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,3
* Animal Genetics and Breeding Unit, University of New England, Armidale NSW 2351, Australia, and
and
Animal Breeding and Genetics Group, Wageningen Institute of Animal Sciences, Wageningen University, 6700 AH Wageningen, The Netherlands
2 Correspondence:
phone: +61 267 733787; fax: +61 267 733266; E-mail:
Susanne.Hermesch{at}pobox.une.edu.au.
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Abstract |
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 Top Abstract IntroductionMethodsResultsDiscussionImplicationsLiterature Cited |
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Key Words: Breeding • Economic Evaluation • Feed Intake • Growth Models • Pigs
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Introduction |
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TopAbstractIntroductionMethodsResultsDiscussionImplicationsLiterature Cited |
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Growth models are used to parameterize biological characteristics of pigs and to evaluate their effect on performance traits and economic profit (e.g., De Lange et al., 2001). Parameters of growth models can also be used as breeding objectives, and their economic weights can then be derived from the change in profit as a result of changing each parameter. De Vries and Kanis (1992) used a growth model to derive economic weights for feed intake capacity in the growing pig. A similar model was used by Skorupski et al. (1995a, b) to derive economic weights based on simulation of life cycle pig production. The model described by De Vries and Kanis (1992) has been developed further (De Greef, 1992; Bikker, 1994) and implications of this change for deriving economic weights have been analyzed by von Rohr et al. (1999). However, the effect of different levels in parameters of the growth model in general and specifically the Technisen Model Varkersvoeding (TMV) model (TMV, 1994) on performance and economic weights has not been demonstrated. The aim of this paper is to derive economic weights for parameters of the TMV growth model, in particular for feed intake and to illustrate the effects of different levels in these parameters on performance traits and economic weights for parameters of the growth model. Changes in these economic weights are compared with the corresponding changes in economic weights for performance traits derived from an economic model as usually used in animal breeding.
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Methods |
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The growth model used to derive performances and economic weights is the TMV model (TMV, 1994). This growth model is based on the concept of a linear-plateau relationship between protein deposition and energy intake. The concept was proposed by Whittemore and Fawcett (1976) and was experimentally demonstrated by Campbell et al. (1983). The concept was then further adapted by De Greef (1992) and has been incorporated in the TMV model. The model assumes that for an animal in a specific weight range an increase in daily energy intake results in a linear increase in protein deposition until a plateau in protein deposition is reached. This plateau, also called maximum protein deposition (Pdmax), is considered an intrinsic factor for each pig. In addition, the model assumes that for each unit of protein deposited a minimum amount of fat is deposited. Each unit increase in energy intake results in extra fat and extra protein deposition, until maximum protein deposition is reached, in a ratio called the marginal ratio (MR) (De Greef, 1992). The marginal ratio increases with live weight of the animal. Within the model the optimum feed intake (FI0) is defined as the minimum amount of feed (energy) required meeting the intrinsic maximum protein deposition (De Vries and Kanis, 1992).
The daily feed intake determines the metabolic energy, which is used for maintenance and protein and fat deposition. The main equations used in the TMV model used to derive the average growth performance over a given weight range are described below. Daily maintenance requirement (M) is assumed to be related to live weight of the animal in kilograms to the 0.63 power (ARC, 1981). Assuming a constant weight gain during the entire growing period the average daily maintenance requirement (Ma) for a specified weight range (wt1 to wt2) can be derived as (Foster et al., 1983):
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The current calculations assume a live weight range of 25 to 115 kg. The average daily maintenance requirement for this weight range is 10.27 MJ ME. Energy not required for maintenance is used for protein and lipid deposition. It is assumed that the energy contents of 1 kg of deposited protein or fat are 23.8 MJ ME and 39.6 MJ ME, respectively. The efficiency of depositing protein and fat are assumed to be 0.45 (Metz et al., 1982) and 0.75 (ARC, 1981), respectively. Therefore, 53 MJ ME are required for deposition of 1 kg of fat or protein. The model described by De Vries and Kanis (1992) assumes that lipid as well as protein deposition are zero when pigs are fed at maintenance level. However, a number of studies (e.g., Black et al., 1986; De Greef, 1992) show that protein is still deposited, while fat is broken down when pigs are fed at maintenance level. Fat deposition is zero at an energy intake of 1.3 maintenance level (Bikker, 1994). Protein deposition at 1.3 maintenance level is derived as 0.3 maintenance requirements (M)/53. Equation 2
represents daily protein deposition (Pd) as a function of feed intake. This formula is only valid when feed intake is insufficient to meet the maximum protein deposition rate. Protein deposition is equal to the maximum protein deposition rate when feed intake is equal or greater than the optimum feed intake. Energy not required for maintenance or protein deposition is used for lipid deposition (Equation 3),
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 | [3] |
where FI is the energy intake in megajoules ME. The energy density was assumed to be 12.72 MJ ME/kg feed. The marginal ratio is defined as b x live weight in the TMV model (TMV, 1994). The default values for b are 0.04, 0.05, and 0.06 for barrows, gilts, and boars, respectively. De Greef (1992) summarized the range of marginal ratios for a given weight range from a number of studies. The corresponding b-values ranged from 0.02 to 0.09. Given the average live weight of 70 kg assumed in this study, values for average marginal ratio analyzed ranged from 1.40 to 6.30. Ash deposition is derived as 0.191 x protein deposition and daily water deposition (Wd) is derived from protein deposition and protein weight as
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where Pm represents protein mass. Assuming a constant rate of protein deposition, the average daily water deposition (Wa) over a given weight range is derived as:
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where Pm1 and Pm2 represent protein mass at the beginning and end of the empty body weight range, respectively. To derive average water deposition for a given weight range, the initial chemical body composition is required. The empty body weight is defined as 95% of the live weight. Based on results of De Greef (1992), the weights of protein, lipid, and ash at 23.75 kg empty body weight are derived as 0.512, 0.385, and 0.103 times the weight of dry matter in the empty body weight, respectively. The water content is derived as 1.05 times 4.9 (protein weight)0.855 (De Greef, 1992). The protein and fat mass are derived iteratively. At 25 kg live weight protein and fat mass are 3.838 and 2.887 kg, respectively. The average daily water deposition and consequently the body composition at an empty body weight of 109.25 kg (115 kg live weight) are derived iteratively. The sum of average daily protein, lipid, ash, and water deposition represents 95% of the average daily gain during the considered body weight range.
In order to obtain lean meat percentage (LMP) of the carcass from input parameters of the linear plateau model a prediction equation is used in the TMV model (unpublished results, De Greef):
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Profit
The profit is derived on a per slaughter pig basis. The payment system assumes that returns per slaughter pig depend on the carcass weight and the price per kilogram carcass weight. A base price of € 1.36 is assumed per kilogram carcass weight given a lean meat percentage of 55%. In addition, a bonus of € 0.0136 per kilogram carcasss weight is paid for each additional per cent increase in LMP from 55 to 65% with no additional bonus being paid if LMP percentage exceeds 65%. Similarly, a penalty of € 0.0136 is imposed per 1% lean meat lower than 55%. A carcass weight of 90 kg is assumed.
Costs per slaughter pig include feed costs, additional costs per day, and fixed costs per pig, which includes piglet price, veterinary costs, etc. The price per kilogram feed is assumed to be € 0.195. Fixed costs are assumed to be € 47.65 per pig and variable costs € 0.113 per day per pig. The number of days required to reach the carcass weight depends on ADG. The profit per pig is then derived from returns minus costs and is derived for a LMP below 65% as
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Economic weights for feed intake are derived in two ways. Using the growth model, economic weights for feed intake and parameters of the growth model are derived by simulating changes in each parameter of the growth model and evaluating the corresponding economic changes in the profit function. Based on the economic model, economic weights for feed intake along with economic weights for further performance traits are directly derived from the first partial derivatives of this profit function with respect to the trait of interest (Eq. 8 to 10.
Breeding Objectives and Economic Weights
A common breeding goal for growing pigs contains the traits feed intake, average daily gain, and LMP, each weighted by its corresponding economic weight, which can be obtained by partially differentiating the profit equation with respect to the parameter
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Economic weights obtained in this (classic) way are called here economic weights derived from an economic model. Alternatively, the driving variables in the growth model, feed intake, MR, and maximum protein deposition determining the performance traits growth rate and LMP can also be considered as breeding objective traits. The respective economic weights can be obtained by changing each of these parameters in the growth model, while keeping the others constant and evaluating the corresponding economic changes in profit.
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Results |
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TopAbstractIntroductionMethodsResultsDiscussionImplicationsLiterature Cited |
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For the growth model, the effect of average daily feed intake and average b-value of the MR on protein deposition is shown in Figure 1. When feed intake is not sufficient to meet maximum protein deposition (Pdmax = 150 g/d), protein deposition increases linearly with rising feed intake (linear part of model). This increase is higher with a lower MR when relatively less lipid but more protein is deposited with each extra kilogram of feed intake. The minimum amount of feed intake to reach this maximum protein deposition, the optimum feed intake, depends on the MR. For a given maximum protein deposition, the optimum feed intake is low in conjunction with a low MR and increases with higher levels of the MR.
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Growth Rate
The increase in growth rate with increasing feed intake follows the pattern of protein and lipid deposition (Figure 2) with protein deposition having a larger effect on growth rate since protein deposition determines ash and, more importantly, water deposition, which constitutes a large part of growth. As a consequence, growth rate increases more per unit increase in feed intake when feed intake is not sufficient to meet energy requirements for the maximum protein deposition in combination with a low MR. Within the plateau part of the model, growth rate is not influenced by the MR and increases with higher feed intake only slightly due to a further increase in lipid deposition.
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Lean meat percentage is highest when both feed intake and MR are low (Figure 3). A higher feed intake reduces LMP to a larger extent once feed intake exceeds the optimum feed intake. The MR does not influence LMP within the plateau part of the model.
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For any given MR, feed conversion ratio is lowest when feed intake equals the optimum feed intake as defined by the linear-plateau model (Figure 4). Within the linear part of the model, feed conversion ratio decreases with higher levels of feed intake. In the plateau part, feed conversion ratio increases with higher levels of feed intake. During the linear part of the model, extra feed intake leads to an increase in protein deposition which mainly determines growth rate through water deposition. The extra gain in growth rate is proportionally higher than the increase in feed intake, resulting in a reduced and therefore improved feed conversion ratio. In contrast, during the plateau part of the model any further increase in feed intake only results in an increase in lipid deposition.
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Different levels of feed intake and different b-values determining MR imply differences in feed conversion ratio, LMP, and growth rate which influence the profit per pig. The effect of feed intake and b-values of the MR on profit per pig is shown in Figure 5, again assuming a maximum protein deposition of 150 g/d. In the linear part of the model, profit per pig increases with lower levels of the MR. However, marginal ratio does not influence profit per pig within the plateau part of the model. In contrast, feed intake affects profit per pig during the plateau part as well as the linear part of the model. For a given MR and maximum protein deposition, profit per pig is maximized when feed intake equals the minimum feed intake required to meet maximum protein deposition. The optimum feed intake based on the growth model, therefore, equals the economic optimum. When feed intake is below this optimum feed intake, profit per pig increases with higher feed intake. Within the plateau part of the model, profit per pig decreases with higher levels of feed intake.
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Economic weights for parameters of the growth model are presented in Table 1 along with economic weights for performance traits derived from the economic model. The breeding goal based on the growth model includes MR, maximum protein deposition, and feed intake. Six different scenarios are presented consisting of two different b-values of the MR (0.03 and 0.05) each with three different levels of feed intake. The economic weight for feed intake based on the growth model is positive, when feed intake is below the optimum feed intake. The magnitude of this economic weight depends on the MR and is higher for a low MR. In contrast, feed intake has zero economic weight when it is optimum with respect to the growth model but has a negative economic weight when feed intake is above its optimum. The magnitude of the negative economic weight for feed intake (€ -13.89 and € -11.34 per pig) is not influenced by the MR but by differences in the absolute feed intake (2.5 vs 2.9 kg). The two remaining breeding objective traits using a growth model are the MR and the maximum protein deposition. Their economic weights also depend on the level of feed intake with respect to the optimum feed intake. The MR has a negative economic weight when feed intake is below or at the optimum level with its magnitude being larger for a smaller MR. In addition, the economic weight for the MR has a larger magnitude in combination with a higher feed intake. Performance traits and, therefore, profit per pig are only influenced by the maximum protein deposition when feed intake is equal to or above the optimum level. Differences in economic weights for maximum protein deposition are small between scenarios.
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Discussion |
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De Lange et al. (2001) describe the MR and maximum protein deposition as two important aspects that characterize pig performance potential. Further, De Lange et al. (2001) summarize the difficulties in predicting feed intake for different groups of pigs managed under different conditions. Despite this, feed intake is often chosen to be a model input parameter. These traits, feed intake, MR, and maximum protein deposition, are included in the breeding objective based on the growth model. Maintenance requirements are assumed to depend on live weight of the animal and are not included in the breeding goal. A similar simplified growth model has been extended to take different maintenance requirements into account which arise from differences in protein turnover (Knap and Schrama, 1996) and thermoregulation (Knap, 1999). Knap (2000) discusses other factors influencing maintenance and concludes that only a limited proportion of the total variance in maintenance is related to body (growth) composition. It may be argued that selection for improved performance at a given feed intake results in an indirect selection response in lower maintenance requirements.
Parameters of the Growth Model Reflect Observed Performances
The level of maximum protein deposition is assumed to be 150 g in this study but differs between genotypes and is higher in entire males than in females (Campbell et al., 1985). The assumed level of 150 g is similar to the maximum protein deposition of 151 g and 156 g observed in Large White and Pietrain castrates by Quiniou et al. (1996). In contrast, Rao and McCracken (1992) and Quiniou et al. (1995) reported a maximum protein deposition of 194 g and 171 g for entire males.
The level of feed intake varies from 19 MJ ME to 38 MJ ME per day in this study and reflects mean voluntary feed intake levels observed in other studies. For example, the mean voluntary feed intake of pigs was 30.0 MJ DE (28.8 MJ ME) per day in a Pietrain population (Quiniou et al., 1995). Bikker (1994) found a voluntary feed intake of around 40 MJ DE (38.4 MJ ME) per day.
A reduction in feed intake causes the largest decrease in protein deposition in conjunction with a low MR. Protein deposition decreased by 6.1, 4.2, and 3.2 g per MJ ME decrease for b-values of the MR of 0.03, 0.05, and 0.07, respectively. Increase of protein deposition averaged 7.3 g/MJ DE in entire boars (Quiniou et al., 1995) in comparison to an increase in protein deposition of 3.8 g/MJ DE in hybrid gilts (Bikker, 1994). This sex effect was further confirmed by Quiniou et al. (1996) who found that each extra megajoule ME intake was associated with 6.1 g of extra protein deposition in entire crossbred Pietrain boars and 4.4 g and 4.7 g in crossbred Pietrain and Large White castrates. Corresponding increases in lipid deposition with higher levels of feed intake are similar in these studies between different types of pigs ranging from 13.2 to 13.9 g/MJ DE. In comparison, in the present study lipid deposition increased by 12.8, 14.8, and 15.7 g/MJ ME for b-values of the MR of 0.03, 0.05, and 0.07, respectively.
Profit Is Maximized for Optimum Feed Intake
The minimum level of feed intake to reach maximum protein deposition has been called optimum feed intake (De Vries and Kanis, 1992). Kanis (1995) showed in a simulation study that the highest returns per pig place per year were accomplished when feed intake was just sufficient to meet requirements for maximum protein deposition. Simulations in this study showed that profit was maximized for the optimum feed intake. The optimal feed intake minimizes feed conversion ratio and maximizes lean meat growth, two traits with a large influence on profit.
Economic Weights
Currently, breeding objectives are usually weighted by economic weights derived from an economic model. It was shown that economic weights for feed intake and growth rate depend on the level of performance in growth rate and feed intake. The use of economic models to derive economic weights for feed intake together with growth rate and LMP always leads to a negative economic weight for feed intake since a reduction in feed intake, assuming no changes in growth rate and LMP, reduces costs of production. Economic weights for LMP are constant as a result of a linear increase in price per kilogram carcass weight with higher LMP. Other payment systems are often nonlinear for lean meat also leading to changes in economic weights with different performance levels. Bioeconomic models as developed by Stewart et al. (1990) allow these nonlinear relationships to be taken into account in genetic improvement schemes.
Economic weights for parameters of the growth model depend, first, on the cost structure assumed, second, on the growth model used, and thirdly, on the level of performance in each parameter. For these reasons it is difficult to compare economic weights with other studies directly. The costs assumed in this study are lower than the costs presented in the studies of De Vries and Kanis (1992) and von Rohr et al. (1999). The influence of modifications in the growth model was analyzed by von Rohr et al. (1999) who showed that economic weights for feed intake are smaller under the modified linear-plateau model, also used in this study, in comparison to the conventional model used by De Vries and Kanis (1992). Changes in the level of parameters of the growth model analyzed in this study showed that economic weights for feed intake and MR (if feed intake is below the optimum feed intake) are of higher magnitude for low levels of each parameter. Other studies have not varied the parameters of the growth model and evaluated the effect of these variations on economic weights. Differences in economic weights for feed intake when feed intake exceeds the optimum feed intake are independent of the MR but depend on the level of feed intake and maximum protein deposition. In this study, the maximum protein deposition was kept constant and economic weights for feed intake (when feed intake exceeds the optimum feed intake) differed only slightly between the two scenarios shown.
Selection for Feed Intake
The use of an economic model to select for feed intake always places a negative economic weight on feed intake. Due to a strong emphasis on feed conversion ratio and leanness, selection has led to a reduction in feed intake (McPhee et al., 1981; Smith et al., 1991; Cameron and Curran, 1994) with the consequence that feed intake is below the optimum in some populations (Eissen, 2000). Different breeding objectives have been suggested to avoid a further reduction in feed intake selecting for lean tissue feed conversion (Fowler et al., 1976), implementing a restricted index on feed intake (e.g., Brandt, 1987) or placing a higher emphasis on growth rate (e.g., Krieter and Kalm, 1989). None of these approaches allows optimization of selection for feed intake directly. The use of a biological model in breeding programs provides an alternative breeding objective to directly select for the optimal feed intake with respect to the animal’s potential for lean meat growth (Kanis and De Vries, 1992). This alternative breeding objective is of special interest in populations where feed intake capacity is below optimum feed intake. Although growth models and input parameters might differ between studies, von Rohr et al. (1999) showed that these differences in models lead to similar expected selection responses and that differences in models have limited implications for breeding objectives and selection strategies.
Implementing Growth Models in Breeding Programs
So far, genetic (co)variances are not available for parameters of growth models describing lipid and protein deposition with varying feed intake at a given live weight. First information on genetic variation is available from stochastic simulation studies as presented by Knap (1996). In addition, Knap and Jørgensen (2000) analyzed data from a serial slaughter trial and concluded that there is animal intrinsic variation in partitioning of body protein and lipid. Serial slaughter trial data, however, are rarely available and guidelines to estimate empirical relationships between "chemical" body composition defined in growth models by lipid and protein deposition and "physical" body composition like backfat and lean meat weight have been developed by De Greef (1995). Transforming current measurements into parameters of the growth model and therefore "inverting" growth models, however, does not provide additional information. Ultimately, recording procedures have to be extended to estimate parameters of the growth model. These recording procedures require performance testing of groups of pigs (i.e., full- or half-sib groups), where individual pigs are recorded on different feeding levels. Records of individual pigs, on different feeding levels, could then be treated as "repeated measurements" of a group of pigs. The level of protein deposition differs with varying feed intake which suggests the use of covariance functions (Meyer, 1998) to describe genetic and phenotypic variation of this longitudinal data. A further aspect is that model parameters depend on live weight and repeated measurements of live weight are necessary (Eissen, 2000). For practical breeding applications, however, live weight measurements have to be taken automatically. Such a technique is available with the development of electronic feeders that allow restriction of feed intake for individual pigs and are equipped with automatic scales (B. G. Luxford, personal communication). Further, video images of pigs obtained from electronic feeders while feeding (Schofield, 1993; Ramaekers et al., 1996) may be employed to obtain repeated measurements of (physical) body composition to obtain a measurement of lipid deposition. These recent developments in automated recording of weight, feed intake, and body composition allow the production of data sets necessary to use growth models for genetic improvement.
The first applications of such models in animal breeding may be to evaluate feed intake capacity of different genotypes with respect to the optimum feed intake. Eisen (2000) has performed such a study providing information about the desired direction of selection for feed intake capacity in each genotype analyzed. The next step may then be to base the breeding objective on parameters of the growth model since only this breeding objective allows emphasis to be placed directly on optimal feed intake.
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Implications |
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TopAbstractIntroductionMethodsResultsDiscussionImplicationsLiterature Cited |
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Footnotes |
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3 Present address: Nutreco, P.O. Box 240, 5830 AE Boxmeer, The Netherlands.
Received for publication November 19, 2001. Accepted for publication September 27, 2002.
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