Technical note: Effects of frozen storage on the mechanical properties of the suspensory tissue in the bovine claw

摘要

It is proposed that a softening of the suspensory tissue in the claw is involved in the development of lameness and claw lesions in cattle. A relatively small amount of research has been carried out to verify this theory. Research in this area would be simplified if mechanical testing of the suspensory tissue could be performed on frozen and stored specimens. The current study tested whether freezing of the specimens changes the suspensory tissues' mechanical properties. Limbs from 3 freshly slaughtered Danish Holstein dairy cows and 6 nonpregnant Angus heifers, without clinical signs of lameness, were allocated to 1 of 2 treatments (frozen or nonfrozen) in such a way that each cow was represented in each treatment group with a frozen limb and a corresponding nonfrozen limb (i.e., frozen left front, fresh right front, and so on). The frozen limbs were kept at - 18℃ for a week before processing and the nonfrozen limbs were processed within 2 h of slaughter. Two samples measuring 8×8 mm were cut from the abaxial side of each claw in such a way that the sample included the horn of the abaxial wall, pedal bone, and the interposed corium. The samples were kept on ice until being mounted in a large deformation rheometer with an extension testing frame, fixed by the horn and the pedal bone, and loaded to failure. During deformation force and displacement data were recorded, from which corresponding stress and strain were calculated. Young's modulus (a measure of tissue elasticity or stiffness) and a measure of physiological support (PS; force needed to displace the sample 1 mm) were calculated from the data. The response variables, Young's modulus and PS, were analyzed separately by a mixed model. The explanatory variables were treatment (frozen or nonfrozen), limb (front or back), claw (medial or lateral), position of the sample (dorsal or palmar-plantar), and group (Angus or Holstein). Interactions between group and treatment and between limb, claw, and sample position were included in the model. Cow identity was included as a random effect. Model reduction was performed by stepwise backward elimination, until all remaining terms were significant at the 5% level or less. Freezing had no effect on the elasticity of the suspensory apparatus or on PS. However, PS was affected by limb (hind legs had higher PS values than front) and the position of the sample (palmar-plantar samples had higher PS values than dorsal). The Angus group had higher PS values than the Holstein group, but the groups differed in age, parity, body weight, lactation, housing, and management, as well as in breed; therefore, further studies are needed to investigate these effects. The results indicate that mechanical testing of bovine claw suspensory tissue can be performed on specimens that have been frozen, thus aiding research in the mechanical aspect of bovine lameness and claw lesions.