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Effect of sward surface height and level of herbage depletion on bite features of cattle grazing Sorghum bicolor swards1

L. Fonseca,*2 P. C. F. Carvalho,* J. C. Mezzalira,* C. Bremm,* J. R. Galli,† and P. Gregorini‡

*Grazing Ecology Research Group, Federal University of Rio Grande do Sul, Porto Alegre, Rio Grande do Sul 91540-000, Brazil; †Facultad de Ciencias Agrarias, Universidad Nacional de Rosario, Departamento de Produccion Animal, Zavalla,

Santa Fe, Argentina; and ‡DairyNZ Ltd. Corner of Ruakura and Morrinsville Roads, 3240, Hamilton, New Zealand

ABSTRACT: To maximize herbage DMI, pregrazing sward surface height (SSH) and level of herbage depletion (HD) must be such that variables determining short-term herbage intake such as bite mass (BM) and bite rate (BR) are optimized. The objective of this study was to determine a SSH target and the level of HD as a proportion of the SSH that optimizes BM and BR of beef heifers grazing Sorghum bicolor swards. Two experiments were conducted using 2 S. bicolor swards and 4 beef heifers (25 mo old; 322 kg BW). Experiment 1 compared the effect of 6 pregrazing SSH, 30, 40, 50, 60, 70, and 80 cm, on BM, BR, and jaw movements. Experiment 2 assessed the effect of HD level as a proportion of SSH (0.17, 0.34, 0.50, 0.67 and 0.84) on BM, BR, and jaw movements using the optimal pregrazing SSH defined in Exp. 1. Short-term herbage DMI was estimated using a double-weighing technique and corrected for insensible BW loss. Herbage DMI was subsequently used to calculate the BM. Net eating time and jaw movements for apprehension and manipulation + mastication during grazing as well as total jaw movements

were determined using the IGER (Institute of Grassland and Envoronmental Research) behaviour recorders. Bite rate and the number of total jaw movements per gram herbage DMI were derived from jaw movement count and measurements of herbage DMI. The results of Exp. 1 showed low and high SSH constraint the ease of herbage harvesting. Greater BM are maintained until a SSH of 50 cm is reached (P < 0.05) and then decline at greater SSH due to herbage dispersion. The nonbiting jaw movement rate increased at greater SSH whereas BR decreased (P < 0.05). For both variables, the turning point was close to a SSH of 50 cm. Experiment 2 showed that such an optimization of BM and BR was maintained until an HD level of 0.34 was reached (P < 0.05). There was a linear increase in both the total jaw movements per unit herbage DMI and the nonbiting jaw movements rate (manipulation + mastication) subsequent to levels of HD greater than 0.34 (P < 0.05). These studies provide, for the first time, sward feature targets to manage grazing and optimize BM and BR, aiming to maximize the short-term herbage DMI of cattle grazing S. bicolor swards.

Key words: cattle, grazing behavior, herbage depletion

© 2013 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2013.91:1–9 doi:10.2527/jas2013-5602

JAS5602

INTRODUCTION

High performance of intensive pasture-based livestock production systems is based on high stocking rates and grazing pressures, which promote pasture growth and use (MacDonald et al., 2008). These systems, however, compromise individual herbage DMI and thereby individual animal performance (Dillon, 2006). In the short term, sward characteristics at intensive stocking rates impose constraints to bite formation, thus reducing bite mass (BM) and bite rate (BR; McGilloway et al., 1999; Gregorini et al., 2011).

1The authors are grateful to the Brazilian Ministry of Education Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for providing financial assistance via CAPES/SPU Project 032/07 and D. Clark, S. Woodward, and Prof. E. Hillerton (DairyNZ Inc.) as well as S. C. Silva (Universidade de São Paulo, Escola Superior de Agricultura Luiz de Queiroz- ESALQ), H. M. N. Ribeiro Filho (Universidade do Estado de Santa Catarina- UDESC), and A. B. Soares (Universidade Tecnológica Federal do Paraná- UTFPR) for their helpful comments to the manuscript.

2Corresponding author: lidianefnc@gmail.com.Received June 25, 2012.Accepted June 5, 2013.

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Given the complexity of grazing behavior patterns, the act of taking a simple bite seems irrelevant. However, due to the high frequency of bites severed per day by grazing ruminants, bite features have major consequences on herbage DMI (Shipley, 2007). Several studies focused on the relationship between sward canopy structure and short-term herbage DMI and have proven BM to be a key determinant of herbage DMI and, thereby, animal performance (Laca et al., 1992; Benvenutti et al., 2009). According to Hodgson (1990), sward surface height (SSH) is the sward structural characteristic that most strongly influences the bite dimensions and decisions of an animal; therefore, management of SSH is critical to facilitate herbage ingestion and achieve greater levels of herbage DMI. The majority of these studies, however, have been conducted with cattle grazing either perennial cool season grasses under temperate grazing situations or perennial and short warm season grasses, with only few under subtropical environments and fewer with cattle grazing tall warm season grasses such as Sorghum bicolor (Burns and Sollenberger, 2002).

Therefore, we hypothesized that to maximize daily herbage DMI, the initial pregrazing SSH and the level of herbage depletion (HD) should optimize the components of short-term herbage DMI (i.e., BM and BR by cattle grazing S. bicolor swards). This hypothesis was tested by determining the pregrazing SSH and the proportion of different levels of HD that maximizing short-term herbage intake by cattle grazing S. bicolor swards.

MATERIALS AND METHODS

Animals and ProceduresAll procedures involving animals were approved

by the Institutional Animal Care and Use Committee of Federal University of Rio Grande do Sul and were conducted in accordance with the Guide for the Care and Use of Agricultural Animals in Agricultural Research and Teaching (FASS, 2010).

Experimental Site

This study was conducted at the Research Station of the Federal University of Rio Grande do Sul on a S. bicolor sward (cultivar BR 501). The swards were sown on December 16, 2009 (4,820 m2), and February 5, 2010 (780 m2), for Exp. 1 and 2, respectively. The swards were direct drilled, with a seed sowing density of 33 kg/ha and a distance of 0.17 m between adjacent rows. At sowing, the swards were fertilized with 20 kg/ha of N and 100 kg/ha of P2O5 and K2O followed by 200 kg/ha of N 20 d later.

Experiment 1 was conducted between January 12 and March 11, 2010. Experiment 2 was conducted

between April 5 and April 15, 2010. Twelve grazing sessions of 45 ± 5 min each were performed in Exp. 1, and 15 sessions (45 ± 5 min each) were performed in Exp. 2. The length of the grazing session was defined as the minimum period of time necessary for the accurate detection of weight fluctuations by the electronic balance used during the evaluation of the short-term herbage DMI rate technique (Penning and Hooper, 1985). All grazing sessions were performed either in the morning or in late afternoon in both experiments.

Animals

Four Angus × Brahman beef heifers (24 ± 2 mo old; 306 ± 56.7 kg of BW) were used in Exp. 1, and 3 of those heifers were used in Exp. 2 (26 ± 2 mo old; 339 ± 45.5 kg of BW). Thirty days before each experiment, heifers were familiarized with the experimental protocol on adjacent areas with S. bicolor swards. Heifers were not fasted before grazing sessions in Exp. 1 because this may influence diet selection (Newman et al., 1994). During the experimental period, animals remained in an adjacent area with S. bicolor and they were handled pulled directly from this area at the beginning of each grazing session. In Exp. 2, diet selection was not the aim of the study and the animals were fasted for 5 h before each grazing session to ensure that they grazed the total percentage intended. The duration of the fasting period was the same in all treatments, which allowed for valid comparisons. The effect of the fasting period on bite dimensions is controversial, though (see Newman et al., 1994; Gregorini et al., 2009b).

Treatments

The treatments in Exp. 1 consisted of 6 SSH (30, 40, 50, 60, 70, and 80 cm) in a randomized complete block design with 2 replicates. The treatments in Exp. 2 were 5 HD levels established from the optimum pregrazing SSH defined in Exp. 1. The levels of HD were 0.16, 0.33, 0.50, 0.67, and 0.84 (proportion of initial SSH). Experiment 2 used a randomized incomplete block experimental design with 3 replicates. In both experiments, the blocking criterion was the time of day, morning or afternoon (times at which the assessment was conducted). Therefore, in Exp. 1, 6 grazing sessions were conducted in the morning and 6 in the afternoon. In Exp. 2, 7 and 8 grazing sessions were performed in the morning and afternoon, respectively. Time of day was blocked in the analyses due to changes in the preferences and ingestive dynamics of animals over the course of the day (Rutter, 2006; Gregorini, 2012). The experimental unit was determined using the average for the 4 animals in Exp. 1 and the 3 animals in Exp. 2. The area for each grazing plot in Exp. 1 was calculated to prevent the pre- and postgrazing SSH from varying by more than

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5%, such that the same sward structure was available throughout the grazing sessions. The grazing scenario in Exp. 1 had areas of 600 m2. In Exp. 2, the grazing plot areas were such that the level of HD was attained within the same time period in all treatments. These grazing plots were 298, 144, 85, 54, and 36 m2 for the 0.16, 0.33, 0.50, 0.67, and 0.84 HD treatments, respectively. The grazing plot areas in both experiments were based on tests that were conducted before each experiment. In Exp. 1, the tests involved letting the animals graze for 45 min in a defined area and then measuring the SSH to ensure that there was no more than a 5% difference between the pre- and postgrazing SSH. This procedure was repeated until the ideal grazing plot size was attained. Similar tests were also conducted in Exp. 2; however, considerably smaller areas were used, and, after 45 min, the high levels of HD were assessed. Subsequently, using a proportionality calculation and taking the HD level of a certain area and the HD level in each treatment into account, the ideal area of each experimental plot was obtained for each treatment.

Sward Measurements

A sward stick was used to measure SSH with 200 readings pre- and postgrazing in each grazing scenario. Pregrazing herbage mass (HM) was determined in both experiments by summing the leaf lamina mass plus the stem + sheath mass for each stratum and expressed as kilograms DM per hectare. Total herbage mass was cut at ground level, using a quadrat of 0.153 m2. In Exp. 1, 5 strata were cut whereas in Exp. 2, 3 stratified strata were cut every 10 cm of SSH per experimental unit. These samples were subsequently separated into their morphological components (leaf lamina and stem + sheath). All samples were oven dried (55°C over a period of least 72 h). The sward bulk density was calculated as the HM divided by the volume of each stratified strata. Because the stems + sheaths and sheaths are structures that play similar constraining roles to herbage DMI during grazing (barriers to the bite depth; Drescher et al., 2006), they were quantified as a single bulk-density variable.

Animal Measurements

Approximately 30 d before the evaluation of each experiment, the animals were familiarized with observers, recording equipment, and the experimental procedures and remained in an adjacent paddock with S. bicolor sward. Before each grazing session, heifers were fitted with feces and urine collecting bags using the methodology described by Penning and Hooper (1985) and with behavior recorders. After the outfit, heifers were weighed and allowed to graze for 45 min and reweighed. After the grazing session, insensible weight loss (H2O evaporation and CO2 and CH4

loss and production) was measured by weighing heifers and moving them to an adjacent nonvegetated area without food or water for 45 min. After this period, heifers were reweighed and the apparatus removed.

A digital scale (MGR-3000 júnior; Toledo, Canoas, Brazil; accurate to 10 g) was used to measure the herbage DMI using the double-weighing technique, as described by Penning and Hooper (1985). The herbage DMI was corrected for the forage DM content in both experiments. The DM content was estimated by collecting 2 samples, composed of 10 subsamples per grazing plot both before and after grazing. In Exp. 1, the samples were collected from the upper halves of the canopy based on the hypothesis that there was a proportional relationship between the herbage removal with each bite and 50% SSH (e.g., Laca et al., 1992; Flores et al., 1993; Cangiano et al., 2002). In Exp. 2, the samples were collected up to the SSH of the HD level for each treatment.

During grazing session, heifers were fitted with IGER (Institute of Grassland and Environmental Research - Ultra Sound advice, London, UK) behavior recorders. Data obtained from the recordings were later analyzed using Graze software (www.ultrasoundadvice.co.uk; Rutter, 2000) to identify and characterize jaw movements [biting (bite severing) and nonbiting jaw movements (manipulation + ingestive mastication)] and to determine the net eating time [time at which heifers were head down and completely involved in severing, manipulating, and masticating bites (grazing = eating + searching times)]. These data were used to calculate the BM, BR, nonbiting jaw movement rate, and total jaw movements (TJM) per gram of herbage DMI. The TJM included nonbiting jaw movements and jaw movements of bites.

The BM was calculated by dividing herbage DMI during the grazing session by the total number of bites, adjusted by BW. The BR was determined by dividing the total number of bites by the eating time. The TJM per unit herbage DMI was calculated by dividing the total number of jaw movements by the herbage DMI intake during the grazing session. The nonbiting jaw movement rate was calculated as the number of nonbiting jaw movements divided by net eating time.

Statistical Analysis

The experimental unit for both experiments was the grazing plot. Heifers were considered as sampling units of each grazing plot. An ANOVA was performed on the sward characteristics (bulk density of the total HM, bulk density of stems + sheaths, bulk density of leaf lamina, and HM), and differences between averages were compared using a Tukey’s session (P < 0.05). For the animal variables (BM, BR, grazing jaw movements, and grazing jaw movements per gram of DM intake), linear

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(yij = a + bx + eij) and segmented (broken line; yij = L + U[(R < x)(R – x)] + eij) regressions were established, in which yij is the dependent variable, a is the intercept, b is the linear coefficient, x is the independent variable, eij represent the experimental error, L is the maximum observed value, U is a constant of the equation, and R is the break point. Regression analyses were performed using JMP software (SAS Inst. Inc., Cary, NC). If the regression equations were significant (P < 0.05), they were compared using coefficients of determination (R2).

RESULTS

Experiment 1The actual SSH were similar to the target SSH

(Table 1). Vertical distribution of HM and morphological

components of sward canopy (leaf lamina and stems + sheaths) are also shown in Table 1. These components, DM weights, and total HM increased with SSH (P < 0.01). Bulk density decreased from the bottom to the upper strata of the sward canopy (Table 1). When the bulk density of the leaf lamina was compared with the bulk density of the stem + sheath, a greater leaf density was detected in the upper strata, particularly in the swards with SSH lower than 50 cm, whereas the taller swards (60, 70, and 80 cm) had more stems + sheaths mass and density in such strata.

The BM remained constant across treatments below 55 cm of SSH, at which point BM decreased as SSH increased (Fig. 1). The TJM rate was constant across treatments, with an average of 71.5 ± 1.53 movements/min. The nonbiting jaw movement rate increased with SSH whereas the BR decreased with SSH (Fig. 2a). For

Table 1. Herbage bulk density, stems + sheaths and leaf lamina by sward canopy strata according to the pregrazing sward surface height in Sorghum bicolor swards

Strata

Sward surface height, cm30 40 50 60 70 80

Herbage bulk density, g DM/m3

>80 23b

80 to 70 30b 353a

70 to 60 37b 345a 434a

60 to 50 63b 379ab 425a 425a

50 to 40 34b 259a 369ab 356a 449a

40 to 30 65b 218b 303a 384a 436a 340a

30 to 20 298a 252ab 311a 376ab 455a 483a

20 to 10 318a 307a 332a 466a 441a 474a

10 to 0 263ab 308a 301a 441ab 589a 516a

Mean ± SEM 232 ± 37.9 237 ± 12.21 261 ± 25.5 336 ± 75.5 353 ± 46.6 370 ± 48.2Leaf lamina bulk density, g DM/m3

>80 23d

80 to 70 30c 334ab

70 to 60 37c 345ab 371a

60 to 50 63bc 374a 403a 324ab

50 to 40 34b 255a 331ab 266abc 265b

40 to 30 65c 218ab 233a 275abc 197abc 152c

30 to 20 298a 125ab 139b 125abc 139abc 112cd

20 to 10 210ab 124ab 102bc 88bc 96bc 114cd

10 to 0 115bc 47ab 26c 39bc 30c 46cd

Mean ± SEM 180 ± 30.0 151 ± 28.7 136 ± 23.4 191 ± 48.9 212 ± 47.7 218 ± 18.8Stems + sheaths bulk density, g DM/m3

>80 0.0f

80 to 70 0.0e 19ef

70 to 60 0.0b 1.7e 62def

60 to 50 0.0c 4.8b 22de 101def

50 to 40 0.0b 4.0c 37b 90d 183cd

40 to 30 0.0b 0.0b 69bc 109ab 239c 188bcde

30 to 20 0.0b 32b 172ab 250ab 315bc 371ab

20 to 10 107a 180a 230a 372a 342b 343abc

10 to 0 148a 260a 264a 387a 537a 424a

Mean ± SEM 51.7 ± 18.9 75.9 ± 16.0 123.3 ± 34.6 143.2 ± 44.4 138.5 ± 13.5 147.1 ± 30.8a,b,c,d,e,fThe averages in the columns that are followed by the same letter are not significantly different, as determined by a Tukey’s test (P < 0.05, n = 10).

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both of these variables, the breakpoint was close to 50 cm of SSH. Therefore, given the inverse relationships exhibited by these variables, the TJM remained constant across the range of SSH studied. In turn, the TJM per unit herbage DMI increased linearly with SSH (Fig. 2b).

Experiment 2

Table 2 shows the characterization of the sward canopy along its vertical profile, with strata determined every 10 cm of SSH during pregrazing (averages of all treatments). The average HM of the treatments was 2,742 ± 148 kg/ha. The bulk density of the leaf lamina was concentrated in the upper strata of the swards (above 30 cm). Conversely, the bulk density of the stems + sheaths was greater in the strata below 30 cm (Table 2).

The BM and BR remained constant up to 32 and 39 HD level, respectively. Thereafter, BM and BR exhibited linear decreases (Fig. 3a and 3b). The TJM were constant across treatments. However, as shown in Fig. 4, there was a linear increase in both the TJM by unit herbage DMI and nonbiting jaw movement rate (manipulation + mastication) after a 0.4 HD (Fig. 4a and 4b).

DISCUSSION

The Influence of Sward Surface HeightThe BM was constant and averaged 4.55 mg DM/kg

BW until SSH reached 55 cm. These values are within the range reported by Gonçalves et al. (2009) using natural grassland forage species. The variables determining BM are bite depth, bite area, and bulk density of the grazed sward canopy stratum (Laca et al., 1992). Area and depth of the bite are less sensitive to variations in the canopy

Figure 1. Bite mass [y = 4.55 + 0.027(55 – x) if x > 55 and y = 4.55, if x < 55; R2 = 0.85, P < 0.0001, SEM = 0.13] according to the pregrazing sward surface heights in Sorghum bicolor cultivar BR 501 swards. Each point corresponds to 1 grazing session (mean of 4 tester animals; n = 15).

Figure 2. (a) Total jaw movements rates ▲ (NS = Non Significant; P = 0.0929), the bite rates ○ (y = 26.8 + 0.62x – 0.0063x2; R2 = 0.74, P = 0.0024, SEM = 1.61), and  the nonbiting  jaw movement  rates ●  (y = 36.6 – 0.35x + 0.004x2; R2 = 0.65, P = 0.0091; SEM = 2.23). (b) Total jaw movements by unit herbage DMI (y = 0.74 + 0.011x; R2 = 0.86, P < 0.0001, SEM = 0.08) according to the pregrazing sward surface height in Sorghum bicolor cultivar BR 501 swards. Each point corresponds to 1 grazing session (mean of 4 tester animals; n = 15).

Table 2. Total herbage mass (HM), bulk density of leaf lamina (LLBD), bulk density of stem + sheath (SSBD), and the pregrazing total bulk density (TBD) of the Sorghum bicolor sward canopy subjected to different levels of herbage depletion as a proportion of sward surface heightStrata, cm

HM, kg/ha

LLBD, g DM/m3

SSBD, g DM/m3

TBD, g DM/m3

50 to 52.3 594a 542.3a 0.0e 542.3a

50 to 40 364c 361.5b 2.9e 364.4c

40 to 30 405c 313.4b 91.3d 404.8c

30 to 20 417bc 184.4c 233.0c 417.4bc

20 to 10 449bc 117.8cd 331.6b 449.4abc

10 to 0 513ab 91.4d 421.5a 512.6ab

Mean ± SEM 457 ± 137.2 268 ± 82.3 180.1 ± 59.0 448.5 ± 101.2a,b,cMeans in the same column that are followed by the same letter are

not significantly different, as determined by a Tukey’s test (P < 0.05, n = 9).

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characteristics because the former is regulated by the tongue of an animal and the latter has a proportional relationship with SSH (Laca et al., 1992; Flores et al., 1993; Gonçalves et al., 2009; Gregorini et al., 2009a). Therefore, in the absence of structural constraints, it can be inferred that the bulk density of the sward canopy kept BM constant up to 55 cm SSH.

The reduction of BM observed when SSH exceeded 55 cm can be attributed to HD, which is characterized by low bulk density in the upper strata of the sward (Stobbs, 1973). Black and Kenney (1984) demonstrated the importance of sward canopy bulk density in the grazing stratum as a determinant of BM. The BM is reduced (and the time spent on each bite increases) when an animal attempts to capture blades that are more scattered

(Palhano et al., 2007). Thus, above a 55 cm SSH, the BR also decreases with an increased need for nonbiting jaw movements, as the BR includes the time spent searching and handling (Prache, 1997). This relationship between sward structure and jaw movements for manipulation is demonstrated by the linear increase in TJM per herbage DMI. This confirms that the number of nonbiting jaw movements executed at greater SSH are intended for capture and manipulation outside the mouth and are more closely related to the canopy structure than to the BM (Hodgson et al.,1994; Ungar and Ravid, 1999).

The presence of stems + sheaths in the grazed stratum may also have contributed to the decrease in the BM and BR at greater SSH (Prache, 1997; Benvenutti

Figure 3. (a) Bite mass [y = 4.19 + 0.027(32.5 – x) if x > 32.5 and y = 4.19 if x < 32.5; R2 = 0.64, P = 0.0006, SEM = 0.38] and (b) bite rates [y = 42.9 + 0.215(39 – x) if x > 39 and y = 42.9 if x < 39; R2 = 0.60, P = 0.0021, SEM = 2.78] according to the level of herbage depletion of a Sorghum bicolor cultivar BR 501 sward. Each point corresponds to 1 grazing session (mean of 3 tester animals; n = 15). SSH = sward surface height.

Figure 4. (a) The total jaw movement per gram of DM intake [y = 1.25 – 0.020(40.8 – x) if x > 40.8 and y = 1.25 if x < 40.8; R2 = 0.75, P < 0.0001, SEM = 0.17] and (b) the nonbiting jaw movements rates [y = 28.8 – 0.241(41.0 – x) if x > 41 and y = 28.8 if x < 41; R2 = 0.62, P = 0.0005, SEM = 2.87] according to the level of herbage depletion of the Sorghum bicolor cultivar BR 501 sward. Each point corresponds to 1 grazing session (mean of 3 tester animals; n = 15). SSH = sward surface height.

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et al., 2008; Gregorini et al., 2009a, 2011). These components may have acted both vertically as a barrier to deep bites (Ginnett et al., 1999; Drescher et al., 2006) and horizontally, limiting the bite area by obstructing the sweeping action of the tongue (Gregorini et al., 2009a). In other words, the animals began to attempt to avoid the stems to capture only the leaf lamina (Ginnett et al., 1999; Drescher et al., 2006), performing more manipulation movements before cropping and resulting in lower BR (Prache, 1997; Benvenutti et al., 2008). In fact, the presence of stems + sheaths in the grazed strata at the greater SSH was a determining factor in the reduction of the BM, given that the bulk density of the stems + sheaths was negatively correlated with this variable (R = –0.58, P = 0.04). Stobbs (1973) and Gregorini et al. (2009a) reported a positive correlation between BM and the proportion of leaf lamina in the grazed stratum. According to Benvenutti et al. (2009), the negative effect of the bulk density of the stems + sheaths on intake herbage rate is a function of the negative effect of the stem + sheaths barrier on BM and BR. When the bulk density of the stems + sheaths in the grazed stratum is taken into account, overall bulk density increased linearly with SSH studied (Fig. 5).

Several studies confirmed the effect of SSH on bite dimensions and thereby herbage DMI (Laca et al., 1992; Hodgson et al., 1994; Ungar and Ravid, 1999; Gonçalves et al., 2009). Therefore, it is clear that SSH associated with a structure that provides the easiest forage harvest is close to 50 cm. The manner by which the leaves are presented to the animals and the degree at which they can be cropped separately from the sheaths and the dead material are extremely significant in pastures based on warm season (C4) species, such as S. bicolor. Therefore,

it is clear that animals must be provided access to swards with large leaf bulk densities and ideal SSH that enables greater BM (Gregorini et al., 2009a; Gregorini, 2012).

Changes in the Harvesting Process during Herbage Depletion

The BM and BR remained constant up to a 0.32 and 0.39 HD, respectively, which shows that there were no changes in sward structure impairing high-mass harvesting. In other words, if it is assumed that, under relatively homogeneous grazing environments, cattle graze by sward canopy strata (Baumont et al., 2004), then, under these conditions, the animals had not yet changed the grazing stratum. This finding corroborates the results obtained by Searle et al. (2005) who argued that BM is constant within each grazing stratum.

In this context, Ungar et al. (2001) and Cangiano et al. (2002) showed that cattle graze in successive strata that are approximately equivalent to half of the SSH, and the removal of these strata reduces bite depth. Therefore, it can be inferred that, at up to 30 cm (above 39 HD), the heifers had not yet reached the grazing stratum with more proportion of stems + sheaths; they still had access to the leaf lamina. Utsumi (2002) found a reduction in the BR when 55 or 36% of initial HM was removed for alfalfa or fescue, respectively. Also, Gregorini et al. (2011) observed a reduction in the number of bites per feeding stations as the extent of HD increased. This is attributed to a change from the first to the second grazing stratum.

Up to a 39% level of HD, the DM of the stems + sheaths, which is considered a constraint to harvesting (Chacon and Stobbs, 1976), was small (94.2 kg/ha). On the other hand, the mass of the leaf lamina, a component preferred by animals, was high (1,269 kg/ha). Under these conditions, the heifers did not need to perform more harvesting movements or manipulate the harvested material, and therefore both the TJM by unit herbage DMI and the nonbiting jaw movements remained constant.

As herbage was depleted, heifers faced changes in the sward structure associated with a decrease in the proportion and bulk density of the leaves and an increase in the number of stems + sheaths (Table 2). This finding agrees with those of Barrett et al. (2001) and Gregorini et al. (2011) for cool season grasses. The BM could have been partially compensated by the increase in the bulk sward density of the lower strata of the sward. However, due to the physical limitations imposed by stems + sheaths (e.g., Ginnett et al., 1999; Drescher et al., 2006), bite depth could have been reduced, which in turn could have negatively affected grams BM. Usually BR and BM show an inverse relationship (Hodgson, 1990). However, a decrease in BM was observed in combination with a decrease in BR at high levels of HD (above 0.39) and was related to

Figure 5. Bulk density of the stems + sheaths in the grazed stratum according to the sward surface height of Sorghum bicolor cultivar BR 501 sward (y = 31.56 + 1.01x; R2 = 0.65, P = 0.0015, SEM = 13.95). Each point corresponds to 1 grazing session (mean of 3 tester animals; n = 15).

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an increase in the number of jaw movements expended during selecting leaves and avoiding stems (Prache, 1997; Searle et al., 2005; Gregorini et al., 2009a). This finding shows that the number of nonbiting movements and the TJM dedicated to harvesting each gram of herbage DM increased after the 0.39 level of HD was reached. The stems interfered with bite formation, and thus more manipulation movements were necessary before biting to avoid them. This may have led to an increase in the time spent capturing and harvesting the herbage, thereby increasing the time expended per bite (Benvenutti et al., 2009). Therefore, when the animal reached the sward canopy strata where the stems were predominant, the BM and BR decreased due to the negative effects of this factor limiting the short-term herbage DMI rate (Ginnett et al., 1999; McGilloway et al., 1999; Gregorini et al., 2011; Fonseca et al., 2012).

In summary, the results reported in this study provide a better understanding of the grazing process when cattle graze swards of annual warm-season grasses such as S. bicolor. The challenge for grazing management lies in identifying SSH and level of HD at which cattle have no difficulty in grazing. In the present study, BM was optimized in swards between 50 and 60 cm of SSH at continuous stocking. For rotational stocking, the sward management target is that in which the sward is less modified during HD (e.g., less proportion of stems + sheaths). Our study indicates that this target is 0.40 HD. At high HD levels, variations in the availability and accessibility of leaves lead to an unbalanced relationship between BM and BR, which highlights the difficulty for cattle maintaining herbage intake rates during the HD (Benvenutti et al., 2008; Gregorini et al., 2011).

LITERATURE CITEDBarrett, P., A. Laidlaw, C. Mayne, and H. Christie. 2001. Pattern of

herbage intake rate and bite dimensions of rotationally grazed dairy cows as sward height declines. Grass Forage Sci. 56:362–373.

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