Effect of sward height on short-term intake by steers grazing winter oat pastures

Livestock Science 225 (2019) 8–14

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Effect of sward height on short-term intake by steers grazing winter oat pastures

T

Laura Nadina, Federico Sánchez Chopaa, María Lorena Agnellib, Julio Kuhn da Trindadec, Horacio Gondad a

Faculty of Veterinary Sciences, National University of the Centre of the Buenos Aires Province, Tandil, Argentina Faculty of Agriculture and Forestry Sciences, National University of La Plata, La Plata, Argentina c Riograndense Rice Institute, Porto Alegre, Brazil d Department of Animal Nutrition and Management, Swedish University of Agricultural Sciences, Uppsala, Sweden b

A R T I C LE I N FO

A B S T R A C T

Keywords: Steers Winter oats Sward height Short-term intake Bite mass Bite rate

Sward structure and animal grazing behaviour are key variables in determining herbage intake. The present study was carried out with the objective of evaluating the effect of different sward heights (SH) on the short-term ingestive behaviour variables, in steers grazing winter oats (Avena sativa cv. Calén). Six Holstein-Friesian steers (196 ± 4 kg LW), grouped into three pairs, grazed on swards with different surface heights (SH): 40 cm (SH40), 50 cm (SH50), and 60 cm (SH60). Short-term intake rates (IR) were estimated during grazing sessions of 1 h by the double-weighing technique with correction for insensible weight losses. The number of grazing jaw movements (GJM) and bite jaw movements were measured using the acoustic recorder technique. Measurements were conducted along three consecutive days. On each day, the pairs of steers grazed on one of the SH (1 pair per SH per d). Sound files were analysed visually and aurally. Number of GJM and BJM were determined during three 5min periods, from minute 5 to 10, from minute 27 to 32 and from minute 50 to 55 of the 1 h grazing session. Bite mass (BM) was calculated as the quotient between IR and the number of bites. Variables of grazing behaviour were analysed by ANOVA according to a 3 × 3 Latin Square design. Classes included in the model were treatment, pair of animals, day, observation time window, and the interaction between treatment and observation time window. Unexpectedly, BM was not affected by the SH (P = 0.97; 0.59, 0.61 and 0.60 g DM bite−1, for SH40, SH50 and SH60, respectively). Similar BM were the result of a numerical, non-significant, decrease in IR (P = 0.65; 30.2, 29.6 and 26.3 g DM min−1, for SH40, SH50 and SH60, respectively), together with a decrease in bite rate (BR; P< 0.001; 51.8, 49.1 and 44.3 bite min−1, for SH40, SH50 and SH60, respectively) with increasing SH. The rate of GJM was similar among treatments (P = 0.50; 88.4, 90.4 and 89.7 GJM min−1, for SH40, SH50 and SH60, respectively). Similar rates of GJM and different BR resulted in increasing numbers of GJM per bite as SH increased (P< 0.001; 1.73, 1.88 and 2.05 GJM per bite, for SH40, SH50 and SH60, respectively) showing than more manipulative grazing jaw movements were needed to form a bite when the steers grazed on the taller swards.

1. Introduction In grazing animals, the short-term intake rate is an important variable for determining the daily herbage intake. Over short periods of active grazing, intake rate is the product of bite mass and bite rate (bites min−1; Allden and Whitakker, 1970). These variables are the result of the interaction between the animal and sward structure and they are not independent from each other. Most of the research aimed at studying the relationship between sward structure and short-term herbage intake has shown bite mass to be a key determinant of the herbage intake (Laca et al., 1992; Gibb

et al., 1997; Benvenutti et al., 2006). Bite mass is largely determined by sward variables, such as surface height (Hodgson, 1990; Gibb et al., 1997) and bulk density (Laca et al., 1992; McGilloway et al., 1999). As the bite mass increases, more grazing jaw movements are used for mastication and bite rate decreases. Conversely, smaller bite mass requires less chewing jaw movements per bite and more grazing jaw movements can be used for herbage harvesting (Spalinger and Hobbs, 1992). However, the fact that there is a need for a minimum time required for each grazing jaw movement (Laca et al., 1994; Newman et al., 1994) imposes a limit to the animal to increase the rate of grazing jaw movements (GJM min−1). Besides the relationship

E-mail address: [email protected] (L. Nadin). https://doi.org/10.1016/j.livsci.2019.04.018 Received 29 November 2018; Received in revised form 23 April 2019; Accepted 24 April 2019 Available online 25 April 2019 1871-1413/ © 2019 Elsevier B.V. All rights reserved.

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grazing management, three areas of winter oats with different SH: 40, 50 and 60 cm, approx., were established: SH40, SH50, SH60 (Table 1). With the objective of allowing the different SH to persist throughout the grazing measurement sessions, the area of each paddock, was adjusted to provide a daily forage allowance of 13 kg DM 100 kg LW−1 steer−1. The resultant areas of the paddocks were approximately 52, 90 and 120 m2, for SH40, SH50, SH60, respectively. Two days before the beginning of the experiment, the steers were weighed, paired according to live weight and randomly assigned to a sequence in which each pair grazed the different treatments. The grazing behavioural measurements were done along 3 consecutive days. The study was performed according to a 3 × 3 Latin square design (3 sward surface heights, 3 pairs of steers and 3 measurement days). The experimental procedure was performed as follows: each day, around 11 am, the animals were moved from the adjacent pasture, where they grazed while not being under study, to the handling area. There, they were kept in pens deprived of feed with access to water, for a period of three hours. This fasting period was established in order to promote active grazing during the 1 h grazing sessions. At 2 pm, the animals were fitted with harnesses for total collection of urine and faeces and weighed at t1 (W1 = initial weight for estimating the rate of insensible weight losses (RIWL) pre-grazing). After being weighed, the animals remained in individual pens for 1 h without access to feed, water and shade, and then they were weighed again at t2 (W2 = final weight for pre-grazing RIWL and pre-grazing weight for estimating IR). Immediately after, the steers were fitted with acoustic recorders (AR) and the three pairs of animals (two steers SH−1) were allotted to their treatment paddocks for the 1 h grazing session. Once the grazing session was finished, the steers were conducted to the handling area, the AR were removed and the animals were weighed at t3 (W3 = post-grazing weight for estimating IR and initial weight for the post-grazing RIWL). The steers then remained in individual pens without access to feed, water and shade for 1 h until being weighed at t4 (W4= final weight for post-grazing RIWL). The harnesses were then immediately removed and the animals returned to the adjacent area. The animals were weighed using an electronic scale (Balcoppan Challeger 576, RS 232, Argentina) with a capacity of 500 kg and a precision of 50 g.

between bite mass and bite rate, differences in short-term intake rate could be explained by the ratio of biting to non-biting GJM (Laca et al., 1994; Gibb et al., 1997). While most of the studies regarding short-term intake rate were conducted in temperate pastures with short surface heights (< 30 cm; Gibb et al., 1997, 1999; Amaral et al., 2013; Gregorini et al., 2009), there is scarce information about grazing dynamics in potentially tall temperate pastures, as Avena spp. In heifers grazing Avena strigosa pastures, Mezzalira et al. (2014) observed that bite mass and short-term intake rate increased as sward height increased from 15 to 40 cm, followed by a significant decrease in both variables when sward height increased from 40 to 50 cm. In Argentina, winter oats (Avena sativa) is one of the most important annual cereal forage and it is well established in grazing systems (Elizalde et al., 1996; Sánchez Chopa et al., 2016). Usually managed under a short-term rotational grazing system, it is not uncommon than the surface height of the winter oats reaches 50–60 cm. The aim of the present study was investigate the effect of sward surface height on the short-term foraging dynamics in steers grazing winter oat (Avena sativa cv. Calén) pastures with surface sward surface heights between 40 and 60 cm. 2. Materials and methods The experiment was conducted at the campus of the National University of the Centre of the Buenos Aires Province (UNCPBA; 37º19´S, 59º07´W), Tandil, Argentina. All procedures regarding the use of animals complied with the requirements of the ethical guidelines published by the International Society for Applied Ethology and were approved by the Animal Welfare Committee of the Faculty of Veterinary Medicine, UNCPBA. 2.1. Animals, treatments and experimental procedure Six Holstein-Friesian steers (196 ± 4 kg LW) were used. During two months prior to the beginning of the experiment, the animals were adapted to grazing winter oats and the proximity of the observers. The treatments under study were three different sward surface heights (SH) which were generated by grazing a winter oats sward (Avena sativa cv. Calén) under a short rotational grazing system, in combination with different periods of regrowth. As a result of the

Table 1 Mean sward surface height (SH, cm; pre- and post-grazing grazed and non-grazed tiller´s height), total herbage dry matter (DM) mass, and proportions of DM present as lamina (L), pseudostem (PS), inflorescence (I) and dead herbage, L:P mass ratio measured to ground level, in steers grazing Avena sativa swards with three different sward surface heights (SH40, SH50 and SH60). .

Sward surface height (cm) Pre-grazing (a) Post-grazing non-grazed tillers Post-grazing grazed tillers* (b) RMSE P Bite depth (a minus b; cm) Bite depth as a proportion of the pre-grazing height Herbage mass (g DM m2 -1) Morphological components (proportion of total DM mass) L PS I Dead L:PS mass ratio

Treatments (SH) SH40

SH50

SH60

RMSE

P=

38.9 c x 34.7 c y 24.7 c z 4.674 <0.0001 14.2 a 0.36 a

53.7 b x 48.6 b y 41.4 b z 4.857 <0.0001 12.1 b 0.23 b

61.1 a x 53.3 a y 46.3 a z 5.890 <0.0001 15.2 a 0.25 b

4.897 6.313 5.595

<0.001 <0.001 <0.001

0.868 0.025

0.030 0.005

99.126

0.009

0.027 0.0145 0.038 0.0056 0.0864

0.046 0.015 0.550 0.032 <0.001

318 0.31 0.46 0.18 0.05 0.70

c

570

a

0.24 0.51 0.21 0.04 0.47

b

a a

Means not sharing common letters (a, b, c) within a row differ by P < 0.05. SEM: standard error of the mean. ⁎ Residual height (Griffiths et al., 2003). 9

b

b a

b b

815 0.23 0.53 0.19 0.05 0.44

a

b a

a b

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using scissors, on each sub plot and on each day. A pooled herbage sample from the four samples was separated manually into lamina, pseudostem, inflorescence and dead fractions, before being dried at 65ºC until reaching a constant weight. The weights of these fractions were used to calculate the total herbage mass (g m−2), as well as the masses of the different morphological components (20 samples SH−1 day−1), representative of the sward horizon grazed by the steers, were collected by hand plucking. Two pooled herbage samples were obtained from the total. One pool was used for determining DM content, immediately after being sampled. The other pool was dried at 65ºC until reaching a constant weight, milled to pass 1 mm screen, and analysed to determine crude protein (CP; Nelson and Sommers, 1973), neutral detergent fibre (NDF; Van Soest et al., 1966) and in vitro DM digestibility (IVDMD; Tilley and Terry, 1963) at the Nutrition Lab of the Faculty of Veterinary Medicine, UNCPBA, Tandil, Argentina.

2.2. Grazing behaviour measurements 2.2.1. Short-term intake rate Short-term intake rate (IR; g DM min−1) was estimated by the double-weighing technique, taking into account the RIWL, according to Penning and Hooper (1985), as:

IR = {[(W2 − W1)/(t2 − t1)]*DM + [(W3 − W4 )/(t4 − t3)]}*[(t2 − t1)/ET )] (1) where IR= short-term intake rate; W2 and W1= animal´s weight postand pre-grazing; t2 and t1= post- and pre-grazing time; DM= proportion of dry matter in the forage; W3 and W4= animal´s weight pre- and post-insensible weight losses; t4 and t3= post- and pre-insensible losses time; and ET= effective eating time. With the aim to determine the effect that grazing may have on the RIWL (Penning and Hooper, 1985), RIWL were estimated pre- and postgrazing sessions. In addition to the AR, 2 observers per treatment counted the number of bites during the grazing sessions. Data recorded by the observers were in turn used to check and validate the bite rate data obtained from the AR, as well as to estimate ET. Due the experimental protocol used and, because throughout the experiment the animals actively grazed along the grazing sessions, resulting in that ET was equal to grazing time, in the present study the short term IR was calculated as:

2.4. Statistical analysis Variables measured on the sward, -initial and final SH, total dry mass and its components: lamina, pseudostem, inflorescence and dead material-, and lamina:pseudostem mass ratio were analysed by ANOVA. Classes included in the model were treatment (sward height), day, and their interaction. Because data of lamina:pseudostem mass ratio were proved not to be normally distributed (Shapiro-Wilks test; P< 0.0001), values were transformed to arcsine before statistical analysis. Differences between the pre- and post-grazing RIWL were evaluated by a paired t-test. The effect of treatments on the post-grazing RIWL was evaluated by ANOVA according to a 3 × 3 Latin square design. Before analysis, ingestive behaviour variables –bite rate, bite mass, intake rate and grazing jaw movements bite−1- were averaged for each pair of animals for each of the three 5-min observation window used for counting total grazing jaw movements and bites. Classes included in the model were treatment, pair of animals, day, observation time window, and the interaction between treatment and observation time window. Before analysis, normal distribution of number of bites (P = 0.43) and total grazing jaw movements (P = 0.26) were tested by the ShapiroWilks test. All the statistical analyses were performed with the SAS software package (version 9.4, SAS Institute Inc., Cary, NC). Mean values were compared using Fisher´s LSD.

IR = {[(W2 − W1)/(t2 − t1)]*DM + [(W3 − W4 )/(t4 − t3)]}*[(t2 − t1)/ET )] (2) According to Dumont et al. (1994), in this study, the values obtained for IR were corrected by RIWL in the same animals in which the ingestive behaviour variables were measured. 2.2.2. Measurement of total grazing jaw movements, bite jaw movements and bite mass Grazing behaviour variables, total jaw movements and bites, were estimated by using the acoustic recorder technique. This equipment consisted of a microphone (Omni directional microphone, ECM-F8, Sony, Japan) connected to a digital recorder (IC recorder, ICD P320, Sony, Japan). Briefly, the microphone was protected by a device made of expanded polystyrene and attached to the steer´s forehead by an elastic band fastened to a halter. The digital recorder was protected by a rubber foam and attached to the halter using an elastic band. After the grazing sessions, the sound files were downloaded to a computer in a MP3 format. This allowed the sound recordings to be interpreted by an operator examining both the digitised sound-wave patterns and aurally by listening to the recordings simultaneously (Sound Forge 9.0, Sony Creative Software Inc., USA; Milone et al., 2009; Nadin et al., 2012). Measurements of total number of jaw movements and those corresponding to biting jaw movements were determined during three 5-min periods, from minute 5 to 10, from minute 27 to 32 and from minute 50 to 55 of the 1 h grazing session. Bite mass was calculated as the quotient between IR and the number of bites.

3. Results 3.1. Chemical composition, sward characteristics and bite depth Herbage dry matter content was similar among treatments (24.7, 26.5, and 25.1, for SH40, SH50 and SH 60, respectively). Winter oats NDF concentration numerically increased, whereas crude protein content and IVDMD decreased, with increasing SH (NDF%: 47.8, 51.5 and 55.3; CP%: 18.5, 16.2 and 15.7; IVDMD%: 81, 76.2 and 70.5; for SH40, SH50 and SH60, respectively). Data of sward surface pre- and post-grazing heights, bite depth, herbage DM mass and the morphological components, are presented in Table 1. The pre-experimental grazing management created three treatments differing significantly (P< 0.001) in pre-grazing SH similar to those intended. Also, the significant differences among SH were consistent in both non-grazed and grazed tillers heights (P< 0.001). Bite depths were similar between SH40 and SH60 and higher (P = 0.030) than in SH50. Bite depth represented a higher proportion of the pre-grazing height in the shortest sward (SH40; P = 0.005) as compared to SH50 and SH60. Herbage mass significantly increased as the SH increased (P = 0.009). The proportion of lamina was higher and that of pseudostem was lower in SH40 (P = 0.05 and P = 0.015, respectively) than in SH50 and SH60. As a consequence, the L:PS ratio was higher in SH40 (P< 0.001) than in the others SH. No differences were observed in the

2.3. Sward measurements Initial SH measurements (n = 40) were taken before the 1 h grazing period, across each of the SH plots. In addition, final SH measurements (n = 40) were taken after the grazing period, distinguishing the tillers that were grazed or not. All the measurements were done using a sward stick similar to the design of the HFRO sward stick (Hill Farming Research Organization, 1986), but in which the ‘contact’ window measured 15 mm × 35 mm. Bite depth was calculated by the difference between pre-grazing SH and post-grazing height of the grazed tillers (Griffiths et al., 2003). Before each grazing session, four samples of herbage per treatment were cut at ground level within quadrats measuring of 20 cm x 25 cm, 10

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Table 2 Effect of sward surface height (SH40; SH50 and SH60) on the rate of insensible weight loss (RIWL), short term intake rate, bite mass, bite rate, and grazing jaw movements (GJM) in steers grazing Avena sativa swards measured during onehour grazing session. . Treatments (SH) SH40 SH50 −1

§ †

RCME†

Observation time§ Time 1 Time 2

P=

−1

min ) RIWL (mg kg LW Pre-grazing* Post-grazing§ RCME† P= Short term intake rate (g DM min−1) Short term intake rate (mg DM kg LW−1 min−1) Bite mass (g DM bite−1) Bite mass (mg DM kg LW−1) Bite rate (bites min−1) Time per bite (s) GJM (GJM min−1) GJM per bite (no.) GJM per g of DM consumed (no.) ⁎

SH60

Table 3 Effect of the time spent grazing on bite rate, grazing jaw movement's rate (GJM) and the proportion of bites to GJM in steers grazing Avena sativa swards along one-hour grazing session. Values (means of three different sward heights) correspond to three 5-min observation windows of sounds files of 60 min. .

−1

82.0 81.8 16.54 0.9824 30.2

89.6 54.7 15.50 0.1110 29.6

85.6 74.16 21.91 0.4080 26.3

22.03 24.74

0.841 0.285

4.98

0.650

154.3

150.8

133.6

25.13

0.632

0.59 3.01 51.8 1.19 88.4 1.73 3.02

0.61 3.10 49.1 1.25 90.4 1.88 3.07

0.60 3.02 44.3 1.37 89.7 2.05 3.59

0.090 0.448 2.91 0.088 3.69 0.091 0.364

0.963 0.968 <0.001 0.002 0.502 <0.001 0.008

a b

c b

a b

b b

b a

a a

Bite rate (bites min ) GJM rate (GJM min−1) GJM per bite (no.)

51.1 91.5 1.81

a

b

48.8 90.9 1.90

a

b

Time 3 45.1 86.5 1.96

b

a

RCME†

P=

3.01 6.15 0.091

0.002 0.211 0.017

§ Time 1: from minute 5 to minute 10, Time 2: from minute 27 to minute 32, and Time 3: from minute 50 to minute 55 of the 60-min grazing session. † RCME: square root of the mean square error.

3.4. Changes in grazing behaviour variables along the grazing session The effect of time of observation (5-min observation window at the beginning, middle and at the end of the one-hour sound recording) on bite rate, the rate of grazing jaw movements, and the ratio grazing jaw movements: bite are presented in Table 3. While the rate of grazing jaw movements was not different among SH (P = 0.211), bite rate decreased (P = 0.0024) and grazing jaw movements: bite ratio increased (P = 0.017) as grazing progressed (Table 3). The same effect occurred in all treatments as the interaction between treatment and observation time for any of the grazing behaviour variables was not statistically significant (P> 0.05). As an example, the decrease in bite rate as grazing progressed for the different treatments is shown in Fig. 1.

Following a 4 h period of food deprivation. Following a 1 h grazing session. RCME: square root of the mean square error.

proportion of inflorescence among treatments (P = 0.550). The proportion of dead material, although different among treatments (P = 0.032), represented only a small amount (4–5%) of the total herbage mass.

4. Discussion In the present study, the steers were deprived of food for a period of 4 h to ensure they graze actively during the following 1-hour grazing sessions. Fasting prior grazing could increase bite mass, bite rate and therefore, intake rate (Chacon and Stobbs, 1976; Chilibroste et al., 1997; Patterson et al., 1998). Patterson et al. (1998), in dairy cows, did observe differences in bite mass and bite rate with a fasting period of 6 h, but not if the fasting period was of 3 h. Chilibroste et al. (1997) also found differences in bite mass, in particular during the first hour of grazing following a fasting period of 16 h. In another experiment with fasting periods of 16 and 2.5 h, Chilibroste et al. (1998) did not report changes in bite mass measured in a grazing session of 138 min after starving. The authors mention that the lack of agreement with the findings reported by Patterson et al. (1998) may be due to the length of the grazing session following fasting suggesting that the effect of fasting on bite mass may be a transient event (Chilibroste et al., 2007). On the other hand, Dougherty et al. (1989) in Angus cows grazing tall fescue, and Dougherty et al. (1988) in cattle grazing lucerne, did not observe any effect of short-term fasting (1–3 h and 4–16 h, respectively) on intake rate and bite rate. Although the occurrence of an effect of the 4 h food deprivation before grazing cannot be precluded, in the present study all treatments were evaluated under the same experimental conditions. Based on visual observation and the sound recordings, the 4 h fasting period proved to promote the occurrence of grazing bouts that lasted 60 min throughout the experiment.

3.2. Insensible weight losses, short term intake rate and bite mass Three post-grazing RIWL estimations, from the SH50 treatment, showed extreme unrealistic values (2; 7 and 411 mg kg LW−1 min−1). Therefore, only fifteen values out of eighteen were used to analyse the effect of treatment on the pre- and post-grazing RIWL, and only three measurements out of six to compare pre- and post-grazing RIWL within the SH50 treatment. No significant differences were observed between RIWL measured pre- and post-grazing, or across treatments (P> 0.05, Table 2). However, because RIWL post-grazing had a greater coefficient of variation (CV= 34.3) than RIWL pre-grazing (CV= 25.3), in the present study the pre-grazing RIWL were used to calculate the intake rate. Neither short term intake rate, nor bite mass were significantly affected by the treatments (P = 0.65 and P = 0.963, respectively; Table 2).

3.3. Grazing jaw movements, bite rate, time per bite No differences were observed in the rate of grazing jaw movements among treatments (P = 0.502, Table 2). However, bite rate showed a significant decrease (P = 0.001), and time per bite a significant increase (P = 0.002) in SH60 as compared with SH40 and SH50 (Table 2). The proportion of total grazing jaw movements classified as bites represented 0.59, 0.54 and 0.49 of the total grazing jaw movements for SH40, SH50 and SH60, respectively. At SH60 treatment more jaw movements were required to form a bite compared with SH40 and SH50. Number of jaw movements bite−1 and time bite−1, increased significantly from SH40 to SH60 (P< 0.001 and P = 0.002, respectively; Table 2). Grazing jaw movements per unit of intake was the highest (3.59 GJM g DM consumed−1, P = 0.008, Table 2) at the tallest sward (SH60).

4.1. Insensible weight losses Insensible weight loss rate is considered the rate at which the LW is reduced in a period of time, due to evaporative and gaseous losses from the animal (Dumont et al., 1994; Gibb et al., 1999). The estimation of RIWL is important for evaluating the herbage intake (Penning and Hooper, 1985; Dumont et al., 1994; Barret et al., 2001; Ginane and Petit, 2005) and its variation with different factors should be considered. Penning and Hooper (1985) in sheep, and Gibb et al. (1999) in dairy cows, reported the occurrence of a significant effect of sward 11

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Fig. 1. Mean bite rates (bites min−1) measured during three 5-min periods: 5 to 10, 27 to 32 and 50 to 55 min, during a 60 min grazing session, in steers grazing on winter oat pastures with three different surface heights: SH40 (▲), SH50 (◊), and SH60 (●). (SH40: y= -0.204 x + 57.85, R2= 0.940; SH50: y= -0.104 x + 52.17, R2= 0.948; SH60: y= -0.091 x + 46.99, R2= 0.922).

with a surface height of 40–50 cm). In the current study, the combination of relatively tall pastures with relatively small size animals may contribute to explain the lack of agreement between the observed results with those reported in the literature. As mentioned by Cangiano et al. (2002), differences in bite depth in relation to sward height are complex to explain due to the many variables involved during grazing, as herbage density of the grazed horizon, leaf and stem proportion, constraints by the resistance posed by the physical properties of the vegetation, and type of animal, among other factors. In line with the numerically small differences in bite depth, the bite mass was almost identical among treatments. Bite mass is calculated as the quotient between the intake rate and bite rate. In spite of that differences in short term intake rate were no significant among treatments (P = 0.650), both the intake rate and the bite rate decreased (P = 0.0024) as sward height increased. As a consequence, the bite mass remained unchanged. While calculated bite mass were within the range of those reported by Mezzalira et al. (2014) in heifers grazing A. strigosa with heights of 40 and 50 cm, these authors observed a significant decrease in bite mass with an increase in the sward height from 40 to 50 cm (4.2 and 2.2 mg DM kg LW−1, respectively). A decrease in bite mass with increasing sward height in tall pastures has also been reported in Panicum maximum by Carvalho et al. (2001), in Sorghum bicolor by Fonseca et al. (2013), and in native grassland by Goncalves et al. (2009) and Bremm et al. (2012). Changes in bite mass occurs when one or more of its characteristics (area, depth and bulk density of the grazed horizon) modifies (Laca et al., 1992). In relatively tall pastures, changes in bite area and depth may be due to presence of stems which may impose a physical barrier, as well as increase the tensile strength of the herbage to be grazed (Benvenutti et al., 2006). It is worth to be noticed that while the bite mass and the rate of grazing jaw movements was similar among treatments (P> 0.05), time per bite and GJM per bite were significant higher in the tallest sward (SH60) as compared to SH40 and SH50 (P = 0.002, P< 0.001, respectively). Based in these variables, as well as on visual observation, it appears evident that when confronted to SH60 the animals performed more manipulative jaw movements during grazing to form a bite than when they grazed on the shorter swards. As a result of that, and in spite of the short term intake rate and the rate of GJM were not significant different among sward heights, the number of GJM per gr of DM consume during the one-hour grazing session was significantly higher in SH60 than in SH40 and SH50 (P = 0.008). This is line with the results published by Mezzalira et al. (2014), who reported an increase, though not statistically significant, in the number of GJM per gr of herbage consumed from 1.2 to 2.0 as sward height increased from 40 to 50 cm. It appears that under the conditions of the present study, different

height on RIWL. In addition, there is evidence showing that RIWL can be affected by the feed intake during the previous hour (Penning and Hooper, 1985; Nuthall et al., 1994). In the present study, RIWL was measured immediately before and after the grazing session and, in contrast to Nuthall et al. (1994), no significant differences were found between pre- and post-grazing RIWL (P> 0.05; Table 2). There are no apparent reasons for the three values of post-grazing RIWL estimations, two extremely low and one extremely high, all from the SH50 treatment. 4.2. Sward characteristics and ingestive behaviour In all treatments, A. sativa was in early reproductive stage with proportions of inflorescences similar among different sward heights (Table 1). Differences in the proportion of lamina and pseudostem between SH40 and the other treatments was reflected, as expected, in a higher L:PS ratio in the shortest sward (P< 0.001). Sward heights used in the present study were very close to those intended, and the differences in height were consistent in pre- and post-grazing, in both grazed and non-grazed tillers. Differences in height between pre- and postgrazing non-grazed tillers (P = 0.0001, Table 1) may have been the result of steers grazing first the tallest tillers they encountered, or of a selective grazing in favour of the emerging inflorescences. It has been shown that bite depth increases with increasing sward height in swards up to 40 cm (Laca et al., 1992, 1994; Hodgson et al., 1994), and 50 cm (Mezzalira et al., 2014); and that it represents a fairly constant proportion of sward height (Laca et al., 1994; Cangiano et al., 2002). In contrast, in the present study bite depth -measured as the difference between pre-grazing SH and post-grazing grazed tillers- was not related to sward height (Table 1), and their magnitude was around 0.5 than those reported by Mezzalira et al. (2014) in heifers grazing on A. strigosa. There is no an apparent reason for the significantly smaller bite depth recorded in the SH50 treatment (P = 0.030). Bite depth represented 0.23–0.36 of the pre-grazing sward height, and the proportion was the highest in the shortest sward (SH40), as compared to SH50 and SH60 (Table 1). Differences among treatments might be explained by the higher proportion of lamina and lamina:pseudostem ratio, and lower proportion of pseudostem, in SH40 than in SH50 and SH60 (Table 1). According to Burns and Sollenberger (2002), the proportion of pseudostems is negatively associated with bite area and bite depth. In addition, in reproductive swards, pseudostems may act as vertical or horizontal barrier to bite formation (Benvenutti et al., 2006). The bite depth expressed as a proportion of the pre-grazing height (Table 1) were smaller than those reported for Laca et al. (1992); Wade et al. (1989; 0.35–0.45) and Mezzalira et al. (2014; 0.62–0.63 in pastures 12

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sward height did not result in sward characteristics capable to affect the bite mass and animals grazed in order to keep bite mass at its possible maximum at expenses of increasing the time and the GJM expended per bite.

Barrett, P.D., Laidlaw, A.S., Mayne, C.S., 2001. Pattern of herbage intake rate and bite dimensions of rotationally grazed dairy cows as sward height declines. Grass Forage Sci. 56, 362–373. Benvenutti, M.A., Gordon, I.J., Poppi, D.P., 2006. The effect of the density and physical properties of grass stems on the foraging behaviour and instantaneous intake rate by cattle grazing an artificial reproductive tropical sward. Grass Forage Sci. 61, 272–281. Bremm, C., Laca, E.A., Fonseca, L., Mezzalira, J.C., Elejalde, D.A.G., Gonda, H.L., Carvalho, P.C., de, F., 2012. Foraging behaviour of beef heifers and ewes in natural grasslands with distinct proportions of tussocks. Appl. Anim. Behav. Sci. 141, 108–116. https://doi.org/10.1016/j.applanim.2012.08.008. Burns, J.C., Sollenberger, L.E., 2002. Grazing behavior of ruminants and daily performance from warm-season grasses. Crop Sci. 42, 873–881. https://doi.org/10.2135/ cropsci2002.8730. Cangiano, C.A., Galli, J.R., Pece, M.A., Dichio, L., Rozsypalek, S.H., 2002. Effect of liveweight and pasture height on cattle bite dimensions during progressive defoliation. Aust. J. Agric. Res. 53, 541–549. Carvalho, P.C., de, F., Marçal, G.K., Ribeiro Filho, H.M.N., Poli, C.H.E.C., Da Trindade, J.K., Oliveira, J.O.R., Nabinger, C., De Moraes, A., 2001. Pastagens altas podem limitar o consumo dos animais. Reun. Anu. da Soc. Bras. Zootec 38, 265–268. Chacon, E., Stobbs, T.H., 1976. Influence of progressive defoliation of a grass sward on eating behaviour of cattle. Aust. J. Agric. Res. 27, 709–727. https://doi.org/10.1071/ ar9760709. Chilibroste, P., Soca, P., Mattiauda, D.A., Bentancur, O., Robinson, P.H., 2007. Short term fasting as a tool to design effective grazing strategies for lactating dairy cattle: a review. Aust. J. Exp. Agric. 47, 1075–1084. https://doi.org/10.1071/EA06130. Chilibroste, P., Tamminga, S., Boer, H., 1997. Effects of length of grazing session, rumen fill and starvation time before grazing on dry-matter intake, ingestive behaviour and dry-matter rumen pool sizes of grazing lactating dairy cows. Grass Forage Sci 52, 249–257. https://doi.org/10.1111/j.1365-2494.1997.tb02355.x. Chilibroste, P., Tamminga, S., Van Bruchem, J., Van Der Togt, P.L., 1998. Effect of allowed grazing time, inert rumen bulk and length of starvation before grazing on the weight, composition and fermentative end-products of the rumen contents of lactating dairy cows. Grass Forage Sci 53, 146–156. https://doi.org/10.1046/j.13652494.1998.5320146.x. Dougherty, C., Smith, E., Bradley, N., Forbes, T., Cornelius, P., Lauriault, L., Arnold, C., 1988. Ingestive behaviour of beef cattle grazing alfalfa (Medicago sativa L.). Grass Forage Sci 43, 121–130. Dougherty, C.T., Bradley, N.W., Cornelius, P.L., Lauriault, L.M., 1989. Short-term fasts and the ingestive behavior of grazing cattle. Grass Forage Sci 44, 295–302. Dumont, B., Penning, P., Orr, R., D'Hour, P., 1994. Effects of some factors on insensible weight loss in grazing sheep. Ann. Zootech. 43, 283. https://doi.org/10.1051/ animres:19940350. Elizalde, J.C., Santini, F.J., Pasinato, A.M., 1996. The effect of stage of harvest on the process of digestion in cattle fed winter oats indoors. II. Nitrogen digestion and microbial protein synthesis. Anim. Feed Sci. Technol. 63, 245–255. https://doi.org/10. 1016/S0377-8401(96)01021-8. Fonseca, L., Carvalho, P.C.de F., Mezzalira, J.C., Bremm, C., Galli, J.R., Gregorini, P., 2013. Effect of sward surface height and level of herbage depletion on bite features of cattle grazing Sorghum bicolor swards. J. Anim. Sci. 91, 4357–4365. https://doi.org/ 10.2527/jas2012-5602. Gibb, M.J., Huckle, C.A., Nuthall, R., Rook, A.J., 1997. Effect of sward surface height on intake and grazing behaviour by lactating Holstein Friesian cows. Grass Forage Sci 52, 309–321. https://doi.org/10.1111/j.1365-2494.1997.tb02361.x. Gibb, M.J., Huckle, C.A., Nuthall, R., Rook, A.J., 1999. The effect of physiological state (lactating or dry) and sward surface height on grazing behaviour and intake by dairy cows. Appl. Anim. Behav. Sci. 63, 269–287. https://doi.org/https://doi.org/10. 1016/S0168-1591(99)00014-3. Ginane, C., Petit, M., 2005. Constraining the time available to graze reinforces heifers’ preference for sward of high quality despite low availability. Appl. Anim. Behav. Sci. 94, 1–14. https://doi.org/10.1016/j.applanim.2005.02.010. Gonçalves, E.N., Carvalho, P.C., de, F., Silva, C.E.G., da, Santos, D.T., dos, Díaz, J.A.Q., Baggio, C., Nabinger, C., 2009. Relações planta-animal em ambiente pastoril heterogêneo: padrões de desfolhação e seleção de dietas. Rev. Bras. Zootec 38, 611–617. https://doi.org/10.1590/S1516-35982009000400004. Gregorini, P., Gunter, S.A, Beck, P.A, Caldwell, J., Bowman, M.T., Coblentz, W.K., 2009. Short-term foraging dynamics of cattle grazing swards with different canopy structures. J. Anim. Sci. 87, 3817–3824. https://doi.org/10.2527/jas.2009-2094. Griffiths, W.M., Hodgson, J., Arnold, G.C., 2003. The influence of sward canopy structure on foraging decisions by grazing cattle. I. Patch selection. Grass Forage Sci. 58, 112–124. Hill Farming Research Organisation, 1986. Biennial report 1984–1985. HFRO, Edinburgh, UK, pp. 29–30. Hodgson, J., 1990. Grazing Management: Science into Practice. Longman Scientific and Technical, 1st ed. Longman Group, London, pp. 203. Hodgson, J., Clark, D.A., Mitchell, R.J., et al., 1994. Foraging behaviour in grazing animals and its impact on plant communities. In: Fahey JrG.C. (Ed.), Forage Quality and Utilization. Lincoln, American Society of Agronomy, Madison, WI, pp. 796–827. 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. https://doi.org/ 10.1016/0168-1591(94)90011-6. Laca, E.A., Ungar, E.D., Seligman, N.G., Ramey, M.R., Demment, M.W., 1992. An integrated methodology for studying short-term grazing behavior of cattle. Grass Forage Sci 47, 81–90. https://doi.org/10.1111/j.1365-2494.1992.tb02250.x. McGilloway, D.A., Cushnahan, A., Laidlaw, A.S., Mayne, C.S., Kilpatrick, D.J., 1999. The relationship between level of sward height reduction in a rotationally grazed sward

4.3. Grazing behaviour along the 1-h grazing session As grazing session progressed, the rate GJM remained similar (P = 0.211), but both across and within treatments the bite rate declined (P = 0.002). On average, the bite rate measured between minute 50 and 55 was 13.3% (SD= 0.06) lower than when measured between minute 5 and 10 of the 60-min grazing period. As consequence of the decline in bite rate, the number of GJM per bite increased over the grazing session (P = 0.017). In cows fasted for 1–3 h prior grazing, Patterson et al. (1998) observed that the mean bite rate declined over the 1-h grazing period. As the same effect was not registered when cows were summited to a longer period of fasting (6 or 13 h), the authors attribute their finding to an increase in the time animals dedicated to search and select the herbage as the degree of hunger decreases as grazing progress (Patterson et al., 1998). In the current study, however, the rate of GJM did not change and the number of GJM per bite increased as grazing progressed suggesting that the declined in bite rate may have been due to changes in the sward structure as a consequence of the grazing down. 5. Conclusion In grazing cattle, it is widely recognised that bite mass is a key determinant of the short-term herbage intake. In studies performed mostly with temperate and relatively short swards, it has been observed that bite mass increases as sward surface height increases. In contrast, in taller pastures bite mass was shown to decrease when SH were higher than 40 cm (A. strigosa; Mezzalira et al., 2014). Results from the present study, however, showed that in steers grazing A. sativa with sward heights from 40 to 60 cm, bite mass remained fairly constant among all sward heights. No changes in bite mass were a product of a numerically, not significant, decrease in intake rate together with a decrease in the bite rate as sward height increased. It appears that under the conditions of the present study, the steers prioritised bite mass as more grazing jaw movements were allocated to each bite as sward height increased. Conflict of Interest None. Acknowledgements We thank María Cecilia Casado and Ramiro Mogni for field assistance. This research was financially supported by the Universidad Nacional del Centro de la Provincia de Buenos Aires, Argentina. The role of the funding source was only to provide the financial support for the conduct of the experiment. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.livsci.2019.04.018. References Allden, W.G., Whittaker, I.A.M., 1970. The determinants of herbage intake by grazing sheep: the interrelationship of factors influencing herbage intake and availability. Aust. J. Agric. Res. 21, 755–766. https://doi.org/10.1071/AR9700755. Amaral, M.F., Mezzalira, J.C., Bremm, C., Da Trindade, J.K., Gibb, M.J., Suñe, R.W.M., Carvalho, P.C., de, F., 2013. Sward structure management for a maximum short-term intake rate in annual ryegrass. Grass Forage Sci. 68, 271–277. https://doi.org/10. 1111/j.1365-2494.2012.00898.x.

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L. Nadin, et al.

Anim. Sci. 66, 299–305. https://doi.org/10.1017/S1357729800009425. Penning, P.D., Hooper, G.E., 1985. An evaluation of the use of short-term weight changes in grazing sheep for estimating herbage intake. Grass Forage Sci 40, 79–84. https:// doi.org/10.1111/j.1365-2494.1985.tb01722.x. Sánchez Chopa, F.S., Nadin, L.B., Agnelli, L., Trindade, J.K., Gonda, H.L., 2016. Nitrogen balance in Holstein steers grazing winter oats: effect of nitrogen fertilisation. Anim. Prod. Sci. 56, 2039–2046. https://doi.org/10.1071/AN141007. Spalinger, D.E., Hobbs, N.T., 1992. Mechanisms of foraging in mammalian herbivores: new models of functional response. Am. Nat. 140, 325–348. https://doi.org/10. 1086/285415. Tilley, J.M.A., Terry, R.A., 1963. A two‐stage technique for the in vitro digestion of forage crops. Grass Forage Sci 18, 104–111. https://doi.org/10.1111/j.1365-2494.1963. tb00335.x. Van Soest, P.J., Wine, R.H., Moore, L.A., 1966. Estimation of the true digestibility of forages by the in vitro digestion of cell walls. In: Proceedings of the 10th International Grasslands Congress. Helsinki, Finland. pp. 438–441. Wade, M.H., Peyraud, J.L., Lemaire, G., Comeron, E.A., 1989. The dynamics of daily area and depth of grazing and herbage intake of cows in a five day paddock system. In: Proceedings of the XVI International Grassland Congress. Nice, France. pp. 1111–1112.

and short-term intake rates of dairy cows. Grass Forage Sci 54, 116–126. Mezzalira, J.C., De Faccio Carvalho, P.C., Fonseca, L., Bremm, C., Cangiano, C., Gonda, H.L., Laca, E.A., 2014. Behavioural mechanisms of intake rate by heifers grazing swards of contrasting structures. Appl. Anim. Behav. Sci. 153, 1–9. https://doi.org/ 10.1016/j.applanim.2013.12.014. Milone, D.H., Rufiner, H.L., Galli, J.R., Laca, E.A., Cangiano, C.A., 2009. Computational method for segmentation and classification of ingestive sounds in sheep. Comput. Electron. Agric. 65, 228–237. https://doi.org/10.1016/j.compag.2008.10.004. Nadin, L.B., Chopa, F.S., Gibb, M.J., Trindade, J.K., Da, Amaral, G.A., Carvalho, P.C., de, F., Gonda, H.L., 2012. Comparison of methods to quantify the number of bites in calves grazing winter oats with different sward heights. Appl. Anim. Behav. Sci. 139, 50–57. https://doi.org/10.1016/j.applanim.2012.03.001. Nelson, D.W., Sommers, L.E., 1973. Determination of total nitrogen in plant material. Agron. J. 65, 1–4. https://doi.org/10.2134/agronj1973.00021962006500010033x. Newman, J.A., Penning, P.D., Parsons, A.J., Harvey, A., Orr, R.J., 1994. Fasting affects intake behaviour and diet preference by grazing sheep. Anim. Behav. 47, 185–193. Nuthall, R., Huckle, C.A., Gibb, M.J., 1994. Factors affecting the rate of insensible weight loss in dairy cattle. Fourth BGS Research Conference. Br. Grassland Soc. Session VII, 159–160. Patterson, D.M., McGilloway, D.A., Cushnahan, A., Mayne, C.S., Laidlaw, A.S., 1998. Effect of duration of fasting period on short-term intake rates of lactating dairy cows.

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