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Effect of leg conformation on survivability of Duroc, Landrace, and Large White sows¹

X. Fernàndez de Sevilla^*, E. Fàbrega^*, J. Tibau^* and J. Casellas^,2

^* Control i Avaluació de Porcí, Institut de Recerca i Tecnologia Agroalimentàries, 17121 Monells, Spain; and Genètica i Millora Animal, Institut de Recerca i Tecnologia Agroalimentàries, 25198 Lleida, Spain

	Abstract

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Sow longevity influences farm economy and can be considered an important indicator of animal welfare. Body features such as leg conformation can play a key role in sow longevity, although little is known about its effect on culling decisions. Within this context, longevity data from 587 Duroc, 239 Landrace, and 217 Large White sows were analyzed with special emphasis on the effect of leg conformation. Sow longevity was analyzed twice for each breed, testing the effect of a subjective overall score for leg conformation, or the presence or absence of 6 specific leg conformation defects. Each preliminary model also included a teat conformation score with 3 levels, farm or origin, backfat thickness at 6 mo of age, and 2 continuous sources of variation, namely the age at the first farrowing and the number of piglets born alive at each farrowing. Overall leg conformation score influenced (P < 0.01) sow longevity in Duroc, Landrace, and Large White sows, with a greater hazard ratio (HR) for poorly conformed sows (1.56, 2.16, and 1.79, respectively) than for well-conformed sows (0.32, 0.66, and 0.68, respectively). Abnormal hoof growth reduced survivability in Duroc (HR = 2.78; P < 0.001) and Landrace sows (HR = 1.88; P < 0.01); the presence of splayed feet (P < 0.05) or bumps and injuries (P < 0.001) increased the risk of culling in Duroc sows (HR = 2.08 and 3.57, respectively), whereas the incidence of straight pastern increased the HR in Large White sows (HR = 2.49; P < 0.01). In all 3 breeds, longevity decreased for plantigrade sows, with a greater HR in Duroc (HR = 3.38; P < 0.001) than in Landrace (HR = 1.53; P < 0.10) and Large White sows (HR = 1.73; P < 0.05). Teat conformation did not influence sow longevity (P > 0.10). Estimates of heritability for longevity in Duroc sows ranged from 0.05 to 0.07 depending on the algorithm applied. Leg conformation had a substantial effect on sow longevity, where an accurate removal of poorly leg-conformed candidate gilts before first mating could improve sow survival and reduce culling costs. These moderate estimates of heritability indicated that survivability of Duroc sows could be genetically improved by direct selection for leg conformation.

Key Words: leg conformation • longevity • sow • survival analysis

	INTRODUCTION

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Over recent decades, genetic improvement of pigs has been focused on productive (growth, lean and meat quality; Van Wijk et al., 2005) and reproductive traits (age at puberty and litter size; Johnson et al., 1994; Noguera et al., 2002). Nevertheless, the economic relevance of functional characters such as sow longevity has also increased (Jalvingh et al., 1992), given its close relation with culling costs. Society’s concern for the well-being of domestic animals has increased in recent years [see Camm and Bowles (2000) for a detailed review of animal welfare standards in the European Union], and sow longevity can be viewed as a critical indicator of animal welfare (Barnett et al., 2001; Engblom et al., 2007).

Sow longevity has been the subject of intensive research (Brandt et al., 1999; Yazdi et al., 2000b; Tarrés et al., 2006a) in relation with both productive (Yazdi et al., 2000a; Serenius and Stalder, 2004; Tarrés et al., 2006b) and morphological characters (Tarrés et al., 2006a). Some morphological traits like leg and teat conformation are genetically (López-Serrano et al., 2000) and phenotypically (Tarrés et al., 2006a) related to sow longevity and could therefore affect culling rates, as suggested by Paterson (1995) and Jørgensen and Sørensen (1992, 1998). Some studies have evaluated the effect of overall leg conformation (LC) on sow survival (Jørgensen and Vestergaard, 1990; Tarrés et al., 2006a), but no currently available research has addressed the influence of specific LC deficiencies of gilts and sows during their productive lives.

The aim of this research was to evaluate the influence of overall LC score and several LC deficiencies on longevity of Duroc, Landrace, and Large White pure-bred sows applying survival analysis techniques. Results should contribute to improving the criteria used to select future gilts and help to optimize their productive lives.

	MATERIALS AND METHODS

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Animal Care and Use Committee approval was not obtained for this study, because the data were obtained under standard farm management without additional requirements. Leg and teat conformations were evaluated without any contact with the sows (visual evaluation) and without moving them out of the growing pen (gilts) or farrowing crate (farrowing sows).

Data Source

We analyzed longevity data of 587 purebred Duroc sows, 239 purebred Landrace sows, and 217 purebred Large White sows from 4 swine companies registered in the Associación Nacional de Criadores de Ganado Porcino Selecto (ANPS; http://www.anps.es, last accessed April 10, 2008) and distributed in the northeast region of Spain. Sows were housed in commercial installations and managed under standard farm conditions. Obviously, this does not discard a certain degree of heterogeneity between farms that must be accounted for by the statistical model. For each breed, data were obtained from 2 nuclei, some of which included a multiplier stage (see Table 1). Sow longevity was defined as the time interval between the first fertile mating and culling or death (complete record), whereas longevity records for sows that were still alive at the end of the data collection period were treated as censored (Cox, 1972). Complete information on pedigree, herd of origin, age at first farrowing, backfat thickness at 6 mo of age, leg and teat scoring (see below), and reproductive records between December 2004 and January 2007 were available for all sows.

Sows were evaluated by visual inspection for leg and teat conformation 6 mo after birth, at 100 kg of BW, and also after their first and second parturitions, as described by Fernández et al. (2005). All evaluations were performed by the same trained technician. General scoring for LC awarded values of 0 (bad conformation), 1 (regular conformation), and 2 (good conformation), according to the absence (2) or presence of several morphological defects and their respective severity (0 or 1). Registered morphological leg defects were excessive or abnormal hoof growth (overgrowth or curved, cracked, or unequal growth of the hoof wall), splayed feet (leg curves outwards at the carpal or tarsal articulations), plantigradism (sow walking or standing with the pastern completely or partially touching the ground), straight pastern (hoof and pastern describing close to a 180° angle), sickle-hooked leg (excessively angled hock moving the rear feet forward), and the presence of bumps or injuries in legs (presence of bumps, open injuries, or inflammatory processes in the legs). These specific defects were scored on a dichotomous scale (presence or absence), and a given gilt or sow could be affected by more than one of these defects at the same time. Although LC scores suffered from a certain degree of subjectivity, they allowed for a straightforward characterization of sow conformation without invasive handling. Scores for morphologic assessment of teat condition, TC_p, are shown in Table 2.

Data were analyzed by the semiparametric proportional hazards model defined by Cox (1972), h (t|x_w) = h₀ (t) exp (x_wβ), where h (t|x_w) = the hazard function of the wth individual at time t conditioned to the appropriate incidence of systematic effects (x_w); h₀ (t) = the baseline hazard function representing the aging process for the whole population (Cox, 1972; Ducrocq et al., 1988); and exp (x_wβ) = a stress-dependent term modeling regression coefficients (β). The standard Weibull assumption for h₀ (t) was discarded by the logarithms test (Ducrocq et al., 1988) on the Kaplan-Meier (Kaplan and Meier, 1958) estimate of the survival function (see Figure 1b).

View larger version (19K): [in this window] [in a new window]   Figure 1. (a) Kaplan-Meier survival functions and (b) logarithms test for each breed.

A stepwise-like approach was adopted to test the significance of the covariates influencing sow longevity. A significance level of P 0.10 was assumed to account for the loss of statistical power due to the high percentage of censored records. At each round, all covariates were independently tested using likelihood ratio tests, and only the most significant remained in the model. After this process, specific models accounted for hoof growth, splayed feet, plantigradism, presence of bumps or injuries, year at first farrowing, backfat thickness, and piglets born alive in the Duroc population; hoof growth, plantigradism, year at first farrowing, backfat thickness, and piglets born alive in the Landrace population; and plantigradism, splayed feet, year at first farrowing, age at first farrowing, and piglets born alive in the Large White population (Table 3). As was specified above, general models had the same structure after replacing the specific leg conformation defects by LC (Table 3).

The between-sires variance component was not estimated for the Landrace and Large White breeds, because the available data were insufficient to support the sire model (Table 1). All computations were performed using the survival kit package (Ducrocq and Sölkner, 1998).

	RESULTS

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Phenotypic Description of Survival Data

In the first reproductive cycle, percentages of failure (culled or dead sows) were 11.9, 16.3, and 17.4% for Duroc, Landrace, and Large White purebred sows, respectively. For the second reproductive cycle, these percentages decreased to 10.4, 8.5, and 11.7%, respectively. Most sows had more than 2 farrowings, so the percentage of complete (and censored) data was 42.1% (57.9%) for Duroc sows, 35.6% (64.4%) for Landrace sows, and 45.4% (54.6%) for Large White sows. A substantial amount of censored data was observed, because the monitoring of productive life only took place over a 2-yr period.

The Kaplan-Meier nonparametrical survival functions for each breed are shown in Figure 1a. Survival curves showed marked slopes beginning at 135 d after the first fertile mating, around the weaning date of the first litter. All the curves then described similar patterns, with sudden reductions every 130 to 160 d, suggesting a cyclical pattern that was more marked in the Duroc and Large White breeds. Note that survival curves were plotted until d 700, because data collection lasted for a 2-yr period. As a result, there were no references from sows with longer productive lives.

Systematic and Genetic Sources of Variation

Statistical significance for all systematic effects and breeds is summarized in Table 3. The overall LC (Figure 2) effect influenced (P < 0.01) on sow survivability in Duroc, Landrace, and Large White sows, with the same pattern being observed in all breeds. The minimum hazard ratio (HR) was associated with a score of 2 (well-conformed sows; HR = 0.317, 0.661, and 0.682, respectively) and the maximum HR related to a score of 0 (poorly conformed sows; HR = 1.56, 2.16, and 1.79, respectively).

View larger version (12K): [in this window] [in a new window]   Figure 2. Survival probability for Duroc sows depending on the overall leg condition score (LC).

View larger version (15K): [in this window] [in a new window]   Figure 3. Survival probability for Duroc sows affected by different leg defects (absence = no leg defects; HG = abnormal hoof growth; SF = splayed feet; PL = plantigradism; BI = bumps or injuries).

View larger version (13K): [in this window] [in a new window]   Figure 4. Survival probability for Landrace sows affected by different leg defects (absence = no leg defects; HG = abnormal hoof growth; PL = plantigradism).

View larger version (14K): [in this window] [in a new window]   Figure 5. Survival probability for Large White sows affected by different leg defects (absence = no leg defects; PL = plantigradism; SP = straight pastern).

	DISCUSSION

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Kaplan-Meier Survival Function

In Figure 1a, the survival curve for all 3 breeds suffered a fall at approximately 135 d after first fertile mating, coinciding with weaning date of the litter. For the Large White and Duroc breeds, important falls were observed every 130 to 160 d, suggesting that most of elimination decisions were taken in the first days after weaning. In a similar way, Yazdi et al. (2000a) reported a similar increase in the risk of culling after weaning for the first 3 litters.

Leg and Teat Conformation

Leg conformation scores evolved over the productive life of each sow, and a unique qualification at the end of the growing period could not provide precise information about the future evolution of the animal. In this research, LC variables for each sow were evaluated at 3 different stages, thereby providing a more accurate description of their incidence and evolution and allowing more precise modeling of their effect on sow longevity.

The general score for LC influenced sow survival. All 3 breeds showed similar patterns, with survivability substantially decreasing with poorer leg conformations. This relationship had previously been reported by López-Serrano et al. (2000), Serenius and Stalder (2004), and Tarrés et al. (2006a). When specific leg defects were analyzed, abnormal hoof growth impaired survival probability in Duroc and Landrace sows with a HR of close to 2. Although sow mobility is limited in current management systems, excessive or abnormal hoof growth could prevent basic movements and hinder access to feed and water. Moreover, this defect is easily detected by farmers, who can preferentially cull these sows. It is important to note that hoof growth has a substantial genetic background in Landrace sows (0.25; Quintanilla et al., 2006), so selective breeding focused on reducing hoof growth could indirectly increase sow longevity.

Plantigradism reduced longevity in all 3 breeds (Tables 4, 5, and 6), although the HR was clearly greater in Duroc (3.38) than in Landrace and Large White sows (1.53 and 1.73, respectively). As for hoof growth, plantigradism could impair the ability of a sow to stand and therefore access resources. This defect should be given special consideration, because its negative effect on sow longevity was observed in all 3 breeds. The remaining defects showed varying effects on sow survival, although inconsistencies between breeds could have been due to limited available data for Landrace and Large White, reducing the statistical power of the analysis. Nevertheless, our results suggest that several defects may influence sow survival, although little is known about their respective temporal evolutions during the productive life or their genetic background. Further studies are necessary to determine the genetic component and to ascertain whether these leg defects could be eradicated by removing affected gilts or if genetic programs would be required.

Sow failure is a clear measure of poor welfare not only because sows that die or are culled obviously have failed to cope, but also because high losses under given conformation defects or in a given environment suggest that even survivors might have serious difficulties (Broom and Johnson, 1993). A currently used definition for animal welfare assumes the 5 freedoms for animal welfare (Farm Animal Welfare Council 1992) involving 1) freedom from hunger and thirst, 2) freedom from thermal and physical discomfort, 3) freedom from pain, injury, and disease, 4) freedom from fear and stress, and 5) freedom to express normal behavior. Effect of significant LC defects on sow longevity could impair sow welfare through the 5 freedoms by 1) limiting sow access to food and water (see above), 2) originating physical discomfort at standing or moving (probably related with pain), 3) directly involving pain and injuries (Gregory 2004), 4) probably originating stress due to pain and discomfort, and 5) modifying the expression of normal behavior due to limitations in movement, respectively. These implications have a substantial effect on the concern of people about animal well-being with considerable criticism from various segments of the society (María, 2006). Within this context, the current swine industry must undergo important modifications, which could then affect production costs (i.e., European Union directive 2001/88/EC). The substantial contribution of leg defects on sow longevity highlighted in this manuscript suggests an appealing research framework to improve sow welfare in intensive management systems.

In this study, sows were kept under standardized crates. The influence of LC (as well as specific defects) on sow survival could increase under the European Union directive 2001/88/EC (which will be mandatory on European farms on January 2013), which will require farmers to keep sows in groups from wk 4 of pregnancy until the week before farrowing, or even in the United Kingdom or Sweden, where gestation stalls are not allowed. Increased freedom of movement could potentially be beneficial to sow leg health, but a sow that developed a disabling leg condition might suffer more effect (such as greater difficulty in accessing food and water) in a group system.

The teat condition score did not influence sow longevity for any of the breeds. Note that gilts were previously selected by teat conformation and that only 4.3% exhibited bad teat condition at the beginning of our research (score 0). Previous studies (Tarrés et al., 2006a) reported the influence of teat condition, so our lack of relevant effect of teat condition on sow longevity could indicate either a lack of influence or low power due to the small incidence of poor teat conformation.

Performance Traits

For all breeds, survivability increased with the number of piglets born, as previously reported by Yazdi et al. (2000a). Backfat thickness at 6 mo of age had affected the Duroc and Landrace breeds. Survival probability was maximum between 16 and 19 mm of backfat thickness at 6 mo of age in Duroc purebred sows, with a substantial increase in culling risk for values of back-fat thickness greater than 19 mm. This optimal interval fits with results reported by Tarrés et al. (2006b). In Landrace sows, survival increased for fatter gilts at 6 mo, which agrees with Tholen et al. (1996), O’Dowd et al. (1997), López-Serrano et al. (2000), and Serenius and Stalder (2004). Finally, survivability increased with the precocity at first farrowing in Large White sows, as previously reported by Holder et al. (1995), Yazdi et al. (2000a), and Serenius and Stalder (2004).

Heritability for Sow Longevity in Purebred Duroc Sows

The variance component for the sire genetic effect was 0.03, and the estimated heritability was 0.07 following Ducrocq (2001), 0.05 following Yazdi et al. (2002), and 0.06 at 699 d according to the binary scale developed by Tarrés et al. (2005). These values were similar to those reported by Tholen et al. (1996; 0.08), López-Serrano et al. (2000; 0.10), Krieter (1995; 0.12), and Yazdi et al. (2000a,b; 0.11 to 0.31). These heritabilities suggested that direct genetic improvement for sow longevity was feasible, although a small genetic trend was expected. Nevertheless, indirect selection programs could also be useful given the high heritabilities reported for hoof growth (Quintanilla et al., 2006).

In conclusion, LC substantially influenced sow longevity. Plantigradism was the only defect that increased the risk of elimination in all 3 breeds, although .the other LC defects also impaired sow survival. These results suggested that an accurate evaluation and the removal of poorly conformed gilts could have a further substantial effect on sow longevity. Although direct heritability was low, selection programs for sow longevity seemed feasible and indirect selection based on some LC defects could also be applied. These results could be partially or completely extrapolated to worldwide intensive systems of pig production where Duroc, Landrace, and Large White breeds are used, although differences in management practices or installations (gestation stalls or sows grouped during gestation) could modulate the final effect of each LC defect on sow survival.

	Footnotes

 
¹ We would like to thank the 4 breeding companies involved in this project, Selección Batallé S.A. (Riudarenes, Spain), UPB Genetic World S.A. (Barcelona, Spain), Agropecuària de Guissona S. Coop. Ltda. (Guissona, Spain), and COPAGA SCCL (Lleida, Spain) for their cooperation. Thanks are also due to J. Piedrafita (Universitat Autònoma de Barcelona, Bellaterra, Spain) for collaboration and to the technicians of the Control i Avaluació de Porcí Center (Institut de Recerca i Tecnologia Agroalimentàries, Monells, Spain) for their assistance. We are also indebted to 2 anonymous referees for their helpful comments on the manuscript. The present study is part of the Welfare Quality research project (FOOD-CT-2004-506508), which has been co-financed by the European Commission within the 6th Framework Programme. The text represents our views and does not necessarily represent a position of the Commission, who will not be liable for the use made of such information.

² Corresponding author: joaquim.casellas{at}irta.es

Received for publication November 26, 2007. Accepted for publication March 26, 2008.

	LITERATURE CITED

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