Prediction of Bite Force from Assessment of Plant Fracture Properties

The translation of plant fracture properties into the bite force exerted appears attractive, but few researchers have attempted to relate fracture properties to bite force. A classic case of integration across the science and engineering disciplines is given in the two studies by Wright and Illius [15] and Illius et al. [27] on the application of the fracture mechanics in a group of broad- and narrow-leaved grasses and measured bite dimensions in goats. These authors determined the predicted bite force by substituting the residual heights from boxed grazed swards into polynomial regression equations summarizing force-canopy structure relationships based on the measured fracture properties. With a similar focus to that of Illius and colleagues, Tharmaraj et al. [47] estimated the resistance of material in situ within a fixed bite area of 100 cm2 (an average value for cattle) progressively down the sward canopy profile using a tensile apparatus. Their polynomial curves showed marked changes in bite force around 0.6 to 0.7 of sward height, which is in agreement with other published literature on pseudostem height in grazed swards [57]. The residual heights from paddock-scale grazing sessions (greater than 1 h) were used to predict bite force after adjustment for the measured bite area.

Despite these inspiring studies, there are limitations with the indirect approach. The most obvious is the assumption involved in modeling bite force as the summation of the strength of leaf and stem, and the number of organs severed in the bite [27,47] although Illius et al. [27] had noted that canopy structure has a greater bearing on bite force than individual plant strength. Nevertheless, the scaling of bite force with number of leaves harvested as determined by the area of sward encompassed within a bite, using a constant, might be ambiguous. A constant scaling factor would assume

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FIGURE 5.3 Relationship between the number of leaves grazed and exerted force per bite (solid circles and fitted linear relationship) by sheep and the predicted bite force using a 1:1 relationship (dashed line) based on the strength of a single leaf from tensile tests.

that all leaves fracture simultaneously under load. From aural observations we know that this characteristic does not hold true; output generated from biomechanical force instruments (see the next section) consistently show that the force-time curve can consist of multiple peaks. Only very recently has there been quantification of the variation between predictions of bite force from fracture properties and the actual forces exerted by the animal. Data from Zhang et al. [26] and unpublished data by W.M. Griffiths and also by E.A. Laca indicate that there is a large discrepancy between the indirect and direct estimates of bite force. Figure 5.3, drawn from the data of Zhang et al. [26], clearly show that the force exerted to sever a bite of a known number of leaves was not proportional to the sum of the strength of individual leaves. However, it is conceivable that the timing of leaf fracture is only one contributor to the force per leaf response curve, and we are left wondering about mechanisms of fracture at the animal level.

A stiff tensile testing machine capable of recording in one dimension is restricted to modeling the action of severance by vertical pull, excluding causes of friction between forage material and the incisors. The procurement action in three-dimensional space has been clearly illustrated in the work of Griffiths and Gordon [33] and Zhang et al. [26]. A plant tiller can essentially be viewed as a moment arm, and creation of an angle perpendicular to the tiller from the rooted position allows for greater friction between the incisors and dental pad, avoiding an action where the incisors would merely scrape against the fibrous veins in the lamina. This leads us to think about the dynamics of vein fracture. According to materials science theory, many grass leaves do not become weaker with the introduction of a notch [41], and, therefore, the biting action of ruminants will not function as a concentrator of stress, making the grass easier to break. Bending flexibility of stems has been assessed as an index for trampling resistance [22], with marked variation between plant species in the angle at which stems break. Zhang et al. [26] reported that plant material was generally severed when the angle was around 60° perpendicular to the vertical plane. While there has been no quantitative evidence for or against the argument of a reduction in strength of plant organs after damage inflicted from being held at an angle over the incisors and exposed to compression stress during the clamping action against the dental pad, it is plausible that the clamping action induces damage to the outer fibers of the lamina from buckling. This would cause the fibers to weaken in much the same way as compression creasing in wood. The implication of such an action is that tensile strength may not be the only plant mechanical parameter determining the ease of bite procurement [39].

Furthermore, compared with the tensile testing instrumentation, animals are soft machines constructed from muscles containing fibers that are extremely elastic and, therefore, have a large capability to store elastic-strain energy [58]. There does not appear to have been any examination of the storage of strain energy in the muscles of ruminant species, but it is conceivable that ruminants may exert sufficient force to create cracks in some of the grasped forage, and the energy stored within muscles is then used to propagate a crack [59]. However, such a mechanism is likely to only be valid if the forage being harvested is considered notch-sensitive (e.g., Stipa gigantea) [60]. Rate-dependent effects have been given as a more likely explanation for notch-insensitive species [61]. King and Vincent [61] suggested that the stretching of grasped leaves allows for the storage of elastic-strain energy in the respective leaves, and the quick jerking motion of the head then facilitates the generation of a shock wave through the leaves. This subsequently influences the fracture of the remaining fibers, most probably as a result of insufficient time for the stress to be redistributed among the remaining fibers, giving rise to an overload in strain energy [15]. This would go some way to explaining the faster rates of head acceleration in small-bodied ruminants, with smaller muscle mass than larger species like cattle [45], and suggests very clearly that acceleration of the head plays a major contributing role in the effort that animals exert in severing a bite.

The fibrous nature of forage material ingested and frequent contamination with soil particles is primarily responsible for tooth wear in ruminants, lending to variation in the incisor-pad configuration [62]. However, there is no consistent evidence of the relation between incisor wear and reduced levels of animal performance [63], although a relationship between dental erosion of the molars and animal growth has been documented [64]. Quantification of the role of dentition on procurement capability and mechanism is largely unexplored, but Hongo et al. [65] showed that sheep that had shed their two central temporary incisors found it difficult to increase the number of leaves procured with increasing sward density, a result that was reversed when the sheep grazed identical swards after the eruption and growth of the two permanent central incisors [65]. Although the study was conducted with a small number of animals (n = 3) that had little opportunity to adapt to foraging without the temporary central incisors, the results clearly suggest that incisors do inflict damage weakening plant tissue and/or forming a point of friction. This result is supported by the finding that in the absence of the two temporary central incisors, the force exerted per grasped leaf was significantly higher than when the animals grazed with complete dentition.

In summary, although it might be argued that the predicted bite forces in the study of Illius et al. [27] were within the range measured directly by Hughes et al. [66] for sheep grazing broad-leaved grasses, the discussion above does not lend support for the indirect approach (sum of individual plant strength and the number of leaves severed) for quantifying bite force. There are clear indications that tensile strength can provide an indication of plant resistance to fracture, but this information cannot be translated using bite dimensions to predict the force exerted by ruminants. Plant fracture properties will, however, continue to provide valuable information on leaf and stem characteristics that can assist with screening for plant traits that lead to improved animal performance.

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