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ANIMAL GENETICS |
,3
* IPG, Institute for Pig Genetics, Beuningen, 6640 AA, The Netherlands;
and
Animal Breeding and Genetics Group, Wageningen University, 6700 AH, Wageningen, The Netherlands; and
and
Premium Standard Farms Inc., Milan, MO 63556
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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Key Words: Carcass Composition • Genetic Parameters • Heritability • Meat Quality • Pig
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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). For a long time, pig breeding programs focused mainly on the reduction of costs. Selection was aimed at increasing litter size, weight gain, decreasing backfat (BF), and improving feed conversion. Now breeding goals are subject to change and are directed much more toward retail carcass yield and meat quality because of the high economic value of these traits. Genetic improvement of valuable cuts of appropriate quality requires estimates of genetic parameters.
Genetic variation in meat quality has received attention in the past two decades (Cameron, 1990; Sellier, 1998; Lonergan et al., 2001). Research on genetic parameters of carcass dissection traits is limited and has received attention recently (NPPC, 1995; Newcom et al., 2002).
The objective of this study is to estimate genetic parameters for carcass and meat quality traits that are of practical relevance in combination with information from current or intended classification systems.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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Live Animal Evaluation and Housing
Piglets were born over a 2-mo period, and at birth, all piglets were individually tagged and birth weights were recorded. Piglets were weaned at an average of 17 d of age, raised in a nursery for 6 to 7 wk, and at an average weight of 22.7 kg, they were randomly allocated to one of three finishing sites (farm). Pigs were raised under commercial finishing conditions with ad libitum access to a corn–soybean diet and water. Metabolizable energy concentration of the diet ranged from 3.340 to 3.475 Mcal/kg of diet (as-fed basis). Males were castrated from 3 to 5 d after farrowing.
Slaughter and Carcass Evaluation
All pigs were slaughtered in one slaughterhouse at an average weight of 118 kg of live weight following a three-stage sampling system. In the first two successive sampling stages, all animals that had reached a visually assessed weight of 118 kg were designated for slaughter; the third stage of sampling contained the remaining individuals from a total of 1,855 pigs evaluated in this study. This system resulted in an average age at slaughter of 164, 172, and 185 d (AGE) for the first, second, and third stages of sampling, respectively. Before slaughter, the animals were held overnight in lairage with ad libitum access to water. Pigs were slaughtered on 17 different days over a 70-d period, and were grouped according to slaughter date (group). Groups could include pigs from different finishing sites as well as from different sampling stages. Whole carcass data were recorded within 45 min postmortem, whereas the remaining measurements were collected 24 h postmortem. After bleeding, scalding, and dehairing, a preeviscerated carcass weight (DCW) was recorded. Final BW was estimated from DCW using the following formula: final BW = 1.065 x DCW. For the purpose of calculating ADG, all pigs were assumed to have a birth weight of 1.36 kg, and ADG was calculated using the following equation: ADG = (final BW – birth weight)/AGE.
Measurements on the carcass were recorded using the left carcass half unless circumstances such as damage required using the right half. A Hennessy grading probe (model 4, Hennessy Grading Systems Ltd., Auckland, New Zealand) was used to record BF and loin depth (LD), both in millimeters, at the 10th rib, and a HCW was recorded after evisceration. The percentage of muscle (PLEAN) was calculated using the following equation (SFK Technologies; Herlev, Denmark): PLEAN = 58.86 – (0.61 x BF) + (0.12 x LD).
Cold carcass weight (CCW) was recorded after temperature equilibration (24 h). Primal cuts of ham and loin were weighed and further dissected into boneless subprimal cuts. The weight of the bone-in loin (LOIN) with skin removed and fat trimmed was recorded along with the bone-in, skin-on ham weight (HAM). Loins were processed to a boneless loin without fat cover (BLOIN). Hams were skinned and defatted and were fabricated into the boneless inside ham (semimembranosus, gracillis, and adductor), outside ham (semitendinosus and biceps femoris), knuckle (vastus intermedius, vastus lateralis, tensor fasciae, and vastus medialis), and light butt (gluteus medius) subprimals. The four subprimal weights added yielded a boneless ham weight (BHAM).
Meat Quality Measurements
Meat quality measurements were taken on both the loin and ham. Loin Minolta L*, a*, and b* (LOINL, LOINA, and LOINB; also referred to as the CIELAB color space) were taken on the fresh cut surface of a 2.5-cm chop removed from the sirloin end of the boneless center cut loin using a Minolta CR 300 colorimeter set at C illuminant (Minolta camera, Osaka, Japan). On the same chop, a subjective color score (1 to 6, with 1 = pale and 6 = very dark) was given to the cut surface (JCScut) using a Japanese color scale. The same system was used to score the rib surface of the loin (JCSrib). A subjective marbling score (LMARB; 1 to 5, with 1 = devoid, 2 = practically devoid, 3 = moderately abundant, 4 = abundant, 5 = overly abundant) was given to the chop based on National Pork Producers Council marbling standards (NPPC, 1991). Cores were taken from a second 2.5-cm chop, directly anterior to the first using a 25-mm coring device to determine drip loss percent (DRIP). The cores were placed in clean, dry, preweighed tubes and placed in a cooler for 24 h. Upon removal from the cooler, the tubes were weighed with and without the sample to determine the amount of exudates (Christensen, 2003). An ultimate pH measurement (pHU) was taken in the boneless loin after the two chops for color and drip loss measurement were removed, approximately 24 to 28 h postmortem. Purge loss (PURGE) was determined by cutting a 7.5- to 10-cm section from the sirloin end of the remaining boneless loin. Sections were placed in sealed plastic bags and refrigerated for 5 d. Upon removal from the cooler, samples were weighed in the bag. The sample was then removed, blotted dry with a paper towel, and weighed again to determine the weight lost, expressed as a percentage of the original sample. Subjective firmness scores (FIRM) on the NPPC (2000; 1 to 3 scale, with 1 = soft [and exudative] and 3 = firm) were assigned to chops based on firmness of the loin and distortion of the loin interface surface on the sirloin end of the loin.
Meat quality measurements taken on the ham included Minolta L*, a*, and b* values on the fresh cut surface of the inside ham muscle (HAML, HAMA, and HAMB). A subjective ham marbling score (HMARB; scale 1 to 4, with 1 = devoid of marbling, 2 = moderate, 3 = abundant, and 4 = overly abundant marbling) was given to the outside ham muscle.
Statistical Analyses
Analysis of variance was performed using the GLM procedure of SAS (SAS Inst., Inc., Cary, NC) to test for significant effects. Significant effects (P < 0.05) of sex (two classes), AGE (covariate), CCW (covariate), and GFP (20 classes) were found on almost all traits, where the latter effect represented the partly nested group (17 classes), farm (three classes), and sampling stage (three classes) combinations. All first-order interactions were examined, but none was found to be significant.
Variance and covariance components were estimated using ASReml (version 1.0; Gilmour et al., 2002). Phenotypic (co)variances were computed by adding genetic common environmental variances and residual (co)variances. The following animal model was used for all traits except for ADG, where AGE and CCW were not used as a covariate:
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Where Yijkl = trait under study; SEXi = fixed effect of ith sex (two classes, barrow or gilt); GFPj = combined fixed effect of jth group, farm, and sampling stages, (20 classes); AGE = age as a covariate; CCW = cold carcass weight (kg) as a covariate; ak = additive genetic effect of kth animal, ak~N(0, A 
); cl = random effect of lth litter, cl~N(0, I
); eijkl = residual effect, eijkl~N(0, I
); b1 = regression coefficient of Y on age; and b2 = regression coefficient of Y on cold carcass weight.
Tests for allometric relationships between CCW and other carcass traits were made; however, no significant improvements in goodness of fit were obtained using log-transformed CCW in the model.
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Results |
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. Heritability estimates and common environmental effects with their standard errors and the genetic variances are presented in Table 2.
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), although the heritability of ADG (h2 = 0.19) was lower than the average of 0.31 presented by Clutter and Brascamp (1998). Meat percent (PLEAN) is a calculated composite measure of BF and LD, and the heritability of 0.43 reflects the weight of the underlying component traits. Moderate to high heritability estimates were obtained for carcass traits (on average h2 = 0.40). Ham primal weight was more heritable (h2 = 0.40) than loin primal weight (h2 = 0.29). Heritability estimates for HAM and ham without bones and fat (BHAM = 0.39) were nearly identical. This was not observed for the loin and BLOIN. The heritability of the individual BLOIN (0.51) was higher than LOIN heritability (0.29). The common environmental effects for the carcass traits averaged 0.14.
Heritability estimates for meat quality traits ranged from 0.08 (DRIP) to 0.31 (LMARB). The Minolta color reading measurements had a heritability of 0.17 on average. The heritabilities of the subjective color measurements, both taken on the loin, JCScut and JCSrib, were higher compared with the objective Minolta color readings and were 0.22 and 0.28, respectively. The heritability estimates for the traits related to water-holding capacity were low (h2 = 0.08, 0.11 and 0.11 for DRIP, PURGE, and pHU, respectively). Marbling in ham and loin was not heritable to the same extent; LMARB (h2 = 0.31) showed a higher heritability than HMARB (h2 = 0.14).
Correlations Among Traits
Phenotypic and genetic correlations are presented in Tables 3 to 5 for growth and fatness, and carcass and meat quality traits. Generally, estimates of genetic correlations were higher in absolute value than phenotypic correlations. In a few cases, the estimated correlation was outside the parameter space (i.e., larger then 1). Phenotypic correlations generally have limited interpretive value and are therefore not discussed.
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. Generally, genetic correlations of ADG with the fatness traits recorded in this study were low. A low correlation was found between LD and PLEAN (0.19). Backfat was not correlated with LD (–0.01). Backfat, LD, and PLEAN had dissimilar correlations with growth rate (0.27, –0.01, and –0.29, respectively). The primal and subprimal cut weights were moderately to highly correlated with each other (range in absolute value = 0.22 to 0.85), with the low correlation of HAM-BLOIN (0.22) being one exception. In contrast, LOIN was higher correlated with the boneless ham (BHAM), 0.58. The HAM was moderately correlated with the BHAM (0.60). The LOIN and the BLOIN were highly correlated (rg = 0.85). A high correlation was found between the BHAM and BLOIN (0.74).
Growth is in general moderate and adversely correlated with the primal and subprimal cut weights, averaging –0.29, except for the correlation between HAM and ADG, which was found to be positive (0.39). Selection for higher growth could have a declining effect on the most valuable primal and subprimal weights. Moderately to highly negative correlations averaging –0.56 were estimated for BF with the ham and loin boneless and primal weight. Correlations of similar magnitude, although positive, were estimated for PLEAN and those primal and subprimal weights. Also loin depth was moderately to highly correlated with the loin and ham primals and subprimals, averaging 0.49.
Correlations Among Meat Quality Traits
In Table 4, the genetic and phenotypic correlations among the meat quality traits are given. The Minolta color measurements on the loin were moderately to highly correlated genetically with the corresponding measurements on the ham, except for the Minolta a* values. Minolta L* measurements were highly correlated with the Minolta b* values for both ham and loin, averaging 0.81. Minolta a* measured at the loin (LOINA) was lowly correlated with Minolta b* and L* values for both ham and loin (averaging 0.11 in absolute value). Minolta a* measured at the ham (HAMA), however, was moderately correlated with the Minolta b* and L* values on both ham and loin, averaging 0.55.
The correlations between the objective Minolta color readings and the subjective JCScut, averaging 0.66 (in absolute value), were generally high, except for HAMA (–0.16). In contrast, JCSrib was generally less correlated with the objective Minolta color readings (averaging 0.26 in absolute value), except for LOINA (0.77).
The genetic correlations estimated between the two water-holding capacity traits, DRIP and PURGE, approached 1. Generally, DRIP and PURGE were both highly correlated with HAML and HAMB (averaging 0.90). Correlations of PURGE with LOINL and LOINB were much lower (0.45 and 0.29). Estimated genetic correlations between DRIP and HAML (1), and DRIP and HAMB (1) were very high, whereas corresponding correlations between DRIP and LOINL, and DRIP and LOINB were high, but less extreme in magnitude (0.81 and 0.74, respectively). The subjective color value JCScut was highly negatively correlated with the water holding capacity traits (averaging –0.82). Correlations with lower magnitudes were found for both DRIP and PURGE, with HAMA and LOINA (averaging 0.17) and JCSrib (averaging 0.37). The correlations between pHU and the color traits showed a similar trend as observed for the water holding capacity traits.
The marbling scores on ham and loin were moderately correlated with each other (0.37). Close to zero or low correlations were estimated between marbling and the JCS values.
Correlations of Meat Quality Traits with Growth and Fatness and Carcass Traits
Table 5 summarizes genetic and the phenotypic correlations of meat quality traits with growth and fatness and carcass traits. High correlations were estimated among the meat quality traits Minolta L* and b*, JCScut, pHU, and the water-holding capacity traits (Table 4). Additionally, unfavorable correlations of high magnitude were estimated between those quality traits and growth rate, averaging 0.70 in absolute value. The correlation of ADG with pHU approaching –1 was noteworthy, as was the high correlation of ADG with DRIP (0.78). This suggests that single-trait selection on fast growth rate may lead to undesirable lower-pH meat, with decreased water-holding capacity and paler color. Daily growth (ADG) had low genetic correlations with both marbling measurements.
Correlations near zero were estimated for BF and PLEAN with both water-holding capacity traits, the subjective meat color traits, and pHU (averaging 0.09 in absolute value). The estimated correlations for BF and PLEAN with the objective Minolta measurements averaging 0.43 were of favorable moderate magnitude. Backfat, LD, and PLEAN were similarly and moderately correlated with both marbling traits and FIRM (averaging 0.30 in absolute value) and were dissimilarly correlated with growth rate (0.35, –0.01, and –0.29 respectively). Firmness (FIRM) was moderately correlated with JCS, DRIP, PURGE, and pHU. Low correlations were obtained between both marbling traits and the traits JCS, DRIP, PURGE, and pHU.
Both marbling traits were in general lowly correlated with the ham and loin primal and boneless weights, averaging 0.12 in absolute value, except for the correlation between HAM and HMARB (0.66). Favorable low to moderate correlations averaging 0.22 in absolute value (range –0.41 to 0.35) were obtained for DRIP, PURGE, and pHU, with both ham and loin primal and subprimal weights. In addition, the correlations for the subjective color traits with the ham and loin primal and subprimal weights averaging 0.44 were of favorable moderate to high magnitude. However, HAM and BHAM were somewhat less correlated (averaging 0.32) with the subjective color readings than LOIN and BLOIN (averaging 0.55). The Minolta color traits were also moderately and favorably correlated with the primal and boneless weights of both, ham, and loin (averaging in absolute value 0.34 and 0.43, respectively), except for HAMA, which was found to be unfavorably correlated with BHAM and BLOIN.
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Discussion |
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Experimental Setup
The experiment was set up to facilitate the identification and validation of QTL. The aim, therefore, was to get approximately 100 offspring per sire. The 20 sires were chosen as representatives for the line based on pedigree information. Pedigree of the dams was not available.
To avoid an overestimation of the additive genetic variances, the random effects of common litter environment were included in the statistical model. The common environmental effect comprised the effect of a shared environment by littermates (i.e., uterine and rearing effects) but also possible effects due to phenomena as dominance and paternal imprinting. Estimates of common environmental effects were in the range of 0.04 to 0.18, which indicates that the contribution of the common environment is noticeable on the ultimate values at slaughter for some traits.
Heritabilities
Growth and fatness traits were in general moderately heritable (Table 2), except for ham marbling and loin depth, for which estimated heritabilities were of lower magnitude. The ADG heritability estimate in this study was low compared with literature values. No clear explanation can be offered for this finding. In addition, published estimates vary considerably, ranging from 0.13 to 0.57 (Lo et al., 1992; NPPC, 1995; Larzul et al., 1997; Gibson et al. 1998; Hermesch et al., 2000a). The heritability estimate of BF in this study is close to the average literature value of 0.49 (Clutter and Brascamp, 1998).
Overall, the estimated heritability for the brightness of meat, measured in an objective way at the loin (LOINL), was in agreement with the estimates found in earlier studies by Cameron (1990), De Vries et al. (1994), Larzul et al. (1997), Hermesch et al. (2000a), and Andersen and Pedersen (2001). For LOINA, the estimated heritability in this study was lower than the range of 0.54 to 0.57 published by Sonesson et al. (1998) and Andersen and Pedersen (2001). The estimated heritability for the meat yellowness of the loin (LOINB) was in agreement with the estimates published by Andersen and Pedersen (2001). Based on a different measurement method, Sonesson et al. (1998) reported a higher heritability (0.54). No heritability estimates were found in the literature for objective color scores measured on the ham primal or subprimals.
Heritability estimates for the subjective color scores, JCScut and JCSrib, were of moderate magnitude. The estimates for both traits correspond to literature values (Hovenier et al., 1993; NPPC, 1995; Andersen and Pedersen, 2001). The heritabilities of the subjective color measurements were higher than those of the objective Minolta color readings. This was not as expected because the environmental effects may be less standardized for subjective measurements. One explanation may be the different methods of measuring: following a continuous scale for the objective (Minolta) measurements or the score in classes for the subjective (JCS) color measurements. Another reason may be that the JCS measurement represents an overall color score, whereas the three Minolta measurements represent single color spectra only. It seems worthwhile to investigate opportunities to combine the three Minolta values.
The estimated heritabilities for the water-holding capacity traits (DRIP and PURGE) were lower than most estimates presented in literature, which ranged from 0.08 to 0.30 (Hovenier et al., 1993; De Vries et al., 1994; Sonesson et al., 1998; Hermesch et al., 2000a). Differences between studies may be due to varying measurement methods and time points. The reviews of Hovenier et al. (1993) and Sellier (1998) reported large differences in heritability estimates due to varying measurement methods for the water holding capacity traits. Moreover, large differences in heritability estimates between breeds for drip loss percent were observed by Hermesch et al. (2000a). Furthermore, the presence or absence of ryanodine receptor gene (halothane gene causing malignant hyperthermia syndrome) segregation influences the heritability estimate.
The estimated heritability for pHU of 0.11 was close to the range of published estimates of 0.13 to 0.20 (Cameron et al., 1990; Lo et al., 1992; De Vries et al., 1994; Larzul et al., 1997; Hermesch et al., 2000a; Andersen and Pedersen, 2001), although several high heritabilities (0.39 to 0.45) for pHU were found in literature as well (Hovenier et al., 1993; NPPC, 1995; Sonesson et al., 1998). In contrast to the published estimates, common environmental effects were corrected for in our study. The low heritability of pHU may limit genetic progress for this trait. When using pHU as an indicator trait, one needs to realize that with restricted amounts of genetic variation, the ability to consistently rank sires for their genetic potential is also limited.
The heritability estimates of HMARB (0.14) and LMARB (0.31) differed from each other, although not significantly. Most literature values are related to loin marbling and range from 0.13 to 0.31 (Lo et al., 1992; NPPC, 1995; Gibson et al., 1998; Sonesson et al., 1998).
The heritability estimates obtained for the carcass traits are among the highest in this study, with similar estimates for the HAM and BHAM, and an increasing value from LOIN and BLOIN. Those high values offer good opportunities to select for higher ham and loin muscle yield, representing the most valuable parts of the carcass. Although high, the estimates in this study were lower than the estimates reported by Newcom et al. (2002). They presented heritabilities of 0.57 and 0.51 for HAM and LOIN, and 0.76 and 0.72 for the boneless subprimals of ham and loin, respectively.
Correlations Among Growth and Fatness and Carcass Traits
The hams and loins are the most valuable parts of the carcass. Although the quality of the meat is of increasing importance, meat yield remains decisive for carcass value, certainly in markets with pricing systems based on primal and subprimal cut weights. Ham weight (HAM) was lowly to moderately correlated (rg = 0.31) with loin weight (LOIN), indicating that selection for high primal ham yield does not necessarily result in a high loin primal yield (Table 3). The BHAM and BLOIN were more highly correlated (0.74). This implies that selection on high ham muscle yield will also lead to increased loin muscle yield. Both primals for ham and loin were highly correlated with their own subprimals, although the estimated correlation between loin primals and subprimals was higher, which may suggest a difference in fat deposition on ham and loin. Little is published about genetic correlations between primals and subprimals. Recently, Newcom et al. (2002) presented correlations between primals and subprimals. They also showed high genetic correlations between ham primal and the boneless ham subprimal (0.89), and between both primal cuts for ham and loin (0.62).
Correlations Among Meat Quality Traits
Generally high correlations were found between most meat quality traits (Table 4). The water-holding capacity traits (DRIP and PURGE), pHU, and the color traits (LOINL, HAML, LOINB, HAMB and JCScut) were especially highly correlated. In line with the phenomena of DFD or PSE meat, the estimates in this study indicate that meat with a high pHU tends to be darker and dry, whereas meat with a lower pHU tends to be more pale and exudative. The high genetic correlations are in agreement with previous studies by De Vries et al. (1994), Gibson et al. (1998), Sonesson et al. (1998), Hermesch et al. (2000b), and Andersen and Pedersen (2001). The high correlations offer possibilities to improve meat quality by measuring a limited number of traits only, which will decrease costs and labor in the slaughterhouse. Ultimate pH is quick and easy to measure and is therefore a good candidate trait that can be used as a quality predictor trait.
The correlation between JCSrib and JCScut of rg = 0.53 is reasonable for subjective measurements. Nonetheless, the differences in correlation of the two traits with the objective color readings suggest that both JCS measurements cannot be considered simply as identical traits.
Marbling scores on both ham and loin were moderately correlated with each other, indicating that marbling in ham or loin may be genetically different; this is supported by the difference in heritability estimates of both traits. However, small differences between both scores may be due to varying measurements, as loin marbling was judged at a chop and ham marbling at the outside surface of a muscle, and the measurements followed different score classes.
Correlations Among Meat Quality and Carcass Traits
Most meat quality traits have a favorable relationship with the primal and subprimal weights of the ham and loin (Table 5), which implies that selection on individual cut weight will also result in improved meat quality. Correlations between subprimals of ham and loin (Table 3) also were favorably high (rg = 0.74), implying that selection on ham only will positively affect loin muscle development or vice versa. The favorable correlations between cut weights and meat quality are striking and in contrast to the general impression (see below). No literature was found presenting correlations of primal or subprimal cut weight with meat quality traits.
Correlations Among Main Selection Traits and Carcass and Meat Quality Traits
Average daily gain and BF have been the main selection traits (among the finishing traits) in the pig breeding industry. Based on estimates obtained in this study, selection on ADG will have a detrimental effect on meat quality traits. In particular, the unfavorable effect of ADG on pHU and DRIP is remarkable (Table 5), which implies that selection for fast growth rate will lead to paler pork with lower pH meat and a decreased water-holding capacity. Comparable estimates from other publications were inconsistent. De Vries et al. (1994) and Hermesch et al. (2000b) showed no clear relationship between growth rate and meat quality traits. Lo et al. (1992) and Hovenier et al. (1993) found a positive correlation between ADG and meat quality traits; however, the general impression is that selection for improved performance goes together with negative effects on meat quality characteristics. Sellier (1998) presented an average genetic correlation of –0.23 between meat quality index (IQV) and carcass lean percent. Rosenvold and Andersen (2003) underlined this conclusion in their review.
The unfavorable correlations between ADG and quality traits approached unity. Checks of the results were performed by estimating the parameters on several subsets of the data. The additional analyses all obtained similar results. In addition, correction of ADG for backfat to create a "fat-free" gain yielded similar results. However, heritabilities of both ADG and meat quality traits were in the range expected, as was the genetic correlation between ADG and backfat.
Backfat depth was not correlated with LD (–0.01) and can be considered as genetically different. Selection for leaner carcasses will not affect meat quality defined by the quality indicators as measured in this study (Table 5). Most of the objective and subjective color traits were favorably correlated with lean meat percent. Only a negative effect of PLEAN on Minolta a* values for both ham and loin was observed. Correlations close to zero were found between both water holding capacity traits and PLEAN. Ultimate pH was favorably correlated with PLEAN. Genetic correlations between PLEAN with pHU and color traits vary substantially in the literature; Hovenier et al. (1993), De Vries et al. (1994), and Sonesson et al. (1998) presented generally negative or close to zero correlations between PLEAN and pHU or meat color traits.
The general impression that selection for improved performance goes together with negative effects on meat quality characteristics was confirmed by this study only when considering growth and fatness traits. When considering primal and subprimal cut weights, strikingly favorable correlations with meat quality were found.
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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Footnotes |
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3 Current address: Nutreco Breeding Research Centre, P.O. Box 220, 5830 AE, Boxmeer, The Netherlands.
2 Correspondence: P.O. Box 43 (phone: +31(0)24 6779999; fax: +31(0)24 6779800; e-mail: rik.van.wijk{at}ipg.nl).
Received for publication June 30, 2004. Accepted for publication November 3, 2004.
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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