is sufficient evidence to demonstrate that subacute ruminal acidosis (SARA) does indeed lead to inflammation of the laminae that results in movement of the pedal bone and development of claw horn lesions, use of the terminology ‘subclinical laminitis’ should be questioned when referring to the development of claw horn lesion.


Body condition score and the ‘fat pad’ Claw horn lesions occur as a result of the production of poor quality horn. This is through damage to the cells producing the claw horn (germinal epithelium) with subsequent impaired growth and keratinization of hoof horn (Hoblet and Weiss, 2001). Therefore, any factor that causes damage to the germinal epithelium may result in claw horn lesions. In more recent years, as the traditional laminitis theory has been challenged and the multifactorial nature of lameness and claw horn lesions has been explored, alternative hypotheses to explain why high producing dairy cows are at increased risk of claw horn lesions have been investigated. In particular the role of the ‘support apparatus’ i.e the tissues within the hoof capsule that support the pedal bone and therefore protect the sensitive corium and germinal epithelium, have been the focus of recent research. The digital cushion or ‘fat pad’ is a key component of the support apparatus and is composed of connective tissue and fat depots (Raber et al., 2004). It is thought to provide force dissipation during claw strike whilst the heel and associated connective tissues serve as a ‘shock absorber’ (Raber et al., 2004). The digital cushion therefore provides a protective function for the corium. The fat content of the digital cushion may change with mobilisation of energy and body reserves as occurs elsewhere in the body; a change in body condition score may therefore also reflect a change in the fat content of the digital cushion. If the fat content of the digital cushion is reduced it is possible that this may impair its functionality in dissipating forces, leading to damage to germinal epithelium and subsequent development of the claw horn lesions. This has been an area of recent research and is discussed in further detail below.


Body condition score A number of recent studies have investigated the relationship between body condition score (BCS) and lameness in dairy cows (Green et al., 2014; Lim et al., 2014; Randall et al., 2015). Because lameness can impact on body condition score itself (through reduced dry matter intake), understanding the relationship between BCS and lameness can be complex; the key question is do lame cows go thin or do thin cows go lame? In order to understand this relationship Randall et al. (2015) explored the association between BCS and first lifetime lameness events. Statistical models were used to control for other confounding factors such as milk yield and days in milk. Low BCS 16 or 8 weeks before a first mild or severe lifetime lameness event, respectively, had a positive association with risk of lameness in cows second lactation or greater. This provided evidence to support the hypothesis that loss in body condition score precedes lameness. In the same study cows with BCS < 2 were at greatest risk of repeated lameness events through their lifetime and that a loss in BCS 0 to 4 weeks post-calving was also


associated with an increased risk of lameness. These findings were consistent with findings from other studies identifying that low BCS is associated with lameness in dairy cows (Green et al., 2014; Lim et al., 2014); cows with low BCS ≤2 (on a scale 0 to 5) are more likely to be treated for lameness in the 2 or >2 to 4 months following such a score (Green et al., 2014).


Digital cushion The association between low body condition score and lameness could be explained by a reduced thickness of the digital cushion as outlined above. Bicalho et al. (2009) demonstrated that this association between low body condition score and reduced digital cushion thickness exists in a cross-sectional study, however it was not possible to show that the loss in digital cushion thickness associated with loss in body condition score preceded the development of claw horn lesions i.e whether a loss in digital cushion thickness was a cause of these lesions. Newsome et al. (2017a) recently demonstrated in a longitudinal study of 179 dairy cows from two herds that sole soft tissue thickness (corium and digital cushion) did change with another measure of body fat (back fat thickness) over time, although other factors such as calving, herd and lesion incidence also have a significant effect on digital cushion thickness. A follow-up study aimed to determine whether a decrease in thickness of the sole soft tissues increased the likelihood of lameness or claw horn lesion development (Newsome et al., 2017b). Findings showed that although a thinner digital cushion was associated with increased lesions and lameness, a change in thickness i.e thinning of the digital cushion was not associated with increased risk. The findings from these two studies suggest that factors other than loss in body fat that are associated with a cow having a thinner digital cushion may be important; for example, hormonal changes associated with calving which reduce the functionality of the suspensory apparatus. The evidence would suggest though that maintaining appropriate BCS is still important in terms of reducing the risk of lameness and this risk factor should not be ignored.


Pathological changes Another aspect to the development of claw horn lesions that has received attention in recent research are the long term pathological changes associated with claw horn lesions that predispose cows to becoming lame again in the future. Numerous studies over the years have reported an increased risk of lameness following a previous lameness event and that risk increases with age. Recently, Randall et al. (2016) reported that severe claw horn lesions in heifers were associated with an increased risk of lameness across all lactations for the duration of time cows spent within the herd investigated, with an associated decrease in overall milk yield during this time. This finding supports a hypothesis highlighted by Hirst et al. (2002) that underlying pathology may be carried over from one lactation to the next. Newsome et al. (2016) provided further evidence to support this hypothesis; micro-computed tomography scanning (CT) was used to demonstrate that claw horn lesions during life were associated with an increase


FEED COMPOUNDER MAY/JUNE 2017 PAGE 27


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