4.1. Gait Characteristics of Sound Gilts and Lame Sows
Although the gaits of SGs and LSs were different in only a few parameters, similar differences between lame and sound sows were already observed in studies using the same equipment [25
], with sound sows presenting a shorter stance time or a wider tarsal angle. An arched back was also more pronounced in lame sows [26
]. On the other hand, several parameters were affected by the week in both types of pig. This could compromise the interpretation of the eventual effects of the different slatted floors tested since the difference might be attributable to the evolution of lameness of the animal or to other unknown parameters. Indeed, the between-days variations (%CV) of some kinematics parameters for the same sow can exceed 10% or even 15% [26
The present study used gilts and small sows, expecting that the anatomy and conformation of these animals could make them more prone to lameness on slatted flooring. Indeed, gilts have smaller feet that may get more easily trapped in wide gaps and be associated with the higher culling rate for lameness observed in first parity sows [27
]. On the other hand, “lame sows” were sows that show a modified gait according to several indicators visually observed such as reduced stride length or avoidance to bear weight on a foot. Generally, these sows do not show an easy, regular and symmetrical gait. Consequently, lameness usually translates into modifications of several gait parameters recorded by kinematics such as a shorter stride length, a longer stance time, a slower walking speed, or a lower joint flexion [25
]. Lameness can be due to various pathologies such as claw lesions, osteochondrosis or arthritis that may affect one or several limbs and that are generally painful [29
]. Therefore, lame sows are a particularly relevant population of animals that may be more sensitive to variations in floor characteristics. Our results showed that 15 gait parameters tended to be or were significantly affected by the slat and gap width treatments in SGs, while only eight parameters were affected in LSs, and six of these on the perpendicular-oriented slats. Therefore, gilts may be more sensitive to slat and gap widths, most likely due to their anatomy.
4.2. Effects of Slats and Gaps Width on Gait Characteristics of the Animals
Of the more than 72 parameters analyzed, only 10 parameters were significantly affected by treatments (p
< 0.05) and another 13 parameters showed tendencies (p
< 0.10). The parameters that were significantly affected were related to the back angle, foot height, stride length, and carpal and tarsal joint angle amplitudes. No significant effects were seen on temporal parameters such as swing time or stance time which were previously reported to be affected by lameness severity [26
] or floor conditions [31
In previous research, the stride length was found as a good indicator of natural or induced lameness [26
] or gait adaptation on a slippery floor [32
], with a reduction of the stride length when compared to control animals or conditions. A narrower back angle, often visually observed as an arched back, was also reported as a good indicator of lameness in sows [26
] and cows [34
]. In the present study, these two parameters were significantly affected in gilts only when walking on slatted floors with 19-mm gaps, with longer strides (on the 105-mm slats only) and a wider back angle when walking on the smaller gap compared to 25-mm gaps.
The effects on some parameters are more difficult to interpret due to less information being available in the literature. For example, the foot height was not shown to be modified by factors such as floor slipperiness [33
], or lameness [25
] in sows. In cows, the hoof height was affected by the floor surface with a higher foot elevation on rubber floor than concrete floor for both cows with and without sole ulcers [35
]. This higher elevation of the stride, along with several other parameters, was interpreted as better comfort and easiness at walking. However, another study found no effect of flooring type (concrete, rubber or resin aggregate) on the foot height [36
]. On the contrary, in horses, induced lameness resulted in a higher elevation of the hoof when trotting [37
], but this effect was less pronounced when walking [38
]. In the present study, the foot elevation was affected differently between gilts and sows depending on the slat orientation. In SGs, both front and rear feet elevations on parallel-oriented slats were similarly affected with a higher elevation when walking on the 105-mm slats, compared to 125-mm slats. On perpendicular-oriented slats, only the front feet height was more elevated on the 105-mm slats. In LSs, the front feet height increased when walking on 25-mm gaps compared to 19-mm gaps of the perpendicular-oriented slats. For sows and gilts on the control floors, the feet elevation was between 4 and 5 cm, thus elevations higher than 5 cm (up to 6 cm) observed in animals walking on 105-mm slats or 25-mm gaps could be interpreted as a deviation from the normal gait. However, other parameters must be considered before a full interpretation can be made.
As for angular parameters, only the amplitudes of carpal and tarsal joint angle were significantly affected by treatments. There is virtually no information available on the biomechanics of walking sows, while information is available for horses [39
]. Carpal and tarsal joint movements involve flexion during the swing phase and extension during the stance phase. Consequently, the angle should reach its minimal value during flexion and maximum during extension. On the other hand, the angle variation and amplitude should be maximal during the swing movement when the limb goes from stretched position to bent and back to stretched position, while the limb stays quite stretched most of the stance phase, especially the front limb [40
]. The values reported in Table 3
for joint angle amplitudes are in accordance with these theoretical principles. In the present study, the locations of reflective markers to assess carpal and tarsal joint variations (front feet/carpal/elbow and rear feet/tarsal/stifle) most likely measured a composite movement of both fetlock and carpal/tarsal joints (see figure 2). Previous research reported that sows lame on a front leg had a lower carpal joint flexion (which would translate in a lower joint angle amplitude during the swing phase), which was indicative of stiffness [28
]. Accordingly, Thorup, et al. [42
] showed that pigs walking on slippery floors have a lower peak joint moment for forelimbs (equivalent to a lower joint amplitude), indicative of gait adaptation. In the present study, joint movements were differently affected between LSs and SGs and between swing and stance phases of the stride. In SGs, only the carpal joint angle amplitude was modified during the stance phase on parallel slats. As reported by Thorup, Laursen and Jensen [42
], gait adaptation to floor conditions was more visible on front limb joints which carry more weight while no change was seen on hind limb joints. Higher carpal joint angle amplitudes during the stance phase could be interpreted as better stretching movement of the limb, and therefore a better ease of movement which was seen for 85-mm slats and for slats with 22-mm gaps in parallel orientation. In LSs, treatments affected both carpal and tarsal joint amplitudes during swing phase with higher flexion on perpendicular-oriented 125-mm slats, indicative of a better ease of movement. Finally, as for gilts, the higher tarsal joint angle amplitude during the stance phase in LSs could be interpreted as a better ease of movement when walking on 22-mm gaps of perpendicular-oriented slats.
Considering all these results together, there was no consistent and very clear effect of slat or gap width on most of the gait parameters studied. Moreover, depending on the parameters considered, effects of gap or slat width differ between SGs and LSs, and between orientation of the slats. Interpretation is therefore difficult, all the more that the information available from previous studies in pigs is scarce. Looking at the values when walking on control floors can further help to look at gait modification in terms of least deviation from the normal gait of the animals. Of the angular parameters significantly affected by slat and gap width, gait characteristics deviated least from solid floor measurements mainly for 105-mm slats for both SGs and LSs. Comparing the results of the current study with and other species, the effects seen tend to show a better ease of walking for the smaller gaps (19 and 22 mm), and the larger slats (105 and 125 mm). Overall, this observation is in accordance with the recommendation of the European Union for a maximum gap width of 20 mm but supports use of a wider slat than the minimum slat width of 80 mm [17
]. Further, it suggests that the commonly used slat of 125 mm in North America could be reduced, with a 19-mm gap being particularly suitable for breeding gilts.