The commercial pork industry's number one challenge is to provide consumers with safe, wholesome food of a high and consistent quality. This food should be produced as efficiently as possible, while considering the impact on the environment, animal welfare, and the well-being of people involved with the production process.

Lean growth - that is lean meat deposition in pigs - is closely associated with the efficiency of pork production for various reasons:

First, it represents the growth of the valuable parts in the pig's body.

Second, lean growth is closely associated with body protein deposition. Body protein deposition, in turn, is the single most important factor that determines dietary amino acid requirements and is one of the main factors determining the dietary requirements for energy in grower-finisher pigs. Energy and protein account for over 85% of the ingredient costs in practical pig diets. Feeding costs are the single most important cost factors in commercial pork production.

Third, lean growth is much more efficient than the accretion of body fat. It requires approximately four times the amount of energy to grow one pound of fat tissue as compared to the energy required to grow one pound of lean tissue.

And fourth, there is considerable variation in lean growth rates between different groups of pigs. For example, the anticipated growth performance and carcass quality of pigs with high, medium or unimproved lean growth rates are presented in Table 1. These performance levels are at energy intake levels considered typical for commercial pig units.

These data indicate that under typical market conditions, an increase in lean growth rate of 5% will translate to at least an additional $1 profit/pig. Therefore, it is important to understand what contributes to differences in lean growth rates between groups of pigs and hopefully to identify the means to increase lean growth rates and the efficiency of pork production.

Calculating Lean Growth Rates Lean growth rates can be calculated from carcass weight at slaughter, the estimated lean content in the carcass, an assumed lean content in a pig's body at the initial weight, and the amount of time required to grow from initial weight to slaughter weight.

Table 2 reviews the steps used to estimate the lean content in pig carcasses and to calculate lean growth rates in different groups of grower-finisher pigs. These calculations were taken from a comprehensive publication of the National Pork Producers Council (NPPC).

Unfortunately, the definition of lean and methods used to quantify the amount of lean that is present in pig carcasses may vary between and among packing plants, pig breeding organizations and research institutions. In particular, the amount of lean present in the belly may be estimated in different ways. Likewise, miscellaneous muscles (in addition to those from the main lean cuts) may or may not be included in total lean. Or, the amount of lean may be standardized to different fat contents.

To complicate matters further, there is a range of carcass measurements used to predict the lean content in the carcass. Relationships between carcass measurements and actual carcass lean contents differ between different genotypes or genetic lines.

Obviously, there is a need to clearly identify how lean is defined and quantified when carcass lean contents and lean growth rates are determined. In addition, we should continue to strive towards a clearly defined, common, unbiased and practical definition and a means to measure lean content in pig carcasses. The Quality Lean Growth Modeling project, coordinated by the NPPC, demonstrates some important leadership in this area.

Lean Growth Curves In addition to average lean growth rates, the change in lean growth rate with increases in body weight should be considered. In particular, for the development of multiple-phase feeding programs, these lean growth curves should be established. Furthermore, for determining the optimum slaughter weight, the change in lean growth rate around the time of slaughter should be estimated, as it determines the rate and cost of producing marginal increments in carcass lean content.

There are three segments to a typical lean growth curve (Figure 1 on page 14). During the early stages of growth, generally up to about 110 lb. body weight, the daily lean growth rate increases.

Between 110 and 176 lb. body weight, the daily lean growth rate is relatively constant.

At approximately 176 lb. body weight, the daily lean growth rate starts to decline towards zero when the pig's mature lean body weight has been reached.

Research conducted at Purdue University, plus observations on commercial pig units, indicates there is considerable variation in the shape of this lean growth curve between different groups of pigs. These variations are particularly noteworthy in pigs over 220 lb. body weight. Results from the NPPC Quality Lean Growth Modeling project will provide important information about lean growth curves in different types of pigs exposed to varying nutritional regimens.

Pig Growth Models Sound information about lean growth curves is vital for the on-farm use of pig growth models. The adjoining sidebar on page 15 reviews the type of information you will need to drive various growth models.

Pig growth models are computer programs that integrate our current knowledge of nutrient utilization for growth and of animal environmental interactions into one system. Lean growth - body protein and body fat deposition - are important drivers of these models because they are increasingly used to finetune feeding and management strategies on individual pig units.

When well-tested models are used properly, they can be valuable tools to improve profitability. In particular, these models can be used to identify what factors determine lean growth at the various stages of growth and thus how lean growth can be manipulated.

What Affects Lean Growth Rates 1. Pig genotype: The pig's upper limit to lean growth or lean growth potential, is determined by genotype, which also includes their sex.

The information contained within the pigs' genes determines lean growth potential. The pigs' lean growth potential and the lean growth potential curves are the results of complex interactions between many different genes. It is thus unlikely that a gene or gene probe will be identified in the near future that will allow us to quickly and reliably identify pigs with high lean growth potentials. However, lean growth potential has a medium to high heritability, so conventional and intensive genetic selection will result in substantial improvements in lean growth potentials of pigs. The practice of crossbreeding is likely to result in only modest improvements in lean growth potentials.

Lean growth potentials are higher in gilts than in barrows. During the grower-finisher phase, the difference in lean growth potential between gilts and barrows is approximately 5%, but ranges from 2% to 15%. This difference appears to vary with genotype.

The difference in lean growth potential appears small up to about 66 lb. body weight, then increases as pigs get heavier. A decline in lean growth potential with increasing body weight begins at a lower body weight in barrows than in gilts. These differences in lean growth potentials should be considered when developing split-sex feeding programs and for determining the optimum slaughter weights for gilts vs. barrows.

Carrying the point one step further, lean growth potentials are higher in boars than in gilts and barrows. Although eliminating castration represents an important means to improve the efficiency of pork production, concern about boar taint and the associated effects on meat quality remains a challenge.

It is important to note that estimates of lean growth potential are usually derived from observed growth rates and carcass characteristics of pigs managed under supposedly ideal, unlimited conditions. However, whether observed lean growth rates match lean growth potential usually remains unconfirmed. To do so would require recording whether a change in nutrient intake or a change of environment altered the oberved lean growth rate. In particular, at lower body weights and in modern pig genotypes, observed lean growth rates are often determined by energy intake and not by lean growth potentials. As mentioned earlier, in order to find a means to improve lean growth rates, it is important to identify factors that determine lean growth at the various stages of growth.

2. Nutrient and energy intake - Sub-optimal intakes of essential nutrients and energy will limit pigs from expressing their lean growth potential. For this reason, practical pig diets are generally over-fortified with the relatively inexpensive nutrients, vitamins and minerals. Furthermore, close attention should be paid to dietary levels of energy, lysine, and the other essential amino acids and to daily feed intakes when managing grower-finisher pigs.

Intake of lysine and other essential amino acids: In the latest version of "Nutrient Requirements of Swine" from the National Research Council (NRC), it is clearly stated that pigs with higher lean growth rates require higher daily intakes of lysine and other essential amino acids. In this publication, it is suggested that pigs require 4.7 g/day of digestible lysine per 100 g/day of fat-free lean gain. In addition, pigs require .036 g/day digestible lysine per 2.2 lb. of metabolic body weight for various body maintenance functions. This indicates that lean growth rate is the main determinant of daily lysine requirements and that failure to meet required lysine intake levels will lower observed lean growth rates and the efficiencies associated with it.

The requirements for other essential amino acids can be derived from lysine requirements based on optimum amino acid to lysine ratios for lean growth and maintenance.

Energy intake: The generalized relationship between energy intake and lean growth is represented in Figure 2. Over a given body weight range and assuming that no other nutrients limit lean growth, there is generally a linear relationship between energy intake and lean growth. Lean growth increases until the animal's energy intake is just sufficient to achieve its lean growth potential. Beyond that point, any further increase in energy intake will not increase lean growth. Rather, the additional energy intake will simply increase the rate of body fat deposition, resulting in a fatter carcass and poorer feed-to-gain ratios (feed conversion). The point at which lean growth is just maximized indicates the level of energy intake at which the efficiency of lean growth is maximized.

Figure 3 shows the considerable variation between genotypes in the relationship between energy intake and lean growth. In fact, in some of today's modern genetic lines, energy intake is insufficient to reach the animal's lean growth potential even when these pigs are fed ad libitum and when they reach 198 to 220 lb. body weight (see LW x LD(A) boars and hybrid gilts in Figure 3).

Yet, in other genotypes, high lean growth potential can be reached at relatively low levels of energy intake (see LW x PP boars in Figure 3).

The differences can largely be attributed to variation in the increase in lean growth per unit increase in energy intake. This is the slope of the linear relationship between energy intake and lean growth in Figures 2 and 3. This variation in the slope is largely due to differing amounts of essential body lipid that different pig genotypes need to deposit, even when energy intake limits lean growth.

Body weight effects, previous nutrition, and differences in maintenance energy requirements between different genotypes further complicate the relationship between energy intake and lean growth. In particular, the effects of body weight should be considered.

The slopes in Figures 2 and 3 decline with increasing body weight. This implies that pigs get fatter when they grow heavier, even when energy intake limits lean growth, and that increasing amounts of energy intake over maintenance are required to maintain a lean growth constant with increases in body weight.

Under commercial conditions, energy intake over maintenance often does not increase in pigs over about 132 lb. body weight. Observed increases in feed intake with increasing body weight are often barely sufficient to satisfy the pigs' increasing energy requirements for maintenance. As a result, observed declines in lean growth rates as pigs reach market weight may be a reflection of a lack of energy intake over maintenance, and not of declines in lean growth potential with increasing body weight (Figure 1).

The discussion about the relationships between energy intake, lean growth and body weight have various implications.

First, these relationships are important for determining the optimum energy intake level over the various body weight ranges.

Second, it supports the need to better characterize these relationships for modern pig genotypes.

Third, in modern pig genotypes, energy intake, and not the animals lean growth potential, is likely to determine lean growth up to 220 lb. body weight or even higher. This implies that close attention should be paid to feed intake levels in finishing pigs, including a means to increase these levels.

And fourth, we have to be very careful with the interpretation of lean growth curves that have been established at one level of energy intake. Changes in energy intake levels will affect lean growth rates at lower body weights but may, or may not, affect lean growth rate at higher body weights. The latter is important as there is considerable variation in feed and energy intake levels among pig units, even for the same genotypes managed on different units.

3. Environmental stresses - Over the last few years, we have become more aware of the negative effects environmental stresses can have on lean growth rates. Crowding of pigs, excessive environmental temperatures, and disease-causing organisms can prevent pigs from expressing their full, genetic performance potential. Researchers have reported that exposure to disease-causing organisms can depress lean growth rates by as much as 30%.

Environmental stresses will affect the animals' expression of lean growth potential, as well as the relationship between energy intake and lean growth. The challenge to producers is to minimize pigs' exposure to environmental stresses. The challenge to researchers is to improve our understanding of the mechanisms whereby these stresses affect animal performance. This should allow producers to quantify what extent pig performance potentials are depressed and to manipulate the pigs' response to environmental stresses.

Factors Affecting Meat Quality As mentioned earlier, we have to consider the quality of pork that we produce.

Pork quality can be evaluated in a variety of objective and subjective ways, although the ultimate quality measure is acceptance by the consumer.

Studies supported by NPPC clearly illustrate that there is considerable variation in consumer acceptance of pork from different groups of pigs, and that there is no clear relationship between lean growth rates and the various aspects of meat quality.

Furthermore, nutritional and management strategies around the time of slaughter can have important effects on pork quality. Whenever alternative strategies to manipulate lean growth rates and lean growth curves are evaluated, the impact of these strategies on pork quality should thus be considered.

Conclusions In growing-finishing pigs, lean growth rate is closely associated with the efficiency of pork production. Plus, lean growth curves should be considered when determining optimum feeding and management strategies for individual pig units.

A proper interpretation of lean growth curves is important as different factors (genotype, sex, amino acid or energy intake, and environmental stresses) may affect lean growth at different stages of growth. The latter is important for the manipulation of lean growth rates and lean growth curves.

T here is a clear need to establish potential lean growth curves for the main pig genotypes and to identify how the expression of lean growth potentials can be maximized. When other strategies to manipulate lean growth rates and curves are evaluated, the impact of these strategies on quality should also be considered.

The benefits of using well-tested, computerized, pig growth models to fine-tune feeding programs for individual grower-finisher pig units can be substantial. For an effective on-farm application of these models, it is important that accurate model inputs are obtained and that good communication is established between the person operating the model and the manager of the pig unit where the model is applied.

The main steps involved in an effective on-farm application of pig growth models are:

1 Establish a lean growth curve. The importance of this is discussed in the "overview" article. Lean growth curves can be established using three different methods:

Calculate the average lean growth rate over the entire grower-finisher phase from initial and final body weight, carcass data, and the number of days required to grow pigs from initial to the final body weight (see Table 2 on page 10). Observations should be obtained from at least 40 representative pigs.* Use this average lean growth rate and the shape of a typical lean growth curve to establish the actual lean growth curve. This is the easiest method and is described in detail by NRC (1998). This is also the least preferred method as it ignores differences in the shape of lean growth curves between different grower-finisher pig units.

Establish a growth curve based on at least four equally spread data points that relate body weight to age. Observations should be obtained from at least 40 representative pigs per data point.* Combine this information with an actual feed intake curve (point 2, below) and run this through a well-tested pig growth model. Then adjust the lean growth curve inside the model to fit the "predicted" growth curve to the "actual" growth curve. The use of models to generate a lean growth curve is more accurate than the first method. However, it is very sensitive to assumptions about maintenance energy requirements and to the accuracy of feed intake measurements.

Directly establish a lean growth curve based on at least four equally spread data points that relate lean tissue mass in the pigs' body to age. Observations should be obtained from at least 40 representative pigs per data point.* The simplest means to estimate the lean tissue mass in the pigs' body is the use of B-mode real time ultrasound equipment. This is the preferred method. However, it requires that accurate relationships between ultrasound measurements (loineye area and backfat thickness) and the actual lean tissue mass in the pig's body are established. Unfortunately, the latter is not always the case because these relationships differ between pig genotypes and the use of incorrect relationships can lead to substantial errors in the established lean growth curve.

2 Establish a feed intake curve. Feed intake affects animal performance. Also, estimates of feed intake are required to establish the target nutrient levels in the various diets. As it is extremely difficult to accurately predict how much feed pigs consume at the various stages of growth, it is essential that feed intake is monitored routinely when pig growth models are applied.

Reasonable feed intake curves can be established when feed intake and average body weights are measured accurately over at least a two-week period, and at least at three different stages of growth that are equally spread over the growing-finisher period. Data should be obtained from at least two feeders and at least 40 pigs per body weight range.*

3 Characterize diets and feeding program. To characterize diets, establish ingredient composition, available energy and amino acid content in the ingredients, fineness of grind, additional processing, and mixing accuracy. To characterize the feeding program, establish what diets are fed over the various body weight ranges.

4 Characterize the main environmental factors known to influence animal performance. These include the effective environmental temperature, stocking density and the presence of disease-causing organisms. If lean growth and feed intake curves are established routinely, an accurate description of the environment is less critical, as these are reflected in observed performance levels.

5 Compare current, observed levels of animal performance to model predicted animal performance. This is to obtain confidence in the predictions generated by pig growth models and in suggested alterations to the current feeding or management program. These comparisons may also be used to make adjustments to model inputs, provided that clear reasons to make these adjustments are identified.

6 Identify alternative feeding and management strategies to be considered. This includes an identification of the potential ranges in nutrient contents in the various diets, formulation and pricing of various diets (to identify diets with the lowest cost/unit of nutrients), and identification of alternative feeding program.

7 Identify production objectives. Is the objective to maximize income per pig, income per pig place per year (considers income/pig as well as throughput), or maximizing the expression of performance potentials (applies when animals are selected to serve as parent stock for the next generation)? In addition, factors such as the cost of disposal of nutrients excreted with pig manure may have to be considered. Each of these objectives requires a different management strategy.

8 Evaluate alternative feeding and management strategies. Use the model to systematically evaluate alternative feeding and management strategies and to identify the optimum solution that meets the desired production objectives and other criteria set out under point 6.

9 Verify projected management and feeding performance changes. Confirm that the suggested changes to the management or feeding strategy do indeed result in the anticipated changes in animal and economic performance (i.e. continue to monitor pig performance and carcass quality).

10 Review feeding and management plan. On a regular basis, review the suggested feeding and management strategies as pig conditions, environmental conditions (changes in lean growth curves or season), or economic conditions (prices) change.

* The actual number of observations required to establish reliable growth, lean growth or feed intake curves will differ between pig units because variability in pig performance differs.

If split-sex feeding is to be applied, these observations should be made for each sex. User-friendly programs such as PorkMa$ter can be used to establish these curves from accurate measurements on representative groups of pigs.

PorkMa$ter is a computerized, performance-monitoring system for grower-finisher pigs based on feed intake and growth curves. Dept. of Animal and Poultry Science, University of Guelph. Internet website address: http://www.aps.uoguelph.ca/~porkm