Heritabilities of osteochondral lesions and genetic correlations with production and exterior traits in station-tested pigs

Abstract

Osteochondrosis might reduce the performance of slaughter pigs, longevity of sows and animal welfare. The aim of the present work was to describe the prevalence in Swiss breeds and to analyse the genetic background of osteochondral lesions. Between January 2002 and December 2005, about 9500 station-tested pigs were examined for several exterior traits before slaughtering at the Swiss pig performance testing station using the Swiss linear description system with a scale from 1 to 7 per trait. The animals belonged to three breeds: Large White dam line, Swiss Landrace and Large White sire line. Additionally, a random sample of these pigs (n = 2622) was examined for osteochondral lesions at seven positions of the carcass after dissection. At first, the surface and shape of the femur, humerus, radius and ulna at the joints were evaluated by a trained person. Afterwards these bones were sawed and the state of the cartilage and the distal epiphyseal cartilage of the ulna was examined at the cutting surface. Osteochondral lesions were scored on a scale from 1 to 6. The prevalence of osteochondral lesions was low at head of humerus, condylus lateralis humeri, radius and ulna proximal and head of femur. Osteochondral lesions at condylus medialis humeri (CMH), distal epiphyseal cartilage of ulna (DEU) and condylus lateralis femoris (CMF) exhibited phenotypic and genetic variance. Their heritabilities ranged from 0.16 to 0.18 using linear mixed animal models. Therefore, it is possible to reduce the prevalence of osteochondral lesions by selection in principle. Exterior traits showed low heritabilities (0.10 to 0.26) but several favourable genetic correlations with osteochondral lesions at CMH, DEU and CMF with low to moderate magnitude. Genetic correlations between osteochondral lesions and production traits were low.

Introduction

Osteochondrosis (OC) is a disturbance of the enchondral ossification in the joint cartilage and in the growth plates (Olsson and Reiland, 1978). Osteochondral lesions may cause or aggravate leg weakness (Lundeheim, 1987; Jørgensen, 2000), reduce longevity of sows and boars and may be painful, if considerable OC lesions occur. Therefore, concerning economic and animal welfare aspects, OC is of noticeable relevance.

It is well known that besides environmental factors genetic effects influence the occurrence of OC lesions (Lundeheim, 1987; Jørgensen and Andersen, 2000; Kadarmideen et al., 2004). The authors estimated low-to-moderate heritabilities for the occurrence of osteochondral lesions and found differences between breeds. But the examination methods, the included cartilages and the prevalence of osteochondral lesions differ between their studies.

Direct selection against osteochondral lesions would be very difficult, because only dissected or X-rayed animals could provide information. Thus, including other genetically related traits − e.g. exterior characteristics − in pig breeding programmes may be a preferable way to control the genetic aspects of OC. Therefore, genetic parameters of these related traits, like heritabilities and genetic correlations, are needed. Lundeheim and Rydhmer (1990), Jørgensen and Andersen (2000) and Kadarmideen et al. (2004) estimated genetic correlations between OC and production traits. They found only few unfavourable associations with moderate magnitude. Some authors (Jørgensen et al., 1995; Jørgensen, 2000) found slight phenotypic associations between OC and several leg weakness symptoms. But to our knowledge, only Jørgensen and Andersen (2000) estimated genetic correlations between exterior traits and OC. They found some favourable, moderate genetic associations.

The aim of the present study is to describe the prevalence of osteochondral lesions at different positions of the carcass in Swiss breeds based on station-tested pigs, which were born at several Swiss breeding farms. Secondly, heritabilities of osteochondral lesions and their genetic correlations with some production and several exterior traits will be estimated.

Material and methods

Station-test

In Switzerland, approximately 3000 growing pigs are performance tested per year. Purebred animals of the Large White dam line (LW_DL), Swiss Landrace (SLR) and Large White Sire line (LW_SL) make up more than 80% of all test animals. Usually, a test group consists of two full-sibs (one female and one male castrate). Animals are housed in 12 barns, each with eight pens and about 10 animals of both sexes per pen. Electronic feed dispensers are used to feed the animals ad libitum. Two different diets are used during the test. At first, each pig obtains approximately 80 kg of a starter feed (19% CP, 13.3 MJ digestible energy (DE), 10 g/kg lysine, 8 g/kg Ca, 5 g/kg P). Afterwards, a finisher feed (15% CP, 12.7 MJ DE, 9 g/kg lysine, 8 g/kg Ca, 5 g/kg P) is used. The test starts at 30 kg live weight and ends with approximately 103 kg. Every week, 40 to 70 animals are slaughtered in a small abattoir at the test station and the left half of the carcass is dissected.

Average daily gain (ADG) from 30 to 103 kg describes the growth of animals. Percentage of premium cuts (PPC) specifies the leanness of carcasses. PPC is defined as the weight of ham, loin and shoulder without fat layer proportional to carcass weight. A full description of the Swiss station-test is given in the annual report of SUISAG (2006).

Examination of bones

Osteochondral lesions were already observed at the Swiss test station in the 1980s and early 1990s (Schwörer et al., 1991). In 2002, more weight was placed on daily gain in the overall breeding goal. SUISAG decided to monitor the occurrence of osteochondral lesions, because unfavourable genetic correlations with daily gain may exist.

Since 2002, osteochondral lesions have been examined at the following bones and joints: head of humerus (HK), condylus medialis humeri (CMH), condylus lateralis humeri (CLH), radius and ulna proximal (RUP), head of femur (FK), distal epiphyseal cartilage of ulna (DEU) and condylus medialis femoris (CMF). Figure 1 shows the corresponding positions at the carcass. At first, a trained person examines the surface and shape of these bones at the joints. Afterwards each bone is sawed with a band saw and the state and shape of the cartilage and the DEU is examined at the cutting surface. Scores from 1 to 4 are used to record the extent of the osteochondral lesion. A score of 1 denotes ‘no visible osteochondral lesion’. Scores 2 to 4 denote ‘mildly to severely affected’, respectively. Osteochondral lesions at the DEU and CMF are scored from 1 to 6, because these lesions seemed to be more variable. More details concerning the Swiss examination of osteochondral lesions are given by Schwörer et al. (1985).

Figure 1 Examinations of osteochondral lesions at the Swiss test station (Schwörer et al., 1985).

Linear description of exterior

A linear description system (LD) has been used to evaluate exterior characteristics of Swiss gilts and boars (on-farm test) and all station-tested pigs since 2000. Five traits are used to describe the rear and fore extremities: X-O posture at rear legs (XORL), side-view angle of rear legs (SVARL), angle of pasterns at rear legs (APRL), size of the inner claw in comparison to the outer claw at rear legs (SICRL) and side-view angle of fore legs (SVAFL). These five traits are scored from 1 to 7 with score 4 as the optimal phenotype and scores 3 to 1 and 5 to 7 as more and more extreme and therefore negative phenotypes. Scores from 4 to 7 are used to describe the regularity at the loin (LOIN). Score 4 denotes ‘regular’ and scores 5 to 7 denote that the animals are increasingly narrow at the loin in comparison to the whole body. Finally, scores from 4 to 7 are used to evaluate the locomotion (LOC) of the animal. Score 4 denotes ‘regular locomotion’ and scores 5 to 7 denote a more and more lurching or stiff locomotion. Figure 2 shows a scheme of the linear description system. Tarrés et al. (2006) recently showed that gilts with an optimum exterior – i.e. only scores of 4 at the linear description – stayed longer in the farms compared with gilts with a suboptimal exterior.

Figure 2 Scheme of the Swiss linear description system (without teat description) (some drawings from Van Steenbergen, 1990).

The present study includes exterior examinations of station-tested animals, which were performed approximately 4 weeks to 1 week before slaughtering. Seven trained persons described the animals during the evaluation period. The length of the carcass (CL) of every animal was measured from the pubis bone to the cervical vertebra after slaughtering. A detailed report of the applied linear description system and the genetic evaluation of exterior traits in Switzerland is given by Hofer (2004).

Data

This study includes 9511 performance-tested animals of the purebred lines LW_DL, SLR and LW_SL that were slaughtered between January 2002 and December 2005 (Table 1). Table 2 presents the average fattening and carcass performance of the station-tested animals. The test animals are progeny of 1051 sires and 19 988 animals are included in the pedigree file.

Table 1 Distribution of station-tested pigs

^† LW_DL = Large White dam line; SLR = Swiss Landrace; LW_SL = Large White sire line.

Table 2 Mean and distribution of performance traits (n = 9511)

^† LW = live weight at slaughtering; ADG = average daily gain (30 to 103 kg); FCR = feed conversion ratio; FI = average daily feed intake; CL = length of carcass; PPC = percentage of premium cuts.

About 27% of all slaughtered animals were examined for osteochondral lesions (n = 2622; Table 3). The number of observations was slightly reduced at RUP and DEU, because these bones were used for another project. Data from linear description were available for nearly all animals. Approximately 50 test animals had no or incomplete exterior data due to different reasons (e.g. clinical lameness at description day, etc.).

Table 3 Distribution of osteochondral lesions within breeds^†

^† For abbreviations see Figure 1.

Statistical analysis

Genetic parameters were estimated with univariate and multivariate linear mixed animal models using VCE5 (Kovač et al., 2002). Due to the categorical characteristic and the very skewed distribution, HK, CLH, RUP and FK were excluded from the final statistical analyse. Preliminary univariate estimations showed nearly no genetic variance for these four traits. CMH, DEU and CMF exhibited a prevalence of more than 10% and remained in the statistical analyses with linear models. The heritabilities on the underlying liability scale might be underestimated according to the formula of Robertson and Lerner (1949) due to the use of linear models, especially for CMH. But according to Gianola (1984) the genetic correlations with other continuously distributed traits are almost not affected. Estimations of heritabilities and genetic correlations of the 13 traits were performed in 14 separate runs. It was not possible to run more than seven traits simultaneously. The results of all runs were averaged. Slightly different linear mixed models were used for different traits.

ADG:

PPC, CL:

XORL, SVARL, APRL, SICRL, SVAFL, LOIN, LOC:

CMH, DEU, CMF:

with:

y

observation of test animal p (p = 1, 2, …, 9511)

μ

overall mean

BREED_h

fixed effect of breed (h = LW_DL, SLR, LW_SL)

BARN_PER_i

fixed effect of barn × period combination (i = 1, 2, …, 149)

SL_DAY_j

the fixed effect of slaughter day (j = 1, 2,…, 213)

SL_PER_k

fixed effect of slaughter period (k = 1, 2, …, 8)

LD_PE_l

fixed effect of the person performing theLD (l = 1, 2, …, 7)

SEX_m

fixed effect of sex (m = female, castrate)

b_mLW

partial linear regression on live weight within sex

farm_y_n

random effect of farm of origin × year (n = 1, 2, …, 249)

litter_o

random effect of the litter (o = 1, 2, …, 4763)

animal_p

random effect of the animal (p = 1, 2, …, 9511)

e

random error of test animal p (p = 1, 2, …, 9511).

All test animals that were fattened in the same barn and period were used as a contemporary group for ADG and exterior traits. Eight slaughter periods (6 months) were used as a contemporary group for osteochondral lesions traits, because only approximately 10 to 15 pigs were examined for OC lesions at each week after slaughtering. The test animals were born at 78 farms within 4 years and belong to 4763 different litters. ADG values were pre-corrected to a constant live weight at slaughtering (103 kg). Therefore, no regression on live weight was included in the model concerning this trait. No correction for live weight was possible for exterior traits, because pigs were not weighed at the description day.

The statistical software S-PLUS (S-PLUS, 1999) was used to test the significance of fixed effects for osteochondral lesions (Table 4).

Table 4 Significance of fixed effects^†

^† For abbreviations see Figure 1 and the text. ***P≤0.001.

Results and discussion

The occurrence of osteochondral lesions varied at different positions of the carcass (Table 3). Very few animals showed osteochondral lesions at HK, CLH and FK. Only approximately 6% of all pigs exhibited mildly osteochondral lesions at RUP. Substantial phenotypic variation was observed at CMH, CMF and especially at DEU.

The distribution of osteochondral lesions in the present study is in good agreement with Kadarmideen et al. (2004). Those authors used a sub-data set of this study that included only test animals slaughtered in 2002 and 2003. Jørgensen et al. (1995) observed a higher prevalence and variation of subchondral lesions at the medial humeral and femoral condyles in Danish Landrace boars. The prevalence at the lateral humeral condyles was similar to the present study and osteochondral lesions at the distal ulna growth plate showed the most phenotypic variation also. The differences might be caused by different definitions of osteochondral lesion scores, different sexes of the animals and differences between the Swiss and Danish environments and populations. Jørgensen and Andersen (2000) X-rayed Danish Yorkshire and Landrace boars. They observed a slightly higher prevalence of OC at the humeral condyles and an explicitly higher prevalence at the femoral condyles. Concerning the distal ulna growth line OC scores varied conspicuously in Danish Landrace boars. But 67% of the Yorkshire boars showed no OC at this growth plate. So, variation was reduced in the Yorkshire breed.

Differences between breeds were moderate and inconsistent at different positions of the carcass in the present study (Table 3). SLR showed proportionally more osteochondral lesions at CMF but slightly less at RUP compared with Large White dam and sire line. In contrast, Jørgensen and Andersen (2000) reported that Danish Landrace boars were significantly more severely affected than Yorkshire boars regarding OC in all localities except of the anconeal process. Differences between Swiss and Danish populations might be a major reason for this disagreement.

Jørgensen (1995) found no significant differences between sexes concerning OC, which is in good agreement with DEU in the present study (Table 4). Otherwise, castrates were significantly more affected at CMH and CMF than females in our study.

Conspicuous phenotypic variation was observed in all linearly described exterior characteristics (Table 5). The scores of XORL showed almost a Gaussian distribution with an average of 3.46. Because 4.0 is the desired average for XORL, it is necessary to reduce the X posture of the rear legs in the Swiss population. Approximately 77% of all animals exhibited an optimal SVARL. But it is still necessary to reduce the proportion of animals with a ‘standing under position’ (score 3). On average, the population is very close to the optimum score of 4 concerning the angle of pasterns but a lot of animals showed slightly too weak (score 3) or slightly too steep (score 5) pasterns. Most of the animals had a smaller inner claw compared with the outside claw. Approximately 70% of all animals showed an optimal angle of the fore legs and the average score is close to the optimum. Nearly 60% of all pigs showed a fully regular locomotion and approximately 4% exhibited conspicuous problems (score 6 or 7) concerning locomotion.

Table 5 Distribution of the scores of linear description^†

^† For abbreviations see Figure 2.

Serenius et al. (2001) analysed exterior data of Finnish progeny and performance tests. Approximately 50% of Finnish Landrace and Large White pigs exhibited small inner claws. Jørgensen (2000) examined 117 Danish crossbred and Yorkshire gilts at 6 months of age for leg weakness symptoms. A high proportion of Danish gilts showed uneven claws especially in crossbred sows. Thus, smaller inner claws seem to be common in different pig populations and pig breeding programmes should aim at equalising the size of both claws in the future.

Approximately 55% of the 117 Danish gilts (Jørgensen, 2000) exhibited a posture of rear legs called ‘turned out’, which is synonym to the Swiss ‘X posture’ and in prevalence very close to our results. About 15% of the Danish gilts showed too weak pasterns and approximately 20% too steep pasterns, which is in good agreement to our findings. In contrast, Jørgensen and Andersen (2000) observed only approximately 5% weak pasterns but about 80% too steep pasterns in Danish Landrace and Yorkshire boars. Most of the Danish gilts showed a normal locomotion (Jørgensen, 2000). But approximately 25% exhibited a more or less stiff gait at the rear legs. In contrast, most of the Danish boars showed a stiff locomotion (Jørgensen and Andersen, 2000). The Danish results suggest that the occurrence of leg weakness symptoms is different between sows and boars especially for pasterns and locomotion. In the present study we did not find differences between females and castrates concerning exterior traits.

Heritabilities and correlations

At first, univariate analyses were performed to estimate the variance components of osteochondral lesion at CMH, DEU and CMF (Table 6). The additive genetic effect explains a noticeable proportion of the total phenotypic variance of osteochondral lesions. Hence, heritabilites for these traits ranged from h ² = 0.16 to 0.18. The corresponding standard errors were low.

Table 6 Variance components and heritabilities (h ²) of osteochondral lesions at different positions of the carcass (univariate estimation)^†

^† c ² = ratio of litter variance, s.e. = standard error of h ² and for other abbreviations see Figure 1.

The random effect farm of origin × year combination explains nearly no variance. This indicates that the farm of origin does not influence the occurrence of osteochondral lesions at the end of the station test significantly. Concerning osteochondral lesions at CMH, the litter effect explains a minor proportion of the total variance. This means that pigs of the same litter show more frequently OC lesions than one would expect due to the identical parentage. Environmental effects, e.g. low milk production of the nursing dam, could be a potential explanation for this observation. But on the other hand no noticeable litter effect was observed for DEU and CMF.

Kadarmideen et al. (2004) estimated very low heritabilities for osteochondral lesions using linear mixed sire models except for RUP and CMF. But the authors used threshold models too. Heritabilities obtained from these threshold models ranged between h ²=0.00 (FK) and h ² = 0.42 (CMH). Transformed to the ‘observed scale’, these heritabilities are in good agreement with the values of the present study except for DEU, which was not significantly different from zero in the study of Kadarmideen et al. (2004).

Jørgensen and Andersen (2000) estimated heritabilities of OC at six different positions of the carcass. The values ranged from h ² = 0.08 to 0.39 and varied sometimes between Danish Yorkshire and Landrace breeds at the same position of the carcass. On average, the heritabilities in Danish populations were higher than in the present study. Differences between Danish and Swiss populations might be a potential reason. Obviously, there were distinctions between Danish breeds. We did not estimate genetic parameters of the three Swiss breeds separately, because less than 500 SLR and SLW_SL test animals had OC data, respectively. Moreover, phenotypic variation of OC was more pronounced in Danish populations compared with the present study. Jørgensen and Andersen (2000) examined both sides of the animal radiologically for osteochondral lesions and analysed the average score statistically. The different methods of examination might have affected the incidence and variation of OC and could be another cause of different heritabilities.

Table 7 presents the results of the multivariate estimation of genetic parameters. Phenotypic correlations of osteochondral lesions to exterior and production traits were close to zero. Heritabilities of osteochondral lesions changed slightly due to the multivariate estimation in comparison to the univariate analyses. Linearly described exterior traits showed low heritabilities (0.10 to 0.20). But all values differed significantly from zero. CL and the PPC exhibited the highest heritabilities of all traits (0.61 and 0.62, respectively). Serenius et al. (2001) estimated lower heritabilities for leg weakness symptoms in comparison to our findings. The present results are in good agreement with Jørgensen and Andersen (2000).

Table 7 Heritabilities (diagonal, bold), phenotypic correlations (above diagonal) and genetic correlations (below diagonal) of multivariate analyses^†

^† For abbreviations see Figures 1 and 2 and Table 2. Underlined values indicate notable genetic correlation is favourable (r_g ⩾ ± 0.15). Italic values indicate notable genetic correlation is unfavourable (r_g ⩾ ± 0.15).

Genetic correlations between osteochondral lesions at CMH, DEU and CMF were low (r_g = −0.14 to 0.13). This indicates that the occurrence of osteochondral lesions at the three positions does not seem to be controlled by identical genes and should be considered as different traits. This conclusion agrees with Jørgensen and Andersen (2000) and Kadarmideen et al. (2004).

The genetic correlations of osteochondral lesions to ADG and PPC were very low. Only the unfavourable genetic correlation between osteochondral lesions at CMF and ADG exceeded r_g = ±0.15 in the present study. Thus, genetic selection that favours leaner and faster growing pigs does not seem to increase the genetic predisposition to exhibit osteochondral lesions considerably in Swiss breeds. Our results are in agreement with Lundeheim and Rydhmer (1990), Jørgensen and Andersen (2000) and Kadarmideen et al. (2004). Those authors estimated few unfavourable genetic correlations with moderate magnitude between OC and production traits. In the same way, Jørgensen (1995) did not find different prevalences of OC between pigs fed restricted v. ad libitum although the pigs fed ad libitum grew faster. In contrast, Busch et al. (2006) reported that a high ADG was associated with an increased risk to exhibit OC.

Genetic correlations of osteochondral lesions at CMH, DEU and CMF to exterior traits showed low to medium values. Nearly all correlations that exceed r_g = ±0.15 were favourable concerning the actual Swiss breeding goal. Especially the occurrence of osteochondral lesions at DEU and the regularity of locomotion seem to be genetically linked (r_g = 0.54). Only the association between regularity at the loin and CMH was genetically unfavourable and exceeded r_g = ±0.15. Therefore, genetically favouring animals with regular locomotion and optimal exterior would reduce the occurrence of osteochondral lesions in the long-term especially at DEU.

In general, our results agree with results by Jørgensen (2000) and Jørgensen et al. (1995). Those authors also found slight phenotypic associations between several leg weakness symptoms and pathological changes of bones and joints. Jørgensen and Andersen (2000) estimated some moderate genetic correlations between leg weakness symptoms and OC at the femoral and humeral condyles too.

Conclusion and implications

At HK, CLH, RUP and CMF even mildly osteochondral lesions are rare in Swiss breeds. More than 10% of the pigs exhibited mostly mildly osteochondral lesion at the CMH, DEU and CMF, respectively. Thus, based on the station-tested pigs osteochondral lesions do not seem to be a severe problem in Swiss breeds at the moment. The occurrence of osteochondral lesions at CMH, DEU and CMF is heritable (h ² = 0.16 to 0.18). Therefore, it is possible to reduce the prevalence of osteochondral lesions by selection in principle.

Direct selection would require examination of all tested pigs for OC lesions, which is too costly. An indirect selection based on exterior traits is difficult due to low heritabilities of OC and exterior traits and low relative weight of exterior in the overall breeding goal. But the low genetic correlations between the most important production traits (ADG and PPC) and osteochondral lesions as well as the favourable genetic correlations with Swiss exterior traits suggest that the prevalence of OC will not increase, if gilts and boars are selected according to the Swiss breeding goal.

Nevertheless, SUISAG will continue the examinations of a random sample of station-tested pigs after slaughtering to observe the further development of osteochondral lesions in the Swiss breeds directly.