| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
ANIMAL GENETICS |
* INRA, UR337 Génétique Quantitative et Appliquée, 78352 Jouy-en-Josas, France;
and
INRA, UE967 Génétique Expérimentale en Productions Animales, 17700 Surgères, France;
and
INRA, UE88 Insémination Caprine et Porcine, 86480 Rouillé, France; and
INRA, UMR1079 Systèmes d’Elevage, Nutrition Animale et Humaine, 35590 Saint Gilles, France
 |
Abstract |
|---|
 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Key Words: carcass composition • growth • meat quality • pig • residual feed intake
|
INTRODUCTION |
|---|
|
TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
) and pigs (Foster et al., 1983). The trait RFI represents the fraction of total feed intake that is unexplained by maintenance requirements and production costs. Selection for low RFI might be helpful for improving feed efficiency while causing no correlated change in production traits (Kennedy et al., 1993; Archer et al., 1999).
Genetic sources of variation in RFI have been investigated in both adult animals (laying hens, lactating cows) and growing animals (cattle, pig, rabbit, and fish; e.g., Korver et al., 1991; Tixier-Boichard et al., 1995; Arthur et al., 2001; Silverstein et al., 2005). There is general agreement among mammalian species that heritability of RFI is of moderate magnitude. As reviewed by Nguyen et al. (2004), most reported estimates of heritability fall within the range of 0.15 to 0.30 for RFI in Large White growing-finishing pigs fed ad libitum and housed in individual pens or in collective pens equipped with electronic feeders.
The aim of the current study was to estimate genetic parameters pertaining to RFI of growing pigs, using data originating from a divergent selection experiment for this trait. Particular attention was paid to the genetic relationships between RFI and carcass and meat quality traits, because information on this topic is thus far limited in pigs.
|
MATERIALS AND METHODS |
|---|
|
TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
This selection experiment was conducted on 2 INRA farms (Rouillé, Vienne, France, and Le Magneraud, Charente-Maritime, France), in accordance with the national regulations for humane care and use of animals in research.
Data were collected during the first 4 generations of upward and downward selection for RFI of purebred French Large White growing pigs (Gilbert et al., 2006). The base population consisted of 30 litters sired by 30 boars standing at French AI centers. From 115 male candidates, 2 divergent lines were established from this common foundation stock. Both lines have been maintained contemporarily with 6 boars selected by line and generation and mated with 35 to 40 sows. The sow herd was partitioned between the 2 farms, and experimental litters were produced from fresh semen (first-parity litters, P1) or frozen semen (second-parity litters, P2) of boars that stood at the INRA-UEICP AI center (Rouillé, Vienne). The same mating design was applied in both parities. Mass selection was applied in males (with a percentage selected of around 7%), whereas no selection was done on the female side, each dam being replaced randomly by 1 daughter. Breeding boars and gilts were chosen among the offspring born in P1 litters. Second-parity litters were produced for estimating direct and correlated responses to selection in females and castrated males.
All animals were raised in the same postweaning unit in Rouillé, whatever their farm of birth, and then were recorded for RFI in pens equipped with 1 single-place electronic feeder ACEMA 64 (Pontivy, France; Labroue et al., 1994; 8 to 12 animals per pen). Facilities available in Rouillé comprised 4 rooms of 4 pens each; a contemporary group is defined as the group of around 40 animals contemporarily tested in a given room with animals penned by line, and sex when necessary. Animals were offered a pelleted diet based on cereals and soybean meal containing 10 MJ of NE/kg and 160 g of CP/kg, with a minimum of 0.80 g of digestible Lys/MJ of NE.
Traits Recorded on P1 Animals
The criterion for selection of the divergent lines, defined as actual minus predicted feed intake, was measured on P1 group-housed boars fed ad libitum from 35 to 95 kg of BW. Daily feed intake (DFI1) was recorded by electronic feeders, and feed conversion ratio (FCR1) was calculated. The 2 traits used to calculate the predicted feed intake by multiple linear regression were ADG (ADG1) from 35 to 95 kg of BW and backfat thickness measured by ultrasonic scanning (UBT), averaged from 6 measures at 95 kg of BW (right and left shoulder, right and left midback, right and left loin), using an ALOKA SSD-500 echograph (Aloka, Cergy Pontoise, France).
Animals were weighed weekly around the intended start-of-test and end-of-test BW for determining the day on which they attained 35 and 95 kg of BW, respectively. Candidates for selection were therefore tested over a fixed BW range, and variation in maintenance requirements, as usually predicted from metabolic BW (BW0.6; Noblet et al., 1999), consequently did not need to be taken into account. The criterion for selection used in this experiment had the following form: DFI1, g –(1.06 x ADG1, g) – (37 x UBT, mm), where ADG1 and UBT are expressed as the deviations of individual values from the contemporary group average value. Coefficients of the multiple linear regression equation used to compute predicted feed intake were derived from a previous data set dealing with 3,539 French Large White boars tested from 35 to 95 kg of BW under ad libitum feeding conditions (Labroue et al., 1999).
For the needs of the current genetic analysis, RFI of P1 boars (RFI1) was computed using their own records for DFI1, ADG1, and UBT and taking into account the fixed effects of contemporary group and pen size (7 or less, 8 to 9, 10, 11, or 12 animals per pen); RFI1 was defined as follows: RFI1, g = DFI1, g – (1.24 x ADG1, g) – (31.9 x UBT, mm). The R2 of the model used to compute the predicted feed intake of P1 animals was 0.66.
Traits Recorded on P2 Animals
For pigs from P2 litters (1 female and 1 castrated male per litter), the entire growing period was considered for the test, lasting from 3 d after the pigs entered the pens equipped with electronic feeders [average BW (BW0) = 28 kg] until the day before slaughter [average BW (BW1) = 107 kg]. Animals were penned by sex and line, and data were collected for ADG (ADG2), daily feed intake (DFI2), and feed conversion ratio (FCR2). At the end of the test, a fasting period of approximately 20 h was used; then, pigs were weighed to obtain slaughter weight. Transport from the farm to the abattoir (Cellessur-Belle, Deux-Sèvres) lasted for around 1 h. Shortly after slaughter, the hot carcass (with head) was weighed (HCW), and dressing percent was defined as the ratio of HCW to slaughter weight. Carcasses were then chilled in a cooling room at 4°C. The day after slaughter, carcass length (from the atlas to the anterior edge of the pubic symphysis), carcass backfat thickness (average of 3 measurements taken on the middorsal line at the level of shoulder, last rib, and hip joint), and weight of head were measured. One-half of each carcass was submitted to a normalized cutting procedure, and weights of ham, loin, belly, shoulder, and backfat were recorded. Lean meat content (LMC) was estimated from a linear combination of the weights of carcass ham, loin, and backfat, expressed as a percentage of the half-carcass weight (Métayer and Daumas, 1998).
Residual feed intake of P2 animals (RFI2) was defined as follows:
 |
where AMBW is the average metabolic BW during the test period (Noblet et al., 1999) and is equal to (BW11.6 –BW01.6)/[1.6 (BW1 – BW0)]. The model used to compute RFI2 included the fixed effects of contemporary group, sex, and pen size. The R2 of the model used to compute the predicted feed intake of P2 animals was 0.76.
Meat quality measurements were performed at 24 h postmortem. Ultimate pH measurements were taken on adductor femoris, semimembranosus (SM), gluteus superficialis (GS), LM, and semispinalis capitis muscles using a Knick Portamess 911 pH meter with a glass electrode (Elvetec Services, Genas, France). Meat color was assessed on GS and gluteus medius muscles concerning the 3 coordinates L*, a*, and b* of the CIELAB color space using a Minolta CR-300 photocolorimeter (Minolta, Carrieres S/Seine, France); L* refers to lightness of meat (a lower value is associated with darker meat), whereas a* and b* values represent the degrees of greenness-redness (redder meat for greater a* value) and blueness-yellowness of the meat, respectively. Water-holding capacity (WHC) was assessed on the GS muscle using a piece of filter paper put on the freshly cut surface of the muscle and measuring the time (in tenths of seconds) required for the paper to become wet (Charpentier et al., 1971); a greater value is associated with better WHC. A linear combination of 3 of the above measurements was used as a global meat quality index (MQI; Tribout and Bidanel, 2000), which is defined as a predictor of the technological yield (ratio of the weight of cooked ham to the weight of defatted and boneless fresh ham) in cured-cooked ham processing.
Statistical Analyses
Variance-covariance components were estimated using REML methodology (Patterson and Thompson, 1971) applied to a multitrait individual animal model. A series of 3- or 4-trait analyses were performed using version 5.0 of the VCE software package (Neumaier and Groeneveld, 1998). All VCE runs included the criterion for selection as 1 of the traits to properly account for the effects of selection for RFI in the upward and downward lines (Hofer, 1998). There were 1,446 animals in the pedigree file tracing back to at least the great grandparents of the foundation males and females.
The fixed effects included in the model were contemporary group (21 levels), farm of birth (2 levels), and pen size (5 levels) for the traits recorded on P1 male candidates for selection. For the traits recorded on P2 females or castrated males, the fixed effects were contemporary group (16 levels), farm of birth (2 levels), pen size (5 levels), and sex (2 levels) for growth and carcass composition traits and the same effects as above with the addition of slaughter day (53 levels) for meat quality traits. Slaughter BW was included as a covariate in the model used for carcass traits, except for weights of cuts, which were analyzed using HCW as a covariate. Approximate standard errors of estimated genetic parameters were obtained from an approximation of the Hessian matrix when convergence was reached.
|
RESULTS |
|---|
|
TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
. Total numbers of P1 and P2 animals were 793 and 657, respectively.
|
Estimated phenotypic and genetic parameters for RFI and growth traits are presented in Table 2 for P1 animals and in Table 3 for P2 animals. Estimates of heritability were 0.14 ± 0.03 and 0.24 ± 0.03 for RFI1 and RFI2, respectively, whereas the estimated genetic correlation between the 2 traits was 0.91 ± 0.08. Heritability estimates were in the range of 0.23 to 0.45 for ADF, daily feed intake, and feed conversion ratio in P1 and P2 animals, whereas the heritability estimate for UBT was 0.63 ± 0.05. As expected, RFI1 was phenotypically uncorrelated with ADG1 and UBT, namely the 2 traits used for calculating the predicted feed intake. Regarding the relationships between RFI and ADG or daily feed intake, there was an excellent agreement between the estimates of phenotypic and genetic correlations in P2 animals but not in P1 animals. No genetic association was found between RFI2 and ADG2, whereas the estimated genetic correlation between RFI1 and ADG1 was –0.35 ± 0.13. The correlation between FCR1 (or FCR2) and RFI1 (or RFI2) was close to 0.70 at both phenotypic and genetic levels. The genetic relationship between RFI and daily feed intake showed a different picture in P1 and P2 animals; the estimated genetic correlation was strongly positive between RFI2 and DFI2 (0.77 ± 0.10) and weakly positive between RFI1 and DFI1 (0.16 ± 0.13).
|
|
Estimated genetic parameters relating to carcass composition traits of P2 animals are given in Table 4. Heritability estimates ranged from 0.23 for shoulder weight to 0.69 for carcass length. The greatest phenotypic correlation (0.20) was found between RFI2 and carcass backfat thickness. There was a tendency for a negative correlation between RFI2 and dressing percent at the genetic level. Estimated genetic correlations between RFI2 and traits related to the amount of back-fat depots were positive (around 0.40). Conversely, the estimate of genetic correlation between RFI2 and carcass LMC was negative (–0.55 ± 0.14), in accordance with similar negative genetic correlations between RFI2 and ham weight or loin weight.
|
Table 5 reports genetic parameter estimates pertaining to meat quality traits measured on P2 animals. Heritability estimates ranged from 0.19 to 0.43 for ultimate pH recorded on 5 muscles and from 0.21 to 0.35 for L*, a*, and b* values of the GS muscle. Heritability estimates were much lower (around 0.10) for L*, a*, and b* values of the gluteus medius muscle and for WHC of the GS muscle. The heritability estimate for the MQI was 0.41 ± 0.06. At the phenotypic level, slightly positive correlations were found between RFI2 and ultimate pH measurements, whereas slightly negative correlations (around –0.10) were obtained between RFI2 and L* values. Regarding estimates of genetic correlations, the positive relationship between RFI2 and ultimate pH, as well as the negative relationship between RFI2 and L* values, are noteworthy. These relationships contributed to the high genetic correlation (0.77 ± 0.14) between RFI2 and MQI, showing that lower RFI is genetically associated with a decrease in meat quality as assessed in this study (lower ultimate pH and lighter color).
|
|
DISCUSSION |
|---|
|
TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Our heritability estimates for ADG, daily feed intake, and feed conversion ratio of both P1 and P2 animals fell within the range of literature values for pigs having ad libitum access to feed (Clutter and Brascamp, 1998). They were in good agreement with heritability values found in the most recent studies pertaining to the French Large White breed (Labroue et al., 1997; Tribout and Bidanel, 2000). Phenotypic and genetic interrelationships among ADG, daily feed intake, and feed conversion ratio in both P1 and P2 animals (results not shown) were also consistent with those previously reported for growing pigs having ad libitum access to feed (Labroue et al., 1997; Clutter and Brascamp, 1998).
As far as RFI is concerned, the heritability estimated here for P1 boars (0.14) is among the lowest values available in the literature, whereas that found for P2 females and castrated males (0.24) is very close to the average literature value quoted by Nguyen et al. (2004). Heritability estimates for DFI1 and DFI2 were similar. The difference in heritability estimates for RFI1 and RFI2 could partly result from the fact that the predicted feed intake was determined more accurately in P2 than in P1 animals (R2 = 0.76 vs. 0.66 for the accuracy of the multiple regression equations used for computing predicted feed intake). Our heritability estimate for RFI1 is comparable to those found by Cameron and Curran (1994) and Johnson et al. (1999) for individually penned pigs and by Von Felde et al. (1996) for pigs tested in collective pens equipped with ACEMA electronic feeders. Heritability values close to 0.25 were reported by Labroue et al. (1999) and Nguyen et al. (2005), whereas values of 0.30 to 0.45 were found by Foster et al. (1983), De Haer and De Vries (1993), and Mrode and Kennedy (1993). A wide range of heritability estimates has similarly been found for RFI in growing beef cattle, as reviewed by Archer et al. (1999) and Herd et al. (2003).
In the current study, estimated genetic correlation between RFI and ADG was zero in P2 females and castrated males, whereas it was moderately negative in P1 boars. A strongly negative genetic correlation (around –0.70) between RFI and lifetime ADG was reported by Nguyen et al. (2005). From a literature review, Nguyen et al. (2004), however, concluded a slightly positive genetic correlation (around 0.20) exists between RFI and growth rate. Altogether, available information suggests that the genetic correlation between RFI and ADG is likely to be of small magnitude.
The positive values found here for phenotypic and genetic correlations between RFI and daily feed intake or feed conversion ratio agree with those reported in the study of Labroue et al. (1999), dealing with French Large White pigs, and other studies reviewed by Nguyen et al. (2004).
From the genetic parameter estimates found in the current and past studies, selection for low RFI in ad libitum-fed growing pigs has the potential to improve feed conversion ratio and reduce feed intake, whereas ADG would be nearly unaffected.
Carcass Composition Traits
Our estimates of heritability for carcass composition traits are in good accordance with average literature values reviewed by Sellier (1998). The phenotypic correlation between RFI1 and UBT in candidates for selection was very close to zero, as expected from the statistical properties of RFI (Kennedy et al., 1993) and because UBT was used for determining the predicted feed intake. Similarly, the phenotypic correlation between RFI2 and LMC was near zero. Estimated genetic correlations between RFI2 and carcass backfat thickness or backfat weight were on the order of 0.40 in our study. A positive genetic correlation between RFI and backfat thickness, averaging 0.24, was also quoted by Nguyen et al. (2004) from earlier published estimates. The negative genetic correlation found here between RFI2 and carcass LMC is in accordance with the genetic correlation of around –0.30 between RFI and loin muscle area found by Johnson et al. (1999) in Large White pigs. In Angus cattle, Arthur et al. (2001) found that RFI is genetically independent of rump fat thickness and LM area. A positive genetic correlation between RFI and carcass fatness (around 0.3 to 0.4) was reported by Renand et al. (1998) in Charolais and Limousin cattle and by Herd and Bishop (2000) in Hereford cattle, whereas a negative value for this parameter (–0.45) was found by Jensen et al. (1992) in dual-purpose breeds.
Another feature of our study is the moderately negative genetic correlation between RFI2 and carcass dressing percent. We are not aware of any published studies on pigs in this respect, but a genetic correlation of around –0.20 between RFI and dressing percent was reported in cattle (Jensen et al., 1992). Selecting for low RFI could therefore result in heavier carcass weight at a given slaughter live weight. Owing to the positive genetic association found here between RFI2 and DFI2, this might be explained by lesser development of the digestive tract in animals having lower feed intake. A negative genetic correlation between daily feed intake and dressing percent (–0.20) was reported by Labroue et al. (1997) in Large White pigs under ad libitum feeding conditions. It may also be hypothesized that high-RFI animals show greater physical activity and aggressiveness when they are mixed with socially unknown animals during transport from the farm to the abattoir and during the resting period at the abattoir, which could induce increased BW loss during this period and, thus, decreased dressing percent.
Meat Quality Traits
Most heritability estimates found here for meat quality traits are within the range of literature values quoted by Sellier (1998). However, some of our estimates tend to be greater than the corresponding values reported for the same breed (French Large White) by Tribout and Bidanel (2000); for example, these authors found a lower heritability (0.24 vs. 0.41) for the MQI used in both studies.
Regarding the genetic relationships between RFI2 and the meat quality traits recorded in the current study, our results point out that RFI2 is positively associated with MQI (ra = 0.77) due to the strong positive correlation with ultimate pH of SM muscle, and, to a lesser extent, the negative correlation with L* value of GS muscle. To our knowledge, only 1 earlier investigation (De Vries et al., 1994) dealt with genetic relationships between RFI and pork quality. Our results are consistent with those of this large-scale study involving Dutch Yorkshire pigs.
Thus, the current study indicates that selecting pigs for low RFI is expected to lead to pork with lower ultimate pH and lighter color. A decrease in oxidative capacity of skeletal muscle might be responsible for these correlated effects, possibly due to changes in the respective proportions of myofibers of glycolytic vs. oxidative metabolic type. Genetic correlations of percentage of glycolytic IIBw myofibers with ultimate pH and L*-value are significantly negative and positive, respectively, in Large White pigs (Larzul et al., 1997). Another explanation could lie in the influence of differential behavior during the preslaughter period (from the beginning of fasting to slaughter). Under the assumption that high-RFI pigs exhibit greater physical activity, these pigs would present at slaughter with greater depletion of muscle glycogen, which would lead to decreased muscle glycolytic potential and, consequently, increased ultimate pH of meat. High-RFI animals would also exhibit darker meat color, because ultimate pH and L* value are negatively correlated (around –0.50) in halothane-negative Large White populations (e.g., De Vries et al., 1994; Tribout and Bidanel, 2000). High-RFI animals could even be prone, in extreme cases, to the DFD meat condition (i.e., ultimate pH greater than 6.20 in SM or LM muscles).
In beef cattle, McDonagh et al. (2001) observed no difference in meat color between high-RFI and low-RFI steers resulting from a single generation of divergent selection for this trait; however, meat from high-RFI steers exhibited lower values of myofibril fragmentation index and greater levels of calpastatin, which would be expected to induce increased toughness of the meat. No adverse relationships between RFI and various beef quality traits were found, at the phenotypic level, by Baker et al. (2006).
On more general grounds, a parallel may be established between the above-mentioned genetic associations between pork quality and RFI, on 1 hand, and the formerly reported genetic associations between pork quality and feed efficiency as classically assessed by feed conversion ratio, on the other hand. Many studies dealing with Large White, Landrace, or Duroc pigs have indeed shown some genetic antagonism (ra on the order of –0.40) between various meat quality traits and feed efficiency. The pork quality traits involved in this genetic antagonism were essentially meat color (Cameron et al., 1999; Hermesch et al., 2000; Tribout and Bidanel, 2000), drip loss (Hermesch et al., 2000; Lonergan et al., 2001), ultimate pH (Tribout and Bidanel, 2000), and shear force value (Lonergan et al., 2001).
In conclusion, the current study indicates that selection for low RFI in growing pigs having ad libitum access to feed makes it possible to improve feed efficiency without compromising growth rate in spite of the reduction in voluntary feed consumption. Carcass composition would be improved (greater muscle to fat ratio), whereas carcass dressing percent would probably be enhanced in low-RFI pigs. Regarding meat quality, selection for low RFI is expected to result in lighter color of meat and, mainly because of lower ultimate pH, decreased technological yield from processing of cured-cooked ham.
1 Corresponding author: helene.gilbert{at}jouy.inra.fr
Received for publication September 1, 2006. Accepted for publication August 3, 2007.
|
LITERATURE CITED |
|---|
|
TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
This article has been cited by other articles:
|
|
 |
|
  R. M. Herd and P. F. Arthur Physiological basis for residual feed intake J Anim Sci, April 1, 2009; 87(14_suppl): E64 - E71. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
