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Development of a model to describe the compositional growth and dietary lysine requirements of pigs fed ractopamine¹

Purdue University, West Lafayette, IN 47907-2054

² Correspondence:
Lilly Hall of Life Science, 915 W. State St. (phone: 765-494-4836; fax: 765-494-9346; E-mail:
aschinck{at}purdue.edu).

	Abstract

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The objective of this research was to use recent ractopamine research data to develop an updated mathematical model to describe the daily compositional growth of pigs fed ractopamine. Mean increases of 18.2, 23.1, and 25.0% for daily protein accretion were assumed for 5, 10, and 20 ppm of ractopamine for an overall gain of 40 kg of BW gain during the feeding period. The relative effect of ractopamine described the rapid increase and subsequent decrease in the effect of ractopamine as a function of BW gain or days on test and ractopamine concentration (RC, ppm). The reduction in ME intake produced by ractopamine was described as 0.036 x (RC/20)^0.7 multiplied by the ME intake for the first 20 kg of BW gain, and then increasing to 0.078 x (RC/20)^0.7 at 40 kg of BW gain feeding period. The ratio of fat-free muscle gain to protein accretion increased by 14 to 16% with the feeding of ractopamine, depending on the dietary lysine/essential AA levels. The ratio of carcass fat gain to empty body lipid gain was increased when lysine and essential AA requirements were met. Daily protein accretion and fat-free lean growth were described as functions of dietary lysine/essential AA intakes. The percentage of lysine in protein accretion increased with the feeding of ractopamine from 6.80 to 7.15%, depending on ractopamine concentration. Equations predicting carcass measurements, such as fat and longissimus muscle depths from carcass weight and composition, were modified to incorporate prediction biases produced by ractopamine. For the four concentrations of ractopamine (0, 5, 10, and 20 ppm, respectively) during a 78 to 110 kg of BW feeding period, the model predicted performance levels for ADG (1.03, 1.15, 1.16, and 1.16 kg/d), gain:feed (kg of ADG/kg of ADFI; 0.360, 0.401, 0.412, and 0.425), dressing percentage (75.1, 76.0, 76.3, and 76.4), percentage fat-free lean (48.7, 51.0, 51.5, and 52.2), longissimus muscle area (38.8, 41.8, 42.5, and 43.5 cm²), 10th-rib fat depth (22.1, 19.8, 19.3, and 18.7 mm), and fat-free lean gain (321, 446, 467, and 495 g/d), comparable to recent research data. The model allows the effect of ractopamine to be added to farm specific pig growth curves. It can be used to evaluate ways to optimize the use of ractopamine, including duration of ractopamine feeding, concentration of ractopamine, and dietary lysine concentration.

Key Words: ß-Adrenergic Agonists • Body Composition • Growth • Models • Pigs

	Introduction

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A goal of pork producers is to efficiently produce lean pork to compete with alternative animal products. The implementation of lean value carcass pricing systems has led to the selection of pigs with increased lean growth rates, increased carcass lean percentages, and increased lean gain:feed (Schinckel, 1994; 1999). Health, nutrition, and facility management strategies have been implemented to increase commercially achievable lean growth rates. Ractopamine hydrochloride (Paylean, Elanco Animal Health, Indianapolis, IN) is a feed additive that increases the rate and efficiency of muscle tissue growth (Watkins et al., 1990; Moody et al., 2000). It has recently been approved to be fed at concentrations of 5 to 20 ppm for up to the last 41 kg before marketing (Elanco, 1999).

Each pork producer must consider the optimal use of ractopamine, including level and duration of use before marketing. Also, the number and composition of diets that include ractopamine must be evaluated. This type of optimization can be achieved through the use of a compositional growth model (Schinckel and De Lange, 1996; De Lange et al., 2001). A simple previous ractopamine model was based on data from older genetic populations of pigs (Millar et al., 1990; Kitts et al., 1991). The objective of this research was to utilize current ractopamine research data to develop an updated growth model.

	Materials and Methods

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General Form of the Pig Growth Model
Data from a 1991 lean growth trial (Thompson et al., 1996; Wagner et al., 1999; Schinckel et al., 2001b) were used to develop prediction equations for empty body protein, empty body lipid, fat-free lean, and total carcass fat tissue mass from real-time ultrasound measurements. Since the regression coefficients in each equation differ with each 20-kg change in BW (Schinckel et al., 2001b; 2002c), multiple prediction equations were used for each 15- to 20-kg live weight interval from 25 kg to market weight. The predicted values of body component mass variables and live weight can be fitted to nonlinear equations to produce compositional growth curves, energy intakes, and dietary essential AA requirements (Schinckel and De Lange, 1996; Smith et al., 1999; Schinckel et al., 2002c). The primary chemical components of body growth were empty body protein and empty body lipid mass through the following formula:

where EBPRO = empty body protein in kilograms and EBLIPID = empty body lipid in kilograms.

The values of the parameters are genetic population- and sex-specific because of differences in the chemical composition of the visceral and fat tissues at similar BW (Wagner et al., 1999; Schinckel et al., 2001b). Pigs from genetic populations selected for increased lean growth have a greater percentage of moisture and a decreased percentage of lipid in the visceral and fat tissues at similar BW (Schinckel, 1999; Schinckel et al., 2001b). The general equations used for terminal cross pigs are presented in Table 1. Genetic population- and sex-specific values are used when empty body protein and empty body lipid are predicted from serial live weight and live real-time ultrasonic scans for the control pigs (Schinckel and De Lange, 1996; Schinckel et al., 2002c). Body weight is predicted as empty BW divided by 0.93. Carcass fat-free lean and total carcass fat mass are predicted from BW, empty body protein, and empty body lipid. Daily ME intake (MEI, Mcal/d) was predicted from the equation ME intake = (0.255 x BW^0.60) + [8.84 x (EBPRO accretion, kg/d)] + [11.4 x (EBLIPID accretion)] where EBPRO = empty body protein (kg/d) and EBLIPID = empty body lipid (kg/d) (Noblet et al., 1999). Daily MEI was then fitted as a nonlinear function of live weight:

where C₁, b₀, b₁, and b₃ are the nonlinear regression parameters.

The efficiency with which digestible lysine is utilized for growth is a function of the ratio (DIP) of daily intake of digestible lysine, above maintenance, relative to the amount contained within the empty body protein accretion when maximal protein accretion is achieved (Moughan, 1989; Gahl et al., 1994; Schinckel, 1994). The proportion of maximal protein accretion achieved (PAP) is a linear, then curvilinear function of DIP. The ratio of PAP:DIP is the efficiency with which the digestible ideal AA intake above maintenance is utilized for growth. The equations of Moughan (1989) were transformed to a function in which the marginal efficiency decreased from 0.70 to 0. The linear part of the function when DIP is less than or equal to 1.2 is:

where PAP is the proportion of maximal protein accretion achieved. This assumes a constant marginal efficiency of 0.70 until approximately 83% of maximal daily protein accretion is achieved. The function for DIP values from 1.2 to 1.6 is:

As DIP approaches 1.6, PAP approaches 1.0, marginal efficiency approaches zero, and overall efficiency (PAP/DIP) approaches 0.625.

The equations assume that the diet primarily contains corn and dehulled soybean meal with a constant concentration of synthetic lysine (0.0 to 0.15% lysine-HCl). Feed intake (kg/d) was predicted using ME concentration of the diet with the lowest lysine/CP concentration that maximized daily protein accretion. The ME concentration (Mcal/kg) of each diet was adjusted for its NE concentration by multiplying the specific diet’s ME concentration by the ratio of its NE concentration to the NE concentration of the diet needed to achieve maximal protein accretion. Because corn has a higher NE (2.395 Mcal of NE/kg; NRC, 1998) than dehulled soybean meal (2.02 Mcal of NE/kg), low-lysine diets (0.55 to 0.70% total lysine) result in greater NE intakes than the high-lysine diets (1.0 to 1.2% total lysine) at the same MEI. The adjustment of ME for NE was needed to reproduce experimental data in which maximal ADG is achieved at dietary lysine levels below that needed to achieve maximal gain:feed and lean growth rate (Cromwell et al., 1993; Usry et al., 1997). Diets that limit protein accretion result in increased lipid accretion and decreased carcass lean percentage. The actual lipid accretion rate was predicted by the following equation:

where EBLIPID = empty body lipid, kg/d; MEI = metabolizable energy intake, Mcal/d; and PA = protein accretion, kg/d (Noblet et al., 1999).

Modeling the Effect of Ractopamine Response
The eight responses taken into account when modeling the effects of ractopamine were: 1) the increase in empty body protein accretion, 2) the change in the response of ractopamine with duration of feeding, 3) the effect of ractopamine to decrease feed intake, 4) the effect of ractopamine on carcass muscle growth relative to empty body protein accretion, 5) increases in the ratio of carcass fat tissue growth to empty body lipid accretion, 6) the increase in carcass growth relative to BW growth, 7) accounting for biases in predicting carcass measurements from carcass composition, and 8) the increase in percentage of lysine in the empty body protein being accreted.

Increased Empty Body Protein Accretion.
Data from six recent trials were used to model the increase in protein accretion as a function of dietary ractopamine concentration (Elanco, 1999; 2001; Herr et al., 2001a,b). A 25% increase in daily empty body protein accretion over the last 40.8 kg of live weight gain was assumed. This increase is based on carcass and visceral chemical composition data (N. H. Williams, T. R. Cline, A. P. Schinckel, and D. J. Jones, unpublished data) and data presented by Stahly (1990), Dunshea et al. (1993), and Elanco (1999). Data from recent trials (Schinckel et al., 2002b) indicate that current high lean gain terminal cross pigs have a greater response to lower concentrations of ractopamine (5 to 10 ppm) than pigs fed 20 ppm a decade ago (Watkins et al., 1990).

The model was used to predict the increase in ADG and gain:feed ratio produced by ractopamine above the controls as a result of increased protein accretion. Barrows and gilts were fed 0, 5, 10, 15, and 20 ppm of ractopamine for a 6-wk period. The predicted daily performance of the pigs fed ractopamine was compared to the daily performance of the same sex fed 0 ppm of ractopamine. The daily increases in protein accretion were fitted to regression equations including the increase in ADG, response in gain:feed weight gain when fed ractopamine (kg), and days fed ractopamine. The most accurate equation was

where RPA is the increase in protein accretion (kg/d), RADG is the increase in ADG (kg/d), and RGF is the increase in gain:feed (g:g) as a result of feeding ractopamine. The equation had a R² of 0.995 and residual standard deviation of 0.91 g/d. All regression coefficients were significant (P < 0.0001). The effect of ractopamine concentration was not significant (P > 0.20) in addition to the four variables included in the regression equation.

The relative effect of ractopamine on protein accretion was based on the weekly effect of ractopamine on ADG and gain:feed ratios in actual research trials. The effect of ractopamine on ADG and gain:feed was calculated as the performance of pigs fed ractopamine minus the sex/week control means. Weekly data used was from trials conducted by Elanco (1999), Herr (2001a,b), and Trapp et al. (2002). The predicted increase in protein accretion produced by ractopamine was divided by the mean overall response of each trial. The mean values are presented in Table 2.

where RC = ractopamine concentration in ppm; a = 0.3495 (SE = 0.004); and b = 0.228 (SE = 0.003) for a 28-d feeding period (Herr et al., 2001b). The nonlinear function predicted 72.8 and 85.2% of the maximal 20 ppm of protein accretion response with the 5- and 10-ppm ractopamine levels respectively. The subsequent data of Webster et al. (2002) also supports the predicted relative response of 5 vs. 10 ppm of ractopamine. In a 28-d trial, carcass protein accretion was increased from 148 g/d (0 ppm) to 186 and 193 g/d with 5 and 10 ppm of ractopamine, respectively.

Change in the Effect of Ractopamine.
The relative effect of ractopamine is the predicted magnitude of the predicted increase in protein accretion relative to the mean predicted increase for the last 40.8 kg of BW gain. The observation for the relative effect of ractopamine was calculated weekly for each pen of pigs fed ractopamine as the predicted increase to protein accretion for that pen for that week based on feed efficiency and the ADG of the pen relative to the mean of the control pigs divided by the overall mean predicted increase in protein accretion for the last 40.8 kg of BW gain. The effect of ractopamine was modeled to describe the rapid increase and subsequent decline in the effect with either increasing time or weight gain on ractopamine (Williams et al., 1994).

The relative effect of ractopamine on protein accretion was modeled as a function of BW gain (kg) on ractopamine:

where RR = relative effect of ractopamine and BWG = body weight gain. The two methods resulted in similar curves from the predicted effect of ractopamine during its duration of use. When the data from a specific trial was analyzed, the function describing weekly effects of ractopamine from BW gain was no more precise (R² = 0.85 vs. 0.83) than the function based on days fed ractopamine (Williams, et al., 1994; Schinckel et al., 2001a; 2002b). However, across trials in different environments, with different performance levels, the effect of ractopamine decreased in a similar manner during wk 5 and 6 on ractopamine (Elanco et al., 2001). Thus, an equal weighting of the BW gain and days fed ractopamine functions was used to describe the effect of ractopamine. The effect of ractopamine observed in recent trials declined more rapidly than that observed in trials a decade ago (Williams et al., 1994; Schinckel et al., 2001a; 2002b).

The Effect of Ractopamine on Feed Intake.
The proportional reduction in MEI (MEIR) was modeled as MEIR = 0.036 for the first 20.4 kg of live weight gain on ractopamine, and then increased to approximately 0.078 at 40 kg of live weight gain by the following equation:

where BWG is the BW gain on ractopamine (Herr et al., 2001a,b). The MEI (Mcal/d) of ractopamine-fed pigs was described by the equation:

at the specified BW based on the results of Herr et al. (2001b) and Elanco (1999).

The Increase in Muscle Gain Relative to Empty Body Protein Accretion.
Increases in dressing percentage and percentage lean produced by ractopamine support an increased ratio of fat-free lean gain to empty body protein accretion. Ractopamine substantially increases muscle growth with little increase in the daily growth of visceral organ or bone tissue (Bark et al., 1992; Elanco, 1999). Pigs fed ractopamine in low-CP diets (8 to 13% CP) had no greater protein accretion based on either chemical analyses or N retention than control pigs, but had an approximate increase of 14% in daily carcass fat-free lean gain or carcass lipid-free soft tissue gain (Elanco, 1999). Pigs fed higher lysine levels approaching that required for maximal daily protein accretion have shown further increases in dressing percentage, fat-free lean percentage, and fat-free lean gain (Herr et al., 2001a). The increase in the ratio of fat-free lean gain to protein accretion increased to approximately 16% based on chemical composition data of pigs (N. H. Williams, T. R. Cline, A. P. Schinckel, and D. J. Jones, unpublished data) and data summarized by Stahly (1990) and Elanco (1999).

The extra amount of fat-free lean gained each day relative to the daily protein accretion achieved (RDFFL, kg) was modeled as a function of the ractopamine concentration, PAP, and daily fat-free lean gain for the control pigs:

where FFL = fat-free lean gain, kg/d and RC is the dietary ractopamine concentration (ppm). The sum of the extra daily amount of fat-free lean is a measure of the accumulated impact of ractopamine on fat-free lean mass above that predicted by the increase in protein accretion.

Increases in the Ratio of Carcass Fat Gain to Empty Body Lipid Gain.
The carcass fat tissue of pigs fed high-lysine (1.1 to 1.2%) diets with ractopamine contains a lower percentage of lipid and a greater percentage of moisture than control pigs or pigs fed low- (0.82%) lysine diets with ractopamine (Schinckel et al., 2002a). Based on predicted lysine requirements of the control and ractopamine-fed pigs and predicted carcass fat tissue and empty body lipid accretion rates, an approximately 4% increase in total carcass fat tissue gain per unit of empty body lipid accretion was predicted when diets approached the essential AA levels needed to achieve maximal daily protein accretion (Schinckel et al., 2002a).

The Increase in Carcass Growth Relative to Live Weight Growth.
Ractopamine increases dressing percentage (Watkins et al., 1990, Moody et al., 2000, Schinckel et al., 2002b). The extent to which dressing percentage is increased by ractopamine feeding is affected by the level and duration of ractopamine feeding and by the level of dietary lysine fed (Watkins et al., 1990; Herr et al., 2001a; Schinckel et al., 2002b). The increased rate of fat-free lean growth relative to visceral organ growth is the primary cause of the increased dressing percentage (Stahly, 1990; Elanco, 1999). The increase in carcass weight above that predicted from BW, empty body lipid, and empty body protein mass was modeled as 0.85 times the sum of the extra fat-free lean gain (kg) above that expected based on the daily protein accretion. This formula is based on chemical composition data (N. H. Williams, T. R. Cline, A. P. Schinckel, and D. J. Jones, unpublished data) and data summarized by Stahly (1990) and Elanco (1999).

Biases in Predicting Carcass Measurements from Carcass Composition and Mass.
Ractopamine changes lean distribution in a way that carcass measurements only partially account for the extra lean mass produced by ractopamine (Mowery et al., 1991; Gu et al., 1992; Schinckel et al., 2003). Prediction equations that included carcass weight, 10th-rib fat depth, and 10th-rib longissimus muscle area predicted only 50 to 60% of the actual increased carcass muscle mass-produced by feeding ractopamione (Watkins et al., 1990; Bark et al., 1992). This underprediction of fat-free lean mass in ractopamine-fed pigs is not constant for any specific prediction equation, but varies depending on the ractopamine concentration, duration of ractopamine feeding, and dietary lysine levels fed (Gu et al., 1992; Schinckel et al., 2003). At the same carcass weight and composition, ractopamine-fed pigs have greater fat depths and longissimus muscle areas than do control pigs. Thus, the prediction of carcass measurements of pigs fed ractopamine must account for these biases. The data from Gu et al. (1992), Herr et al. (2001a), and Schinckel et al. (2003) were used to develop equations to predict carcass measurements from carcass fat-free lean and total carcass fat tissue composition. Each carcass measurement is a function of carcass weight, fat-free lean mass, fat tissue mass, and b times the sum of the extra fat-free lean gain above that expected based on the daily protein accretion, where b is specific for each carcass measurement.

Increase in Percent Lysine in the Protein Accreted.
The lysine content of the protein accretion was modeled to increase when ractopamine is fed. Ractopamine increases the ratio of fat-free lean gain to protein accretion (Stahly, 1990). Muscle tissue has high concentrations of lysine and other essential AA (Riis, 1983; Wünshe et al., 1983). The equation used was:

where RC is the dietary ractopamine concentration fed (ppm). This equation is based on the following assumptions: 1) fat-free muscle contains 22.5% CP (Bark et al., 1992; Wagner et al., 1999), 2) muscle protein contains 8.9% lysine (Lloyd et al., 1978; A. P. Schinckel, unpublished data), 3) empty body protein of the control pigs contains 6.8% lysine (Bikker et al., 1994; NRC, 1998), and 4) the ratio of fat-free lean gain to protein accretion increases from approximately 2.40 to 2.64, 2.68, and 2.74 with 5-, 10-, and 20-ppm dietary concentrations of ractopamine, respectively, when pigs are grown from 69 to 110 kg BW. Using these assumptions, the nonmuscle tissues contain approximately 4.33% lysine. Because ractopamine increases the ratio of fat-free lean gain (8.9% lysine) to protein accretion, the percentage of protein accretion deposited in the carcass muscle tissues increases from 54.0 to 59.4, 60.3, and 61.65%, and the lysine content of the protein accretion increases from 6.80 to 7.05, 7.09, and 7.15% as the ractopamine concentration increases from 0 to 5, 10, and 20 ppm, respectively.

Example Data
The compositional growth of gilts fed ractopamine was predicted for gilts reared in two environments: 1) segregated early weaning with all-in/all-out management in modern facilities and 2) an older, continuous-flow facility (Kendall et al., 2000). Compositional growth curves and MEI of the control pigs were developed from serial real-time scans and live weights (Schinckel and De Lange, 1996; Smith et al., 1999; Schinckel et al., 2002c). The effect of ractopamine was modeled for four levels of ractopamine (0, 5, 10, and 20 ppm) from 78 to 110 kg of live weight. Gilts were fed dietary lysine levels predicted to achieve maximal protein accretion from 78 to 110 kg of BW; 0.80, 1.00, 1.07, and 1.14% lysine for 0, 5, 10, and 20 ppm of ractopamine, respectively. The predicted values assume that dietary lysine/essential AA concentrations are adequate for maximal effect from feeding ractopamine to increase protein accretion and fat-free lean gain for ractopaine concentration.

The joint effects of lysine/CP concentrations and ractopamine on compositional growth were also evaluated. The segregated early-weaned (all-in/all-out management) gilts were assigned diets containing 0.55, 0.70, 0.85, 1.00, or 1.15% lysine and either 0 or 10 ppm of ractopamine upon reaching 78 kg of BW. Compositional growth, final BW, and carcass composition were evaluated on a constant 28-d test period and on a 78 to 110 kg of BW interval.

Out of Sample Validation.
A trial was used which included 456 PIC 337-sired terminal cross gilts and barrows fed either 0 or 10 ppm of ractopamine (Boyd et al., 2001a, b). The pigs started the 28-d test at an initial mean weight of 92 kg. There were eight experimental treatments: 1) 0 ppm of ractopamine, 0.78% lysine, and 15.4% CP; 2) 10 ppm of ractopamine, 0.78% lysine, and 15.4% CP; 3) 10 ppm of ractopamine, 0.90% lysine, and 17.1% CP; 4) 10 ppm of ractopamine, 1.02% lysine, and 18.8% CP; 5) 10 ppm of ractopamine, 1.14% lysine, and 20.6% CP; 6) 10 ppm of ractopamine, 0.90% lysine with added synthetic AA containing 14.98% CP; 7) 10 ppm of ractopamine, 1.02% lysine with added AA containing 16.62% CP; and 8) 14 d of diet five and 14 d of diet three.

A total of six pens and 57 pigs (nine barrows or ten gilts per pen) were assigned to each treatment. Pigs were weighed and feed data collected weekly. The model was parameterized to reproduce as closely as possible the biweekly ADG and gain:feed ratio data of the control pigs (Table 3).

	Results

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View larger version (17K): [in this window] [in a new window]   Figure 1. Effect of ractopamine concentration on predicted ADG (kg/d) of segregated early-weaned gilts reared via all-in/all-out management. Pigs were fed corn-soybean meal-based diets predicted to achieve maximal daily protein accretion rates. Data for gilts fed 0 ppm are from Kendall et al. (1999).

View larger version (19K): [in this window] [in a new window]   Figure 2. Effect of ractopamine concentration on predicted fat-free lean gain (g/d) of segregated early-weaned gilts reared via all-in/all-out management. Pigs were fed corn-soybean meal-based diets predicted to achieve maximal daily protein accretion rates. Data for gilts fed 0 ppm are from Kendall et al. (1999).

View larger version (18K): [in this window] [in a new window]   Figure 3. Effect of ractopamine concentration on predicted protein accretion (g/d) of segregated early-weaned gilts reared via all-in/all-out management. Pigs were fed corn-soybean meal-based diets predicted to achieve maximal daily protein accretion rates. Data for gilts fed 0 ppm are from Kendall et al. (1999).

View larger version (18K): [in this window] [in a new window]   Figure 4. Effect of ractopamine concentration on predicted lipid accretion (g/d) of segregated early-weaned gilts reared via all-in/all-out management. Pigs were fed corn-soybean meal-based diets predicted to achieve maximal daily protein accretion rates. Data for gilts fed 0 ppm are from Kendall et al. (1999).

View larger version (19K): [in this window] [in a new window]   Figure 5. Effect of ractopamine concentration on predicted fat-free lean gain (kg/d) relative to hot carcass weight gain (HCWT, kg/d) for segregated early-weaned gilts reared via all-in/all-out management. Pigs were fed corn-soybean meal-based diets predicted to achieve maximal daily protein accretion rates. Data for gilts fed 0 ppm are from Kendall et al. (1999).

View larger version (18K): [in this window] [in a new window]   Figure 6. Effect of ractopamine concentration on predicted gain:feed (kg/d) for segregated early-weaned gilts reared via all-in/all-out management. Pigs were fed corn-soybean meal-based diets predicted to achieve maximal daily protein accretion rates. Data for gilts fed 0 ppm are from Kendall et al. (1999).

View larger version (18K): [in this window] [in a new window]   Figure 7. Effect of ractopamine concentration on predicted lean gain:feed (kg/d) for segregated early-weaned gilts reared via all-in/all-out management. Pigs were fed corn-soybean meal-based diets predicted to achieve maximal daily protein accretion rates. Data for gilts fed 0 ppm are from Kendall et al. (1999).

With the 0.55 and 0.70% lysine diets, the difference in growth rate, protein accretion, and gain:feed between the 0- and 10-ppm ractopamine diets was very small. These low-lysine diets severely limited the effect of ractopamine. Pigs fed ractopamine had lower predicted protein accretion rates when fed the 0.55% lysine diet than pigs not fed ractopamine for two reasons: 1) ractopamine decreased feed intake and thus grams of lysine intake, and 2) the percentage of lysine in the protein accretion was increased by ractopamine. At the 0.55% lysine level, gilts fed ractopamine were predicted to have greater fat-free lean growth rates and dressing percentages than control gilts. As lysine levels increased, the effect of ractopamine was predicted to increase such that protein accretion, fat-free lean gain, gain:feed, and to a lesser extent, dressing percentage increased. The model predictions are similar to actual research results (Anderson et al., 1987; Jones et al., 1988, Dunshea et al., 1993) that substantiate that pigs fed ractopamine require greater dietary lysine/CP concentrations to achieve maximal ADG, gain:feed, and protein accretion rates than do pigs not fed ractopamine.

The predicted differences in carcass measurements between the ractopamine and control gilts fed 1.15% lysine are similar to recent research results (Weber et al., 2002). Weber et al. (2002) fed 1.10% lysine corn-soybean meal diets (3.35 Mcal of ME/kg) or 1.17% lysine corn-soybean meal diets with 5% added animal fat (3.59 Mcal of ME/kg) to 192 gilts in the same facility as the segregated early-weaned gilts. After 28 d of 10 ppm of ractopamine, the gilts fed ractopamine had 3.9 kg more BW, 3.9 kg more carcass weight, 5.1 cm² greater longissimus muscle area, 0.12 cm less 10th-rib backfat depth, and 1.4% greater dressing percentage.

The actual and predicted biweekly and overall actual least squares mean and predicted values are shown in Table 8. For this trial, the ADG of the pigs fed ractopamine was slightly underpredicted for the first 2 wk (0.02 to 0.06 kg/d) and overpredicted (0.01 to 0.13 kg/d) for wk 3 to 4. Overall, the model predicted that the 0.78% lysine diet would limit the ADG and gain:feed response to ractopamine. It also predicted a 0.03-kg/d greater ADG for pigs fed the 0.78 lysine diet compared to an actual increase of 0.04 kg/d. The model also predicted a 0.021 unit increase in gain:feed for the pigs fed the 0.78% lysine plus ractopamine diet compared to pigs fed the control diet and compared to an actual increase of 0.029 in gain:feed. The model predicted within 0.01 kg/d the overall increase in ADG as a result of feeding 0.90% lysine and 1.14% lysine diets. The model overpredicted (0.155 vs. 0.105 kg/d) the increase in 4-wk ADG produced by the ractopamine 1.02% lysine diet. Pigs fed 10 ppm of ractopamine with 1.02% lysine had lower overall ADG than pigs fed either the 0.90 or 1.14% lysine. The model predicted 0.052, 0.059, and 0.063 increases in gain:feed for the 28-d period in comparison to actual increases of 0.049, 0.052, and 0.065 for the pigs fed 10 ppm of ractopamine with 0.90, 1.02, and 1.14% lysine, respectively.

For this trial, the actual gain:feed and ADG were lower during wk 3 and 4 for the pigs fed the 1.02 and 1.14% lysine diets than predicted. The predicted maximal daily protein accretion of pigs fed ractopamine was determined by two factors: 1) daily protein accretion rates of the control pigs, which is a function of BW, decreased as BW increased over 90 kg, and 2) the relative effect of ractopamine decreased as the BW gain achieved while being fed ractopamine increased. Because the pigs fed ractopamine had the highest ADG during wk 1 and 2, the pigs achieved greater BW gain while being fed ractopamine during wk 3 and 4. This resulted in lower predicted protein accretion, ADG, and gain:feed for pigs fed ractopamine. Also, the model predicts decreased dietary NE concentrations for the high-lysine/CP diets, which also decreases ADG at the same MEI. It is possible that some alternative factor or variable needs to be taken into account to more accurately predict the performance of pigs fed diets with high-lysine/CP concentrations, which allow 95 to 100% of maximal protein accretion to be achieved vs. pigs fed lower lysine/CP diets during wk 3 to 4.

Herr et al. (2001a) also found a more rapid decrease in the effect of ractopamine for pigs previously phase-fed high-lysine diets (1.08 and 1.22% lysine) during wk 1 to 4 than for pigs fed lower lysine (0.82 or 0.97%) diets with ractopamine. During wk 5, the pigs previously fed 0.82 or 0.97% lysine diets had ADG of 0.85 and 0.84 kg/d and gain:feed of 0.311 and 0.324 compared with an ADG of 0.76 kg/d and gain:feed of 0.279 for the pigs previously fed high-lysine diets.

	Discussion

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The ractopamine model predicts a number of changes in the pigs’ compositional growth. Also, the effect of ractopamine is dependent on the dietary essential AA levels fed. The lysine requirements of both control and ractopamine-fed pigs increased with genetic selection for increased lean growth and lean efficiency (Schinckel, 1994; Herr et al., 2001a).

Early ractopamine research was primarily conducted with diets containing 16% CP, which would not maximize the effect of ractopamine in modeling high-lean-gain pigs (Watkins et al., 1990; Herr et al., 2001a). The development of a daily compositional growth model of the effect of ractopamine allows the optimization of management, nutrition, and marketing strategies (Millar et al., 1990; Kitts et al., 1991). A number of issues can be resolved with the use of such a model; for example, should producers target specific days of ractopamine feeding or a specific weight range of ractopamine feeding? Will the recommendations change with the growth performance achieved (environmental conditions), the marketing system, or input costs, such as the cost of ractopamine and soybean meal? Should one diet be fed during the duration of the ractopamine feeding period or should pigs be fed a higher lysine diet (approximately 1.10 to 1.15% lysine) followed by a lower lysine diet (0.85 to 0.90% lysine)?

The objective of pork producers is to maximize daily returns above feed costs. The optimal use of ractopamine depends on a number of factors. The most important factor is the relative value of carcass lean to carcass fat. A previous ractopamine model identified that both the optimal ractopamine concentration and duration of ractopamine use were substantially less for carcass weight marketing (1:1 ratio of lean to fat value) than marketing systems with higher (2:1 to 4:1) ratios of lean value to fat value (Kitts et al., 1991).

Pork producers will likely only receive partial payment for the increased carcass cut out value produced by ractopamine. Since ractopamine primarily increases muscle mass and alters muscle distribution, prediction equations from instruments used by pork processors will underpredict the lean content of ractopamine fed pigs (Mowery et al., 1991; Gu et al., 1992; Schinckel et al., 2003). The model accounts for these biases in the prediction of the carcass measurements. The increase in lean percentage predicted from optical probe measurements will depend on the accuracy of the data used to develop the prediction equation and accuracy of the measurements during commercial pork processing (Boland et al., 1995; Lofgren et al., 2000). The magnitude of biases predicted by the model assumes a low level of measurement errors. An increase in the level of measurement errors would decrease the magnitude of the effect of ractopamine predicted by the optical probe measurements (Lofgren et al., 2000).

The model assumes a consistent percentage increase in protein accretion and fat-free lean gain based on the commercially achieved performance levels, which includes genetic and environmental effects. Trials in which different genetic populations of pigs were fed ractopamine in the same environment support the consistent percentage improvement in fat-free lean growth rate (Gu et al., 1991; Bark et al., 1992; Schinckel et al., 2002b). The change in the effect of ractopamine relative to environmental effects has not been extensively researched. Pigs reared under average commercial conditions have live weight growth rates from 0.70 to 0.80 kg/d compared to 0.95 to 1.15 kg/d when reared under less limiting, almost ideal conditions in research facilities (Holck et al., 1998; Schinckel, 1999). Genetic populations selected for increased lean gain:feed, increased carcass lean percentage, and decreased feed intake can be substantially more sensitive to disease challenges and higher stress environments in terms of growth rate, lean growth rate, gain:feed, morbidity, and mortality (Frank et al., 1997; Kendall et al., 1999). Currently, two genetic populations with similar commercially achieved performance levels are predicted to have identical responses to ractopamine. However, a high-lean-gain-low-feed-intake genetic population may be at 60 to 65% of its maximal protein accretion, whereas the medium-lean-growth-high-feed-intake genetic population may be achieving 75 to 85% of its maximal genetic potential. The effects of the pigs’ maximal genetic potential for lean growth or protein accretion, the environmental effects, and genetic x environmental interactions are not taken into account.

	Implications

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Ractopamine substantially increases muscle growth and percentage of carcass lean tissue. The dietary concentration of ractopamine, duration of use, and dietary lysine levels affect the compositional growth changes and benefits produced by feeding ractopamine. The model is based on adding the effect of ractopamine to the compositional growth of pigs not fed ractopamine. The development of a daily compositional growth model including the response to ractopamine allows the optimization of management, nutritional, and marketing strategies.

	Footnotes

 
¹ Purdue University Agricultural Research Paper No. 16913.

Received for publication July 11, 2002. Accepted for publication November 20, 2002.

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