Claw quality is an important factor that influences the welfare and productivity of sows. Claw quality is evaluated by visual scoring for claw shape, shape dimensions, lesion scoring, and measurement of structural, physical, and biochemical properties of the claw horn.1,2 In sows, mainly claw lesion scores are evaluated to define claw quality, but other measurements, including claw conformation, horn growth and wear, and mechanical claw characteristics, are rarely evaluated. More recently, effect of diet on histological claw characteristics were assessed with or without partially substituting inorganic zinc (Zn), copper (Cu), and manganese (Mn) sources with their organic forms.3,4 Claw lesions are a common multifactorial disorder in sows, with malnutrition and floor type, among others, noted as predisposing factors.5,6 Claw lesions influence claw quality, but claw quality also influences the occurrence and severity of claw lesions. Furthermore, claw quality depends on the internal characteristics of the claw, including optimal horn production, which is influenced by a diffuse nutrient supply from the dermis to the avascular epidermis.2,7,8 An insufficient nutrient supply results in a disturbed diffusion of nutrients to the avascular epidermis. This negatively affects horn production, thereby increasing the susceptibility of the claw to damage from the environment.7,8 The structural, regulatory, and catalytic functions of Zn are related to horn production.7 However, results from previous studies, mainly in cattle, are inconclusive: reports range from no effect or a reduction in claw lesion and lameness scores with varying dietary Zn concentrations.9-11 In weaned pigs, claw quality was affected by dietary Zn level.12 Studies in sows did not assess the impact of increased dietary Zn concentration, but showed similar claw lesion scores, neither deterioration nor better scores, or better scores with (partial) substitution of inorganic Zn, Cu, and Mn sources by organically bound Zn, Cu, and Mn, 3,13-18 except in one study.19 Study duration may be important for detecting differences between treatment groups. A lack of effect of dietary Zn may have been related to an excessively short study duration: 12 months is the threshold reported in the literature.10,11,20 Claw quality may be affected internally by dietary Zn concentration, while floor type is an external factor in development of claw lesions.6 An increased occurrence of claw lesions is observed when sows are housed on fully or partly slatted concrete floors, whereas straw bedding seems to be positively associated with fewer and (or) less severe claw lesions.6,11,21 A rubber top layer on concrete floors appears to protect claws due to a cushioning effect.22,23 In a long-term study in sows, however, the risk of more severe claw lesions increased when sows were housed on a rubber floor.24

Therefore, it was hypothesized that both dietary Zn concentration and floor type would influence claw quality in sows. The objective of this longitudinal study conducted over three reproductive cycles was to evaluate the effect of dietary Zn supplementation on claw quality characteristics in sows housed on two different floor types during gestation.

Materials and methods

All experimental procedures involving these animals were approved by the Institute for Agricultural and Fisheries Research (ILVO) Ethics Committee for animal experiments.

This longitudinal 2 × 3 factorial experiment was conducted according to the institutional and national guidelines for the care and use of animals.

Animals and management Six groups of non-lame primiparous sows (n = 131 gilts, RA-SE Genetics, RA-SE Genetics NV, Ooigem, Belgium; http://www.ra-se.com/nl/) entered the study when their locomotion score was ≤ 60 mm on a 150-mm tagged visual analogue scale (tVAS) developed by Nalon et al.25 These gilts were purchased per group (21 ± 4 sows per group) and quarantined for 4 to 6 weeks before their first insemination at days 233 ± 12 of age. The experimental period started 10 days before the first insemination (day 0) and the six successive production groups were monitored during three reproductive cycles (3-week interval between groups). When a sow was removed before the end of the experiment (n = 36), a new gilt replaced her.

Body weight, backfat thickness, and body condition score (BCS) at the start of the study were 149 ± 21 kg, 16 ± 4 mm, and 3.0 ± 0.5, respectively (mean ± SD). Sows were vaccinated at day 55 of gestation against porcine reproductive and respiratory syndrome virus (Porcilis; MSD Animal Health, Boxmeer, The Netherlands), 7 and 4 weeks before parturition against neonatal diarrhea caused by Escherichia coli (Neocolipor; Merial, Lyon, France), and 1 week postpartum against parvovirus and erysipelas (Parvoruvax; Merial). The sows were dewormed 17 days before parturition.

After weaning of the third reproductive cycle (end of the study), sows that had participated for at least 12 months in the experiment (ie, sows that had completed at least two reproductive cycles; n = 95) were transported to a commercial abattoir after the left and right front claw were marked with a color-coded tie wrap. After slaughter the following morning, both front claws were removed at the carpal joint and collected before the sows entered the scalding vat to preserve the metacarpal bones and claw structures. After the claw structures were collected for examination, the remaining parts of the front claws, including metacarpal bones, were frozen at -20°C.

Housing facilities

During the quarantine period (4 to 6 weeks), each purchased sow group (21 ± 4 sows per group) was housed as a static production group on concrete floors with straw bedding. The quarantine unit consisted of two pens with natural ventilation, no climate control, and artificial light.

Sows were housed in individual gestation crates during the insemination period (eg, shortly before the first insemination [day -10] to day 28 [insemination day 0] and from weaning [day -7] until 4 weeks after insemination [day 28] in their successive reproductive cycles). Housing and management conditions were similar for all sows. Housing was naturally ventilated, with artificial light and with temperature adjusted according to the outdoor temperature. In the first week after weaning, lights were on between 8:00 am and 8:00 pm. Crates had partly slatted concrete floors (14.4%; 1.38 m2 per sow).

During mid- to late gestation (day 28 to day 108), the sows were housed again in their static production groups. An electronic feeding system was used, with individual sow recognition through an electronic transponder in the sow’s ear. Ventilation was maintained at 73 m3 d-1·sow-1 and indoor temperature was set at 20°C. Light was artificial, and in each pen, one light source was illuminated during the night so that sows could easily access the feeding system. The group-housing facility consisted of four pens (4.45 m × 18.75 m; 40.4% slatted floor). Two of the four pens, oriented diagonally to each other, had a similar floor type: either concrete slats and solid concrete lying areas or concrete slats covered with rubber (EasyFix; Rubber Products Ltd, Galway, Ireland) and rubber lying mats (Gummiwerk Kraiburg Elastik GmbH & Co Kg, Tittmoning, Germany) on 50% of the solid concrete floor area (“concrete” or “rubber” floor type, respectively).26 At the start of the study, production groups were alternately assigned to the concrete or rubber floor type during group housing and returned to the same floor type for the duration of the experiment. Per pen, two rubber balls and a static and dynamic rotating grooming brush were provided as environmental enrichment.

From 1 week before the expected parturition date until weaning (day 108 to day 143), sows were housed individually in units with 10 farrowing crates. Housing and management conditions were similar for all sows. Ventilation for the farrowing units was adapted to the temperature. The indoor temperature was set to 23°C and light was artificial. The crates within the pen had a steel slatted floor (crate size 1.33 m2), which had a nonslip section, and the pen had a PVC-slatted floor (pen size 3.60 m2). Floor heating (0.60 m × 0.45 m) and heat lamps were provided in the pen for the piglets at the beginning of lactation.

Dietary treatment

All primiparous sows (gilts) in quarantine (4 to 6 weeks) were fed a pre-experimental gestation diet formulated according to NRC recommendations27 and commercial standards for gestating sows. The pre-experimental diet was fed ad libitum and contained 895 g per kg dry matter (DM), 127.3 g per kg crude protein, 301.5 g per kg neutral detergent fiber (NDF), 155.4 g per kg acid detergent fiber (ADF), 23.5 g per kg acid detergent lignin (ADL), 27.9 g per kg crude fat, 70.2 g per kg crude ash, 128.6 g per kg starch, 66 g per kg sugar, 121 mg per kg total Zn (ie, ± 21 mg Zn per kg originating from ingredients and 100 mg per kg added Zn as ZnO via premix), and 4.7 g apparent ileal digestible lysine per kg diet.

Throughout the experimental period, sows were fed a gestation and lactation diet formulated according to NRC recommendations27 and commercial standards (Tables 1 and 2), except for Zn. Phytase was added via the premix to simulate practical conditions. The gestation diet was provided 7 days before the first insemination, or after weaning of the preceding reproductive cycle (day -7) until 1 week before parturition (day 108). The sows were fed twice daily from day -7 to the end of the first 4 weeks of gestation (day 28), in total 2.3 kg per day, whereas during mid-gestation and up until the end of gestation (day 28 to day 108), sows were fed 2.6 kg per day. The lactation diet was provided from 1 week before parturition until weaning (day 108 to day 143). The sows were fed twice daily and received 3 kg feed provided in two equal portions during the week before parturition (day 108 to day 115). After parturition, 0.25 kg of feed per suckling piglet was gradually supplemented in addition to 3 kg feed, also provided in two equal portions daily.

Throughout the experiment, all sows had ad libitum access to drinking water, except in the first 4 weeks of gestation, when water was automatically provided through nipple drinkers for 15 minutes every hour and for 45 minutes while feeding to reduce water spillage.

Within each static production group, equal numbers of sows were randomly allocated to one of three dietary treatment groups, depending on the number of sows. The dietary treatments differed in Zn concentration: Zn not supplemented, originating from ingredients only; 50 mg Zn per kg supplemented; and 100 mg Zn per kg supplemented. Thus, total dietary Zn concentration was expected to remain below the European Union (EU) upper limit of 150 mg per kg (EU regulation 2016/1095). The Zn supplement comprised 50% inorganic Zn as ZnO (75% Zn) (33.3 or 66.6 g ZnO per 1000 kg feed, INVE Belgium NV, Baasrode, Belgium), and 50% organic Zn as Availa Zn containing 10% Zn in an amino-acid complex: single amino acids from hydrolysed soy proteins (molar ratio 1:1, 250 or 500 g Availa Zn per 1000 kg feed, Zinpro Corporation, Eden Prairie, Minnesota).

Claw quality measurements

On all sows, claw lesion scores, claw conformation, and horn growth and wear were determined. For some claw quality measurements (horn wall Zn concentration and histological and mechanical claw characteristics), 36 sows (12 from each dietary treatment group and at least one from each static production group) were selected. These sows were selected according to three criteria: three reproductive cycles completed; remained in their group of origin, ie, the group the sow was allocated to at the start of the experiment (repeat breeders that were transferred to another group allocated to the same treatment group and floor type did not meet this criterion); and housed in their group during the entire gestation period (eg, not separated from the group during the group housing period).

Claw lesion scoring, measurements of claw conformation and horn growth and wear were performed at the start of the experiment (day -10, baseline) and then on day 50 and day 140 of every cycle. For these measurements, sows were placed in a sow chute (FeetFirst sow chute; Zinpro Corporation, Eden Prairie, Minnesota), and lifted off the ground for maximum 15 to 20 minutes. At day 140, the supporting beam of the sow chute was disinfected between lactating sows to prevent pathogen transmission. After cleaning (water, brush and [or] hoof knife) and drying the claws with paper towels, lateral and medial claw digits of front and hind claws were scored for eight types of claw lesions using a tagged visual analogue scale (tVAS) of 160 mm (Figure 1). This scoring system was based on the “Zeugenklauwen check” (Wageningen University) and the method of FeetFirst by Zinpro Corporation.28 All claw digits were scored for seven claw conformation measurements12 using a digital calliper (Mitutoyo Belgium NV, Kruibeke, Belgium) following a methodology adapted from Calabotta et al29 and Vermunt and Greenough.2 These dimensions were subsequently used to calculate the distal toe angle, sole area, claw volume, claw horn size, and toe:heel ratio as described in van Riet et al.12 For horn growth and wear, a superficial reference point was incised into the dorsal horn wall of both claw digits of the left front and right hind claw by carving a small indentation with a hoof knife 0.5 cm below the periople and colored with Indian ink.12 At the subsequent evaluation (day 50 or day 140), the displacement above and below this reference point was measured using a digital calliper to determine horn growth, wear, and net horn growth.12 A new superficial reference point was incised into the dorsal horn wall and colored with Indian ink. For each above-mentioned claw quality variable, a mean score per sow per parameter per scoring day (day 50 or day 140) was calculated and used for further analysis.

Histological and mechanical horn characteristics were determined after slaughter. For histological examination, a claw horn wall (abaxial) sample, including the periople, and heel horn sample closest to the heel horn-sole junction (containing both the epidermal and dermal layer) of each claw digit of the front claws was collected using a scalpel. The left lateral and right medial claw digit samples were fixed in a 3.5% buffered formaldehyde solution. Subsequent preparation steps are described in van Riet et al.12 After H&E staining, standardized photographs (n = 5 positions per section; 10× magnification) of the sagittal heel horn sections (perpendicular to the bearing surface) and transverse horn wall and heel horn sections (parallel to the bearing surface) were examined. Each photograph of the transverse horn wall and sagittal heel horn sections was alternately assessed once by one of two observers. All transverse heel horn sections were assessed once by one observer. To calculate the histological claw characteristics, 93.4% of the transverse horn wall sections, 98.7% of the transverse heel horn sections, and 71.1% of the sagittal heel horn sections were included. These characteristics included the number of dermal papillae for the sagittal heel horn or dermal lamellae for the transverse horn wall sections per 1000 μm, width, distance, and length of longest dermal papillae or lamellae, and horn tubules density.12 The remaining sections were excluded because of broken samples and absence of the dermis layer. To assess differences between the two observers, histological measurements of 14 sagittal heel horn sections were conducted by both observers, using an average of five photographs per section. A paired t test and Pearson correlation were used to analyze the differences and correlations between observers.

For mechanical horn examination, an abaxial horn wall sample of the lateral and medial digits of the right front claw (mean length, width, thickness: 27 × 17 × 4.5 mm) was sawn using an oscillating saw, after which the underlying tissues were removed with a scalpel. The angle adjacent to the dorsal border and periople was marked. The horn wall samples were weighed and individually stored in a vacuum (to prevent fluctuations in moisture content) at -20°C until analysis. Horn wall samples of the left front claw could not be used to test the mechanical horn characteristics because of the incised superficial reference point for horn growth and wear measurements. The lateral or medial abaxial horn wall samples (n = 36 sows, 1.7 ± 0.4 g) were defrosted during 24 hours at 4°C and weighed (1.8 ± 0.4 g). The horn wall samples were cut into the required dimensions: two subsamples of 20 mm length and 6 mm width with a variable thickness using a mitre cutter with lever transmission (LOWE 3140/HÜ, Original LOWE, Gebr. Schröder GmbH, Kiel, Germany) and were weighed. On the first day, the two subsamples of the lateral or medial sample were tested. The subsample with the marked angle adjacent to the dorsal border and periople was tested first, followed by the subsample without marking. The two subsamples of the other lateral or medial sample from the same sow were tested the next day in the opposite order to that used the first day. The horn wall samples were tested with a three-point bending test (Texture Analyzer; Stable Micro Systems Ltd, Surrey, United Kingdom) according to the methodology described by Franck et al.30 The test characteristics (stress area and strain height) were adjusted for each sample because the sample thickness and weights differed. The span between the two supports was set to 15 mm and the sample was compressed over a distance of 5.5 mm (eg, maximal deformation) using a force transducer (load cell, 30 kg) exerted in the middle of the span distance of 15 mm. Each sample was tested with a loading velocity of 1 mm per minute and 15 mm per minute to determine visco-elastic properties of the claw horn. The time between the velocities ranged between 1 and 1.5 hours. A force-deformation curve was generated and converted (Exponent Software; Stable Micro Systems Ltd) to a stress-strain diagram. Then Young’s modulus, yield stress, and maximal stress were determined.30 Young’s modulus is a measure of the rigidity and stiffness of the horn and is represented as the slope of the linear phase of the initial line. Yield stress is the point on the stress-strain diagram in which the material starts to lose its mechanical function and material properties begin to change at further loading. Yield stress is represented as the point of the line where the line becomes nonlinear, using a parallel straight line with the same slope as the initial line (strain equal to 1%), where the intersect with the stress-strain diagram is defined as the yield stress.30 Maximal stress is represented as the maximal load a sample can withstand.30 The abaxial horn wall samples were stored in a vacuum at -20°C post testing until further analysis. Abaxial horn wall samples were dried at 103°C to a constant weight and analyzed for Zn content.

Chemical analysis

Feed samples of the gestation and lactation diets were collected from each batch and ground to pass through a 1-mm sieve for Near Infrared Spectroscopy evaluation, then pooled per dietary treatment group every 3 months for proximate analysis according to international standard methods accredited by ISO 17025.31 Dry matter, crude ash, crude protein, crude fat, calcium, and phosphorus content were determined according to 71/393/EEC, ISO 5984, ISO 5983-2, ISO 6492, ISO 6490/1, and ISO 6491, respectively. The American Oil Chemists’ Society (AOCS) approved procedure Ba 6a-05 was used to determine crude fiber content, and the procedures described in Van Soest et al32 were used to determine ADF, NDF, and ADL.

The homogenized feed sample was further ground to pass through a 0.5-mm sieve, and three of five samples per dietary treatment were subjected to Zn and Cu analysis. Copper was analyzed to assess possible antagonistic effects of Zn on Cu metabolism. Feed samples (1 g) were ashed and digested with HNO3 on a hot plate (150°C) for at least 30 minutes, then transferred to a 50-mL flask. The Zn and Cu concentrations were determined by inductively coupled plasma optical emission spectrometry (ICP-OES, Vista MPX; Varian Inc, Palo Alto, California).

The Zn concentration in abaxial horn wall samples (± 0.8 g) was determined by ICP-OES, Vista MPX after the samples were diluted in 10 mL 6N HNO3 for 12 hours, heated on a hot plate at 150°C for approximately 2 hours, and transferred to a 50-mL flask.

Statistical analysis

Linear mixed models were used for the outcome variables (Y) mean claw-lesion scores, claw conformation, horn growth, horn wear, and net horn growth. Dietary Zn supplementation (XZn), floor type (XFT), phase within the reproductive cycle (Xphase), parity (Xpar), digit (Xdig, lateral or medial), claw (Xclaw, front or hind), the two-way interactions between dietary Zn supplementation or floor type and phase within the reproductive cycle and interaction between dietary Zn supplementation and floor type were included as fixed effects. Reproductive cycle (Bcycle), sow (Bsow) and group (Bgroup) were included in the models as random effects to correct for the repeated measurements.

An example of a model is given in the following equation:

Y = β0 + β1XZn + β2XFT + β3Xphase + β4Xpar+ β5Xdig + β6Xclaw + β7XZnXphase + β8XFTXphase + β9XZnXFT + Bcycle + Bsow + Bgroup

Similar linear mixed models were used to analyze the histological and mechanical claw characteristic data. Fixed and random effects included in the models differed according to the sampled structures (eg, only right front claw was used for mechanical claw characteristic versus all claws included for claw-lesion scoring) and time of sampling (eg, claw structures collected after slaughter versus multiple observations throughout the reproductive cycle). For the histological claw-characteristic data, dietary Zn supplementation, floor type, leg (left or right front), digit, and interaction between dietary Zn supplementation and floor type were included as fixed effects. Sow and group were included in the model as random effects. For the mechanical claw-characteristic data, dietary Zn supplementation, floor type, digit, and interaction between dietary Zn supplementation and floor type were included in the linear mixed model as fixed effects, and sow and group as random effects per test velocity. Differences between test velocities and between two samples of the same digit of the same sow were analyzed using a paired sample t test.

Non-significant interactions were excluded from all final models and P values of the main effects are presented. In case of a significant interaction, partitioned post-hoc P values are presented. Partitioned post-hoc tests are tests of the simple effects of one variable for each level of the other variable. In the case of non-significant interactions, an all pairwise comparisons post-hoc test was performed. The P values of all post-hoc tests were corrected for multiple comparisons using the Tukey-Kramer method.

The analyzed data were considered to be sufficiently normally distributed, on the basis of the graphical evaluation (histogram and QQ-plot) of the residuals. All analyses were performed using SAS 9.4 (SAS Institute Inc, Cary, North Carolina).

Results

The interaction between dietary Zn supplementation and floor type was not significant for any of the outcome variables (P > .05). Effects of parity, claw (front and hind), and claw digits (lateral and medial) on claw quality measurements are presented in the supplementary data and supported by Suppl tables 1 and 2.

Throughout three reproductive cycles, 36 sows (27.5% of 131 gilts that entered the study), of which 9, 16, and 11 sows from the 0-, 50-, and 100-mg supplemented Zn per kg treatment group, respectively, were removed from the experiment, and 21 of them were replaced by non-lame primiparous sows. Sows were removed for several reasons: spontaneous death (n = 10), euthanasia (rectal or uterine prolapse, severe locomotion disorders; n = 7), or reproductive failure after multiple attempts (n = 19). In total, 70 sows remained in their group of origin, whereas 26 sows (repeat breeders) were transferred to another group allocated to the same dietary treatment and floor type. In total, 92 of 95 sows were slaughtered. Two sows died after their third parturition and front claws from one sow were collected post mortem after euthanasia at the ILVO experimental farm directly after the other sows of the group were loaded for transport.

Claw lesion scores

Dietary Zn supplementation influenced the mean heel horn erosion score (P = .01), showing a 3.48-mm higher (worse) mean heel horn erosion score for the non-supplemented sows compared to the 100-mg Zn per kg supplemented sows (Table 3). Other types of claw lesions did not differ between dietary treatment groups (P > .05). Floor type did influence some claw lesion scores.26 Mean claw lesion scores were lower at day 50 than at day140 of the reproductive cycle, except for horizontal wall cracks (Table 3).26

Claw conformation

Claw dimension measurements. An interaction was found between dietary Zn supplementation and phase within the reproductive cycle for the mean sole (base) length (P = .03), showing shorter sole lengths for the non-supplemented sows than for the 50-mg Zn per kg supplemented sows at day140 (Table 3). Mean heel height was influenced by dietary Zn supplementation (P = .01), with lower heel heights for the non-supplemented and 100-mg Zn per kg supplemented sows compared with the 50-mg Zn per kg supplemented sows (Table 3).

Independent of dietary Zn supplementation, the mean toe height and mean claw length of sows were lower and shorter, respectively, for the rubber floor than for the concrete floor (P = .001 and P = .04, respectively) at day 50 (Table 4). Mean heel height tended to be lower for sows housed on the concrete floors at day 50 (P = .06) and was lower for sows housed on the rubber floors (P = .04) at day 140. The interaction between floor type and phase within the reproductive cycle was not significant for claw width. Mean claw width did not differ between floor types.

Mean length of the dorsal border, diagonal claw length, toe height, heel height, and claw length were all shorter at day 50 than at day 140; claw width was the only exception (Table 3).

Claw morphology calculations. The mean claw volume was less (P = .03) for non-supplemented sows and tended to be less (P = .06) for the 100-mg Zn supplemented sows than for 50-mg Zn per kg supplemented sows. The mean toe:heel ratio was influenced by dietary Zn supplementation (P = .02), with a higher ratio for the non-supplemented sows compared with the ratio of the 50-mg Zn per kg supplemented sows. Distal toe angle, sole area, and claw horn size did not differ (Table 3).

The mean distal toe angle, sole area, and toe:heel ratio were lower for sows housed on rubber floors than for sows housed on the concrete floors at day 50 (Table 4). Mean claw volume was lower for the rubber floor type compared with the concrete floor type at day 140 (P = .01). The mean claw horn size did not differ (Table 4).

Mean distal toe angle and toe: heel ratio were lower, while mean sole area, claw volume, and claw horn size were higher at day 140 than at day 50 of the reproductive cycle (Table 3).

Horn growth and wear. Horn growth and wear did not differ between dietary treatment groups (Table 3). At day 50, horn growth and wear were lower for sows housed on rubber floors than on concrete floors. At day 140, horn growth and wear did not differ between floor types (Table 4). Net horn growth (horn growth minus wear) did not differ between dietary treatment groups nor between floor types at day 50 and day 140 (tables 3 and 4).

Net horn growth differed between day 50 (-4.2 mm, horn wear dominated) and day 140 (+4.4 mm, horn growth dominated) (P < .001).

Histological claw characteristics. Length of the longest dermal papillae (P = .83) and width of papillae (P = .13) did not differ between observers. Differences between observers were found for sample length (P = .03; confidence interval [CI] 3.3-57.8 µm, mean difference 30.5 µm), number of dermal papillae (P = .003; CI -0.9 to -0.2, mean difference 0.6), number of dermal papillae per 1000 µm (P = .003, CI -1.0 to -0.2, mean difference -0.6), and distance between papillae (P = .02, CI 9.1-100.3 µm, mean difference 54.7 µm). Correlation coefficients of inter-observer reliability were 0.95 for sample length, 0.79 for dermal number, 0.96 for longest length, 0.43 for distance between papillae, and 0.94 for papillae width. On the basis of the high inter-observer reliability (except distance between papillae) and the numerically irrelevant (but significant) differences, observer was not included in the final statistical models.

The number of dermal lamellae per 1000 µm, distance between lamellae, width of the lamellae, or length of the longest lamellae of the transverse horn wall did not differ between dietary treatment groups nor between floor types (Table 5).

The distance between the dermal papillae of the sagittal heel horn tended to be shorter for the non-supplemented sows (P = .08) and was shorter for the 50-mg Zn per kg supplemented sows (P = .003) compared with the 100-mg Zn per kg supplemented sows (Table 5). The number of dermal papillae per 1000 µm, width of the papillae, or length of the longest papillae of the sagittal heel horn did not differ (P > .05) between dietary treatment groups.

Independent of dietary Zn supplementation, the length of the longest papillae of the sagittal heel horn tended to be shorter and the distance between dermal papillae was shorter for sows housed on rubber floors than sows housed on concrete floors (Table 5). The number of dermal papillae per 1000 µm or width of the papillae did not differ between floor types (Table 5).

The density of the heel horn tubules of the transverse heel horn, expressed as the number of horn tubules within a defined surface area of 1 mm2, did not differ between dietary treatment groups nor between floor types (Table 5).

Mechanical horn characteristics. Horn wall Zn concentration did not differ between dietary treatment groups (Zn concentration in DM was 128, 122, 120 mg Zn per kg for non-supplemented, 50-mg per kg- and 100-mg per kg supplemented sows, respectively; P = .39) nor between floor type treatments (Zn concentration in DM was 123 and 124 mg Zn per kg for concrete and rubber floor types, respectively; P = .86).

Thickness of the abaxial horn wall sample did not differ with dietary Zn supplementation (P = .94) nor with floor type (P = .77). None of the mechanical abaxial horn wall characteristics were significantly affected by dietary Zn supplementation or floor type (Table 6). Young’s modulus differed between 1 mm per minute and 15 mm per minute test loading velocities (P = .01); tended to differ for maximal stress (P = .07); and did not differ for yield stress (P = .22), showing visco-elastic properties of the horn wall. Differences between the two horn wall samples per claw digit per sow for both test velocities were found for Young’s modulus, yield stress, and maximal stress (P < .001).

Discussion

The welfare and productivity of sows can be affected by poor claw quality. In the present study, multiple claw characteristics were determined to evaluate claw quality in response to dietary Zn supplementation. Minor effects on claw quality measurements were found irrespective of floor type. Possible factors that overruled the potential influence of Zn in the present study are the overall good claw lesions score – with a mean lesion score close to the upper threshold of 40 mm for healthy claws – and the unexpectedly high background Zn supply via premix in the basal diet during lactation. The impact is, however, not quantifiable. Differences in claw quality for floor type were observed under these similar study conditions, but floor type is documented as an external predisposing factor for claw lesions, whereas dietary Zn concentration may affect claw quality internally6,18 by influencing horn production, which may be more time dependent. Therefore, study duration is an important factor to take into account. Claws require a minimum duration for the horn capsule to be produced. In cattle, this was determined as 12 months.10,11,20,33 On the basis of the length of the dorsal border and horn growth in the present study, the horn capsule in sows may have been produced in 5 to 6 months. The present study lasted 14.8 months, thus it seems that study duration is unlikely to have influenced the lack of response to Zn observed here.

Claw quality is determined by multiple claw characteristics and depends on an optimal horn production.1,2,7,8 In the literature, Zn has been shown to have an important function in horn production.7 If Zn supplementation influences horn production, changes in multiple claw quality measurements should become visible. In the present study, however, only heel horn erosion, heel height, claw volume, toe:heel ratio, and the distance between dermal papillae were influenced. These differences were not linear and not constant between different treatment groups for all characteristics. For instance, the distance between dermal papillae was longer for the 100-mg per kg supplemented sows, indicative of a lower quality of the structure, whereas the heel horn erosion score was better. In accordance with the present study, Anil17 observed differences only for vertical wall cracks in group-housed sows and only for heel-sole junction lesions in stall-housed sows. Other claw lesion scores assessed in their study were not improved over time. Some differences in claw quality measurements were observed only at day 50 or only at day 140 of the reproductive cycle, suggesting that phase within the reproductive cycle is important for claw quality. This is supported by the substantial fluctuation in net horn growth (horn growth minus wear) between gestation (-4.2 mm between day 140 and day 50, horn wear dominant) and lactation (+4.4 mm between day 50 and day 140, horn growth dominant). This difference in net horn growth between gestation and lactation can be caused by housing conditions and differences in physiology. Sows were housed in groups for 80 days during gestation and were more able to move around, whereas during lactation the sows were individually housed.

Comparing results of the present study with other studies in sows is difficult, because those studies did not assess the impact of Zn supplementation but did investigate (partially) substituted inorganic Zn, Cu, and Mn sources with organically bound sources at high mineral levels. Similar or better claw lesion scores were reported with organically bound Zn, Cu, and Mn supplementation compared with inorganic sources.13-18 Bradley19 found no difference between mineral sources for claw size and shape, however. Results of the present study do not agree with those of some other studies. This may reflect differences in breeds and test conditions, but also the difference in background levels of Zn in the basal diet. While the results derived from the chosen test conditions in our study should be extrapolated with caution, yet the detail of our measurements, together with the three-cycle duration of the study, rendered important insights that are new to the field. Nevertheless, the type of Zn supplement used may be an interfering factor, as other studies in sows did find positive results of Zn source on claw lesion scores.13-17 Perhaps Zn alone does not trigger the processes required to optimise claw quality, and other minerals or dietary components are required as well.

Other studies in sows defined claw quality mainly by claw lesion scores, but claw conformation, horn growth and wear, and mechanical horn characteristics are rarely evaluated. Histological claw characteristics have been assessed more recently.3,4 Claw conformation did not differ between treatment groups except for heel height, claw volume, and toe:heel ratio, which is in accordance with Bradley.19 Horn growth and wear rate also did not differ in the present study. Lisgara et al18 also found no effect of partially substituted mineral sources on toe and dewclaw length in one herd, whereas the score did improve over time in each of the other two herds. Herd-specific characteristics might have influenced the outcome in that study. In the same study,18 claw (front or hind) did interfere with the results. In the present study, horn growth and wear rate differed between front and hind claws as well as for other claw lesion scores and claw conformation. Net horn growth did not differ between claws, however. There is a need to further determine the impact of these factors. For histological claw characteristics, Varagka et al3,4 found that lamellar hyperplasia was most frequently observed and sows had a higher claw lesion score with lamellar hyperplasia. This histological condition is also described in bovine and equine laminitis and may cause low quality horn production. Partially substituting inorganic Zn, Cu, and Mn sources with organic sources resulted in fewer histological changes than in the control group with inorganic mineral sources.3 These interesting findings were deduced from a sample at the dorsal wall halfway between the periople and tip of the toe, whereas in the present study a sample from the periople was collected, where horn production is initiated. In the present study, no differences were found in histological characteristics of the horn wall or in mechanical horn strength, although the sample for mechanical horn strength was more closely located to the sample location of Varagka et al.3 Testing for mechanical horn characteristics has not been conducted previously and was based on test conditions used in bovine horn wall without dietary interventions.30 For sows, the extrapolation of this test and its conditions needs to be further explored.

In the present study, most sows had one or more claw lesions, but only the mean heel horn erosion score was better for the 100-mg Zn per kg supplemented sows compared with the non-supplemented sows at day 50 of the reproductive cycle. This result may favor Zn supplementation; however, it is questionable whether the measured difference is relevant for the sows’ welfare and performance and whether this difference is distinguishable during visual claw scoring. Furthermore, it is remarkable that the non-supplemented sows were able to maintain claw quality at the same level as the supplemented sows. While the non-supplemented group had a high background Zn supply during lactation, overall dietary Zn concentration was well below commercial practice and EU regulation of 150 mg per kg total Zn (EU regulation 2016/1095), without negative results, which questions whether the systematic supplementation of Zn, in addition to the amount of Zn (and phytase) present in the basal diet, should be revisited. Further reductions of the EU upper limit for total dietary Zn concentration when using phytase, as suggested by the European Food Safety Authority in 2014, should be tested in relation to claw quality in future studies.

The effect of floor type on claw quality measurements in the present study appeared to be substantial, irrespective of dietary Zn supplementation. Sows housed on a rubber floor had better scores for some claw lesion types, but worse scores for vertical horn wall cracks, white line separation, and overgrown claw at day 50 of gestation, which changed to better scores on rubber at day 140.26 At day 140, scores for white line and claw length were better for sows housed on a rubber floor during gestation.26 Other types of claw lesions did not differ between floor types. This finding is in contrast to results of another study24 in sows that found an increased risk of poor scores for toe overgrowth, heel sole cracks, and horn wall cracks in first-parity sows housed on rubber slat mats, as compared to sows housed on a concrete slatted floor. In the second parity of that study, sows housed on rubber slat mats had an increased risk of poor scores for toe overgrowth, heel sole cracks, and white line damage.24

Discrepancies between studies may relate to the proportion of solid floor area, as the present study had a higher proportion of solid floor area as compared to a fully slatted floor covered with rubber in the study of Calderón Díaz et al.24 Another explanation may be the quality of the floor as characterised by the slip resistance, abrasiveness, hardness, wear resistance, and age of the floor.6 Rubber slat mats may be less abrasive 24,34 and may be softer than concrete slats.24,35-39 The influence of floor type on claw conformation in the present study can be a result of this softer rubber floor. Softer floors can reduce natural horn wear,6,40-42 which was also found in the present study in which sows housed on rubber floors had less horn wear at day 50 compared with sows housed on concrete floors. However, some claw lesion scores were lower (better) on rubber. It seems possible that a higher risk for claw lesions does not depend on insufficient wear alone: the balance between horn growth and wear and load bearing (eg, the pressure the floor exerts on the claw and how the load is distributed over the weight-bearing surface) may also be important factors.43 In the present study, horn growth was lower for sows housed on rubber floors at day 50 and net horn growth was not affected, indicating that the balance between horn growth and wear could be maintained and that therefore claws were less prone to develop lesions. Similarly, the distal toe angle was lower on rubber floors, which indicates that there was less wear, less growth, and a smaller sole area.44 Load bearing may have been better in the sows housed on rubber floors in the present study, because the histological claw characteristics showed a shortened distance between dermal papillae in the sagittal heel horn, indicating a stronger structure.

Slat properties are also important.6,45-47 The rubber top layer in the present study was attached to the concrete floor in a fashion similar to that reported in Calderón Díaz et al.24 One advantage of the rubber top layer may be a reduction in the risk of claws getting entrapped between the slats. An important disadvantage in the study of Calderón Díaz et al24 was that the manure could not pass through the slats easily. This observation was not included in the present study. Floors covered with liquid and manure have been reported to soften and irritate the claw, resulting in diminished claw strength.6 This may not have been the case in the present study, where moisture content of the horn wall and horn wall strength were not affected by floor type.

In conclusion, no interaction effects between dietary Zn supplementation and floor type were found for claw quality measurements. Dietary Zn supplementation to a typical basal diet had only minor influences on claw quality in sows. Floor type affected multiple claw quality measurements positively. The rubber top layer does improve claw quality and can be implemented for prevention of claw lesions, but more research is warranted. Apart from dietary Zn supplementation and floor type, other factors affected claw quality measurements: differences between lateral and medial claw digits and between front and hind claws were observed, and the reproductive phase had an important effect on claw quality. Worse claw lesion scores were found at day 140, a period in which horn growth dominates and claw conformation characteristics changes, compared with day 50.

Implications

• Dietary Zn supplementation seems not to be the major factor affecting claw quality in the sows in this study.
• Floor type affects claw quality in sows.
• Phase within the reproductive cycle influences claw quality measurements and needs to be considered when claw quality is assessed.

Acknowledgements

This study was supported by the Institute for Promotion of Innovation through Science and Technology in Flanders (IWT, grant number 090938), and co-funded by Orffa, VDV Beton, Boerenbond, AVEVE, INVE, and Boehringer Ingelheim. The authors thank the technicians M. van Yperen and T. Martens, animal caretakers at the ILVO experimental farm, ILVO colleagues and students, J. Buist, J.-M. Muijlaert, laboratory personnel of participating departments, and personnel of the abattoir in Eeklo (Belgium) for their much appreciated assistance and support. Thanks also to M. Levenson for English-language editing.

Conflict of interest

None reported

Disclaimer

Scientific manuscripts published in the Journal of Swine Health and Production are peer reviewed. However, information on medications, feed, and management techniques may be specific to the research or commercial situation presented in the manuscript. It is the responsibility of the reader to use information responsibly and in accordance with the rules and regulations governing research or the practice of veterinary medicine in their country or region.

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