For the identification of the genetic performance potentials in grower-finisher pigs it is essential that pig performance is monitored under closely defined, "optimal" conditions and that potential animal-environmental interactions are considered. If, in pig breeding programs, these interactions are ignored, then this can have important consequences for the specific performance traits that are genetically altered and the rate at which genetic progress is made (e.g. Stern et al., 1994; Knap, 1995; Schinckel and de Lange, 1996).
In this contribution performance potentials of grower-finisher pigs are discussed in terms of partitioning of energy and protein intake for growth and carcass quality. This will allow for the interpretation of potential interactions between nutrition and (genetic) performance potentials in pig breeding programs (e.g. Schinckel and de Lange, 1996; Black et al., 1995; de Lange and Schreurs, 1995).
In this paper, interactions between nutrients other than energy and protein (amino acids) and grower-finisher pig performance are ignored; it is assumed that other nutrients do not limit the expression of performance potentials in grower-finisher pigs.
INTERACTIONS BETWEEN NUTRIENT INTAKE AND NUTRIENT PARTITIONING
When characterizing pig (geno-) types in terms of nutrient partitioning for growth, five major aspects of nutrient partitioning can be recognized. Potential interactions between nutrition and each of these aspects of nutrient partitioning will be discussed when appropriate.
1. THE UPPER LIMIT TO BODY PROTEIN DEPOSITION (Pdmax)
Body protein deposition (Pd) is closely related to the accretion of muscles, or lean growth. In addition, Pd is the main factor that determines the daily requirements for essential amino acids (e.g. lysine, methionine, methionine + cysteine, threonine, tryptophan, isoleucine) and one of the main factors determining requirements for dietary energy (e.g. Black and de Lange, 1995). The upper limit to body protein deposition (Pdmax) is ultimately determined by the pigs' genetic potential (e.g. Moughan and Verstegen, 1988).
In a stress and disease-free environment, the observed Pd is determined by one of three factors: 1) protein intake (i.e. the first limiting dietary amino acid), 2) energy intake (see the next section), or 3) Pdmax. In order to confirm that the observed Pd is Pdmax it should be demonstrated that the dietary amino acid intake is adequate and that a further increase, or a slight decrease in energy intake does not alter Pd.
It should be noted that environmental stresses such as pig density and the exposure to disease can prevent pigs from expressing their true Pdmax (e.g. Williams, 1994). As it is not (yet) possible to predict the extent to which the true Pdmax of pigs is reduced due to these stresses, the term "operational Pdmax" was introduced. The operational Pdmax represents the upper limit to Pd that pigs can achieve under practical conditions (de Lange and Schreurs, 1995; Moughan et al., 1995). Depending on the environments to which pigs are exposed the operational Pdmax may thus vary for a particular pig (geno-) type.
Pdmax and amino acid intake
As there are no reported interactions between pig (geno-)types and the efficiency with which dietary amino acids are used for Pd, there is a very close relationship between Pd and amino acid requirement across different pig (geno-)types (Susenbeth, 1995; de Lange and Mohn 1996). In other words, there are very clear interactive effects between dietary protein (lysine) levels and pig (geno-)types on Pd and other aspects of animal performance (Table 1)(Campbell and Taverner, 1988; Stahly et al., 1989; Bikker, 1994).
Table 1. Interactive effects of pig genotype and dietary lysine levels on lean gain in G/F pigs (Stahly et al., 1988; Interactive effect P<.05).
|Dietary lysine, %|
The adequacy of dietary amino acid intake can be derived from the efficiency with dietary lysine is retained in body protein (if this is less than 55% in corn-soybean meal based diets then lysine intake is unlikely to limit Pd; it can be assumed that body protein contains approximately 7% lysine)(Susenbeth, 1995; de Lange and Mohn, 1996). For the other amino acids, their ratio to lysine should be monitored (these should exceed the minimum ratios, i.e. the ratios in the ideal, balanced, amino acid profile)(Fuller et al., 1989; de Lange and Mohn, 1996).
Pdmax and body weight
As we move to more advanced phase and split-sex feeding programs and/or as the need to more closely determine the optimum slaughter weight increases, it becomes more critical that the (slight) changes in Pdmax with changes in body weight be determined in various pig (geno-) types. If Pdmax curves are established, it should be confirmed that Pdmax is reached at all stages of growth. Great care should be taken in the interpretation of Pd curves that have been established in pigs that are fed at only one level of energy intake. This is because different factors may determine Pd over different body weight ranges (e.g. energy intake at the lower body weights and Pdmax at the higher body weights). In these instances, complex mathematical functions are required to relate Pd to body weight (to reflect that different factors determine Pd at different body weight ranges) and part of the fitted Pd curves will change as soon as energy intake is changed (Figure 1).
It may well be that the actual Pdmax curves in different pig (geno-)types follow a predictable pattern, i.e. that Pdmax is largely constant up to approximately 80-90 kg body weight and that relatively simple mathematical equations can be used to relate Pdmax to body weight as pigs approach maturity (e.g. Moughan and Verstegen, 1988; Whittemore et al., 1988; Ferguson and Gous, 1993). For example, in a recent study at the University of Guelph we have demonstrated that the Pdmax is basically constant between 20 and 75 kg body weight in gilts from the University of Guelph herd (Mohn and de Lange, unpublished).
Figure 1. Estimated Pd and Pdmax in grower-finisher pigs that are fed at two levels of energy intake (80 or 90% of daily voluntary energy intake according to NRC, 1988; estimates are generated with a dynamic model that simulates growth in the pig; Ralpig96).
According to Moughan and Verstegen (1988), observed Pd in pigs between 50 and 90 kg body weight and that are fed high quality diets ad libitum will provide a reasonable estimate of the pigs' Pdmax (Figure 1). The extent to which the average Pd measured between 25 and 90 kg body weight under-estimates the average Pdmax over this body weight range will be determined by the difference between achieved Pd and Pdmax at the lower body weights (i.e. when energy intake rather that Pdmax determines Pd)(Figure 1; see also the next section). If slaughter weights are substantially higher than 90 kg body weight, then it becomes critical that the pattern of decline in Pdmax as pigs approach maturity be determined as well. According to Schinckel (1994) there is considerable variation between different pig (geno-)types in the pattern of decline of Pdmax.
2. THE RELATIONSHIP BETWEEN ENERGY INTAKE AND PD WHEN ENERGY INTAKE IS LESS THAN THAT REQUIRED TO REACH Pdmax.
It is important to determine the relationship between energy intake and Pd in various pig (geno-) types for two reasons: 1) carcass quality and feed efficiency are optimized when energy intake is just sufficient to reach Pdmax (e.g. de Lange and Schreurs, 1995), and 2) there is considerable variation in this aspect of nutrient partitioning between different pig (geno-) types (Figure 2) (e.g. Campbell et al., 1985; Campbell and Taverner, 1988; de Greef, 1992; Bikker, 1994; Quiniou et al., 1995).
Figure 2. Relationship between energy intake and Pd in various pig (geno)types (Quiniou et al., 1995).
Differences in the relationship between energy intake and Pd between different pig (geno-) types can be attributed to the fact that different pigs have different requirements for deposition of (essential) body lipid associated with Pd even when energy intake limits Pd (Whittemore, 1983; Black and de Lange, 1995).
As Pdmax in growing pigs continue to improve (through genetic selection or better health management) energy intake is more likely to become the limiting factor for Pd. Three recent studies indicate that in pigs with high genetic capacities for lean growth, Pdmax cann't be reached below about 80 to 90 kg body weight (Campbell and Taverner, 1988; Rao and McCracken, 1991; Bikker, 1994).
Figure 2 also illustrates some important interactive effects of energy intake and pig (geno-) type on Pd. For example different amounts of energy intake are required to just reach Pdmax (the plateau in Pd) in different pig (geno-)types and there is no clear interaction between Pdmax and the amount of energy required to just reach Pdmax.
The relationship between energy intake and Pd changes with body weight (Figure 3) and is affected by nutritional history (Black et al., 1986; Whittemore et al., 1988; de Greef, 1992; Bikker, 1994). This further complicates the interpretation of the effects of energy intake on Pd in various pig populations. If, for example, energy intake is reduced (due to heat stress or crowding) then energy intake may limit the expression of Pdmax, even during the late finisher phase.
Figure 3. Relationship between energy intake and Pd at various body weights in a defined population of pigs (Quiniou et al., 1995).
3. MAINTENANCE ENERGY REQUIREMENTS
It is important to consider maintenance energy requirements; maintenance contributes to about one third of total energy requirements in growing pigs (Black and de Lange, 1995). Major energy-demanding processes for maintenance of an animal include those associated with blood flow, respiration, muscle tone, ion balance, tissue turnover, animal activity and the ingestion of feed (Black and de Lange, 1995). Traditionally, energy requirements for maintenance have been related to body weight via mathematical allometric functions (ARC, 1981). However, studies such as those conducted by Koong et al. (1983) and Noblet et al. (1991) indicate that such relationships only explain part of the variation of maintenance energy requirements that are observed between different groups of pigs (Table 2).
|Sex/Genotype||Metabolizable energy requirements for Maintenance (kJ/Kg bodyweight.60)|
|MS x LW||
Differences in maintenance energy requirements between different groups of pigs are quite closely related to residual feed intake, i.e. feed intake that can't be explained based on "average" requirements for growth and maintenance. According to Kennedy et al. (1993) residual feed intake is a trait with a medium to high heritability.
The main factor that is likely to explain differences in maintenance energy requirements between different pig (geno-)types is animal activity. Animal activities associated with feeding and animal interactions can account for 20% or more of maintenance energy requirements (Halter et al., 1980, Verstegen et al., 1987). The distribution and energy requirements of major tissue groups (muscle, fat, visceral organs) in the pigs' body should be considered as well. As fat tissue is metabolically a relatively inactive tissue, maintenance energy requirements are lower in fat pigs than in lean pigs. For this reason maintenance energy requirements are better expressed per kg body protein than per kg (metabolic) body weight. It should, however, be noted that energy requirements in the visceral organs (gastro-intestinal tract, liver, kidneys, spleen, heart, lungs, reproductive tract) is much larger on a per kg protein basis than that in muscle tissue. Recent studies suggest that there is considerable variation in the size of visceral organs between groups of pigs (Koong et al., 1983; de Greef, 1992; Bikker, 1994; Quiniou and Noblet, 1995). Indications are that at least some of the variation in visceral organ size is determined by pig (geno-)type (Table 3).
Table 3. Contribution of lean tissue mass and visceral organs to empty body weight in different types of pigs (Quiniou and Noblet, 1995; corrected to an average body weight of 47.2 kg).
|Tissue (% of empty body weight)|
|Sex / Genotype||Muscle||Visceral organs*|
For determining maintenance energy requirements in various pig (geno-) types it is important to consider the environmental factors that may affect energy requirements for maintenance as well. These include the thermal environment (in pigs under cold stress maintenance energy requirements increase by about 4% per 1 oC decrease in temperature), presence of disease-causing organisms and environmental factors that induce extreme levels of animal activities (e.g. poor management of the feeder and the thermal environment) or extreme sizes of the visceral organs (e.g. feeding diets that are high in anti-nutritional factors).
4. FEED INTAKE CAPACITY
As mentioned in the previous section energy (feed) intake is more likely to limit Pd and up to higher body weights when lean growth potentials in pigs continue to increase. It will thus become increasingly important to maintain high levels of energy (feed) intake, especially in selection programs.
Both ARC (1981) and NRC (1988) suggested empirical relationships between body weight and ad libitum daily intake of digestible energy (DE). It was recognized that diet DE content and body weight are two of the main factors determining voluntary feed intake in pigs.
However, studies such as those conducted at Purdue University indicate that there is a tremendous amount of variation (20 to 30%) in voluntary feed intake between different (geno-) types of pigs even though these pigs were managed under similar conditions and fed similar diets (Schinckel, 1994). The effect of sex on feed intake appears to differ between pig (geno-)types as well (it may vary between 3 and more than 10%). Furthermore, feed intake is related to a large number of feed and environmental factors.
An approach to understanding ad libitum feed intake in different groups of pigs is to recognize that voluntary feed intake is driven by the pig's requirements for nutrients, and that feed intake is reduced because of various constraints imposed on the animal (Black et al., 1986; Emmans and Kyriazakis, 1989). These constraints relate to diet characteristics (bulk density, nature and rate of digestion of fiber, water holding capacity, nutrient and anti-nutrient contents), environment (thermal, social, physical, presence of disease-causing organisms) and the pig's physical capacity to ingest feed. Important first steps in characterizing voluntary feed intake in different pig (geno-) types are to determine the rates of Pd and body lipid deposition (Ld) in a "non-limiting" environment (e.g. Ferguson and Gous, 1993), and the physical capacity to ingest feed (e.g. Black, 1995) at the various body weight ranges and in various pig (geno-) types.
According to Black et al. (1986), the pig's physical capacity to ingest feed (gut fill) is likely to limit performance in growing pigs up to approximately 40 kg body weight. In these pigs, an increase in the nutrient density of the diet will thus not affect feed intake but it will increase the daily nutrient intake. At body weights higher than approximately 40 kg body weight, the daily energy intake is more likely to determine feed intake; in this situation pigs tend to compensate for changes in diet energy content with changes in feed intake in such a manner that the daily energy intake is constant.
It speaks for itself that the use of unpalatable feed ingredients, feed ingredients contaminated with toxins, nutrient imbalances, and improper feed preparation should be avoided as these factors will reduce voluntary feed intake. For example, if the amount of protein in the diet is larger than required by the pigs, some energy is required to excrete this excessive protein. This will reduce the amount of (net) energy supplied by the diet and that is available for growth. In addition, it will increase body heat production and may reduce voluntary feed intake (especially if pigs are under heat stress; e.g. Whittemore, 1983).
Environmental factors that are likely to affect feed intake under practical conditions are pig density, the effective environmental temperature, and feeder design and management. It should be noted that as the lean growth potentials in pigs increase, pigs are more prone to heat stress. As there is more heat production associated with Pd than with Ld, pigs with high lean growth potentials will perform better when environmental temperatures are reduced (towards the low end of the thermoneutral zone).
5. RELATIONSHIPS BETWEEN CHEMICAL BODY COMPOSITION AND PHYSICAL CARCASS CHARACTERISTICS.
The four main chemical body components in the pigs body are water, protein, ash and lipid. As there are close relationships between body water, ash and protein, the total body protein and body lipid contents are important determinants of physical carcass characteristics and thus carcass value. Furthermore, the manipulation of the distribution of total body protein and lipid over the various body tissues will affect the efficiency with which dietary protein and energy are used for the production of lean meat.
At market weight between 50% and 60% of the total body protein weight is present in the lean tissue; approximately 15% is present in the visceral organs (gastro-intestinal tract, liver, kidneys, spleen, heart, lungs, reproductive tract) and the remainder is present in non-lean carcass parts (skin, hair, dissectable bones, dissectable fat; Table 4). About 30% of total body lipid is present in lean tissue and visceral organs. As there are pig (geno-)type effects on the contribution of various tissues to empty body weight (Table 3), it is very likely that the partitioning of whole body protein is affected by pig (geno-)types as well.
Table 4. Distribution of total body protein in lean tissue and visceral organs in pigs at approximately 85 kg body weight and fed at two levels of energy intake (Bikker, 1994).
|2.2 x Maint.||3.7 x Maint.|
% of whole body protein present in:
Composition of lean tissue:
The results summarized in Table 4 clearly illustrate that with nutrition the distribution of protein in the pig's body can be manipulated. At higher energy intake levels, the fraction of whole body protein that is deposited in lean tissue is reduced. Moreover, the (intra-muscular) lipid content is increased when feed intake levels are increased.
Furthermore, one of our recent studies at the University of Guelph suggests that the relative contribution of the main lean cuts to the total amount of lean in the pig's body can be manipulated by nutrition. It appears that feeding pigs diets limited in protein (amino acids) during the finisher phase (as illustrated by a reduction in Pd), limits the size of the trimmed ham to a larger extent than the size of the other cuts (Table 5). This is probably a reflection of the fact that the ham is a relatively late maturing muscle group in the pig's body.
These results suggest that energy and amino acid intake levels should be considered when evaluating differences in carcass lean yield and carcass composition between pig (geno-)types and when identifying the optimum feeding strategy to optimize carcass quality in specific groups of pigs.
Table 5. Effect of dietary amino acid levels during the finisher phase on whole body protein deposition rates and the size of trimmed cuts in the carcass of pigs slaughtered at approximately 100 kg body weight (Tuitoek et al., 1996).
Dietary methionine +
|Whole body protein deposition, g/d||117||118||105||11.5|
|Weights of main cuts,
When characterizing pig (geno-) types in terms of nutrient partitioning for growth, five major aspects of nutrient partitioning can be recognized : 1) the upper limit to body protein deposition, 2) the relationship between energy intake and body protein deposition, 3) maintenance energy requirements, 4) feed intake capacity, and 5) relationships between chemical body composition and physical body characteristics. Potential interactions between pig (geno-)types and these aspects of nutrient partitioning should be considered when evaluating pig (geno-)types or when feeding programs are developed for specific populations of pigs.
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