Análisis del rendimiento por planta y parcela

In the 7 weeks of this trial, routine foot trimming did not result in a significant reduction in prevalence or incidence of lameness or disease (ID, FR or granuloma) when compared to the control group. Treatment allocation was also not associated with a reduced risk of becoming lame in any of the three models even when the addition of treatment with topical antibacterial at trimming was accounted for. There was however a higher proportion of locomotion score 3 sheep in the control compared with the treatment group between T1 and T2 and this was significant. There may be four reasons for this observation. Firstly, routine foot trimming may have reduced the prevalence of higher locomotion scores but not the prevalence of lameness overall. Secondly, the initial stratification did not include locomotion score or heritability of resistance to footrot (see discussions below) and an element of error may have been introduced. Thirdly, an increase in severity of locomotion score is associated with an increased severity of FR lesion scores even prior to being visible (Kaler et al., 2011). Although the frequency distribution of ID and FR lesion severity at T1 was

not significantly different between groups, the slightly higher number of ewes with FR lesions (4 cf. 2) at T1 in the control group may have been enough to produce a difference in higher locomotion scores between groups. Finally, the application of topical antibacterial to 11 treatment ewes at trimming compared with no control ewes may have reduced the prevalence of higher locomotion scores in the treatment group.

The higher numbers of locomotion score 3 control ewes were the result of a sharp rise in the prevalence and incidence of mild cases of lameness in the control group which was not seen in the treatment group following a gathering event on the 29th June. The flock had been gathered to remove faecal-matted wool from hindquarters, worm and weigh lambs during which there were a large number of lame lambs that were examined and treated by the farm shepherd for lameness, specifically FR lesions. Gathering results in an increase in the transmission of D. nodosus through environmental contamination due to the increase in stocking

density and may lead to increased susceptibility of the flock to disease (Wassink et al., 2003a; 2003b; 2004). This does not however explain why the steep rise in

lameness was only seen in one group when the treatment groups were run as a single flock. Ewes are at increased risk of becoming lame when one or more of her offspring are lame and vice versa (Kaler et al., 2010b) presumably as a consequence of increased proximity from the maternal bond. Lambs were not assessed as part of the trial. Identity of lamb and dam pairs was not possible because when the study began lambs were >14 weeks, fairly independent and any original identifying management markings applied at birth were unreadable. It is therefore plausible that lambs of control group ewes had more disease (specifically FR) than those of treatment ewes leading to a spike of lameness in

Chapter 2 Page 84 control ewes only. An alternative explanation is that routine foot trimming reduced the susceptibility of ewes to disease at gathering events. This explanation is however highly unlikely given that the flock was gathered more than once during the trial and the uni-lateral rise in lameness was not repeated. It is therefore recommended that future studies be conducted after weaning or include an assessment of lambs. Resistance to footrot varies between phenotypes, and research by Nieuwhof et al. (2008) indicates that effective selection for resistance to footrot in the UK is possible. Consequently, an alternative plausible explanation for the differences in higher locomotion scores observed between treatment groups could be differences in individual’s heritability of resistance to footrot between animals in the groups.

It was estimated that with hoof horn growth rates of ~3.6 mm per month (Shelton et al., 2012), routine foot trimming would have little effect after 7 weeks and this

was chosen as the period for re-examination of all ewes’ feet. There was no significant improvement in foot conformation of those sheep that were routinely foot trimmed after 7 weeks. In addition, there was no significant difference in foot conformation detected between sheep that were routinely foot trimmed and those that were not or in the general welfare of the sheep, as measured by changes to body condition score. Feet of control ewes with ID/FR and ‘poor’ foot conformation at T1 were however, significantly more likely to have ID/FR lesions compared with treatment feet between T1 and T2 or at T2. Routine foot trimming allows shepherds to examine all the feet of all their ewes, and where disease is found presents an opportunity for treatment. One possible explanation is that in this study, 7 of 11 ewes received treatment with topical antibacterial by the shepherd for disease at trimming. Although treatment at trimming was not

significantly associated with a reduced risk of lameness in any of the models, the 7 ewes treated in the treatment group cf. 0 in the control group may have been sufficient to produce this result. Alternatively, this may have been an aberration of the small numbers involved (5/6 cf. 0/4).

Of those feet that were trimmed, there were significantly more feet in the poorer foot conformation categories (score 2 and 3) that received a poor trim (i.e. toe horn cut perpendicular to the sole, bleeding or sensitive tissue exposure) and vice versa. The thickness of sole and wall horn is only 2-3mm thick (EBLEX, 2008a)

and therefore there is only slight room for error to avoid over trimming. It may be more difficult to trim hoof horn that is very overgrown or ‘abnormal’ in shape compared to that which is only slightly overgrown or misshapen without damaging the integrity of the horn because more hoof horn has to be trimmed to return it to a ‘normal’ shape. In addition, sheep were significantly more likely to be lame during the 7 week period after trimming, if at trimming they bled or had sensitive tissue exposure and this association was confirmed by model 3. Current recommendations are that feet only be trimmed where necessary (EBLEX, 2008a). However, this study highlights that it is exactly these sheep that are at increased risk of an accidental overtrim and perhaps therefore should not be trimmed at all.

Similar to findings by Kaler et al. (2010b), in this study sheep with poor foot conformation or FR at T1 were significantly more likely to become lame, regardless of whether or not they received a routine trim. Treating lame sheep within 4 days of becoming lame and using parenteral antibiotics reduces the risk of developing poor foot conformation (Kaler et al., 2010b). If 50% of farmers are

Chapter 2 Page 86 using foot trimming as a means to improve the foot conformation of their flock (Wassink et al., 2005), they should be advised to stop routine foot trimming and instead to appropriately and promptly treat lame ewes within 1-4 days of becoming lame, with parenteral antibiotics for cases of FR (Wassink et al., 2010a; Kaler et al., 2010a; Hawker, 2008).

At foot level, there were a significantly higher number of feet with ID only lesions in the treatment and trimmed group than in the control group at T1. There was however no significant difference in the presence of ID lesions between feet of treatment compared with feet of control sheep. This finding is as a consequence of the shepherd’s subjective trimming criteria, i.e. he was more likely to trim feet if they had ID lesions. This is not unlikely given that 62% of farmers self-report to carry out therapeutic foot trimming to treat ID (Wassink et al., 2010b).

Overall, the results suggest that the 6 hours spent by this shepherd routine foot trimming 85 ewes (less than half of his flock) was not beneficial to individuals or the flock as a whole and was time ill spent, even if one considers that the small differences in lameness were entirely attributable to routine foot trimming.