue to improve.
Over long time spans using very short generation intervals, it has been inevitable
that the various hybrid products have also diverged in characteristics.
Genetic Technologies
Genetic selection programmes have been based on large nucleus populations and
structured breeding pyramids with male and female lines to produce a first cross
female and a pure sire line; each with different qualities. The nucleus level
populations are often 10-20,000 females and the use of embryo transfer and artificial
insemination has enabled these nucleus herds to exist as breeding entities but
actually to be physically located in many parts of the world. Multiple trait selection
indices have been used to facilitate overall economic merit selection and more
recently BLUP (Best Linear Unbiased Prediction) technologies have taken this a
step further allowing varying environments and varying locations for nucleus level
selection. Massive computer power is utilised for this process and most companies
have their own in-house geneticists and statisticians to both design the programmes
and to develop the breeding lines.
Taking into account that the various companies started off with different foundation
stock and the differences in their selection objectives and weighting factors
used in their indices, it seems inevitable that after 25-30 years of selection
there will be significant differences in the basic phenotypic expression for these
commercial products.
The hybrid companies for obvious commercial reasons do not divulge the details
of their selection indices and each will place a different level of emphasis on
different selection objectives using various weighting factors. The outcome of
this has been that lean growth traits are likely very different in commercial
hybrid products and therefore nutritional requirements are also very different
Nutritional Requirements
The need for lean growth data and other information on specific hybrids stems
from the complexities that nutritionists are faced with in practice in formulating
diets for different growing and finishing pigs. This process is nowadays carried
out with great precision to utilise expensive feed ingredients. Feed programmes
are designed using knowledge of the growth potential and the derived lysine-calorie
ratios and available nutrient requirements. These parameters in some feeding systems
are altered on a weekly basis. It is however necessary to know the lean growth
characteristics for the specific genotype to achieve the accuracy required. Figure
3 illustrates the likely range in growth characteristics between unimproved genotypes
and highly selected lines. These growth curves are modeled on Gompertz equations
to give slaughter ages that are significantly different. If these lines were given
exactly the same diet programmes, there would be gross inefficiency and waste
of nutrients for some of the lines.
As pigs grow they change their body composition and lean tissue growth remains
relatively constant through a large portion of the growth curve to slaughter.
Lean tissue deposition requires 14 MJ Gross Energy for 1 kg tissue accretion whereas
fat deposition requires 49 MJ per kg of gain. As the animals grow and the rate
of fat deposition accelerates, then inevitably this changes dietary energy and
protein requirements.
Figure 3 - The range growth for modern
genotypes
Figure 4 presents data to illustrate
for an improved pig growing at about 850 g/d day from 25 kg to 90 kg showing how
energy and lysine/energy ratios change. These relationships however will change
gradually as the animals get leaner genetically and hence express higher lean
gains.
Commercial Evaluations
In the United Kingdom, the Meat & Livestock Commission have published limited
data to allow comparisons but there is still a dearth of hard facts. Close (1994)
has provided some analysis in relation to unimproved versus improved pigs using
commercial data from his own experience and this provides some insight into the
likely variation that exists in commercial practice. The MLC however in the mid
1990s embarked on an exercise using their own Stotfold Farm unit to evaluate the
growth characteristics of 4 hybrid products against a genetic control. This has
produced some valuable information for the industry although the published report
does not label the specific companies. Table 1 presents a sample of these data
to illustrate the observed range.
Table 1
Growth characteristics for 4 different hybrid products (MLC Stotfold Data)
|
Company
|
J
|
K
|
L
|
M
|
Control
|
| Daily Gain |
841 |
841 |
834 |
804 |
811 |
| FCR |
2.53 |
2.84 |
2.68 |
2.67 |
2.69 |
| P2 |
11.2 |
16.2 |
12.8 |
11.9 |
13.1 |
| Lean % |
58 |
53 |
56 |
57 |
55 |
| Lean Growth |
401 |
318 |
369 |
363 |
348 |
The swine industry in some parts of the world has relatively recently moved towards
a large corporate farming structure particularly in North America and this has
brought putative economies of scale. Some of these companies run many hundreds
of thousands of breeding females and finish millions of pigs per year. They demand
a high level of efficiency and accordingly carry out their own assessments on
lean growth characteristics to help them make selection decisions on the various
hybrids available. At this level of production it is economically justified for
them to commission their own growth evaluations on various hybrids to facilitate
the selection of the appropriate product for their own production conditions.
In figure 5 are given the results of one such evaluation that was carried out
using many thousands of growing pigs with a serial slaughter programme to generate
very accurate lean growth characteristics. In figure 5 also is presented the comparable
data from a published study carried out in the public domain. This latter study
was implemented with less animals and also used ultrasound back fat levels to
estimate lean gain. The contrast presented in figure 5 illustrates how much error
can hence be built into nutritional programmes. If the diet programme was based
on the university data or the wrong hybrid data then there would be significant
waste of nutrients in either case.
Figure 5 - Commercial evaluation of 2 hybrid
lines for optimum lean gain and nutrient requirements
Conclusions
It is self-evident from the foregone discussion that hybrid pig products over
recent years have changed in a manner driven by the selection index parameters
and there will have been divergence in lean growth characteristics. Nutrition
technology has also moved on at a pace and with phase feeding systems and sophisticated
software systems in use, then the level of accuracy demanded has also increased.
It is imperative if accuracy and efficiency is to be sustained to understand the
genetic product that is being fed and its growth characteristics. The only way
that this can be done is for geneticists and nutritionists to work much more closely
together than they have in the past. The North American experience is pointing
to the fact that unless the genetic parameters are provided then the production
companies will set up their own evaluations to produce their own comparative data.
References
Close, W.H. 1994 Feeding new genotypes. In Principles of Pig Science. Ed D.J.A.
Cole, J. Wiseman and M.A. Varley. Nottingham University Press. 123-140.
MLC, 1997 Stotfold Pig Development Unit. First Trial Results. MLC Report
MLC, 1998 Stotfold Pig Development Unit Second Trial Results. MLC Report
Author: Dr. Mike Varley, SCA Nutrition Ltd, North Yorkshire, UK
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