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The implications of compound chew–bite jaw movements for bite rate in grazing cattle
Applied Animal Behaviour Science 98 (2006) 183–195 www.elsevier.com/locate/applanim
The implications of compound chew–bite jaw movements for bite rate in grazing cattle Eugene David Ungar a,*, Nir Ravid a, Tomer Zada a, Ezra Ben-Moshe a, Rafi Yonatan a, Hagit Baram a, Avraham Genizi b a
Department of Agronomy and Natural Resources, Institute of Field and Garden Crops, The Volcani Center, P.O.B. 6, Bet Dagan 50250, Israel b Department of Statistics and Experiment Design, The Volcani Center, P.O.B. 6, Bet Dagan 50250, Israel Accepted 13 September 2005 Available online 11 October 2005
Abstract Many aspects of the management of grazing systems are directly or indirectly related to the rate of herbage intake achieved by the animal. Intake rate depends, in part, on the time budget of the process, which derives from the basic behavioural component—the jaw movement. Chewing and biting jaw movements have generally been considered mutually exclusive, but acoustic monitoring has demonstrated the existence of the compound chew–bite jaw movement. We used a simple model to examine the implications of chew–bites for the forage-intake time budget, and tested it empirically. The model defines the total number of jaw movements per bite (a) and the number of chew actions per bite (b) in relation to the allocation of jaw movements between chews, bites and chew–bites. We examined empirically the variation among animals in the allocation of jaw movements for cattle (Bos taurus L.) grazing leafy, uniform swards in two separate studies. Grazing sessions were recorded on video, with the sound track originating from a forehead microphone. Each sound burst produced by a jaw movement was classified as a bite, chew or chew–bite. Jaw movements in these segments generated a virtually uninterrupted, regular succession of bite, chew and chew–bite sounds. Among individual animals, the variation in the rate of jaw movement was extremely low (Study 1: mean = 78.9 min1, CV = 6%; Study 2: mean = 77.9 min1, CV = 4%), but variation in the allocation of jaw movements was high (CV of proportion of jaw movements allocated to bites, chews and * Corresponding author. Tel.: +972 3 9683411; fax: +972 3 9669642. E-mail address: [email protected] (E.D. Ungar). 0168-1591/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.applanim.2005.09.001
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chew–bites = 58, 21 and 50%, respectively, for Study 1; 32, 12 and 32%, respectively, for Study 2). The proportions of chews and bites traded off directly against chew-biting. As the proportion of chew–bites increased, the number of jaw movements per bite declined and therefore the bite rate increased. Different combinations of the three types of jaw movement conformed to an isocline of a constant number of chew actions per bite (CV = 11 and 9% for Studies 1 and 2, respectively). Furthermore, this ratio was close to unity (1.27 and 1.24 chews per bite for Studies 1 and 2, respectively), and was similar for different levels of herbage mass in the range 145–255 g m2. The possible implications of these findings for the regulation of bite weight and diet quality are discussed. # 2005 Elsevier B.V. All rights reserved. Keywords: Acoustic monitoring; Bite dimensions; Intake; Mastication
1. Introduction A better understanding of the factors that determine herbage intake rate by ruminants remains an important theme in improving the management of grazing systems (Wales et al., 2005). Intake rate can be defined as the product of bite weight and bite rate. While many studies have been conducted of the relations between sward characteristics and bite weight, less attention has been devoted to the time budget of the intake process, which determines bite rate. The fundamental behavioural unit that determines the time budget of intake is the jaw movement. Classically, two jaw movements have been defined, that ‘‘compete’’ for the animal’s time—the bite and the chew. Biting and chewing have been assumed to be mutually exclusive in intake models (e.g., Ungar and Noy-Meir, 1988; Gross et al., 1993). However, a new window on intake opened when a microphone was pressed against the forehead of a grazing cow (Bos taurus). Not only did this enable both bites and chews to be heard distinctly, but also revealed the compound jaw movement (Laca et al., 1992b; Laca and WallisDeVries, 2000). In the ‘‘chew–bite’’, herbage already in the mouth is chewed as the jaws close to grip and sever fresh herbage. The phenomenon has since been reported in giraffes (Ginnett and Demment, 1995) and sheep (Rutter et al., 2002). We examined the implications of the chew–bite for the time budget of intake via a simple conceptual model and the analysis of empirical data. Specifically, we examined the allocation of jaw movements among the three types (bite, chew and chew–bite), and its relationships to jaw movements per bite, chews per bite and bite rate. The analysis draws from two studies that employed acoustic monitoring of cattle as they grazed pristine, continuous and homogeneous swards. 2. Materials and methods 2.1. The conceptual model Intake models typically define the time budget of the intake process in terms of searching, handling and processing components (Stephens and Krebs, 1986). For large ungulates grazing dense, leafy and continuous swards, the potential bites can be regarded
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as being readily apparent, and the foraging environment as being food-saturated. Thus, the searching component does not constrain the intake rate, and can be ignored. If herbage is abundant, the processing component – ingestive chewing – constrains the intake rate by limiting the rate at which boli are formed and cleared from the mouth, and by competing with biting (cropping) jaw movements—the handling component. Because the animal moves as it grazes, the foraging process can be viewed as a progressing front of similar bites, i.e., depletion without depression, in a non-patchy environment (Ungar et al., 1992). Intake rate (I; definitions of all symbols used in the model are given in Table 1) under such grazing conditions can be constructed from the familiar definition based on bite weight (W) and bite rate (R): I ¼ WR
(1)
Micro-sward studies have illustrated the mechanistic link, via bite dimensions, between bite weight and basic sward features such as height and bulk density (Laca et al., 1992a,b; Ungar et al., 1991, 2001). Bite rate derives from the time budget of the intake process, which encompasses the biting and chewing actions, both of which are performed by jaw movements. If chewing requirements were directly related to the amount of herbage ingested, there would be a clear functional linkage between W and R. Acoustic monitoring of grazing cattle reveals that jaw movements generate a virtually uninterrupted, regular succession of sounds (biting or chewing) when the sward is abundant and of high quality (Ungar, 1996). Thus, we define a constant, J that represents the rate of jaw movement during bouts of active grazing. If we know J and the total number of jaw movements that are invested by the animal per bite taken (a), we can derive the bite rate (R): R¼
J a
(2)
Table 1 List and definition of symbols Symbol
Definition
Units
Bn B0n Bp B0p Cn Cn0 Cp C 0p I J N Qn Qp R W a b
Number of bite jaw movements Number of biting jaw movements Proportion of bite jaw movements Proportion of biting jaw movements Number of chew jaw movements Number of chewing jaw movements Proportion of chew jaw movements Proportion of chewing jaw movements Instantaneous intake rate Rate of jaw movement Total number of jaw movements in sample Number of chew–bite jaw movements Proportion of chew–bite jaw movements Bite rate Bite weight Jaw movements per bite Chews per bite
– – – – – – – – g min1 min1 – – – min1 g – –
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If the only possible types of jaw movement are chew or bite, then: a¼1þb
(3)
where b is the chews per bite. Ingestive chewing requirements in grazing ungulates have not been widely studied because of practical limitations, but the use of acoustic monitoring enabled Laca et al. (1994) to classify the jaw movements of cattle unequivocally. For the vegetation they were studying, they found that 71% of the variation was explained by a linear relationship between the number of chews per bite (b) and bite weight. With the inclusion of the chew–bite, the grazing animal has a repertoire of three options from which to create the stream of jaw movements: chews, bites and chew–bites. Bite and chew–bite jaw movements may include tongue sweeping to bring herbage into the mouth. We assumed that a chewing action is equally effective when performed as part of a chew–bite or as a chew. Similarly, we assumed that the bite part of a chew–bite is as effective as a bite. When N is the total number of jaw movements performed in the course of a bout of active grazing, Cn, Bn and Qn are the numbers of jaw movements allocated to chews, bites and chew–bites, respectively. The number of chews per bite and the number of jaw movements per bite are then: b¼
Cn þ Qn Bn þ Qn
(4)
a¼
Cn þ Bn þ Qn Bn þb ¼ Bn þ Qn ðBn þ Qn Þ
(5)
In the absence of chew-biting Eq. (4) reduces to Eq. (3), and in the absence of biting a = b. Although the animal is constrained to perform a certain number of chewing jaw movements per bite removed, this can be achieved by several different combinations of the three types of jaw movements. For example, if b = 1, the animal could allocate half of the jaw movements to bites and half to chews; alternatively, it could allocate all jaw movements to chew–bites and thereby double the rate of intake, provided that each bite requires only one chew. If many chews need to be performed per bite, then the benefits of chew-biting are obviously smaller. We now express the allocation of jaw movements as the proportions Cp, Bp and Qp, and re-define b and a accordingly: Cp þ Bp þ Qp ¼ 1
(6)
b¼
Cp þ Qp Bp þ Qp
(7)
a¼
1 Bp þ Qp
(8)
Thus, the intake equation can be formulated as: I ¼ WJðB p þ Q p Þ
(9)
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The use of proportions enables the allocation of jaw movements between the three types to be shown graphically by means of the unit-sum triangle (Fig. 1). If bites and chew–bites are represented by the two base corners of the triangle, then constant levels of total jaw movements per bite (a) are represented by horizontal lines on the triangle. The number of jaw movements per bite is unity along the base of the triangle (no chews), and it increases geometrically to infinity as the proportion of chews approaches unity (i.e., the number of bites approaches zero). Isoclines of number of chews per bite (b) have differing orientations and are not parallel lines. Chew-biting enables the animal to reduce the total number of jaw movements per bite without reducing the number of chews per bite. 2.2. Empirical studies Grazing sessions were conducted at the Volcani Center, Bet Dagan, Israel (328590 N, 348480 E), using Israeli–Holstein dairy heifers. In each study season, animals of similar ages were halter-and-rope trained and accustomed to the field procedures. The animals were experienced grazers and hunger was not induced prior to the observations. 2.2.1. Acoustic monitoring The allocation of jaw movements was derived from the sound track of video (analogue VHS) recordings of the grazing sessions. Sounds were collected by a small inward-facing microphone pressed against the forehead of the animal. The animal’s head acts as a sound box, whereby biting and chewing actions can be heard clearly as ripping and grinding sounds, respectively. There has been unanimous agreement by all who have listened to such recordings that the sound bursts correspond to bite, chew and chew–bite jaw movements. The acoustic signal was captured on video via a transmitter and receiver. The acoustic signal was interpreted aurally to classify each sound burst produced by a jaw movement. A distinction was made between sounds that could be classified
Fig. 1. The unit-sum triangle for the allocation of jaw movements among bites (Bp), chews (Cp) and chew–bites (Qp). Each point within or on the triangle may be represented by three triangular coordinates that sum to unity. Feint lines show increments of 0.1 in the proportions. Solid lines show isoclines for the number of chewing jaw movements per bite (b), dashed lines show isoclines for the total number of jaw movements per bite (a).
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unequivocally and those that could be classified with less than complete certainty. This yielded the three definitive classes of chew, bite and chew–bite, the five uncertain classes of probable chew, probable bite, probable chew–bite, chew or chew–bite and bite or chew– bite and an additional class of unknown. The non-definitive classes combined accounted for less than 5% of total jaw movements. Hence, for simplicity, each ‘‘probable’’ class was combined with its corresponding ‘‘definitive’’ class, with ‘‘chew or chew–bite’’ being classed as chew and ‘‘bite or chew–bite’’ being classed as bite. 2.2.2. Study 1 An area of 0.6 ha was planted with oats (Avena sativa L.) in April 1996 with a commercial drill (between-row spacing 15 cm) and irrigated. The grazing sessions were conducted 6 weeks later, between the third-leaf phase and tillering, using nine animals of similar ages (590 8 days; mean S.D.) and live weights (496 26 kg). An ungrazed, uniform expanse of herbage of approximately 50 m2 was selected before each grazing session, and the unextended plant height was determined at 20 points. The observation animal was fitted with the acoustic equipment, led calmly to the selected area and allowed to graze. The animal could meander freely as it grazed, with a handler who trailed the animal keeping to its side and rear, with the rope slack. A grazing session of up to 10 min was recorded on video by a second person standing to the front and side of the animal, and 20 measurements of grazed plant height were made during the session (by a third person). This cycle was repeated for each of the nine animals. A total of five samples were taken of herbage mass (0.5 m2 cut per sample), vertical mass distribution and dry matter content. A 5-min session of uninterrupted grazing (i.e., no pauses in the rhythm of jaw movements of more than 5 s) was extracted from the video recordings for each animal, and the sound track was sequenced. The durations of the grazing sessions, the numbers and durations of headup chewing bouts and the numbers and durations of pauses or breaks in jaw activity were also determined from the video recording. 2.2.3. Study 2 In April 1998, two areas, each of 60 m 4 m, were planted with oats with a precision drill at a between-row spacing of 8 cm, and were irrigated. The grazing sessions were conducted in the first week of July, during the early tillering phase. Four animals (age 567 8 days, weight 458 33 kg) grazed herbage at two initial sward heights of 20 and 25 cm. The four animals were observed in two cycles, two animals per observation day. Animal sequence was randomized within cycle, and initial sward height was randomized within animal. Each grazing session was assigned to a strip 4 m long and 0.5 m wide (six sowing rows). At the start of each observation day, the two strips to be grazed were defined by sampling part of the surrounding vegetation for herbage mass, vertical mass distribution and dry matter content, and by clipping the remainder. Sampled vegetation was trimmed to the designated treatment height before sampling, and the strips to be grazed were similarly trimmed. Each observation animal was allowed to make one grazing pass of a fresh herbage strip at its own pace, and the acoustic signal was captured on video. After the animal had grazed, a measurement of plant height (grazed or ungrazed) was made every 10 cm along each of the six sowing rows of the herbage strips (240 measurements per strip). The sound track for each grazing session was sequenced, and the video record was used to measure
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overall duration, and the numbers and durations of head-up chewing bouts and pauses as in Study 1. 2.2.4. Analysis The proportions of each type of jaw movement (Bp, Cp, Qp) were computed relative to the total number of sequenced sounds, with ‘‘unknown’’ excluded (N). We defined the number of biting jaw movements (B0n ) as Bn + Qn, and the number of chewing jaw movements (Cn0 ) as Cn + Qn. Note that if Qn > 0, B0n þ Cn0 > N. The proportions of biting jaw movements (B0p ) and chewing jaw movements (C 0p ) were similarly computed. If Qp > 0, B0p þ C0p > 1. The numbers of jaw movements per bite (a), chews per bite (b) and bite rate (R) were computed according to Eqs. (8), (7) and (2), respectively. The rate of jaw movement (J) was based on N and the duration of the grazing session. In both studies, head-up chewing time was included and total pause/break time was deducted. Chewing requirements were analyzed by linear regression of Cn0 on B0n . In Study 2, a two-way analysis of variance was applied to J, b, a and R, with the terms animal (as a random factor) and initial sward height (as a fixed factor) in the model, and the experimental unit being the animal initial sward height combination.
3. Results 3.1. Study 1 The mean herbage mass (S.E.) was 144.5 12.1 g m2 for a mean initial sward height of 20 cm. The mean proportions of the herbage mass contained in 5-cm horizons, from the ground upwards, were 0.36, 0.26, 0.22 and 0.16. The dry matter content of the herbage was 133 g kg1 dry matter (DM). On average, a grazing session lasted 4.5 min and contained 358 jaw movements, i.e., a jaw movement rate (J) of 78.9 min1 (Table 2). When the total number of jaw movements was normalized to unity chew–bites and chews each accounted for a proportion of 0.39 and bites for the remaining 0.22. There was high variability among animals in the allocation of jaw movements (see CV in Table 2), but very low variability in J (CV = 6%). The mean bite rate (R) was 48.2 min1, and the animals executed 1.66 jaw movements per biting action (a) and performed 1.27 chewing actions per biting action (b). Fig. 2 shows the allocation of jaw movements for the nine grazing sessions on the unitsum triangle. The points appear to be oriented along an isocline of b. The line represents the b isocline equal to the group mean (1.27). Linear regression of the number of chewing jaw movements (Cn0 ) on the number of biting jaw movements (B0n ) yielded the following equation (showing S.E.): Cn0 ¼ 33:9ð66:5Þ þ 1:117ð0:298ÞB0n ;
r 2 ¼ 0:62
(10)
The intercept was not significantly different from zero (P = 0.6082) and the slope – an estimate of b – was significant (P = 0.0074).
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Table 2 Mean, standard error (S.E.) and coefficient of variation (CV, %) of the number of chew, bite and chew–bite jaw movements and derived variables, performed by dairy heifers grazing young, vegetative oat swards in May 1996 (Study 1: nine animals) and June 1998 (Study 2: three animals at two initial sward heights, H) Measure
Symbol Study 1 Mean
Total number of jaw movements Number of bites Number of chews Number of chew–bites Proportion of bites Proportion of chews Proportion of chew–bites Number of biting jaw movements Number of chewing jaw movements Proportion of biting jaw movements Proportion of chewing jaw movements Chews per bite Jaw movements per bite Duration (s) Rate of jaw movement (min1) Bite rate (min1) Herbage mass (g m2) Initial height (cm) Residual height (cm) Bite depth (cm)
Study 2 S.E.
CV H = 25 cm
H = 20 cm
Mean
S.E.
CV Mean
S.E.
CV
N
358
16.4
14
245
82.0
58
247
67.7
47
Bn Cn Qn Bp Cp Qp B0n
79 138 141 0.220 0.387 0.393 220
15.2 10.8 22.3 0.043 0.027 0.066 13.7
58 23 47 58 21 50 19
63 103 79 0.246 0.402 0.352 142
26.8 37.6 26.1 0.045 0.029 0.065 44.5
73 63 57 32 12 32 54
63 96 88 0.256 0.400 0.343 151
16.2 20.6 32.1 0.014 0.024 0.034 47.6
45 37 63 9 11 17 55
Cn0
279
18.8
20
182
60.3
57
184
51.9
49
B0p
0.613
0.027 13
0.598
0.029
8
0.600
0.024
7
C0p
0.780
0.043 16
0.754
0.045 10
0.744
0.014
3
b a – J R – – – –
1.27 1.66 272 78.9 48.2 144.5 22.8 9.9 13.0
0.047 0.074 11.0 1.5 2.0 12.1 1.4 0.8 1.0
11 13 12 6 13 19 18 25 23
1.26 1.68 192 78.0 46.8 255 25 11.1 13.9
0.065 9 0.077 8 66.7 60 3.38 8 3.77 14 38.1 26 – – 0.6 9 0.6 7
1.24 1.67 192 77.9 46.6 179 20 8.8 11.2
0.039 5 0.066 7 56.4 51 1.80 4 0.94 3 5.17 5 – – 1.2 23 1.2 18
Fig. 2. The allocation of jaw movements between bites (Bp), chews (Cp) and chew–bites (Qp) for nine dairy heifers grazing young, vegetative oat swards in May 1996 (Study 1), shown on the unit-sum triangle. The line is the isocline for the mean number of chews per bite (b).
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The proportions of chews and bites traded off directly against chew-biting (Fig. 3b and g). As the proportion of chew-biting increased, the bite rate increased (Fig. 3o) and the number of jaw movements per bite declined (Fig. 3t). The number of chews per bite showed no clear response to any of the variables (Fig. 3c, h, l, p, q and r). 3.2. Study 2 One heifer was clearly distracted during both its grazing sessions; its considerable headup chewing resulted in atypical allocations of jaw movement. Data for this animal were excluded. The mean herbage masses were 255.3 38.1 and 178.3 5.2 g m2 for initial sward heights of 25 and 20 cm, respectively. The mean proportions of herbage mass contained in
Fig. 3. The relationships among the proportions of jaw movements allocated to bites (Bp), chews (Cp), chew–bites (Qp), chews per bite (b), jaw movements per bite (a), rate of jaw movement (J) and bite rate (R) for nine dairy heifers grazing young, vegetative oat swards in May 1996 (Study 1). Values are the Pearson product-moment correlation coefficient (r) and the significance probability (P).
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5-cm horizons, from the ground upwards, were 0.30, 0.26, 0.22, 0.13 and 0.09 for an initial sward height of 25 cm, and 0.33, 0.29, 0.23, 0.16 for a height of 20 cm. The dry matter content of the herbage was 125 g kg1 DM. On average, grazing sessions lasted 3.2 min and contained 246 jaw movements (Table 2). The jaw movement rate (J) was 78.0 min1. Chew–bites, chews and bites accounted for proportions of 0.35, 0.40 and 0.25, respectively, of jaw movements. The mean bite rate (R), number of jaw movements per bite (a) and number of chews per bite (b) were 46.7 min1, 1.68 and 1.25, respectively. Variability was greater for the allocation of jaw movements than it was for the derived variables of J, a and b. Fig. 4 shows the allocation of jaw movements for the six grazing sessions on the unit-sum triangle. The line represents the b isocline equal to the group mean (1.25). Neither animal nor initial sward height showed any clear influence on jaw movement allocation, and these factors were not significant in the analysis of variance of J, b, a and R. Linear regression of the number of chewing jaw movements (Cn0 ) on the number of biting jaw movements (B0n ) yielded the following equation (S.E.): Cn0 ¼ 7:39ð17:5Þ þ 1:198ð0:109ÞB0n ;
r 2 ¼ 0:96
(11)
The intercept was not significantly different from zero (P = 0.6940) and the slope was significant (P = 0.0004). The relationships among the key variables showed a positive correlation between the proportion of chew–bite jaw movements (Qp) and bite rate (R) (Fig. 5o), a negative correlation between Qp and jaw movements per bite (a) (Fig. 5m), and no clear trends with chews per bite (b) (Fig. 5c, h, l, p, q and r).
Fig. 4. The allocation of jaw movements between bites (Bp), chews (Cp) and chew–bites (Qp) for three dairy heifers grazing young, vegetative oat swards at two initial heights in June 1998 (Study 2), shown on the unit-sum triangle. Circle, initial sward height of 20 cm; triangle, initial sward height of 25 cm. The line is the isocline for the mean number of chews per bite (b).
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Fig. 5. The relationships among the proportions of jaw movements allocated to bites (Bp), chews (Cp), chew–bites (Qp), chews per bite (b), jaw movements per bite (a), rate of jaw movement (J) and bite rate (R) for three dairy heifers grazing young, vegetative oat swards at two initial heights in June 1998 (Study 2). Circle, initial sward height of 20 cm; triangle, initial sward height of 25 cm. Values are the Pearson product-moment correlation coefficient (r) and the significance probability (P).
4. Discussion Studies 1 and 2, which were conducted in different seasons with different animals, yielded remarkably similar estimates of the mean jaw movement rate (J). The finding that J was constant is consistent with our simple conceptual model of intake rate. Although we have no rigorous way of testing the assumption that biting and chewing actions are equivalent if performed as pure or compound jaw movements, it is at least supported by the absence of any trend between the proportion of chew–bites and chewing requirement (b) in either study. Our results were also consistent with the assumption that although different animals are constrained by a similar chewing requirement (b), they can allocate jaw
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movements differently. Consequently, the number of jaw movements per bite (a) did respond to allocation; a declined and bite rate increased as chew-biting increased. The implications of the chew–bite may, however, extend beyond bite rate alone. If biting and chewing jaw movements ‘‘competed’’, bite weight and intake rate should be positively related. However, when chew-biting is possible, this is not necessarily the case, since nothing is gained by loading the mouth with more herbage than can be processed by a single chewing jaw movement. In such a case, the animal would need to intersperse chews among the stream of chew–bites and thereby reduce the bite rate. If, as a result of chewbiting, the animal has no need to maximize bite weight, it can restrain its bite depth (‘‘skim’’) and thereby, on most swards, gain in diet quality. According to this conception, the ideal grazer would perform only chew–bites and regulate its bite weight (via bite dimensions) to the amount processed by a single chew. It is hence noteworthy that not only did variation among animals in jaw movement allocation align along isoclines of chews per bite (b), but also that the b values obtained were close to unity. Furthermore, similar b values were obtained on swards that offered quite different levels of herbage mass: 145 g m2 in Study 1, 178 g m2 in Study 2 at a height of 20 cm and 255 g m2 in Study 2 at a height of 25 cm. That finding is more indicative of regulation of bite weight than of maximization. This raises a question regarding the relevance of micro-scale studies of the relation between sward structure and intake to the phenomena that occur on larger spatial scales. Indeed, a recent analysis, based on acoustic monitoring, of jaw movement allocation and bite rate for cattle grazing single feeding stations of various sizes showed that the steady-state behaviour typical of extended grazing bouts was not attained at the spatial scale of a single feeding station (Ungar et al., 2005). Further experimentation that includes the accurate estimation of bite weight will be required to test rigorously the hypothesis of bite weight regulation versus maximization. In conclusion, chew–bites account for a major share of jaw-movement allocation in cattle grazing in a non-patchy foraging environment. Variation in bite rate between animals is largely accounted for by differences in jaw movement allocation rather than jaw movement rate. Variability in jaw movement allocation among individual animals is large, and appears to be oriented along an axis of constant chews per bite, but varying numbers of jaw movements per bite and, therefore, of varying bite rate.
Acknowledgements Contribution no. 132/2004 from the Agricultural Research Organization, Institute of Field and Garden Crops, Bet Dagan, Israel. This research was supported by Research Grant Award No. IS-2331-93C from BARD, the United States–Israel Binational Agricultural Research and Development Fund.
References Ginnett, T.F., Demment, M.W., 1995. The functional response of herbivores—analysis and test of a simple mechanistic model. Funct. Ecol. 9, 376–384.
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Gross, J.E., Shipley, L.A., Hobbs, N.T., Spalinger, D.E., Wunder, B.A., 1993. Functional response of herbivores in food-concentrated patches, tests of a mechanistic model. Ecology 74, 778–791. Laca, E.A., WallisDeVries, M.F., 2000. Acoustic measurement of intake and grazing behaviour of cattle. Grass For. Sci. 55, 97–104. Laca, E.A., Ungar, E.D., Seligman, N.G., Demment, M.W., 1992a. Effects of sward height and bulk density on bite dimensions of cattle grazing homogeneous swards. Grass For. Sci. 47, 91–102. Laca, E.A., Ungar, E.D., Seligman, N.G., Ramey, M.R., Demment, M.W., 1992b. An integrated methodology for studying short-term grazing behaviour of cattle. Grass For. Sci. 47, 81–90. Laca, E.A., Ungar, E.D., Demment, M.W., 1994. Mechanisms of handling time and intake rate of a large mammalian grazer. Appl. Anim. Behav. Sci. 39, 3–19. Rutter, S.M., Ungar, E.D., Molle, G., Decandia, M., 2002. Bites and chews in sheep: acoustic versus automatic recording. In: 10th European Intake Workshop, Reykjavik, Iceland, 22–24 August. Stephens, D.W., Krebs, J.R., 1986. Foraging Theory. Princeton University Press, Princeton. Ungar, E.D., 1996. Ingestive behaviour. In: Hodgson, J., Illius, A.W. (Eds.), The Ecology and Management of Grazing Systems. CAB International, Wallingford, pp. 185–218. Ungar, E.D., Noy-Meir, I., 1988. Herbage intake in relation to availability and sward structure: grazing processes and optimal foraging. J. Appl. Ecol. 25, 1045–1062. Ungar, E.D., Genizi, A., Demment, M.W., 1991. Bite dimensions and herbage intake by cattle grazing short handconstructed swards. Agron. J. 83, 973–978. Ungar, E.D., Seligman, N.G., Demment, M.W., 1992. Graphical analysis of sward depletion by grazing. J. Appl. Ecol. 29, 427–435. Ungar, E.D., Ravid, N., Bruckental, I., 2001. Bite dimensions for cattle grazing herbage at low levels of depletion. Grass For. Sci. 56, 35–45. Ungar, E.D., Ravid, N., Zada, T., Ben-Moshe, E., Yonatan, R., Brener, S., Baram, H., Genizi, A, 2005. The importance of patch size in estimating steady-state bite rate in grazing cattle. In: Murphy, J.J. (Ed.), Utilisation of Grazed Grass in Temperate Animal Systems. Proceedings of a Satellite Workshop of the XXth International Grassland Congress, Cork, Ireland, July 2005. Wageningen Academic Publishers, Wageningen, The Netherlands, p. 154. Wales, W.J., Stockdale, C.R., Doyle, P.T., 2005. Plant and sward characteristics to achieve high intake in ruminants. In: Murphy, J.J. (Ed.), Utilisation of Grazed Grass in Temperate Animal Systems. Proceedings of a Satellite Workshop of the XXth International Grassland Congress, Cork, Ireland, July 2005. Wageningen Academic Publishers, Wageningen, The Netherlands, pp. 37–47.
The implications of compound chew–bite jaw movements for bite rate in grazing cattle Eugene David Ungar a,*, Nir Ravid a, Tomer Zada a, Ezra Ben-Moshe a, Rafi Yonatan a, Hagit Baram a, Avraham Genizi b a
Department of Agronomy and Natural Resources, Institute of Field and Garden Crops, The Volcani Center, P.O.B. 6, Bet Dagan 50250, Israel b Department of Statistics and Experiment Design, The Volcani Center, P.O.B. 6, Bet Dagan 50250, Israel Accepted 13 September 2005 Available online 11 October 2005
Abstract Many aspects of the management of grazing systems are directly or indirectly related to the rate of herbage intake achieved by the animal. Intake rate depends, in part, on the time budget of the process, which derives from the basic behavioural component—the jaw movement. Chewing and biting jaw movements have generally been considered mutually exclusive, but acoustic monitoring has demonstrated the existence of the compound chew–bite jaw movement. We used a simple model to examine the implications of chew–bites for the forage-intake time budget, and tested it empirically. The model defines the total number of jaw movements per bite (a) and the number of chew actions per bite (b) in relation to the allocation of jaw movements between chews, bites and chew–bites. We examined empirically the variation among animals in the allocation of jaw movements for cattle (Bos taurus L.) grazing leafy, uniform swards in two separate studies. Grazing sessions were recorded on video, with the sound track originating from a forehead microphone. Each sound burst produced by a jaw movement was classified as a bite, chew or chew–bite. Jaw movements in these segments generated a virtually uninterrupted, regular succession of bite, chew and chew–bite sounds. Among individual animals, the variation in the rate of jaw movement was extremely low (Study 1: mean = 78.9 min1, CV = 6%; Study 2: mean = 77.9 min1, CV = 4%), but variation in the allocation of jaw movements was high (CV of proportion of jaw movements allocated to bites, chews and * Corresponding author. Tel.: +972 3 9683411; fax: +972 3 9669642. E-mail address: [email protected] (E.D. Ungar). 0168-1591/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.applanim.2005.09.001
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chew–bites = 58, 21 and 50%, respectively, for Study 1; 32, 12 and 32%, respectively, for Study 2). The proportions of chews and bites traded off directly against chew-biting. As the proportion of chew–bites increased, the number of jaw movements per bite declined and therefore the bite rate increased. Different combinations of the three types of jaw movement conformed to an isocline of a constant number of chew actions per bite (CV = 11 and 9% for Studies 1 and 2, respectively). Furthermore, this ratio was close to unity (1.27 and 1.24 chews per bite for Studies 1 and 2, respectively), and was similar for different levels of herbage mass in the range 145–255 g m2. The possible implications of these findings for the regulation of bite weight and diet quality are discussed. # 2005 Elsevier B.V. All rights reserved. Keywords: Acoustic monitoring; Bite dimensions; Intake; Mastication
1. Introduction A better understanding of the factors that determine herbage intake rate by ruminants remains an important theme in improving the management of grazing systems (Wales et al., 2005). Intake rate can be defined as the product of bite weight and bite rate. While many studies have been conducted of the relations between sward characteristics and bite weight, less attention has been devoted to the time budget of the intake process, which determines bite rate. The fundamental behavioural unit that determines the time budget of intake is the jaw movement. Classically, two jaw movements have been defined, that ‘‘compete’’ for the animal’s time—the bite and the chew. Biting and chewing have been assumed to be mutually exclusive in intake models (e.g., Ungar and Noy-Meir, 1988; Gross et al., 1993). However, a new window on intake opened when a microphone was pressed against the forehead of a grazing cow (Bos taurus). Not only did this enable both bites and chews to be heard distinctly, but also revealed the compound jaw movement (Laca et al., 1992b; Laca and WallisDeVries, 2000). In the ‘‘chew–bite’’, herbage already in the mouth is chewed as the jaws close to grip and sever fresh herbage. The phenomenon has since been reported in giraffes (Ginnett and Demment, 1995) and sheep (Rutter et al., 2002). We examined the implications of the chew–bite for the time budget of intake via a simple conceptual model and the analysis of empirical data. Specifically, we examined the allocation of jaw movements among the three types (bite, chew and chew–bite), and its relationships to jaw movements per bite, chews per bite and bite rate. The analysis draws from two studies that employed acoustic monitoring of cattle as they grazed pristine, continuous and homogeneous swards. 2. Materials and methods 2.1. The conceptual model Intake models typically define the time budget of the intake process in terms of searching, handling and processing components (Stephens and Krebs, 1986). For large ungulates grazing dense, leafy and continuous swards, the potential bites can be regarded
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as being readily apparent, and the foraging environment as being food-saturated. Thus, the searching component does not constrain the intake rate, and can be ignored. If herbage is abundant, the processing component – ingestive chewing – constrains the intake rate by limiting the rate at which boli are formed and cleared from the mouth, and by competing with biting (cropping) jaw movements—the handling component. Because the animal moves as it grazes, the foraging process can be viewed as a progressing front of similar bites, i.e., depletion without depression, in a non-patchy environment (Ungar et al., 1992). Intake rate (I; definitions of all symbols used in the model are given in Table 1) under such grazing conditions can be constructed from the familiar definition based on bite weight (W) and bite rate (R): I ¼ WR
(1)
Micro-sward studies have illustrated the mechanistic link, via bite dimensions, between bite weight and basic sward features such as height and bulk density (Laca et al., 1992a,b; Ungar et al., 1991, 2001). Bite rate derives from the time budget of the intake process, which encompasses the biting and chewing actions, both of which are performed by jaw movements. If chewing requirements were directly related to the amount of herbage ingested, there would be a clear functional linkage between W and R. Acoustic monitoring of grazing cattle reveals that jaw movements generate a virtually uninterrupted, regular succession of sounds (biting or chewing) when the sward is abundant and of high quality (Ungar, 1996). Thus, we define a constant, J that represents the rate of jaw movement during bouts of active grazing. If we know J and the total number of jaw movements that are invested by the animal per bite taken (a), we can derive the bite rate (R): R¼
J a
(2)
Table 1 List and definition of symbols Symbol
Definition
Units
Bn B0n Bp B0p Cn Cn0 Cp C 0p I J N Qn Qp R W a b
Number of bite jaw movements Number of biting jaw movements Proportion of bite jaw movements Proportion of biting jaw movements Number of chew jaw movements Number of chewing jaw movements Proportion of chew jaw movements Proportion of chewing jaw movements Instantaneous intake rate Rate of jaw movement Total number of jaw movements in sample Number of chew–bite jaw movements Proportion of chew–bite jaw movements Bite rate Bite weight Jaw movements per bite Chews per bite
– – – – – – – – g min1 min1 – – – min1 g – –
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If the only possible types of jaw movement are chew or bite, then: a¼1þb
(3)
where b is the chews per bite. Ingestive chewing requirements in grazing ungulates have not been widely studied because of practical limitations, but the use of acoustic monitoring enabled Laca et al. (1994) to classify the jaw movements of cattle unequivocally. For the vegetation they were studying, they found that 71% of the variation was explained by a linear relationship between the number of chews per bite (b) and bite weight. With the inclusion of the chew–bite, the grazing animal has a repertoire of three options from which to create the stream of jaw movements: chews, bites and chew–bites. Bite and chew–bite jaw movements may include tongue sweeping to bring herbage into the mouth. We assumed that a chewing action is equally effective when performed as part of a chew–bite or as a chew. Similarly, we assumed that the bite part of a chew–bite is as effective as a bite. When N is the total number of jaw movements performed in the course of a bout of active grazing, Cn, Bn and Qn are the numbers of jaw movements allocated to chews, bites and chew–bites, respectively. The number of chews per bite and the number of jaw movements per bite are then: b¼
Cn þ Qn Bn þ Qn
(4)
a¼
Cn þ Bn þ Qn Bn þb ¼ Bn þ Qn ðBn þ Qn Þ
(5)
In the absence of chew-biting Eq. (4) reduces to Eq. (3), and in the absence of biting a = b. Although the animal is constrained to perform a certain number of chewing jaw movements per bite removed, this can be achieved by several different combinations of the three types of jaw movements. For example, if b = 1, the animal could allocate half of the jaw movements to bites and half to chews; alternatively, it could allocate all jaw movements to chew–bites and thereby double the rate of intake, provided that each bite requires only one chew. If many chews need to be performed per bite, then the benefits of chew-biting are obviously smaller. We now express the allocation of jaw movements as the proportions Cp, Bp and Qp, and re-define b and a accordingly: Cp þ Bp þ Qp ¼ 1
(6)
b¼
Cp þ Qp Bp þ Qp
(7)
a¼
1 Bp þ Qp
(8)
Thus, the intake equation can be formulated as: I ¼ WJðB p þ Q p Þ
(9)
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The use of proportions enables the allocation of jaw movements between the three types to be shown graphically by means of the unit-sum triangle (Fig. 1). If bites and chew–bites are represented by the two base corners of the triangle, then constant levels of total jaw movements per bite (a) are represented by horizontal lines on the triangle. The number of jaw movements per bite is unity along the base of the triangle (no chews), and it increases geometrically to infinity as the proportion of chews approaches unity (i.e., the number of bites approaches zero). Isoclines of number of chews per bite (b) have differing orientations and are not parallel lines. Chew-biting enables the animal to reduce the total number of jaw movements per bite without reducing the number of chews per bite. 2.2. Empirical studies Grazing sessions were conducted at the Volcani Center, Bet Dagan, Israel (328590 N, 348480 E), using Israeli–Holstein dairy heifers. In each study season, animals of similar ages were halter-and-rope trained and accustomed to the field procedures. The animals were experienced grazers and hunger was not induced prior to the observations. 2.2.1. Acoustic monitoring The allocation of jaw movements was derived from the sound track of video (analogue VHS) recordings of the grazing sessions. Sounds were collected by a small inward-facing microphone pressed against the forehead of the animal. The animal’s head acts as a sound box, whereby biting and chewing actions can be heard clearly as ripping and grinding sounds, respectively. There has been unanimous agreement by all who have listened to such recordings that the sound bursts correspond to bite, chew and chew–bite jaw movements. The acoustic signal was captured on video via a transmitter and receiver. The acoustic signal was interpreted aurally to classify each sound burst produced by a jaw movement. A distinction was made between sounds that could be classified
Fig. 1. The unit-sum triangle for the allocation of jaw movements among bites (Bp), chews (Cp) and chew–bites (Qp). Each point within or on the triangle may be represented by three triangular coordinates that sum to unity. Feint lines show increments of 0.1 in the proportions. Solid lines show isoclines for the number of chewing jaw movements per bite (b), dashed lines show isoclines for the total number of jaw movements per bite (a).
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unequivocally and those that could be classified with less than complete certainty. This yielded the three definitive classes of chew, bite and chew–bite, the five uncertain classes of probable chew, probable bite, probable chew–bite, chew or chew–bite and bite or chew– bite and an additional class of unknown. The non-definitive classes combined accounted for less than 5% of total jaw movements. Hence, for simplicity, each ‘‘probable’’ class was combined with its corresponding ‘‘definitive’’ class, with ‘‘chew or chew–bite’’ being classed as chew and ‘‘bite or chew–bite’’ being classed as bite. 2.2.2. Study 1 An area of 0.6 ha was planted with oats (Avena sativa L.) in April 1996 with a commercial drill (between-row spacing 15 cm) and irrigated. The grazing sessions were conducted 6 weeks later, between the third-leaf phase and tillering, using nine animals of similar ages (590 8 days; mean S.D.) and live weights (496 26 kg). An ungrazed, uniform expanse of herbage of approximately 50 m2 was selected before each grazing session, and the unextended plant height was determined at 20 points. The observation animal was fitted with the acoustic equipment, led calmly to the selected area and allowed to graze. The animal could meander freely as it grazed, with a handler who trailed the animal keeping to its side and rear, with the rope slack. A grazing session of up to 10 min was recorded on video by a second person standing to the front and side of the animal, and 20 measurements of grazed plant height were made during the session (by a third person). This cycle was repeated for each of the nine animals. A total of five samples were taken of herbage mass (0.5 m2 cut per sample), vertical mass distribution and dry matter content. A 5-min session of uninterrupted grazing (i.e., no pauses in the rhythm of jaw movements of more than 5 s) was extracted from the video recordings for each animal, and the sound track was sequenced. The durations of the grazing sessions, the numbers and durations of headup chewing bouts and the numbers and durations of pauses or breaks in jaw activity were also determined from the video recording. 2.2.3. Study 2 In April 1998, two areas, each of 60 m 4 m, were planted with oats with a precision drill at a between-row spacing of 8 cm, and were irrigated. The grazing sessions were conducted in the first week of July, during the early tillering phase. Four animals (age 567 8 days, weight 458 33 kg) grazed herbage at two initial sward heights of 20 and 25 cm. The four animals were observed in two cycles, two animals per observation day. Animal sequence was randomized within cycle, and initial sward height was randomized within animal. Each grazing session was assigned to a strip 4 m long and 0.5 m wide (six sowing rows). At the start of each observation day, the two strips to be grazed were defined by sampling part of the surrounding vegetation for herbage mass, vertical mass distribution and dry matter content, and by clipping the remainder. Sampled vegetation was trimmed to the designated treatment height before sampling, and the strips to be grazed were similarly trimmed. Each observation animal was allowed to make one grazing pass of a fresh herbage strip at its own pace, and the acoustic signal was captured on video. After the animal had grazed, a measurement of plant height (grazed or ungrazed) was made every 10 cm along each of the six sowing rows of the herbage strips (240 measurements per strip). The sound track for each grazing session was sequenced, and the video record was used to measure
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overall duration, and the numbers and durations of head-up chewing bouts and pauses as in Study 1. 2.2.4. Analysis The proportions of each type of jaw movement (Bp, Cp, Qp) were computed relative to the total number of sequenced sounds, with ‘‘unknown’’ excluded (N). We defined the number of biting jaw movements (B0n ) as Bn + Qn, and the number of chewing jaw movements (Cn0 ) as Cn + Qn. Note that if Qn > 0, B0n þ Cn0 > N. The proportions of biting jaw movements (B0p ) and chewing jaw movements (C 0p ) were similarly computed. If Qp > 0, B0p þ C0p > 1. The numbers of jaw movements per bite (a), chews per bite (b) and bite rate (R) were computed according to Eqs. (8), (7) and (2), respectively. The rate of jaw movement (J) was based on N and the duration of the grazing session. In both studies, head-up chewing time was included and total pause/break time was deducted. Chewing requirements were analyzed by linear regression of Cn0 on B0n . In Study 2, a two-way analysis of variance was applied to J, b, a and R, with the terms animal (as a random factor) and initial sward height (as a fixed factor) in the model, and the experimental unit being the animal initial sward height combination.
3. Results 3.1. Study 1 The mean herbage mass (S.E.) was 144.5 12.1 g m2 for a mean initial sward height of 20 cm. The mean proportions of the herbage mass contained in 5-cm horizons, from the ground upwards, were 0.36, 0.26, 0.22 and 0.16. The dry matter content of the herbage was 133 g kg1 dry matter (DM). On average, a grazing session lasted 4.5 min and contained 358 jaw movements, i.e., a jaw movement rate (J) of 78.9 min1 (Table 2). When the total number of jaw movements was normalized to unity chew–bites and chews each accounted for a proportion of 0.39 and bites for the remaining 0.22. There was high variability among animals in the allocation of jaw movements (see CV in Table 2), but very low variability in J (CV = 6%). The mean bite rate (R) was 48.2 min1, and the animals executed 1.66 jaw movements per biting action (a) and performed 1.27 chewing actions per biting action (b). Fig. 2 shows the allocation of jaw movements for the nine grazing sessions on the unitsum triangle. The points appear to be oriented along an isocline of b. The line represents the b isocline equal to the group mean (1.27). Linear regression of the number of chewing jaw movements (Cn0 ) on the number of biting jaw movements (B0n ) yielded the following equation (showing S.E.): Cn0 ¼ 33:9ð66:5Þ þ 1:117ð0:298ÞB0n ;
r 2 ¼ 0:62
(10)
The intercept was not significantly different from zero (P = 0.6082) and the slope – an estimate of b – was significant (P = 0.0074).
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Table 2 Mean, standard error (S.E.) and coefficient of variation (CV, %) of the number of chew, bite and chew–bite jaw movements and derived variables, performed by dairy heifers grazing young, vegetative oat swards in May 1996 (Study 1: nine animals) and June 1998 (Study 2: three animals at two initial sward heights, H) Measure
Symbol Study 1 Mean
Total number of jaw movements Number of bites Number of chews Number of chew–bites Proportion of bites Proportion of chews Proportion of chew–bites Number of biting jaw movements Number of chewing jaw movements Proportion of biting jaw movements Proportion of chewing jaw movements Chews per bite Jaw movements per bite Duration (s) Rate of jaw movement (min1) Bite rate (min1) Herbage mass (g m2) Initial height (cm) Residual height (cm) Bite depth (cm)
Study 2 S.E.
CV H = 25 cm
H = 20 cm
Mean
S.E.
CV Mean
S.E.
CV
N
358
16.4
14
245
82.0
58
247
67.7
47
Bn Cn Qn Bp Cp Qp B0n
79 138 141 0.220 0.387 0.393 220
15.2 10.8 22.3 0.043 0.027 0.066 13.7
58 23 47 58 21 50 19
63 103 79 0.246 0.402 0.352 142
26.8 37.6 26.1 0.045 0.029 0.065 44.5
73 63 57 32 12 32 54
63 96 88 0.256 0.400 0.343 151
16.2 20.6 32.1 0.014 0.024 0.034 47.6
45 37 63 9 11 17 55
Cn0
279
18.8
20
182
60.3
57
184
51.9
49
B0p
0.613
0.027 13
0.598
0.029
8
0.600
0.024
7
C0p
0.780
0.043 16
0.754
0.045 10
0.744
0.014
3
b a – J R – – – –
1.27 1.66 272 78.9 48.2 144.5 22.8 9.9 13.0
0.047 0.074 11.0 1.5 2.0 12.1 1.4 0.8 1.0
11 13 12 6 13 19 18 25 23
1.26 1.68 192 78.0 46.8 255 25 11.1 13.9
0.065 9 0.077 8 66.7 60 3.38 8 3.77 14 38.1 26 – – 0.6 9 0.6 7
1.24 1.67 192 77.9 46.6 179 20 8.8 11.2
0.039 5 0.066 7 56.4 51 1.80 4 0.94 3 5.17 5 – – 1.2 23 1.2 18
Fig. 2. The allocation of jaw movements between bites (Bp), chews (Cp) and chew–bites (Qp) for nine dairy heifers grazing young, vegetative oat swards in May 1996 (Study 1), shown on the unit-sum triangle. The line is the isocline for the mean number of chews per bite (b).
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The proportions of chews and bites traded off directly against chew-biting (Fig. 3b and g). As the proportion of chew-biting increased, the bite rate increased (Fig. 3o) and the number of jaw movements per bite declined (Fig. 3t). The number of chews per bite showed no clear response to any of the variables (Fig. 3c, h, l, p, q and r). 3.2. Study 2 One heifer was clearly distracted during both its grazing sessions; its considerable headup chewing resulted in atypical allocations of jaw movement. Data for this animal were excluded. The mean herbage masses were 255.3 38.1 and 178.3 5.2 g m2 for initial sward heights of 25 and 20 cm, respectively. The mean proportions of herbage mass contained in
Fig. 3. The relationships among the proportions of jaw movements allocated to bites (Bp), chews (Cp), chew–bites (Qp), chews per bite (b), jaw movements per bite (a), rate of jaw movement (J) and bite rate (R) for nine dairy heifers grazing young, vegetative oat swards in May 1996 (Study 1). Values are the Pearson product-moment correlation coefficient (r) and the significance probability (P).
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5-cm horizons, from the ground upwards, were 0.30, 0.26, 0.22, 0.13 and 0.09 for an initial sward height of 25 cm, and 0.33, 0.29, 0.23, 0.16 for a height of 20 cm. The dry matter content of the herbage was 125 g kg1 DM. On average, grazing sessions lasted 3.2 min and contained 246 jaw movements (Table 2). The jaw movement rate (J) was 78.0 min1. Chew–bites, chews and bites accounted for proportions of 0.35, 0.40 and 0.25, respectively, of jaw movements. The mean bite rate (R), number of jaw movements per bite (a) and number of chews per bite (b) were 46.7 min1, 1.68 and 1.25, respectively. Variability was greater for the allocation of jaw movements than it was for the derived variables of J, a and b. Fig. 4 shows the allocation of jaw movements for the six grazing sessions on the unit-sum triangle. The line represents the b isocline equal to the group mean (1.25). Neither animal nor initial sward height showed any clear influence on jaw movement allocation, and these factors were not significant in the analysis of variance of J, b, a and R. Linear regression of the number of chewing jaw movements (Cn0 ) on the number of biting jaw movements (B0n ) yielded the following equation (S.E.): Cn0 ¼ 7:39ð17:5Þ þ 1:198ð0:109ÞB0n ;
r 2 ¼ 0:96
(11)
The intercept was not significantly different from zero (P = 0.6940) and the slope was significant (P = 0.0004). The relationships among the key variables showed a positive correlation between the proportion of chew–bite jaw movements (Qp) and bite rate (R) (Fig. 5o), a negative correlation between Qp and jaw movements per bite (a) (Fig. 5m), and no clear trends with chews per bite (b) (Fig. 5c, h, l, p, q and r).
Fig. 4. The allocation of jaw movements between bites (Bp), chews (Cp) and chew–bites (Qp) for three dairy heifers grazing young, vegetative oat swards at two initial heights in June 1998 (Study 2), shown on the unit-sum triangle. Circle, initial sward height of 20 cm; triangle, initial sward height of 25 cm. The line is the isocline for the mean number of chews per bite (b).
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Fig. 5. The relationships among the proportions of jaw movements allocated to bites (Bp), chews (Cp), chew–bites (Qp), chews per bite (b), jaw movements per bite (a), rate of jaw movement (J) and bite rate (R) for three dairy heifers grazing young, vegetative oat swards at two initial heights in June 1998 (Study 2). Circle, initial sward height of 20 cm; triangle, initial sward height of 25 cm. Values are the Pearson product-moment correlation coefficient (r) and the significance probability (P).
4. Discussion Studies 1 and 2, which were conducted in different seasons with different animals, yielded remarkably similar estimates of the mean jaw movement rate (J). The finding that J was constant is consistent with our simple conceptual model of intake rate. Although we have no rigorous way of testing the assumption that biting and chewing actions are equivalent if performed as pure or compound jaw movements, it is at least supported by the absence of any trend between the proportion of chew–bites and chewing requirement (b) in either study. Our results were also consistent with the assumption that although different animals are constrained by a similar chewing requirement (b), they can allocate jaw
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movements differently. Consequently, the number of jaw movements per bite (a) did respond to allocation; a declined and bite rate increased as chew-biting increased. The implications of the chew–bite may, however, extend beyond bite rate alone. If biting and chewing jaw movements ‘‘competed’’, bite weight and intake rate should be positively related. However, when chew-biting is possible, this is not necessarily the case, since nothing is gained by loading the mouth with more herbage than can be processed by a single chewing jaw movement. In such a case, the animal would need to intersperse chews among the stream of chew–bites and thereby reduce the bite rate. If, as a result of chewbiting, the animal has no need to maximize bite weight, it can restrain its bite depth (‘‘skim’’) and thereby, on most swards, gain in diet quality. According to this conception, the ideal grazer would perform only chew–bites and regulate its bite weight (via bite dimensions) to the amount processed by a single chew. It is hence noteworthy that not only did variation among animals in jaw movement allocation align along isoclines of chews per bite (b), but also that the b values obtained were close to unity. Furthermore, similar b values were obtained on swards that offered quite different levels of herbage mass: 145 g m2 in Study 1, 178 g m2 in Study 2 at a height of 20 cm and 255 g m2 in Study 2 at a height of 25 cm. That finding is more indicative of regulation of bite weight than of maximization. This raises a question regarding the relevance of micro-scale studies of the relation between sward structure and intake to the phenomena that occur on larger spatial scales. Indeed, a recent analysis, based on acoustic monitoring, of jaw movement allocation and bite rate for cattle grazing single feeding stations of various sizes showed that the steady-state behaviour typical of extended grazing bouts was not attained at the spatial scale of a single feeding station (Ungar et al., 2005). Further experimentation that includes the accurate estimation of bite weight will be required to test rigorously the hypothesis of bite weight regulation versus maximization. In conclusion, chew–bites account for a major share of jaw-movement allocation in cattle grazing in a non-patchy foraging environment. Variation in bite rate between animals is largely accounted for by differences in jaw movement allocation rather than jaw movement rate. Variability in jaw movement allocation among individual animals is large, and appears to be oriented along an axis of constant chews per bite, but varying numbers of jaw movements per bite and, therefore, of varying bite rate.
Acknowledgements Contribution no. 132/2004 from the Agricultural Research Organization, Institute of Field and Garden Crops, Bet Dagan, Israel. This research was supported by Research Grant Award No. IS-2331-93C from BARD, the United States–Israel Binational Agricultural Research and Development Fund.
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