INTRODUCTION

The pork industry is driven by consumer demand for consistent quality lean pork products. To meet current and future consumer demand, the pork industry must continue to improve the efficiency of lean pork production. The two primary approaches for improving the efficiency of pork production are through genetic and environmental-management changes.

Each commercial producer must first decide which genetics he should use on his production facility. After making the key genetic decisions, each pork producer must consider cost-effective management changes to optimize the expression of genetic potential of their pigs. A number of alternative management decisions must be evaluated which are influenced by production costs (i.e., feed, facilities, labor and interest costs) and the relative rate of payment for carcass lean vs. fat in the marketing system.

Swine growth models are an integration of our current knowledge of the effects of genetic potential, nutrient intake, and environmental conditions on pig growth. These models can be used to identify alternative means to improve the efficiency of pork production and to estimate daily nutrient requirements for pigs of different ages and genetic groups when managed under a range of environmental conditions (de Lange and Schreurs, 1995). Recently, models have been developed that identify means of improving the efficiency of nitrogen and phosphorus retention in growing-finishing pigs and minimizing the potential contribution of pork production to environmental pollution.

DEFINING THE GENOTYPE

For an effective application of swine growth models, the maximum growth potential of pig genotypes must be accurately characterized. The three primary growth parameters required to characterize a genotype of pigs are: daily protein accretion, potential partitioning of energy intake above maintenance between protein and lipid accretion and daily energy intake (Schinckel and de Lange, 1996). The major hindrance to the implementation of swine growth models is the lack of accurate, yet economical, methods for estimating these parameters (Schinckel, 1994b).

PROTEIN ACCRETION RATES AND POTENTIALS

The protein accretion rates determine the pig's nutrient requirements for growth, composition of growth, and response to nutrition or management changes. Protein accretion curves have been developed for several commercially available genotypes under optimal conditions (Schinckel et al., 1995; Thompson et al., 1996). Protein accretion curves for four genotypes of terminal cross barrows representing high (A), medium-high (C and D) and medium (B) lean growth genotypes are presented in figure 1. The major differences between genotypes are the overall mean protein accretion rate and the rate in which protein accretion declines after 90 kg live weight. In general, genotypes with overall high protein accretion rates maintain higher protein accretion rates at heavier weights than average or below average protein accretion genotypes.

Figure 1. Protein accretion for barrows of four genotypes.

To develop maximum protein accretion curves for a genotype, pigs must be fed nutritionally non-limiting diets. The dietary protein levels can alter the shape of the observed protein accretion curves (Friesen et al., 1996). Feeding protein levels below requirements for the genotype will limit protein accretion. Also, if the protein restriction is removed, compensatory growth may occur.

There are currently two economically feasible methods for estimating protein accretion curves (Schinckel and de Lange, 1996). The first method is to obtain serial B-mode (real-time) ultrasonic backfat depth and loin muscle area measurements to predict body component mass from 20 to 120 kg live weight. The second method to economically predict on-farm empty body protein accretion curves is from an estimate of the mean fat-free lean gain from 20-120 kg (Schinckel et al., 1996).

ENERGY PARTITIONING

The second parameter needed to characterize a genotype is its ability to partition different amounts of available energy to either protein (lean) or lipid (fat) growth. As feed intake increases, a linear response in lean growth and fat rate occurs. The change in protein accretion (or lean growth) per unit increase in energy intake is called the slope. As energy intake increases, protein accretion increases until a plateau occurs (Figure 2). The plateau is achieved when the energy required for maximum protein accretion is reached. Energy supplied in excess of a pig's maximum lean growth rate requirements will be utilized for fat deposition (Figure 3). This causes a rapid increase in lipid accretion and the ratio of lipid to protein accretion (Figure 4). Pigs with high lean growth rate potentials respond to higher energy intakes by increasing lean growth rate.

Figure 2. Predicted daily protein accretion rates for three genotypes at 53 kg live weight-50 kg empty body weight at different energy intakes. Maximum daily protein accretion rates of 110, 150 and 150 g, slope of protein accretion (g) per Mcal ME intake of 21.3, 24.8 and 28.3.

Figure 3. Daily lipid accretion rates at different energy intakes.

Figure 4. Ratio of lipid gain to protein gain at different energy intakes.

The slope of lean gain (or protein accretion) on energy intake determines the extent to which energy intake is partitioned into lean versus fat gain. Pigs from 10-40 kg. live weight with moderate feed intakes during this time deposit a high proportion of lean and little fat. For this reason, improving management to increase feed intake at these live weights can be very cost effective because the additional nutrients will efficiently be used to increase lean gain. As a pig grows, the slope becomes less steep and the partitioning of energy changes so that at moderate energy intakes 70-80% of the intake is needed for maximum lean growth, the ratio of lean gain to fat gain declines. The slope of lean gain to energy intake decreases rapidly at heavier live weights (70-120 kg) as the pig's maximum lean growth rate declines. In other words, once a pig matures, and its lean growth rate declines, the partitioning of energy shifts so that even at moderate energy intakes the ratio of fat gain to lean gain increases (de Lange et al., 1995).

Lean lines depositing a high proportion of lean due to steep slopes of protein accretion on energy intake require less energy to achieve the same lean growth rate. These genotypes have the potential to achieve very efficient lean growth due to their high ratio of lean to fat growth. However, these lean genotypes will also respond differently to environments which limit feed intake. Low feed intake genotypes which are gaining a higher proportion lean will respond to feed intake limiting environments with larger absolute and percentage drops in live weight and lean growth.

ENERGY INTAKES

Swine growth models require an estimate of daily energy intake to estimate the other aspects of animal performance and nutrient requirements. Nonlinear equations have been primarily used to describe the curvilinear relationship between daily food or energy intake to either age or body weight (Schinckel and de Lange, 1996).

If the empty body protein and body lipid accretion rates are determined for pigs given ad libitum access to feed, energy intake can be predicted largely based on the energetic cost of empty body protein and lipid deposition, estimated at 10.54 and 12.64 Mcal ME/kg, respectively (Smith et al., 1997). This approach requires an estimate of maintenance energy requirements which are assumed to be determined by empty body protein mass. Daily energy intakes at each live weight were predicted within 4% of observed feed disappearance for an on-farm example. Using this approach, it became obvious that the energy intake curves are driven by the pigs' energy demands for protein (lean) and lipid (fat) accretion.

The use of different selection criteria has caused large genotype differences in feed intake. When feed cost is a high percentage of total cost and carcass value programs are widely implemented, lean efficiency and carcass merit become relatively much more important than growth rate. Feed intakes of pigs primarily selected for a combination of leanness and lean efficiency have substantially decreased feed intakes(Ellis et al., 1983; Brandt, 1987; Cameron and Curran, 1994).

Importation of seedstock has added genetic variation for lean growth, fat accretion and feed intake. Thirty percent differences in feed intake exist between different genotypes (Schinckel, 1994a).

EVALUATION OF ENVIRONMENTAL LIMITATIONS

Environmental factors including disease exposure, social stress and less than optimal stocking density limit growth such that pigs managed under commercial conditions are unlikely to express their maximum potential protein accretion, even when allowed ad libitum access to a high quality nutrient dense diet. For this reason, the term operational protein accretion was introduced. The operational protein accretion is the maximum protein accretion rate that pigs can achieve under specified commercial conditions. This definition implies that, for a given genotype, the maximum achievable protein accretion potentials are limited by environmental conditions under which it is determined.

Protein accretion curves can be used to estimate the magnitude in which environmental conditions (health status, facilities, stressors) limit the expression of the pigs genetic potential for lean growth. A substantial difference between on-farm and maximum achievable protein accretion or lean growth rates are indicative of major environmental limitations. In such cases, the commercial producer must evaluate the benefits and costs of environmental-management changes.

A trial recently evaluated the difference in growth of pigs raised under average commercial and optimal environmental conditions (Holck et al., 1997). Pigs weighed an average of 32.5 kg at 77 days of age. The barrows were blocked by weight and allotted to either a commercial grow-finish facility (.74 m²/pig, 24 pigs/pen, COMM) or a test station (2.23 m²/pig, 3 pigs/pen UNRES).

Table 1. Comparison of commercial versus optimal commercial conditions
	Environment
	Commercial	Optimal	% Optimal/ Commercial	P value
Daily gain, kg/d	.730	1.038	142	<.001
Days to 118 kg	192	160	83	<.001
Daily fat-free lean gain, g/d	240	342	142	<.001
Daily fat gain, g/d	240	353	148	<.001
Daily feed intake, kg/d	2.45	3.00	122	<.05
Feed/gain	3.29	2.85	87	<.05
Feed/lean gain	10.39	9.30	90	ns
Backfat, cm	2.48	2.74	112	ns

Holck et al., 1997.

Dramatic and significant (p<.0001) differences in bodyweight gain were observed between pigs raised in the two different environments. The least-square mean ADG from 30 to 120 kg bodyweight was .73 and 1.04 kg/day for the COMM and UNRES groups, respectively (Table 1). The UNRES group had significantly higher mean weights by two weeks after allotment compared to the COMM group and those differences in body weights were magnified during the growout period. The increased body weights consisted of both fat and lean gain.

Average daily gain showed a very small increase with live weight, reached a maximum (780 gm/day at 59.0 kg live weight in the commercial and 1070 gm/day at 42 kg live weight in the unrestricted environment) and decreased slightly as live weight increased (Figure 5). Pigs reared in the unrestricted environment immediately grew faster than pigs reared in the commercial environment. The advantage in live weight growth at each live weight was consistent during the finishing period.

Figure 5. Average daily gain for 30-120 kg in the environments (Holck et al., 1997).

The daily fat-free lean growth rates increased slightly from start of test to a maximum rate achieved at 52 kg live weight in both environments and then declined (Figure 6). The rate of decline from 52 kg to 120 kg live weight was greater for pigs reared in the commercial environment (311 to 162 g/day commercial environment vs. 391 to 282 g/d in the unrestricted environment). At 120 kg live weight, the commercial pigs achieved 52% of their maximum daily lean growth versus 72% for pigs reared under the unrestricted environment. The daily fat-free lean growth rates of the unrestricted pigs were substantially greater than the commercial pigs up to 35 days on test. Daily fat growth rates increased as live weight increased for pigs reared in either environment (Figure 7). Daily fat growth was consistently 50-60 g/day higher for the pigs reared under the unrestricted environment.

Figure 6. Daily fat-free lean growth rate of pigs reared in two environments (Holck et al., 1997).

Figure 7. Daily fat gain of pig reared in two environments (Holck et al., 1997).

Feed disappearance increased almost linearly in the UNRES group with a feed intake of 3.74 kg/hd/day recorded during the final 2 weeks prior to slaughter. The increase in feed intake was not linear in the COMM group which experienced considerable variation, especially in the finishing building (Figure 8).

Figure 8. Daily feed intake of pigs reared in two environments (Holck et al., 1997).

The unrestricted environment was designed to provide optimal physical (thermal, stocking density, feed/water access), social (pigs per pen, movement), and microbial (sanitation, segregation) conditions to minimize physiological and immunological stress. Performance of pigs raised in this environment was used as an estimate of genetic potential may be considered for this genotype. Differences in growth rates between pigs raised in commercial and unrestricted grow-finish environments were clearly demonstrated in this trial. The commercial environment reduced rate of live weight gain, lean and fat accretion to approximately 70% of the potential growth demonstrated in the unrestricted environment. While many researchers have shown performance differences of this magnitude using segregated-early-weaning techniques (Walker and Wiseman, 1993; Williams et al., 1994; Schinckel et al., 1995), it is important to note that in this research trial the pigs were not segregated pigs until 11 weeks of age.

EFFECTS OF DISEASE ON GROWTH

Research conducted in two health status environments, medicated early weaning and continuous-flow commercial conditions, indicate that disease status affects lean growth to a greater extent than fat growth (Williams et al., 1993). Conventional health status pigs, although consuming less feed and growing more slowly, also had a lower percent lean than the SEW pigs.

Research in several species indicate that an animal's immune system response to disease organism antigens is a partial cause of reduced growth rates. Antigen induced cytokines cause a cascade of physiological changes including decreased growth hormone and insulin-like growth factor-1 (IGF-1).