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Technical Note: Effects of tethering on herbage selection, intake and digestibility, grazing behavior, and energy expenditure by Boer x Spanish goats grazing high-quality herbage¹

A. K. Patra^*, R. Puchala^*, G. Detweiler^*, L. J. Dawson, T. Sahlu^* and A. L. Goetsch^*,2

^* E. (Kika) de la Garza American Institute for Goat Research, Langston University, Langston, OK 73050; and and College of Veterinary Medicine, Oklahoma State University, Stillwater 74078

	Abstract

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Twenty-four yearling Boer x Spanish goats were used in a crossover experiment to determine the effects of tethering on herbage selection, intake and digestibility, grazing behavior, and energy expenditure (EE) with high-quality herbage. Four 0.72-ha paddocks of wheat (Triticum aestivum) and berseem clover (Trifolium alexandrium) were grazed in the spring. Each paddock hosted 6 animals, 3 with free movement and 3 attached to a 3-m tether that was moved daily and provided access to an area of 28.3 m². One animal of each treatment and paddock was used to determine herbage selection, fecal output, or grazing behavior and EE. Herbage DM mass in tethered areas before grazing averaged 2,649 and 2,981 kg/ha in periods 1 and 2, respectively. The CP concentration in ingesta was greater (P < 0.05; 23.1 and 20.3 ± 0.82%) for free vs. tethered animals, although in vitro true DM digestion (75.7 and 76.5 ± 1.20%, respectively) did not differ (P > 0.05) between treatments. Intake of ME based on in vitro true DM digestion and fecal output was greater (P < 0.05) for free vs. tethered animals (12.7 and 10.4 ± 0.89 MJ/d). No treatment effects were observed (P > 0.05) for time spent ruminating or grazing (405 and 366 ± 42.5 min/d, respectively), although mean EE was greater (P < 0.05) for free vs. tethered animals (633 and 512 ± 27.4 kJ/kg of BW^0.75 for free and tethered, respectively), with differences (P < 0.05) between treatments at each hour of the day. Tethering animals may be acceptable to model those with free movement for some measures such as ingesta composition but appears inappropriate for others, such as energy metabolism.

Key Words: energy • goat • grazing • tethering

	INTRODUCTION

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Much nutrition and physiology research has been conducted with confined ruminants because of difficulties in applying some procedures with unrestrained grazing animals. If tethering adequately simulates conditions of free-moving grazing animals, then many basic measures could be made to more fully characterize the physiology of grazing and responses of ruminants to different herbage conditions. An example is use of multiple blood vessel catheters and marker infusion to partition the grazing energy cost into that attributable to metabolism by the gastrointestinal tract and liver vs. peripheral tissues for locomotion.

Effects of tethering on nutrient intake, requirements, and utilization have not been extensively studied, but some reports are available. Moniruzzaman et al. (2002) noted that feed intake and growth of Black Bengal goats grazing for an 8-h period were not affected by tethering. Rumination time was longer for tethered animals, which was attributed to greater selection of herbage relatively high in stems vs. leaves, although nutritive value, level of stem, and herbage mass were not described. Romney et al. (1996) tethered Tanzanian goats in a 19.2-m² area moved daily with 8 h of grazing. Herbage intake and time spent grazing were not different between tethered animals and those with free movement, though grazing by tethered animals gradually decreased as the grazing period progressed and herbage mass declined. Muir and Massaete (1996) and Nguluve and Muir (1999) noted lower ADG by growing animals tethered for 7 h daily with 3-m ropes (access to approximately 28 m²) compared with others having free movement during that same period of time.

The hypothesis of this experiment was that tethered goats would serve as an appropriate model for unrestrained grazing animals. The objectives were to determine how tethering of meat goats influences herbage selection, intake and digestibility, grazing behavior, and energy expenditure (EE).

	MATERIALS AND METHODS

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Paddocks, Animals, and Treatments
The experiment was conducted at the American Institute for Goat Research in April 2005 and was approved by the Langston University Animal Care Committee.

Four 0.72-ha paddocks were used, which in the fall had been fertilized and seeded with 168 kg/ha of wheat (Triticum aestivum) and 22.4 kg/ha of berseem clover (Trifolium alexandrium). Twenty-four Boer x Spanish goats were used (approximately 1.5 yr of age when the experiment began). Sixteen goats were wethers, and 8 were doelings previously fitted with rumen cannulas (5.1-cm i.d.). For 2 wk before the experiment began, goats grazed an adjacent paddock with similar herbage.

The experiment was a crossover, with two 2-wk periods. Period length was minimized to prevent large changes in the herbage conditions within and between periods. Four wethers and 2 doelings were randomly assigned to paddocks, with 2 wethers and 1 doeling randomly chosen for each of the 2 treatments. Treatments were free or unrestrained movement (free) and restraint (tethered). Goats grazed with continuous stocking during the experiment. Based on herbage mass estimates described later, the rate of herbage growth was slightly but not markedly greater than the rate of harvest.

Tethering involved allowing the goats to move only within a circle. A chain that was 3-m long was attached to a metal ring placed around a steel post at one end and to the animal at the other, resulting in a grazing area of 28.3 m². Attachment to the animal was via a collar on the neck for 1 wether and the doeling in each paddock. For the other wether [used for the monitoring of heart rate (HR) and grazing behavior], because of the nature of the equipment, the attachment was to a leather collar made from belts situated around a front leg.

The location of the tethered goats was changed each day. They were placed in a row though not with overlapping areas of paddock access. Each morning at 0700, the tethered goats were moved forward to the next post, and they were not allowed to graze an area that had been previously used for tethering. The area available to each tethered animal was chosen to allow predicted removal of herbage of no more than 30% of that available. For example, with an herbage mass of 2,000 kg/ ha, 50 kg of BW, and DMI of 3% BW, of the 5.66 kg of herbage DM available daily, removal would be 1.5 kg or 26.5%. Tethered goats had access to a small plastic hut or enclosure (0.6 x 1.2 m) for shelter and that was placed at the periphery of the accessible area and was moved each day as well. The enclosure was fitted with 2 containers, one on the inside for a small, trace mineralized salt block and the other on the outside for a water bucket. Free goats had access to a larger enclosure that was in each paddock for shelter, an automatic waterer, and a large, trace mineralized salt block. However, free goats generally grazed near the tethered goats and sometimes shared the small enclosures with the tethered goats. After period 1, the goats were switched to the other treatments and remained in the same paddocks.

Measures
Goats were weighed full or unshrunk at the beginning and end of each period at 1100 h. Shrunk BW was not determined so that period length could be minimized. It was assumed that digesta fill did not markedly differ between treatments. Herbage mass was determined by using 2 methods. The first was a weekly measure by clipping herbage to a height of approximately 1.3 cm in 5 randomly placed 0.25-m² quadrats in each paddock. The second method of addressing herbage mass was to use a disk meter or plate (Bransby et al., 1977) to make an estimate of biomass in the area grazed by the tethered goats. The physical nature of the herbage as influencing the distance between the ground and the disk plate after its release did not seem to vary during the experiment; hence, calibration (establishment of the relationship between disk height and herbage mass) occurred once at the end of period 1, with 10 points. The relationship was strong (R² = 0.947). The disk meter was used daily on 6 mornings before moving the tethered goats on days on which fecal output was determined. Readings were taken at each time in the areas to be grazed the next day or the areas that had been grazed in the previous 24 h, or both. However, postgrazing measures were later omitted from the analyses, because they appeared to have been influenced by the movement of goats in these areas on the preceding day (i.e., trampling).

Each of the 3 free and 3 tethered goats in the paddocks were used for different measures. This was to minimize the length of the experiment to incur relatively small changes in herbage and other environmental conditions. Also, this averted potential effects of 1 measurement procedure on another. With 4 paddocks and 2 periods, there were 8 observations per treatment.

One wether per paddock and treatment was used to determine EE by use of HR. To do so, before the pretrial grazing adaptation period, the 8 goats were placed in a head-box respiration calorimetry system (Sable Systems, Henderson, NV) while consuming ad libitum, coarsely ground alfalfa hay for quantification of oxygen consumption and production of CO₂ and methane. The Brouwer (1965) equation without urinary N excretion was used to predict EE. At the same time, HR was measured with a Polar S610 monitor (Polar, Woodbury, NY). The ratio of EE to HR for each animal was then used to predict EE from HR measured when grazing, which was on d 8 of each period. Use of HR to measure EE by grazing ruminants was recently reviewed by Brosh (2007). The effects of cold temperatures on the EE:HR ratio [comparable to the oxygen pulse discussed by Brosh (2007)] were considered to be minor. This is relevant to the present grazing experiment conducted using paddocks in the spring compared with EE:HR determined under controlled environmental conditions in a building. Furthermore, Puchala et al. (2005, 2007) noted similar EE:HR between forage and mixed concentrate-forage diets, and Berhan et al. (2006) did not observe any effects on EE:HR of various HR, comparable to ones in grazing conditions, achieved by standing and walking on a treadmill at different speeds with or without forage consumption. Goats used for HR measures were also employed to assess grazing behaviors using IGER grazing behavior monitoring system units (Ultrasound Advice, London, UK) over a 24-h period on d 8.

The rumen-cannulated doelings were used to assess herbage selection or diet composition, as described by Lesperance et al. (1960) and Olson (1991). It was assumed that physiological state (i.e., wethers and doelings) did not markedly influence treatment differences. Collections began at approximately 0800, 1200, and 1600 h on d 8, 12, and 10, respectively. Thus, there was approximately 2 d between ingesta collections. First, digesta in the reticulorumen were removed, and warm water was added several times to ensure total evacuation. Goats were allowed to graze for 30 to 60 min, after which time the ingesta were sampled and frozen. All digesta were then returned to the rumen. The second wether of each paddock and treatment was used to determine fecal output. For this, fecal bags were used over a 5-d period (d 8 to 12). Daily aliquots of feces (20%) were used to form composite samples, which were stored frozen.

Laboratory Analyses
Quadrat herbage samples and calibration samples for the disk meter were dried in a forced-air oven at 55°C for 24 h, followed by immediate weighing. Ingesta samples were also dried at 55°C and ground to pass a 1-mm screen. A partial DM concentration in feces was assayed by drying at 55°C, followed by grinding to pass a 1-mm screen. Ground ingesta and feces were analyzed for DM (100°C), ash, Kjeldahl N (AOAC, 1990), NDF, and ADF (filter bag technique; ANKOM Technology Corp., Fairport, NY). Ingesta samples were analyzed for in vitro true DM digestibility (IVTDMD; filter bag technique; ANKOM Technology Corp.), with NDF as the end-point measure. Ruminal fluid for IVTDMD was collected from 3 mature Boer crossbred wethers grazing a grass-based pasture and supplemented with approximately 0.75% BW of a pelleted concentrate containing 20% CP (DM basis). This method (http://www.ankom.com/09_procedures/Daisy%20method.pdf) is similar, except for the end-point measure, to the procedure of Tilley and Terry (1963).

Calculations and Statistical Analysis
Though the effect of ingesta sampling time or day was evaluated as noted below, for calculating DMI, the values were averaged over time. The IVTDMD for ingesta samples was used to determine a calculated in vivo total tract DM digestibility, first by assuming metabolic fecal DM excretion of 11.9% DMI (Van Soest, 1994). However, this adjustment was recommended for cattle and sheep and may not have been evaluated with goats. Also, in the study on which this adjustment was based (Van Soest et al., 1966), considerable variability existed among different forages in the comparison between yield of insoluble residue obtained from the Tilley and Terry (1963) method of second-step acid-pepsin digestion and the neutral detergent method. The 2-stage Tilley and Terry (1963) method results in microbial cell debris that, as a proportion of the initial substrate, is similar to metabolic fecal DM in vivo, therefore not requiring an adjustment (Minson, 1990; Van Soest, 1994). However, the initial adjustment of Van Soest (1994) resulted in lower recovered energy than expected from the change in BW (i.e., negative recovered energy for free and tethered goats despite no observed decrease in BW). Hence, a second method of adjustment was employed, which is of importance only for absolute magnitudes of ME intake (MEI) and recovered energy and does not affect relative differences between treatments. Metabolic fecal CP was assumed to be 2.67% DMI (Moore et al., 2004). Because most metabolic fecal CP in ruminants appears to be of microbial origin (Van Soest, 1994), this value was divided by a bacterial CP concentration of 48.56% (Ørskov, 1992), resulting in an adjustment of 5.5% to convert IVTDMD to a corrected in vivo apparent total tract DM digestion basis. Values averaged over paddock were applied to fecal output estimates for the corresponding treatment and period to estimate DMI. Metabolizable energy intake was estimated from herbage OM concentration and by assuming 19.33 kJ/g of digestible OM intake (NRC, 1981) and a ME concentration of 82% of DE (Garrett et al., 1959). Metabolizable energy intake relative to BW^0.75 of the goats used for fecal output for the 4 treatment-period combinations were applied to those used for EE, with recovered energy determined as the difference between MEI and EE.

Data were analyzed using the MIXED procedure (SAS Inst. Inc., Cary, NC). Most data were analyzed with a model consisting of animal (used for the different measures), period of the crossover, and treatment, with a repeated measure of period and a random effect of goat. To evaluate the temporal patterns of the behavior measures and EE, values were averaged for the 24 one-hour periods. These data were also analyzed with PROC MIXED, with random effects of animal and animal within treatment x period and a repeated measure of period x hour. Diet composition data were analyzed in a similar manner, considering time of ingesta collection.

	RESULTS

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Herbage Mass, Ingesta Composition, and Herbage Intake
Disk meter estimates of herbage mass were similar to final quadrat estimates for period 1 but were lower for period 2 (Table 1). Based on herbage mass by disk meter, tethered goats had 7.5 and 8.4 kg of herbage DM available for grazing in periods 1 and 2, respectively.

View larger version (20K): [in this window] [in a new window]   Figure 1. Effects of tethering (T) compared with free movement (F) on the hourly pattern of time spent ruminating by Boer x Spanish goats. Tethering involved attachment to a 3-m tether for access to an area of 28.3 m2 that was moved daily. In the free movement group, goats grazed unrestrained in 0.72-ha paddocks.

View larger version (21K): [in this window] [in a new window]   Figure 2. Effects of tethering (T) compared with free movement (F) on the hourly pattern of time spent eating by Boer x Spanish goats. Tethering involved attachment to a 3-m tether for access to an area of 28.3 m2 that was moved daily. In the free movement group, goats grazed unrestrained in 0.72-ha paddocks.

View larger version (17K): [in this window] [in a new window]   Figure 3. Effects of tethering (T) compared with free movement (F) on the hourly pattern of energy expenditure (EE) by Boer x Spanish goats. Tethering involved attachment to a 3-m tether for access to an area of 28.3 m2 that was moved daily. In the free movement group, goats grazed unrestrained in 0.72-ha paddocks.

	DISCUSSION

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Grazing Behavior, EE, and RE
Osuji (1974) suggested that the activity cost of grazing is primarily related to the total time spent in the activity rather than rate of consumption, and rate of MEI consumption was similar for free and tethered animals [0.57 vs. 0.55 kJ/(kg of BW x min)]. Conversely, based on different numbers of bites per minute by cattle in 2 experiments, Di Marco et al. (1996) suggested that rate of herbage consumption has a marked effect on the effect of grazing on EE. However, conditions in these 2 studies may have influenced findings. That is, in the first experiment in which EE was considerably greater when grazing vs. resting, during the 1-wk adaptation period, cattle did not graze and were fasted for 24 h before grazing. In the second experiment, in which EE was only slightly greater when grazing than resting, the 1-wk adaptation period included 2 h/d of grazing without fasting before grazing measures.

Greater MEI by free vs. tethered animals would have contributed to the difference in EE because of the effect of metabolic workload on EE by metabolically active splanchnic tissues (Johnson et al., 1990). However, the difference in EE between free and tethered animals (121 kJ/kg of BW^0.75 or 24%) was slightly greater than in MEI (88 kJ/kg of BW^0.75 or 17%). This suggests that the difference between grazing treatments in EE was not simply because of greater MEI. Another factor that would have contributed to the difference is the presumed greater distance traveled by free animals. Although neither distance traveled nor number of steps was assessed in this experiment, with some assumptions, distance traveled can be addressed. Based on the average cost of movement for the 6 walking treatments without herbage ingestion of Berhan et al. [2006; 3.82 J/(kg of BW x m)], the difference in EE between free and tethered animals equates to a predicted distance traveled of 14 km. Given the small size of these paddocks and the high nutritive value of the herbage, this distance traveled is very unlikely. Therefore, factors other than or in addition to distance traveled or metabolic workload probably contributed to greater EE by free than tethered animals.

Speakman and Selman (2003), in a review of physical activity and exercise mainly based on human subjects and some laboratory animals, indicated that exercise increases resting metabolic rate, with a much greater effect on net energy balance than the direct energy cost of exercise. Similarly, Di Marco et al. (1996) noted that EE in cattle estimated by CO₂ production rate remained 32% greater the night after grazing than when in a corral before grazing compared with a 52% difference during grazing. Di Marco and Aello (1998) did not note carryover effects of walking on the EE of cattle. But, the 1-wk adaptation period entailed 2 h daily of grazing, which preceded the 2-d measurement period with different walking treatments subsequent to 2 h of grazing. Effects of free movement while grazing on resting metabolic rate may be attributable to changes in physiological processes influencing resting metabolism and in lean tissue mass. It is difficult to recommend whether such an energy cost for support or capability of activity should be classified as ME_m or partitioned to activity per se. Nonetheless, these findings imply that systems to predict the grazing activity energy cost based on factorial approaches (horizontal and vertical distances traveled, number of position changes, grazing time, etc.) should rely on estimates from animals well-adjusted to such conditions or activities.

Fairly consistent differences in EE between free and tethered animals throughout the day suggest potential to study physiological processes affected by free movement by temporarily restraining grazing animals. Examples of this approach are experiments of Goetsch (1998) and Hersom et al. (2003), in which grazing animals fitted with blood vessel catheters were temporarily restrained and infused with a blood flow marker to characterize energy use by splanchnic tissues. Although, performing such measures at different times of the day would certainly be desirable to ensure that possible variation in contributions of various physiological processes to total EE are adequately depicted.

Results of this experiment indicate that free grazing compared with tethering with daily access to relatively small paddock areas can affect EE throughout the day regardless of whether grazing, ruminating, or idle, which suggests influence of free movement on basal metabolic rate rather than only affecting EE associated with unrestrained movement or being solely attributable to MEI, distance traveled, or grazing time, or all of these. The elevation of EE due to free grazing can be compensated for by increased MEI when mass of herbage of high nutritive value is moderate to high; thus, an advantage of tethering over free grazing of less energy used for activity would be restricted to a greater efficiency of herbage utilization and not a greater level of production. Tethering as a model for free movement may offer a reasonable means of studying factors of foraging behavior such as ingesta composition but appears inappropriate for other ones such as EE and efficiency of energy metabolism.

	Footnotes

 
¹ This project was supported by National Research Initiative Competitive Grant no. 2004-35206-14166 from the USDA Cooperative State Research, Education, and Extension Service.

² Corresponding author: goetsch{at}luresext.edu.

Received for publication October 9, 2007. Accepted for publication January 21, 2008.

	LITERATURE CITED

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