The genetics of lean tissue growth in the pig

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)

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