Heterosis for litter size and growth in crosses of four strains of Iberian pig

Livestock Science 147 (2012) 1–8

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Heterosis for litter size and growth in crosses of four strains of Iberian pig J.M. Garcı´a-Casco a,n, A. Ferna´ndez b, M.C. Rodrı´guez b, L. Silio´ b a b

´rico, INIA, Ctra. EX-101, km 4.7, 06300 Zafra, Spain Centro de I þD en Cerdo Ibe ˜a, km 7, 28040 Madrid, Spain Departamento de Mejora Gene´tica Animal, INIA, Ctra. La Corun

a r t i c l e i n f o

abstract

Article history: Received 7 September 2011 Received in revised form 9 March 2012 Accepted 12 March 2012

Data collected from a complete diallel cross scheme with four ancient lines of Iberian pigs have been analyzed in order to estimate genetic line and heterotic effects on litter size and growth traits. Records of the number of piglets born alive (NBA) and total number of piglets born (TB) from 2768 litters of 817 dams were analyzed using different animal models fitting crossbreeding parameters. Estimated values of genetic correlations between NBA or TB records at early and later parities were close to 0.60. Significant differences in litter size at first and second parities were observed between some dam lines, but not at the third and later parities. Negligible progeny specific heterotic effects on litter size traits were estimated in early parities, but they were significant at the third and later parities (ranging from þ 0.5 to þ 1.0 alive piglets and from þ0.6 to þ 0.9 born piglets). All these results reinforce the evidence supporting the hypothesis of a partially different genetic basis of litter size at early and later parities. Growth performance in the final fattening period (100–160 kg) of purebred and crossbred pigs was analyzed with a Bayesian procedure modeled by a linear growth function. Data consisted of 2103 weight records from 579 pigs of 352–493 days of age. Relevant heterotic effects on weight at 420 days of age (intercept of the growth function) were inferred for the different crosses with mean values ranging from þ 12.7 to þ 17.7 kg. Heterotic values for daily growth rate (slope of the linear function) were not significantly different from zero for five out of the six combinations of lines but an important heterosis (þ 66 g/d) was estimated between the remaining combinations. The implications of this remarkable heterosis between Iberian pig lines on breeding schemes are discussed in the context of the current breeding structures of this breed. & 2012 Elsevier B.V. All rights reserved.

Keywords: Litter size Growth Heterosis Bayesian analysis Iberian pig

1. Introduction The Iberian breed had its origin prior to the development of European breeds, which were mainly based on breed standards and registered in herdbooks by breed societies. For centuries, this breed was extensively farmed in the sparse woodlands of the Southwest of the Iberian peninsula to satisfy the high demand for animal fats. Without selective

n

Corresponding author. Tel.: þ34 924563400; fax: þ34 913478743. E-mail address: [email protected] (J.M. Garcı´a-Casco).

1871-1413/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.livsci.2012.03.005

preponderance of any group of breeders, its genetic singularity reflects two particular features: the effect of adaptation to alternating environmental conditions derived from seasonal availability of feeding resources and semiarid continental climate (Silio´, 2000), and the absence of introgression of genes of Asian breeds usual in the Western pig breeds (Alves et al., 2003; Ojeda et al., 2008). Selective breeding, demographic fluctuations and scarce genetic flow among herds led to the development of local varieties showing phenotypic and genotypic differences (Fabuel et al., 2004). Animal breeders are aware that crossbreeding of two breeds or lines has usually positive effects on the

2

J.M. Garcı´a-Casco et al. / Livestock Science 147 (2012) 1–8

productive performance of F1 offspring. Heterosis and breed/line effects together are the primary genetic components of efficiency in pig crossbreeding systems. The extent of heterosis will depend on the differences in gene frequencies between the populations at crossing (Falconer, 1981). Contrary to plant breeding, diallel mating schemes have been scarcely performed in farm animals, and focused on identifying the most productive genetic combinations (Comerford et al., 1988; Orengo et al., 2009). The genetic heterogeneity of most livestock populations poses an additional difficulty for estimating crossbreeding parameters because the distribution of progenies of sires and dams is unbalanced across crossbreeding groups. Komender and Hoeschele (1989) have shown the advantages of animal models, accounting for all the genetic relationships among individuals, to avoid biased estimates in the analysis of unbalanced crossbreeding experiments with undesirable associations of genotypes and crosses. Repeatability animal models have been commonly used for the genetic evaluation of litter size, assuming that the litters farrowed in different parities are under the same genetic control. But strong evidence of a different additive genetic basis for litter size across the reproductive lifespan of the sow has been reported for several pig breeds (Irgang et al., 1994; Noguera et al., 2002; Roehe and Kennedy, 1995; Serenius et al., 2003). A previous study confirmed in Iberian pigs a partially different genetic control of litter size and weight in the first two and in later parities (Ferna´ndez et al., 2008). Consequently, heterosis and line effects may also differ in successive parities requiring the use of a multi-trait animal model. Growth has been modeled using production functions with a reduced number of parameters related to the breeding goal. Varona et al. (1997) have implemented a Bayesian procedure which takes into account all the available weight records and allows a joint analysis of parameters of production function, (co)variance components, systematic effects and breeding values. This approach, modeled by a linear growth function, was applied to the analysis of growth performances up to 95 kg using two different datasets of progenies from a diallel cross among Iberian pig lines (Ferna´ndez et al. 2002). In the present study, we extend the use of this Bayesian technique for analyzing growth performance of finishing pigs from 100 to 160 kg of body weight. The objective of this study is to estimate heterosis and genetic line effects on litter size and growth from a full diallel cross involving the four main traditional varieties of the Iberian pig breed. 2. Material Data were collected in an experimental farm (CIA Deheso´n del Encinar, Oropesa, Spain) from a complete diallel cross scheme with four ancient lines of Iberian pigs: the Portuguese Red strains Ervideira (RE) and Caldeira (RC) and the Black Hairless strains Campanario (BHC) and Puebla (BHP) from Extremadura (Spain). The number of piglets born alive (NBA) and the total number of fully formed piglets plus stillborn (TB) were

recorded on 2768 litters born along 15 years from the 16 possible crosses among 193 sires and 817 dams of the quoted Iberian pig strains. Only natural matings were performed, being 8 months the approximate age of gilts at first service. The size and structure of data and pedigree, and the mean and standard deviation of litter size traits recorded in the first and second parities and in the third and subsequent parities are presented in Table 1. Note that all the non-founder animals of each line are related. The mean values of the inbreeding coefficient (F) were similar for each line: F¼0.06, 0.07, 0.03 and 0.04 for RE, RC, BHC and BHP, respectively. The distribution of litter records per cross and the number of sires and dams of each line are shown in Table 2. Although 79 out of the 193 boars only sired purebred progenies, the remaining 114 sired contemporary purebred and crossbred litters. In order to test the growth performance of different genetic types, two or three full-sib piglets were sampled at weaning from each one of 288 litters farrowed in three batches along successive autumns from mating of 32 boars and 208 sows. The distribution of the tested pigs per cross and the number of sires and dams of each line are showed in Table 2. A total number of 579 piglets (287 males and 292 females) were penned in groups of about 40–50 individuals of the same sex. From weaning up to about 12 months of age they were fed twice daily with a

Table 1 Size and structure of data and pedigree, and mean and standard deviation (between brackets) of litter size traits. Animals in pedigree Sows with records Litters In first and second parity In third and later parities Number born alive in first and second parity Number born alive in third and later parities Total number born in first and second parity Total number born in third and later parities

972 817 2768 1454 1314 6.63 (2.16) 7.87 (2.29) 6.90 (2.20) 8.29 (2.33)

Table 2 Number of litters and animals with records and number of sires and dams per line (between brackets) in the two analyzed data files from the crossbreeding scheme among four lines of Iberian pigs: Ervideira (RE), Caldeira (RC), Campanario (BHC) and Puebla (BHP). Sire lines (No. of sires)

Dam lines (No. of dams) RE

RC

BHC

BHP

Total

Litter size RE (56) RC (50) BHC (39) BHP (48) Total (193)

(200) 406 68 66 115 655

(213) 82 428 119 66 695

(183) 80 144 351 66 641

(221) 148 82 79 468 777

(817) 716 722 615 715 2768

Growth RE (8) RC (9) BHC (7) BHP (8) Total (32)

(48) 64 26 29 18 137

(48) 31 71 18 25 145

(54) 31 24 70 18 143

(58) 30 29 28 67 154

(208) 156 150 145 128 579

J.M. Garcı´a-Casco et al. / Livestock Science 147 (2012) 1–8

diet containing 3450 kcal of digestible energy (DE) and 186 g of crude protein (CP) per kg of dry matter (DM). The average daily ration increased along this period from 1.0 to 1.5 kg per pig. In the final period of fattening, pigs were hand-fed to appetite with an average daily ration increasing from 2.0 to 4.5 kg of a diet containing 3680 kcal of DE and 146 g CP per kg DM. The analyzed data file consisted of 2103 available weight records from 579 pigs (272 purebred and 307 crossbred) with 352–493 days of age, and with 2–4 records per animal. The genetic relationships among all the tested animals were known up to the 68 founders of the herd and the pedigree consisted of 980 animal–sire–dam entries. 3. Methods 3.1. Standard mixed-model analysis of litter size data The analyses of reproductive data (NBA and TB) were performed using two different animal models: a bivariate analysis under a model with repeatability, and a multivariate analysis considering the litter size records (NBA or TB) at two parity classes (first and second parities and third and subsequent ones) to be different traits a) Repeatability bivariate animal model, expressed in a general matrix form "

yNBA yTB

#

" ¼

#"

XNBA

0

0

XTB

" þ

#

"

bNBA ZNBA þ bTB 0

WNBA

0

0

WTB

#"

pNBA pTB

#

" þ

0

#"

ZTB

eNBA

uNBA

#

uTB

#

eTB

where the column vectors yNBA and yTB represent the number of piglets born alive and the total number of fully formed piglets plus stillborn (NBA and TB), respectively. The vectors bNBA and bTB fit systematic effects on both traits, including effects of four parity classes (t ¼1, 2, 3 and Z4), two farrowing seasons (spring and autumn) and, according to the model of Dickerson (1969), four dam lines, specific litter heterotic effects for each combination of lines (six levels) and reciprocal effects for each one of these combinations (six levels).The vectors uNBA and uTB, pNBA and pTB, and eNBA and eTB are vectors of additive genetic effects, permanent environmental effects of each sow, and random residuals, respectively. The incidence matrices Xi, Zi, and Wi associate elements of bi, ui, and pi with the records in yi (i¼NBA or TB), respectively. The expectation of yi is Xibi and the variance– covariance structure of random effects of this bivariate animal model is as follows: 2 3 As2uNBA uNBA 6 6 7 6 AsuNBATB 6 uTB 7 6 6 7 6 6p 7 6 0 6 NBA 7 6 V6 7¼6 6 pTB 7 6 0 6 7 6 6 eNBA 7 6 0 4 5 6 4 eTB 0 2

AsuNBATB 2 uTB

As

0

0

0

0

0

0

0

Is2pNBA

IspNBATB

0

0

IspNBATB

Is2pTB

0 2 eNBA

0

0

0

Is

0

0

0

IseNBATB

0

where A is the numerator relationship matrix; s2uNBA, s2uTB, s2pNBA, s2pTB and s2eNBA and s2eTB are variances of direct additive genetic, permanent environmental, and residual effects, respectively, and suNBATB, spNBATB and seNBATB are the covariances between the respective effects for both traits. b) Multitrait animal model, expressed in a general matrix form 2 3 2 32 3 b1 y1 0 0 X1 0 6 7 6 76 7 0 76 b 2 7 6 y 2 7 6 0 X2 0 6 7¼6 76 7 6 y3 7 6 0 6 7 0 X3 0 7 4 5 4 54 b 3 5 b4 y4 0 0 0 X4 2 32 3 Z1 0 0 0 u1 6 76 7 0 76 u2 7 6 0 Z2 0 76 7 þ6 6 0 6 7 0 Z3 0 7 4 54 u3 5 u4 0 0 0 Z4 2 32 3 2 3 p1 W1 0 0 0 e1 6 76 7 6 7 0 0 76 p2 7 6 e2 7 W2 6 0 76 7 þ 6 7 þ6 6 0 6 7 6 7 0 7 0 W3 4 54 p3 5 4 e3 5 p4 e4 0 0 0 W4 where y1, y2, y3 and y4 represent litter size records at the first and second parities (NBA r 2, TB r 2) and at the third and subsequent parities (NBA Z 3 and TB Z 3), respectively. The vectors of systematic effects for the four traits (b1–b4) include the effects of parity order (expressed as the deviation from the first parity of each trait), two seasons (spring and autumn), four dam lines, specific heterotic effects of progenies for each combination of lines (6 levels) and reciprocal effects for each combination (6 levels) as proposed by Dickerson (1969). The vectors u1–u4, p1–p4, and e1–e4 are random additive genetics, permanent and residual effects, respectively. The incidence matrices X1–X4, W1–W4 and Z1–Z4 associate the records with the elements of the previously described vectors. The variance–covariance structure of random effects of the multivariate animal model is as follows: 3 2 As2 u1 u1 6 : 7 6 : 6 7 6 6 7 6 Asu14 6 u4 7 6 6 7 6 6p 7 6 0 6 17 6 6 7 6 7 6 : V6 6 : 7¼6 6p 7 6 0 6 47 6 6 7 6 6 e1 7 6 0 6 7 6 6 6 : 7 6 4 5 4 : e4 0 2

::

Asu14

0

::

0

0

::

::

:

:

::

:

:

::

::

As2u4

0

::

0

0

::

::

0

Is2p1

::

Isp14

0

::

::

:

:

::

:

:

:: ::

::

0

Isp14

::

Is2p4

0

::

0

0

::

0

Is2e1

::

::

:

:

::

:

:

::

::

0

0

::

0

Ise14

::

0

3

7 7 7 7 7 7 0 7 7 : 7 7 7 0 7 7 7 Ise14 7 7 : 7 5 :

0

Is2e4

All analyses were performed using the VCE-6 software (Groeneveld et al., 2008; Neumaier and Groeneveld, 1998).

3

7 7 7 7 7 7 7 0 7 7 7 IseNBATB 7 5 Is2eTB 0

3

3.2. Bayesian analysis of a linear growth function

0

The Bayesian procedure described by Varona et al. (1997) to analyze performance data from production functions was applied to the data. In the analyzed period of final fattening, the weight of n tested pigs was modeled by

J.M. Garcı´a-Casco et al. / Livestock Science 147 (2012) 1–8

4

a linear growth function, with parameters a (intercept) and b (slope), that corresponds to the weight at 420 days of age (W420D) and the rate of daily growth (GR), respectively. The kth recorded weight of the jth animal controlled at day xjk was considered as a sample from the normal distribution yjk 9aj ,bj , s2 t  Nðaj þ bj xjk , st 2 Þ 2

where rt is the variance of the performances given the production function parameters. The analysis of the parameters of the growth function requires another set of variables related to genetic and environmental relationships between production functions of n tested animals. These sources of variation were incorporated in the following bivariate animal model w ¼ X b þZu þ e where x ¼[wa, wb] is the matrix of order n  2 indicating the values of intercept and slope (a and b); b ¼[ba, bb] is a matrix of systematic effects on parameters a and b: sex (two levels), testing batch (three levels), four parental lines and specific heterotic effects for each combination of lines (6 levels) and reciprocal effects for each combination (6 levels); u¼[ua, ub] and e¼[ea, eb] are matrices of additive genetic and residual effects for both parameters, being G and R the additive and residual (co)variance matrices; X and Z are known incidence matrices. The matrix X includes, as covariates, the proportion of genes from the four parental lines of each individual (with values 0, 1/2 or 1) and the coefficients of heterotic and reciprocal effects for each combination of lines according to the model of Dickerson (1969). Under a hierarchical Bayesian interpretation, the joint posterior distribution given the performances can be explained as the likelihood of performances given the growth function parameters multiplied by the prior distribution of these parameters f ða,b, st 2 , b,u,G,R9yÞpf ðy9a,b, st 2 Þf ða,b9b,u,RÞf ðst 2 Þ f ðu9GÞf ðbÞf ðGÞf ðRÞ A detailed description of all these distributions can be found in Ferna´ndez et al. (2002) and Varona et al. (1997). The marginalization of the posterior distribution for each variable of interest was achieved using a Gibbs sampling algorithm (Geman and Geman, 1984; Wang et al., 1994). The method provides the marginal posterior distribution of all the unknowns in the pool. The inference was focused on the mean and standard deviation of the marginal posterior distributions and on the posterior probability intervals of the parameters of interest. The convergence was assessed by the double-chain method (Garcı´a Corte´s et al., 1998), doubling the sampling from different initial values of the additive variance of the slope. The length of the Gibbs sampling was 1,000,000 after discarding the first 500,000 (warm-up) and saving only one sample from each 100 iterations. 4. Results and discussion 4.1. Litter size

(S.D.¼ 2.36) total born piglets, respectively. The main results obtained from the analysis using the repeatability animal model are summarized in Table 3. The estimated values of the heritability and the permanent environmental coefficient correspond to repeatability values of r2 ¼0.15 and 0.17, for NBA and TB respectively. These values are similar to those obtained in other pig populations (Ferna´ndez et al., 2008; Rothschild and Bidanel, 1998). Parity effects on these traits (expressed as deviations from the first parity) show a remarkable increase of litter size up to the fourth parity. Previous studies with larger data sets of Iberian sows showed that litter size in this breed increases up to the fourth or fifth parity, and then slowly decreases (Ferna´ndez et al., 2008; Pe´rez-Enciso and Gianola, 1992). The estimated dam line effects expressed as deviations from the effect of the Ervideira line (lRE) show minor differences for litter size between lines. Significant litter heterotic effects on NBA were estimated for four out of the six analyzed combinations of lines, with values ranging from þ0.4 to þ0.7 piglets born alive per litter. The magnitude of estimated specific heterotic effects on TB is lower, being statistically significant

Table 3 Analysis of the number of piglets born alive (NBA) and total number of piglets born (TB) under the repeatability animal model: estimated values of heritability (h2), coefficient of permanent environmental effects (p2), genetic and phenotypic correlations (ru, rp) and effects of parity order, season (Spring minus Autumn) and crossbreeding parameters (dam line (l), specific heterosis between lines (h) and reciprocal effects (r)).

Variance ratios h2 p2 ru rp

TB mean (S.E.)

0.06 (0.02) 0.09 (0.03)

0.07 (0.02) 0.10 (0.03) 0.96 (0.02) 0.99 (0.01)

Parity order 2–1 3–1 Z 4–1

0.96 (0.11)nnn 1.23 (0.12)nnn 1.78 (0.11)nnn

1.00 (0.11)nnn 1.28 (0.12)nnn 1.99 (0.11)nnn

Season effect SA

0.30 (0.08)nnn

0.29 (0.08)nnn

Crossbreeding parameters lRC  lRE lBHC  lRE lBHP  lRE

0.30 (0.30) 0.15 (0.28) 0.07 (0.27)

0.57 (0.31) 0.29 (0.29) 0.27 (0.28)

hRERC hREBHC hREBHP hRCBHC hRCBHP hBHCBHP

0.74 0.29 0.48 0.43 0.36 0.53

rRERC rREBHC rREBHP rRCBHC rRCBHP rBHCBHP

 0.21 (0.39)  0.01 (0.39) 0.50 (0.30) 0.17 (0.30)  0.47 (0.39) 0.34 (0.39)

P o0.005. P o 0.01. Po 0.05.

nnn

Means and standard deviations of NBA and TB records were 7.22 (S.D. ¼2.31) piglets born alive and 7.56

NBA mean (S.E.)

nn n

(0.19)nnn (0.20) (0.15)nnn (0.15)nnn (0.19) (0.20)nn

0.62 0.18 0.42 0.35 0.10 0.34

(0.20)nnn (0.20) (0.15)nn (0.15)n (0.20) (0.20)

 0.25 (0.39) 0.02 (0.40) 0.29 (0.31) 0.07 (0.31)  0.53 (0.40) 0.19 (0.40)

J.M. Garcı´a-Casco et al. / Livestock Science 147 (2012) 1–8

for only three combinations of lines, with values from þ0.3 to þ0.6 piglets born per litter. These values of litter heterosis between pairs of Iberian lines are equivalent to the largest reported for litter size at birth between Western pig breeds (Cassady et al., 2002; Rothschild and Bidanel, 1998). Due to the lower litter size of the Iberian breed, they are even greater when expressed as a percentage of the respective mean traits (about 7.6% for NBA and 6.0% for TB). The results of the analysis using the multitrait animal model allow the estimation of parameters for each one of the four considered litter size traits (NBA r 2, TB r 2, NBA Z 3 and TB Z 3). Homogeneous heritabilities for all parity classes and high values of genetic correlations would be expected if most of the additive genetic effects affecting litter size at early and later parities were the same. However, some differences between h2 values are observed both for NBA and TB (Table 4). Moreover, for each one of these traits the values of genetic correlation between the first two parities and the third and later ones are clearly less than one. Similar results have been reported from other pig populations (Hanenberg et al., 2001; Serenius et al., 2003) and also in other Iberian pig lines (Ferna´ndez et al., 2008).

5

Furthermore, line and heterotic effects estimated in this study reinforce the evidence supporting the hypothesis of a partially different genetic basis of litter size at early and later parities. Thus, significant differences in litter size at the two first parities are observed between the lines Ervideira and Caldeira (lRC–lRE), but are not observed at the third and later parities. These results are consistent with previous findings in other pig breeds. In fact, significant genetic line  parity interactions for litter size and weight have been reported in a comparison of six maternal genetic lines developed by U.S. seedstock suppliers, most of them originated from Yorkshire/Large White and Landrace breeds (Moeller et al., 2004). Finally, the effects of specific heterosis on litter size traits are also conditional to parity order. Heterotic effects on NBA and TB in the two first parities (NBA r 2) were only significant between the Red Iberian lines Ervideira and Caldeira (hRERC). But the significant specific heterosis estimated for NBA Z 3 in five out of the six pairs of lines is noteworthy, with values ranging from þ0.6 to þ1.0 piglets born alive per litter. Heterotic effects on TB Z 3 are significant in three combinations of lines, with values from þ0.6 to þ0.9 piglets born per litter. Reciprocal effects for litter sizes traits were not statistically significant in any of

Table 4 Analysis of the number of piglets born alive (NBA) and total number of piglets born (TB) under the multitrait animal model: estimated heritabilities (diagonal), genetic correlations (above diagonal), coefficient of permanent environmental effects (p2) and effects of season (Spring minus Autumn) and crossbreeding parameters (dam line, specific heterosis and reciprocal effects).

Variance ratios h2 NBA r 2 TB r 2 NBA Z 3 TB Z 3 p2

NBA r 2

TB r 2

NBA Z 3

TB Z 3

0.06 (0.02)

0.99 (0.01) 0.07 (0.03)

0.59 (0.14) 0.48 (0.17) 0.11 (0.04)

0.09 (0.03)

0.10 (0.04)

0.09 (0.04)

0.68 0.61 0.93 0.10 0.10

0.94 (0.10)nnn

0.98 (0.10)nnn

0.52 (0.12)nnn

0.68 (0.13)nnn

0.31 (0.10)nnn

0.31 (0.10)nnn

0.27 (0.11)n

0.24 (0.12)n

0.98 (0.34)nnn 0.58 (0.33) 0.57 (0.32)

 0.18 (0.42)  0.12 (0.39)  0.32 (0.38)

0.06 (0.42)  0.04 (0.39)  0.07 (0.38)

(0.12) (0.15) (0.04) (0.03) (0.04)

Parity ordera

Season effect SA

Crossbreeding parameters lRC  lRE 0.69 (0.32)n lBHC  lRE 0.37 (0.32) lBHP  lRE 0.48 (0.30) hRERC hREBHC hREBHP hRCBHC hRCBHP hBHCBHP

0.57 (0.26)n  0.01 (0.26) 0.09 (0.20) 0.19 (0.20) 0.13 (0.25) 0.32 (0.25)

0.45 (0.26)  0.07 (0.26) 0.05 (0.20) 0.13 (0.21)  0.07 (0.25) 0.16 (0.25)

0.98 0.61 0.92 0.69 0.48 0.68

(0.28)nnn (0.29)n (0.23)nnn (0.23)nnn (0.32) (0.31)n

rRERC rREBHC rREBHP rRCBHC rRCBHP rBHCBHP

 0.28 (0.52)  0.62 (0.51) 0.29 (0.40) 0.53 (0.41)  0.52 (0.49)  0.20 (0.50)

 0.29 (0.53)  0.65 (0.52) 0.27 (0.40) 0.54 (0.41)  0.45 (0.50)  0.11 (0.50)

 0.28 (0.57) 0.60 (0.59) 0.66 (0.47)  0.24 (0.45)  0.39 (0.63) 1.07 (0.62)

P o0.01. nnn P o 0.005. n Po 0.05. a Deviated from the first or the third parity for the parity classes t r 2 and t Z 3, respectively.

nn

0.85 0.43 0.88 0.59 0.21 0.46

(0.29)nnn (0.30) (0.24)nnn (0.23)n (0.32) (0.32)

0.46 (0.58) 0.67 (0.60) 0.33 (0.48)  0.46 (0.46)  0.54 (0.64) 0.72 (0.64)

6

J.M. Garcı´a-Casco et al. / Livestock Science 147 (2012) 1–8

the performed analyses. All these results indicate the usefulness of Caldeira (RC) as a dam line in crossbreeding schemes, and of its crosses with the Ervideira (RE) and Puebla (BHP) as the most adequate ones according to the estimated values of heterosis.

Table 5 Estimated statistics of marginal posterior distributions of heritabilities (h2), genetic correlation (ru) and effects of crossbreeding parameters for body weight at 420 days (W420D) and rate of daily growth (GR) in finishing Iberian pigs (100–160 kg). W420D (kg)

4.2. Growth The Bayesian procedure provides an estimate of the variance of performances given the production function 2 parameters (st ) that includes measuring errors or physiological variations of the animals along the analyzed fattening period. In this study, estimation of this error variance when modeling weight performance was carried out under the assumption of constant variance throughout the analyzed growth period. The posterior mean and 2 standard deviation (PSD) of st were 10.98 and 0.52, respectively. Note that the Bayesian procedure provides estimates of the intercept (aj) and the slope (bj) of the linear growth function for each animal (j ¼1, 579), taking into account their own performances and, through prior distributions, information from relatives and other unrelated animals sharing the same systematic effects (Varona et al., 1997). These estimates are even obtained for animals with only one record of performance and they are less influenced by outliers than the standard REML analysis. The main statistics of variance ratios, genetic line and specific heterotic effects on the intercept (a¼W420D) and slope (b¼GR) of the growth linear functions obtained from the analysis of growth performances are summarized in Table 5. Knowledge on the posterior distribution of the parameters allows for the construction of confidence intervals, under the assumptions of correct priors and reaching convergence. Both heritability of W420D and GR present posterior means of 0.27, with a posterior mean of genetic correlation of 0.40, but all these distributions show large values of the dispersion parameters (PSD and 95% HPD). The posterior means of differences between males and females for W420D and GR are þ12.52 (PSD 1.22) kg and þ58 (PSD 7) g/d, respectively. The inferred line effects on W420D indicate that the Ervideira line (lRE) is the heaviest, being significantly different from the Caldeira (lRC) and Campanario (lBHC) lines (posterior mean values: lRC lRE ¼  12.2 and lBHC  lRE ¼  15.6 kg, respectively). A smaller difference for this trait is found between the Black Hairless Iberian lines (posterior mean: lBHP lBHC ¼ þ8.8 kg), with a significant value of the posterior probability of this difference below zero (Prob (lBHP  lBHC o0)¼0.027). Most of the differences between lines for GR in the analyzed period (100–160 kg) are small, and their respective 95% HPD intervals include the value zero. Relevant differences for this trait are found between the Red Iberian lines (posterior mean: lRC  lRE ¼  57 g/d; Prob (lRC  lRE o0)¼0.975) and between the BHC and RC lines (posterior mean: lBHC  lRC ¼ þ52 g/d; Prob (lBHC lRC o0)¼0.030). The statistics of posterior distributions of specific heterotic effects are evidence of significant positive heterosis on W420D between the six pairs of lines, with values ranging from þ12.7 (hBHCBHP) to þ17.7 kg (hRCBHC).

Mean Variance ratios h2 0.27 ru 0.40

PSDa

0.10 0.35

Crossbreeding parameters m 132.70 3.52 lRC  lRE  12.18 5.28 lBHC  lRE  15.59 4.62  6.82 4.70 lBHP  lRE lBHC  lRC  3.42 5.09 lBHP  lRC 5.35 5.24 lBHP  lBHC 8.77 4.54

GR (g/d) 95% HPDb

Mean PSDa 95% HPDb

0.07/0.46  0.27/0.97

0.27

0.13

0.04/0.53

126.08/139.81  22.73/ 1.71  24.63/ 6.58  15.67/2.66  13.19/6.85  4.44/16.21  0.26/17.61

566  57 5  13 52 43 9

20 29 26 26 28 29 25

527/603  116/  1  53/48  66/36  3/107  11/102  61/38

hRERC hREBHC hREBHP hRCBHC hRCBHP hBHCBHP

13.96 17.17 13.41 17.73 14.06 12.74

2.76 2.73 2.97 2.93 2.79 3.03

8.68/19.43 11.79/22.44 7.63/19.27 11.93/23.34 8.78/19.68 6.61/18.51

21 19 11 66 25 19

16 16 17 17 16 18

 12/51  15/47  26/42 33/99  8/56  16/52

rRERC rREBHC rREBHP rRCBHC rRCBHP rBHCBHP

0.63  2.17 5.88 0.97 5.07  4.28

4.24 4.15 4.76 5.04 4.51 4.77

 7.65/8.92  10.37/5.89  3.18/15.51  8.77/10.79  3.68/14.04  13.86/4.85

21 5  30 21  19  36

24 24 27 29 26 27

 28/68  43/48  84/20  37/76  92/16  69/31

a b

PSD, posterior standard deviation. 95% HPD, 95% highest posterior density interval.

Although positive mean values of specific heterotic effects on GR are also inferred for the six combination of lines, only the cross between lines RC and BHC shows a significant heterosis for this trait (hRCBHC ¼ þ66 g/d). Note that reciprocal line effects for both growth traits (W420D and GR) are not different from zero. Among the studied lines, the use of Ervideira (RE) as a sire line in Iberian pig crossbreeding schemes seems advisable according to the reported effects on growth traits. From a production point of view, a more relevant growth trait is body weight recorded immediately before the slaughter. To evaluate the line and heterotic effects on this trait, an additional analysis was performed with a similar model fitting the weight at 465 days of age (W465D) as a new intercept (a) of the linear growth function. The results again showed significant mean effects on the final body weight (W465D) of parental lines lRC and lBHC relative to the heaviest line lRE of 14.3 and 15. 8 kg, and a significant difference between the Black Hairless Iberian lines (posterior mean: lBHP  lBHC ¼ þ8.4 kg; Prob (lBHP  lBHC o0)¼ 0.050). All the specific heterotic effects on W465D differ significantly from zero, and their posterior means range from þ13.5 (hBHCBHP) to þ20.3 kg (hRCBHC). Taking into account the small line effects on GR in the analyzed period, most of the quoted differences between the lines for W420D and W465D could be attributed to effects on the initial weight of the tested pigs. To check this hypothesis, another analysis was performed fitting the weight at 365 days of age (W365D) as an intercept.

J.M. Garcı´a-Casco et al. / Livestock Science 147 (2012) 1–8

The results confirmed a lower body weight at the start of the fattening trial of pigs originating from lRC and lBHC parental lines. Significant effects on W365D of lines lRC and lBHC expressed as mean deviations from the lRE line are  8.8 and  15.3 kg respectively, and the mean difference for this trait between the lines BHP and BHC is þ9.3 kg (Prob (lBHP  lBHC o0)¼0.013). All the specific heterotic effects on W365D differ significantly from zero, and their posterior means range from þ11.5 (hBHCBHP) to þ16.0 kg (hREBHC). In order to facilitate their comparison at the same scale of measurement, the heterosis values are usually expressed as percentages of the mean (h/m  100). The posterior means of the scaled values of specific heterosis for W420D range from 9.6 (PSD 2.3)% for the scaled hBHCBHP to 13.4 (PSD 2.2)% for the scaled hRCBHC. Moreover, the values of scaled heterosis on weight by age for the six combinations of lines decrease during the analyzed fattening period. The extreme example of this tendency corresponds to the scaled heterosis between the RE and BHC lines for W365D, W420D and W465D, with values of 16.3, 13.0 and 11.6%, respectively. The smallest changes of scaled heterosis are observed between the RC and BHC lines with values of 14.0, 13.4 and 13.1%. This common diminishing tendency suggests that the heterosis values estimated in the final fattening period may be mainly attributed to the previous heterotic effects on the growth of pigs with restricted feed intake from weaning to the start of the trial. Heterotic effects have been related in some cases to resistance to stressful environments (Barlow, 1981). A restriction of feeding level may be considered a source of environmental stress, and consequently, large heterotic effects on growth traits could be expected along the early growth period (up to 98 kg). This result was obtained in a previous comparison of the growth performance of two sets of Iberian pigs with restricted feeding and hand-fed to appetite (Ferna´ndez et al., 2002). A high heterotic effect on the growth of animals with restricted feeding level is meaningful for the production of Iberian pigs, because a severe restriction of feeding (until a body weight of 100 kg) is a very usual practice in this extensive production system (Lo´pez-Bote, 1998). 4.3. Implications for Iberian pig breeding schemes The production of Iberian pigs increased along the last two decades providing the raw material of meat products of high sensorial quality. Most of the three million pigs slaughtered per year and labeled as Iberian are crossbred Duroc  Iberian pigs reared in conventional farms, although purebred Iberian pigs fattened with acorns and pasture are the source of the most expensive dry-cured products (Lo´pezBote, 1998). Genetic improvement of the low reproductive performance of Iberian sows is necessary because maternal Iberian origin is mandatory for labeling Iberian products. Moreover, selection of Iberian sire lines is required for elite production based on purebred pigs. The four main productive traits are daily growth along the final fattening period and percentages on carcass weight of trimmed hams, forelegs and loins. The values of heterosis for reproductive and growth traits found in this study confirm the practical interest of crosses between sire and dam lines of Iberian pigs. Two or

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three way crossbreeding schemes among Iberian pig lines are particularly necessary to improve the economic efficiency of low and medium input production systems based on purebred Iberian pigs. The implementation of these crossbreeding schemes requires the use of adequate purebred lines. According to our results, the Red Caldeira and Black Hairless Campanario strains may be useful as dam and sire lines, respectively. However, these and other old strains are nowadays endangered or blended. In fact, diverse studies of the Iberian population based on genetic markers show evidence of several cases of admixed lines resulting from genetic flow between some of the traditional Iberian pig varieties (Alves et al., 2003, 2006; Fabuel et al., 2004). This may complicate the success of the proposed crossbreeding strategy.

Conflict of interest statement Authors declare that they do not have any actual or potential conflict of interests including any financial, personal or other relationships with other people or organizations within three years of beginning the submitted work that could inappropriately influence, or be perceived to influence, their work.

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