Energy needs of the free-ranging goat

Small Ruminant Research 60 (2005) 111–125

Energy needs of the free-ranging goat夽 M. Lachica ∗ , J.F. Aguilera Animal Nutrition Department, Estaci´on Experimental del Zaid´ın (CSIC), Camino del Jueves s/n, 18100 Armilla, Granada, Spain Available online 24 August 2005

Abstract The new tendency in livestock production systems invites all people involved in ruminant management to pass from intensive to semi-extensive and extensive systems. To accomplish this goal a deep knowledge in several aspects of the animal behaviour and nutrition is needed. The availability of energy is the main limiting factor in animal production for an efficient utilization of resources and for the achievement of acceptable levels of animal performance compatible with resource preservation. This paper deals with the assessment of the energy requirements of goats with particular reference to the free-ranging animal. As a first step, a headline is dedicated to bring the digestion capacity to light with some very particular characteristics in goats. It makes reference to the energy requirements for maintenance and lactation. Attention is paid to the efficiency with which the animal utilizes the available energy of feeds in these processes. Data are given for the energy cost of the two main activities in grazing: eating and walking. Emphasis is placed on the estimation of total energy expenditure of goats in open range including methods (factorial and isotopic) and the information that, to our knowledge, has appeared in the literature. Applications of direct estimations seem a better approach than to use theoretical values to get energy expenditure in grazing animals. Further investigation is needed to develop and improve new and existing techniques to quantify the total energy expenditure of animals in grazing systems. © 2005 Elsevier B.V. All rights reserved. Keywords: Goat; Heat production; Locomotion; Eating; Grazing

1. Introduction In the last decades a great progress has been experienced in knowledge of nutrition and metabolism of the animals which provide food to human beings, particu夽 This paper is part of the special issue entitled Plenary Papers of the 8th International Conference on Goats, Guest Edited by Professor Norman Casey. ∗ Corresponding author. Tel.: +34 958 572757; fax: +34 958 572753. E-mail address: [email protected] (M. Lachica).

0921-4488/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.smallrumres.2005.06.006

larly of the ruminants. Many well-established concepts have been changed. Improvements in our understanding of relevant processes in these fields of animal science, in part as a result of the increased availability of new technical powerful tools, have been incorporated to animal production systems, particularly to those proposed in developed countries. In parallel, a growing interest in goat production has spread over the world, not only due to its perfect adaptation of this animal to semi-extensive or extensive production systems, but also as recognition of its ability to thrive in dry areas with advantage over other ruminants, being able to

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obtain an adequate diet even when forage is scarce and the terrain inaccessible. Goat production fulfils current priorities in agriculture both in developed countries, where objectives are focused to sustainable development, animal welfare, high quality animal products, etc., and in no developed countries, where the day to day survival imposes a goal: an efficient use of available land resources for the achievement of an increased production. Comparatively with cattle and sheep there has been little research on the nutrition and metabolism of the goat. The dearth of relevant scientific and technical support by the end of the eighties was widely recognised and has encouraged the revision of existing information (i.e. AFRC, 1997) or practical assumptions, some of them likely adopted without sufficient scientific support. A considerable amount of work has been done at our laboratory to contribute to improve our state of knowledge in specific areas of the digestive physiology, nutrition and metabolism of the goat where the need for more information was noted. There is little information in the literature concerning energy requirements of the goat, as a result of the aforementioned minor economical importance of goats in developed countries. Most of the recommendations have been derived from other ruminant species (cattle and sheep) even when there is consistent evidence to show that goats seem to have developed unusual physiological features in comparison with other ruminants, as a better ability to use nitrogen (Alam et al., 1985; Domingue et al., 1991) and water (Wahed, 1984; Howe et al., 1988) under stress conditions. That means that these extrapolations may be misleading. Most studies to assess energy requirements have been performed at the laboratory and several techniques to estimate the energy expenditure in open range conditions have been set up (double labelled water (DLW), CO2 -entry rate (CERT), heart rate and factorial method). However, practically nobody has later applied them to assess energy expenditure in the free-ranging animal, so that more accurate estimations of its energy requirements, on which to base pattern of management according to the availability of herbage, could be obtained. In this paper an account is given on the contribution of our laboratory to the assessment of the energy needs of the goat with emphasis in the animal in open range; also it includes the information that, to our knowledge, has

been published about energy expenditure in grazing goats.

2. Digestion capacity Given the dearth of direct comparisons of goats and sheep (the usual model in nutrition research) concerning the digestive physiology comparative studies were made at our laboratory in which low to medium (Isac et al., 1994; Garc´ıa et al., 1994, 1995; Molina Alcaide et al., 1997) or high quality diets (Molina Alcaide et al., 2000) were offered. Partially this home information refers to non-producing animals fed at maintenance level and confined in metabolic crates (Isac et al., 1994; Molina Alcaide et al., 2000) and, therefore, minimizing differences in selective behaviour between animals which might account for unequal digestive efficiencies. In these experiments we failed to find inter-species differences in digestibility, degradation rate and fractional rate of passage of the digesta out of the rumen, when medium or high quality forages were offered. From these trials we concluded that goats and sheep show at stall equal capacities of digestion of medium- to good-quality diets. This would validate extrapolations of feed evaluations, what implies a unique energy value for both goats and sheep. Furthermore, from 102 individual energy balances carried out in adult goats fed on pelleted lucerne hay and barley grain in widely variable relative proportions, at intakes ranging between 0.80 and 3.02 times maintenance level, 32 out of them in castrated males (Prieto et al., 1990) and 70 in lactating animals (Aguilera et al., 1990), the following highly significant (P < 0.001) regression equation was found: MEI (kJ/kg0.75 per day) = 0.836 (±0.0016) DEI (kJ/kg0.75 per day), n = 102,

r = 0.998

where MEI and DEI mean metabolizable energy intake and digestible energy intake, respectively. This equation indicates that in goats energy losses in urine plus methane attained 16.4% DE intake. The regression coefficient 0.836 (CV = ±1.93%) is in the middle of the range of ME/DE values for ruminants (0.81–0.86; AFRC, 1993). Energy losses due to methane produc-

M. Lachica, J.F. Aguilera / Small Ruminant Research 60 (2005) 111–125

tion attained 6.66 kJ/100 kJ GE or 10.32 kJ/100 kJ DE close to values published for sheep and cattle. Methane production in goats could be predicted as a function of the digestibility of the dietary energy (D, %) by the equation: CH4 (kJ/100 kJ GE) = −2.58 + 0.151 (±0.0147)D, n = 32,

r = 0.883,

P < 0.001

Consequently, medium- to high-quality feeds seem to have the same digestible energy value (DE, kJ/g DM) or metabolizable energy value (ME, kJ/g DM) for both goats and sheep, a fact of the most importance for evaluation of energy needs or application of feeding strategies to meet requirements in semi-extensive production systems. However, most of the data from our laboratory were obtained in non-producing animals, wethers and goats, grazing in a single herd low- to medium-quality pastures in semi-arid lands of the South of Spain (Garc´ıa et al., 1994, 1995; Molina Alcaide et al., 1997). In these trials we observed that differences in selective behaviour between animal species could account for unequal digestive efficiencies. These studies were also clear to demonstrate that goats had higher energy intakes than sheep, which could be explained by a faster rate of passage of particles out of the rumen and a systematic tendency for a higher rate of degradation of the material when placed in bags suspended in the rumen. Also, the size of the rumen content (g DM) was found to be larger in goats than in sheep. So, we concluded that while goats seem to have a similar capacity to digest medium- to high-quality forages than sheep, they were able to maintain a larger rumen fill without increasing ruminal distension. This fact would also contribute to increased voluntary intakes with respect to sheep and might be crucial to enable goats to meet their higher energy requirements for maintenance in comparison to sheep (Aguilera et al., 1986, 1991; Prieto et al., 1990).

3. Energy requirements for maintenance and lactation Comparatively with goats, there are a lot of data on energy needs which have been published for farm ruminants based on calorimetry experiments. However, available values from respirometry trials carried out in

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Table 1 The fasting heat production (fasting HP, kJ/kg0.75 per day) of goats measured by calorimetric experiments Physiological state

Fasting HP

References

Various Males Castrated males Castrated males

315 269–326 275 231

AFRC (1997) Haque et al. (1996, 1998) Tolkamp et al. (1994) Tovar-Luna et al. (2003)

goats to determine their fasting heat production (fasting HP) are very scarce. They are shown in Table 1. It has been usually assumed that goats have similar energy requirements for maintenance than sheep and similar energy needs for milk production than cows. While Tolkamp et al. (1994) did not find differences between both species, the values obtained of parallel measures of fasting HP made with adult castrated male goats and wethers using respiration chambers (Aguilera and Prieto, 1986) were 324 and 272 kJ/kg0.75 per day, respectively. This suggests a metabolic rate 20% higher in goats than in sheep (Aguilera et al., 1986; Prieto et al., 1990). In our laboratory two methods were chosen to predict the energy requirements for maintenance according to the physiological state and level of production of the goat: (a) We used two feeding levels (maintenance and fasting) and measured the energy balance to predict the energy intake at zero energy retention, using a linear regression of energy retention (RE) versus MEI: RE (kJ/kg0.75 per day) = −a + b MEI (kJ/kg0.75 per day) where RE = MEI − total HP (kJ/kg0.75 per day); b is km and a is fasting HP (kJ/kg0.75 per day). The values obtained appear in Table 2. (b) A similar approach was used for the assessment of the maintenance energy needs of the lactating goat: energy balance measurements were performed in animals subjected at ME intakes leading to positives ER values. An overall efficiency of use of ME for production (body retention plus milk) of 66.9% was obtained while for MEm a value of 401 kJ/kg0.75 per day was estimated. This value clearly differs from those reported for sheep of

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Table 2 Estimates of energy requirements for maintenance (MEm , kJ/kg0.75 per day), fasting heat production (fasting HP, kJ/kg0.75 per day) and efficiency of utilization of ME for maintenance (km ) of Granadina goats under different physiological states Physiological state

Equation of regression

Fasting HP

km

MEm

Adult castrated males Growing females Lactating females

RE = 0.732MEI − 324 RE = 0.760MEI − 320 RE + YEa = 0.669MEI − 268

324 320 –

0.732 0.760 –

443 421 401

a

YE: milk energy yield (kJ/kg0.75 per day).

similar body weight (318 kJ/kg0.75 per day; ARC, 1980) and for milking cows (510 kJ/kg0.75 per day, Moe et al., 1970; 523 kJ/kg0.75 per day, van der Honing and van Es, 1974). A mean estimate of 422 kJ/kg0.75 per day was adopted for MEm taken from a pool of experimental values (Table 2). Our preferred value equals the average figure calculated by the NRC (424 kJ/kg0.75 per day) and is somewhat lower than the one preferred by the AFRC (438 kJ/kg0.75 per day). It is also in the range of published values on fattening cattle and dry cows ARC (1980) but higher than that reported by the ARC (1980) for sheep, as a result of the higher fasting metabolism observed in goats (Aguilera et al., 1986; Prieto et al., 1990) in comparison with sheep. The efficiency of use of ME for lactation in the absence of change in body-energy stores (kl ) was calculated as assumed by ARC (1980): Energy mobilized from body stores in support of milk synthesis is used with an efficiency of 0.84 while concomitant fat deposition during lactation has an energetic efficiency 0.95 times that of milk secretion. Consequently, we estimated the corrected milk energy yield (YEc ) as milk energy (milk E) + (0.84 × negative ER) + (1.05 × positive ER). Eqs. (1) and (2) were obtained by regressing YEc on MEI, using values from both positive and negative energy retentions or from concomitant positive energy deposition, respectively: YEc (kJ/kg

0.75

which can be derived from biochemical considerations (0.60–0.65). Armstrong and Blaxter (1965) found a value of 0.691. When YE and apparent body ER were both expressed as a percentage of MEI above maintenance (EMp ) assuming for MEm the value of 401 kJ/kg0.75 per day, and then RE was regressed versus YE, as depicted by Armstrong and Blaxter (1965) in their experiments with lactating goats, Eq. (3) was obtained: RE −0.907YE = + 67.6 100 kJ MEp 100 kJ MEp

The intercept of this equation indicates that a change in milk production to zero YE/100 kJ ME given above maintenance would result in an increase of body energy retention of 67.6 kJ. Alternatively, at zero body ER/100 kJ MEp , the secretion of milk energy would be 74.5 kJ/100 kJ MEp . Also, Eq. (3) indicates that the efficiency of fat deposition during lactation is 0.907 times that of milk synthesis, and, therefore, that the energetic efficiency of fat deposition is higher in the lactating animal. The ARC (1980) has assumed an efficiency of 0.95 times that for YE. On the other hand, the energy in goat milk can be estimated with high precision as a function of the nutrients concentration and its corresponding energy value according with this equation: EL (kJ/kg) = 38.00F + 24.44CP + 16.45L, CV = ±2.1%

per day)

= 0.667MEI (kJ/kg

per day) − 258

(1)

= 0.657MEI (kJ/kg0.75 per day) − 237

(2)

0.75

YEc (kJ/kg0.75 per day)

From these equations efficiency values for the use of ME for lactation (kl ) of 0.667 and 0.657 are obtained. These figures are slightly higher than those

(3)

(4)

where F, CP and L are the concentration of fat, crude protein (total N × 6.38) and lactose, respectively, as g/kg. From the practical point of view, when it is not available the equipment for infrared reflectance analysis, the determination of some particular nutrient can be awkward. For these cases it could be very useful the Eq. (5) constructed from the results of 17 average milk samples of Granadina goats. It allows the calculation

M. Lachica, J.F. Aguilera / Small Ruminant Research 60 (2005) 111–125

of milk energy as a function of the dry extract (DEx, g/kg), with a CV equal than that of Eq. (4): EL (kJ/kg) = 25.69DEx − 164.8,

r = 0.882,

CV = ±2.1%

(5)

None of the isolated nutrients gave a good estimation of the total energy content in milk. The samples came from Granadina goats in the third month of their second lactation (Aguilera et al., 1984).

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production, HP) by the grazing animal, once partial energy expenditure of each of the activities it performed is known. The energy cost of each activity is quantified by calorimetry, and then the total extra energy daily expended is calculated by summation. Walking and eating are the main contributors to the increased muscular effort in the grazing goat in comparison with the confined one, so an accurate assessment of their energy costs and a reliable methodology for accurate recording under field conditions are of utmost importance to avoid unsatisfactory estimations of the total energy needs of the free-ranging goat.

4. Energy requirements for physical activities 4.1. Locomotion Detailed accounts of this subject have been recently given by us (Lachica and Aguilera, 2003, 2005) so it is dealt with briefly here. The factorial method is widely accepted to estimate the total energy expenditure (heat

The energy cost of locomotion was relatively well defined in domestic sheep and cattle (Table 3), although most of the available data refer to the locomotion on

Table 3 Net energy cost (J/(kg BW m)) for horizontal and vertical locomotion on ascent and descenta and apparent energetic efficiencyb (%) of walking in various ruminant species Species

Gradient (◦ )

Energy cost

Domestic sheep

0 2.7 5.1 0 N/A 0 3 9 0

2.47

0 0 6 0 5 10 6 0 0 0 0

2.00 2.09

0 0 2.86/5.71 −2.86/−5.71

3.63 3.35

Horizontal

Cattle

Domestic goat

a

Efficiency

Reference

Vertical 25.4 27.0

39 36

32.0

31

27.7 37.7

35 26

2.83 2.30

4.65

26.0

38

24.0 30.3 26.6

41 32 37

1.54

1.91 0.63 1.47 0.38

31.7 13.2

31 135

Clapperton (1964) Clapperton (1964) Clapperton (1964) Farrell et al. (1972) Farrell et al. (1972) Brockway and Boyne (1980) Brockway and Boyne (1980) Brockway and Boyne (1980) Taylor et al. (1982) Brody (1945) Ribeiro et al. (1977) Ribeiro et al. (1977) Shibata et al. (1981) Shibata et al. (1981) Shibata et al. (1981) Thomas and Pearson (1986) Lawrence and Stibbards (1990) Mendez et al. (1996) Dijkman and Lawrence (1997) Di Marco and Aello (1998) Taylor et al. (1974) Lachica et al. (1997a) Lachica et al. (1997a) Lachica et al. (1997a)

Energy recovered. Efficiency of vertical ascent calculated as the ratio of 9.81 J to the energy cost of doing this work, and that of vertical descent as the ratio of the energy recovered to 9.81 J. b

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the level and very few studies have investigated the energetic efficiencies associated with vertical ascent or descent. The situation has been even more unsatisfactory concerning the goat, as to our knowledge, Taylor et al. (1974) published the single paper in which a value for goats was reported. Lachica et al. (1997a) determined the HP of Granadina goats while standing at rest on each of five different slopes (−10, −5, 0, 5 and 10%), and while walking at each treatment combination of speed (10, 20 and 30 m/min) and slope (−10, −5, 0, 5 and 10%) on a treadmill inside a confinement-type respiration chamber (Lachica et al., 1995). O2 consumption and CO2 - and CH4 production was used to calculate HP according to Brouwer (1965). Within slopes the energy costs of locomotion (ECw, J/(kg BW m)) were estimated from the coefficient of linear regressions of HP (J/(kg BW h)) on distance travelled (Dt , m). Average values over each slope were 1.91, 2.33, 3.35, 4.68 and 6.44 J/(kg BW m) for −10, −5, 0, 5 and 10%, respectively. Then an exponential relationship was constructed, which allows the calculation of the energy cost of locomotion (ECw , J/(kg BW m)) as a function of slope (Sl, %): ECw = 3.39 e0.063 Sl

(6)

The intercepts of the regression equations mentioned above are estimates of the goats’ metabolic rate standing at rest. Their overall mean value was 6332 J/(kg BW h), equivalent to a daily HP of 405 kJ/kg0.75 . Our estimate of 3.35 J/(kg BW m) for the net energy cost of locomotion on the level in Granadina goats is somewhat lower than that of 3.63 J/(kg BW m) which can be calculated from the data reported by Taylor et al. (1974). Mean values of the energy cost of walking were calculated by separation of the horizontal (Dh ) and vertical (Du and Dd ) components by multiple regression equation of HP (J/(kg BW h)) on the horizontal and vertical distances travelled (m) in ascent or descent: HP = 6724 + 3.31Dh + 31.7Du − 13.2Dd

(7)

The regression coefficients of Dh , Du and Dd in Eq. (7) indicate values for the net energy cost (J/(kg BW m)) of horizontal (ECh ) and vertical locomotion on ascent (ECu ) and on descent (ECd ), respectively. The net

energy cost of upslope locomotion is higher than that for moving on the level due to the energy expended to work against gravity. During downslope travel the situation is just the opposite, potential energy is recovered as kinetic energy and, in our trials, the energy expenditure was lower than on the horizontal, leading to a decrease in energy expenditure relative to the horizontal costs. The efficiency of upslope locomotion, calculated as the ratio of work done to the energy cost of doing it and expressed as a percentage, averaged 30.9%. The ARC (1980) publishes a preferred value of 28 J/(kg BW m) for the energy cost of the vertical movement for both sheep and cattle, equivalent to an energetic efficiency of 35%, but our results suggest that the efficiency of locomotion on positive slopes appear to decline as the gradient increases (see Table 3). The information available on the energy cost of downslope locomotion is controversial and is not our intention to get in discussion about the so-called “negative work”. Instead, we just will make some comments. The ARC (1980) states that there is a lack of available data on this cost of descent in ruminant animals and assumes that it is similar to that of walking on the level. It is believed that downslope movements are less expensive because gravitational energy is recovered as kinetic energy during descent. We observed that the average amount of energy recovered when moving downslope, calculated by Eq. (7) was 13.2 J/(kg BW m). Consequently, the energetic efficiency of this activity, calculated as the ratio of energy recovered to energy stored and expressed as a percentage, was found to be 135% and, therefore, unacceptable. As discussed earlier (Lachica and Aguilera, 2003, 2005), our results indicated that the amount of energy recovered during downslope locomotion exceeds the maximum transfer in potential energy per meter vertical movement, being impossible to conceive a satisfactory explanation in terms of saving of negative work. Consequently, the coefficient of the vertical component (Dd ) in the regression Eq. (7) is only an index of likely energetic saving effect of negative work and has no other meaning, such as the recovery of gravitational potential energy. 4.2. Eating Prehension, mastication and propulsion of food in the alimentary tract involve muscular activities which have a significant contribution to the energy expendi-

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Table 4 Comparison of estimates of rates of ingestion (g DM/min) and energy cost of eating in ruminants Species

Feed or diet

Rate of ingestion

Energy cost of eating J/(kg BW g DM)

J/(kg BW min)

As a percent of the MEI

Cattle (mean LW: 353 kg)a

Fresh cut forage Long, dried forage Chopped, dried forage Pelleted food and grain

20–25 17–37 39 130–138

1.95 (1.36–2.44) 1.48 (1.04–2.21) 0.79 0.23 (0.22–0.24)

40.2 (36.3–44.0) 32.5 (27.8–45.3) 27.6 23.8 (19.4–28.9)

5.9 4.9 2.5 1.0

Cattle (mean LW: 617 kg)b

Fresh cut forage Long, dried forage Chopped, dried forage Pelleted food and grain

38 46 29–39 119

0.85 0.55 0.46 (0.45–0.52) 0.13

31.3 24.4 16.3 (11.9–20.2) 15.6

5.0 3.2 4.4 0.6

Sheep (mean LW: 52 kg)c

Fresh cut forage Long, dried forage Chopped, dried forage Pelleted food and grain

4–7 8–9 4–14 8–58

5.12 (3.09–6.20) 6.95 4.01 (1.91–7.95) 1.07 (0.30–2.51)

36.2 (29.3–46.4) 54.9 (45.8–61.5) 38.1 (33.8–43.1) 36.6 (18.4–63.4)

2.6 3.9 2.5 0.4

Sheep (mean LW: 27 kg)d

Long, dried forage

2

10.9 (9.8–11.9)

Goat (mean LW: 35 kg)e

Fresh cut forage Long, dried forage Chopped, dried forage Pelleted food and grain

7 8 8–15 46–99

7.03 12.04 6.50 (4.77–8.23) 1.80 (1.44–2.28)

a b c d e

19.3 (19.0–19.5)

2.9

44.3 97.1 66.7 (63.8–69.5) 118.3 (75.7–143.6)

3.2 4.7 3.1 0.7

Adam et al. (1984). Susenbeth et al. (2004). Osuji et al. (1975). Suzuki et al. (2003). Lachica et al. (1997b).

ture in the free-ranging animal. The amount of food eaten, the length of time spent feeding and the nature and physical form of the feed consumed are important factors influencing the amount of energy required to do this work. Prior to our experiments (Lachica et al., 1997b) no information had been published concerning the energy cost of eating in the goat. The available data referred to cattle and sheep (Table 4). Table 5 shows both the rate of ingestion and the energy cost of eating in goats offered seven feeds of different nature and physical form. The animals were individually placed in a confinement-type respiration chamber and their heat production was determined before and during eating each one of seven feeds assayed. The energy cost of eating was calculated from the increment in HP between both periods and related to the type and amount of feed consumed as well as to the time spent eating. The nature and physical form of the diet have a deep influence on the rate of ingestion, as previously found in cattle and sheep (see Table 4), thereby affecting the energy cost of this activ-

ity (Table 5). The cost of eating was much lower for concentrates than for forages. Within forages it varied with their physical form. When expressed as a function of the time spent eating, the energy cost of this activity ranged according to the physical structure of the feed. A higher cost of eating was found to be related to a lower rate of ingestion and vice versa, as found by Osuji (1971) in sheep. The rate of ingestion (R, g DM/min) was negatively correlated to both the ADF and ADL concentrations (g/100 g ss): R = 105.9 − 2.4ADF ln R = 4.84 − 1.03ADL Also, significant relationships between the energy cost of eating (CE, J/g DM) and the rate of ingestion (R, g DM/min) or the concentration of ADF or ADL (g/100 g DM) were also established: ln CE = 6.78 − 0.40 ln R

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Table 5 Rate of ingestion and energy cost of eating in goats offered feeds which differ in physical structure Feed and physical form Barley grain Rate of ingestion (g DM/(kg BW min)) Energy cost of eating J/(kg BW g DM) J/(kg BW min) J/g DOMa Percent MEb a b

Field bean grain

Pelleted lucerne hay

Chopped lucerne hay

Vetch straw

Olive leaves and twigs

Fresh cut lucerne

2.64

1.21

1.59

0.38

0.20

0.24

0.15

1.45 143.8 63.6 0.4

1.65 75.8 78.0 0.5

2.24 135.9 179.6 1.2

4.75 69.8 348.8 2.2

8.20 63.9 606.2 3.9

11.78 97.8 735.2 4.7

7.08 44.6 492.2 3.2

DOM: digestible organic matter. Calculated from the chemical composition of the feed and the digestibility of organic matter.

ln CE = 3.40 + 0.054ADF ln CE = 3.63 + 0.201ADL Expressed as J/(kg BW min) our estimates of the energy cost of eating are higher than those published for cattle and sheep (see Table 4). However, no inter-species differences were observed when comparisons are established in terms of J/g DM ingested or as kJ/100 kJ ME ingested (see Table 4). In conclusion, our results suggest that in the goat the energy cost of eating depends of the type of food consumed and accounts from 1 to 5% of the intake of ME. The energy cost of eating cannot be neglected in the free-ranging animal. Extrapolation of data obtained in confined animals to grazing animals may be misleading. Osuji (1974) with sheep and Webster (1978) with cattle estimated an increase in HP with respect to indoor animals of 17 and 5–9.3%, respectively. Prieto et al. (1992) observed in free-ranging goats that the annual mean increase in energy expenditure above maintenance due to the act of feeding attained 8.8% of their total heat production.

5. Energy expenditure in grazing conditions Energy expenditure is much higher in open range than in confined animals. In the fields, animals spend more time walking, eating and foraging for food, therefore increasing their energy requirements several folds. Such increase can be even greater in arid lands where the ruminants have to travel long distances look-

ing for food and water, implying a big deal for the animals. To quantify the total energy expenditure (heat production) of free-ranging animal is not at all an easy task. Several methods have been applied aiming at this objective. Still the available information is rather meagre. In Table 6 a reference is given to the methods that have been used to estimate the energy expenditure of the grazing goat.

5.1. Factorial method It can be applied whenever the energy costs of various activities are known. The total energy expenditure by grazing animal equals a polynomic function, each term of which is the energy cost of each activity multiplied by the total time spent on it. It should be noted that the increase in heat production (HP) of the animal in open range as a result of its increased physical activity with respect to the confined animal must be met with the available energy provided by food resources (ME) and is considered as a component of the total energy transfer related to the maintenance of body function in the absent of any net gain or loss in tissue. The efficiency of utilization of ME for maintenance (km ) is accepted to be close to 0.70. That means that the increment in the animal’s energy requirements due to its physical activity will be given by the expression
ME/
HP/0.70. Seasonal variations in grazing activities might affect the distance travelled by the free-ranging goat and therefore its productive performance. The results from two surveys carried out in two different areas of the

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Table 6 The energy requirements for maintenance (MEm ) in ruminants. Comparison between estimates made with animals housed indoors and those of similar animals at pasture MEm (MJ per day)

Increase (%)

References

Indoors

At pasture

Cattle – 50.6 50.6 50.6 50.6 49.3–50.5

– 50.6 77.8 88.3 77.0–104.6 99.9–125.3

15.0 0 53.8 74.4 52.2–106.7 102.6–148.1

Blaxter (1967) a Corbett et al. (1961) a Reid (1958) a Wallace (1955) a Hutton (1962) a Ca˜nas et al. (2003) b

Sheep – 6.7 5.9 5.9 – 5.3 10.2–10.6

– 8.4 9.6–11.3 8.8 – 7.0 15.0–15.9

11.0 25.4 62.7–91.5 49.2 60.0–70.0 32.1 41.5–55.9

Blaxter (1967) a Langlands et al. (1963) a Coop and Hill (1962) a Lambourne and Reardon (1963) a Young and Corbett (1972) c Osuji (1974) d Ca˜nas et al. (2003) b

Goat – 6.3 6.3 6.3 7.7

– 7.0 7.9 9.1 9.4–11.7

25–50–75 11.1 25.4 44.4 22.1–51.9

NRC (1981) e Lachica et al. (1997c) f Lachica et al. (1999) f Lachica et al. (unpublished data)g Lachica et al. (2003) g

a b c d e f g

Estimates of feed intake for constant live weight. Calculated as: MEgrazing = MEI − (MEm + MEp ). Direct measurements at pasture (CERT and MIC). Calculated from the energy cost of different activities and their duration at pasture. Maintenance plus low, medium or high level of activity. Taking into account the extra costs associated to locomotion at pasture. Direct measurements at pasture (CERT-13 C).

semi-arid lands of southern Spain along the year are well illustrative at this respect (Lachica et al., 1997c, 1999). The areas were rather similar as far as geographical location, topographic and climatic characteristics, but differences in vegetation communities were outstanding. The hypothesis was that differences in available food would have an important effect on grazing activities and on the animals’ behaviour. In both areas the topography was rugged and the experimental flock had a type of goat management considered as semiextensive. The total distance walked was measured by direct observation (using pedometers), and also other grazing activities. Vertical ascent or descent, by the use of altimeters. The energy expenditure of locomotion was calculated from both the horizontal and vertical (ascent and descent) components of each goat’s travel

and the corresponding energy cost and energy recovery according to Eq. (7) (Lachica et al., 1997a). The calculated energy expenditure was compared with an assumed value of MEm for the confined lactating goat (401 kJ/kg0.75 per day; Aguilera et al., 1990). Increases in heat production above maintenance of 10.8 and 26.3% were found as annual mean for the two areas under study. Our observations are in the range of values obtained by other authors (Table 6). In our survey the average value for the increase in energy expenditure over maintenance found in one of the areas was 2.4 times higher than in the other. This fact indicates that the extrapolation of theoretical allowances for physical activity even to apparently similar conditions may lead to big errors in the estimation of the animal’s total energy expenditure. At this respect it should be men-

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tioned that the NRC (1981) considers that the increased muscular activity of the goat under grazing conditions accounts for a 25% increment in the case of light activity; a 50% increment on semiarid rangeland pasture and on slightly hilly land; and a 75% increment in case of long-distance travel on sparsely vegetated grassland or on mountainous transhumance pasture. The factorial method appears to be adequate to simulate and quantify grazing activities of goats on open range by means of direct observation. It is simple and easy to apply to field conditions, requiring only one observer for locomotion studies. At this point we would like to draw attention to the spectacular development of computing, electronic and telemetry systems. From the pioneer studies of Penning (1983) in sheep, developing a chewing recording, or those of Roberts et al. (1995), using the Global Position System (GPS) with an unusual approach to post-processed differential GPS (DGPS) for monitoring the positions of sheep at 1 min intervals with an accuracy of 5 m, to the actual situation where such devices are available from some commercial companies (ruminating monitors and GPS collars) not to many years have passed but the currently technology has qualified the factorial method as a very powerful tool to estimate energy expenditure in open-range since right now. In fact, nowadays the position can be determined with an accuracy lower than 2–3 m for Wide Augmentation Area SystemGPS (WAAS-GPS, for U.S.) or European Geostationary Navigation Overlay Service-GPS (EGNOS-GPS, for Europe). 5.2. CO2 entry rate technique (CERT) A detailed account on the application of this technique to the assessment of total heat production in the free leaving animal has been reported in this journal (Lachica and Aguilera, 2003) and the reader is kindly addressed there. Several methods have been proposed to estimate carbon dioxide (CO2 ) production and subsequently to assess total energy expenditure of animals in free living conditions. A popular approach that relies on the measurement of the apparent entry rate of CO2 into the body pools using H13 CO3 − as a tracer, is the so-called carbon dioxide entry rate (CER) technique. This approach, among other isotopic methods, offers the possibility of relatively simple analytical procedures and lower cost in terms of isotope and mass

spectrometry facilities. It requires a constant infusion of labelled bicarbonate and frequent collection of body fluids to determine the dilution of the isotope at equilibrium. When the animal is in steady state, a few samples of body CO2 should yield a satisfactory estimate of entry rate. When it is expected that entry rate does not markedly vary from the daily mean, an estimate of mean rate could be obtained from a sample obtained by continuous collection or from a number of samples taken at appropriate intervals. Most of kinetic studies in the past decades used radioactive tracers. Since 1970s an increased awareness of the biohazards of radioactivity, as well as a greater availability of stable isotopes and development of mass spectrometry techniques, stimulated the use of stable, non-radioactive, isotopes. However, as stable isotopes are naturally occurring in the body pool it is necessary to determine their natural abundance before the start of the tracer infusion. In a broad sense, two main problems arise when 13 C-labelled tracers are used: First, carbon 13 naturally contributes approximately 1.1% of the carbon pool. However, plants which fix CO2 by way of the Calvin C3 cycle pathway differ in natural abundance from plants which fix CO2 through the C4 -dicarboxylic acid pathway (Jones et al., 1979). The intake of C4 plants will significantly increase breath 13 CO2 above natural abundance to easily detectable levels. Furthermore, such isotopic fractionation also occurs within components of animal tissue. Specifically, differences in natural abundance of 13 C in fat, carbohydrate and protein if different metabolic fuels are used during various physiological and nutritional conditions will alter the enrichment in H13 CO3 − produced. The variation in natural abundance of 13 C must be taken into account in the design and subsequent analyses involved in the tracer study; and concerning the second problem it must say that a serious drawback of CER technique concerns the sequestration of labelled carbon in body metabolites which prevents quantitative recovery of the isotope and results in overestimation of CO2 production. No consistent data exist to document where the label is lost. It is claimed that fixation of CO2 may occur in biosynthetic pathways, e.g. formation of urea, in other reactions of the TCA cycle, especially those catalysed by pyruvate carboxylase and malate dehydrogenase, or diverted into slow-turnover pools, i.e. bone carbonate (Elia et al., 1988; Rocha et al., 1994).

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It seems obvious that a detailed evaluation of all factors which may induce changes in either natural abundance or isotope sequestration is needed so these techniques can be accurately used in studies on the energy metabolism of unrestrained animals. Prieto et al. (2001) determined in goats the effect of exercise on natural abundance of 13 C. Values of natural abundance varied with a change in physical activity. These values (at.% 13 C) were as average 1.09656 and 1.09363 for the standing at rest and some intermittent exercise conditions, respectively. Other factors, such as level of feed intake and cold exposure, have also been studied (Lachica et al., unpublished data). It was observed a significant decrease of 0.00147 in atoms percent excess (APE, %) in the natural abundance of 13 C after a three days fasting in comparison with that found in samples taken from goats fed ad libitum. A significant change in background enrichment was also noticed as a result of cold exposure (shearing versus no shearing). This change was associated with a shift in relative rates of oxidation of metabolic substrates for thermogenesis. The observed values were 0.00158 lower in APE for the sheared versus unsheared goat. This indicates a preferential oxidation of fat and liberation of CO2 of lower 13 C content. Changes in natural abundance should be taken into account to avoid bias when calculating 13 C enrichment in animals subjected to a variable work load or stress, particularly if the levels of enrichment expected after H13 CO3 − infusion are small. The effect in goats of a mild physical activity – imposed by intermittent locomotion in a treadmill enclosed in an open-circuit respiration chamber – on the apparent recovery of 13 CO2 and the subsequent estimation of the energy expenditure of the animal was addressed in a survey carried out in our department with the objective of validating this technique for application to free living conditions (Prieto et al., 1997). Preliminary results were revised (Prieto et al., 2001). The goats were continuously infused with the NaH13 CO3 solution via the jugular vein catheter since 15 h before starting saliva sampling from the parotid gland (time required to reach isotopic equilibrium was estimated to be 8–16 h (Prieto et al., 1991; Sahlu et al., 1992; Lachica, 1993; Herselman et al., 1998) when saliva samples are taken). The CO2 production was recorded simultaneously. There was a significant relationship between the observed values for CO2 (mL/kg BW h) production determined simultaneously by respirom-

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etry (CO2r ) and those estimated by CERT (CO2cert ) when the goats were standing at rest and doing intermittent exercise (CO2r /CO2cert = 0.72 and 0.78, respectively). Therefore, in our trials values of CO2 entry rate exceeded the observed values for CO2 production determined by respirometry, indicating the sequestration of labelled carbon into body metabolites occurred and that dilution of 13 C as a result of the moderate increase in CO2 production due to the mild exercise tends to reduce the fixation of the labelled isotope in pools which turn over slowly, thus favouring a free exchange at the sampling sites. An increased exercise may also lead to enhanced release of 13 C previously sequestered in metabolites which are then used as oxidative substrates. Therefore, the CO2 entry rate value must be corrected by the apparent recovery of 13 CO to achieve the real value of CO production 2 2 (CO2 = CER value × apparent recovery of 13 CO2 ). In these trials, average total HP was estimated according to Eq. (8) and attained 7736 and 8429 J/(kg BW h) for the goats standing at rest and subjected to intermittent exercise, respectively. The CO2 production, which has been estimated from CER values and mean recovery, can now be related to the animal’s energy expenditure. From own results, based in respiration trials (Aguilera et al., 1990; Lachica, 1993) we concluded that the following regression equations could be used to calculate total HP from the CO2 produced by the unrestricted goat: lactating goats: HP (J/(kg BW h)) = 19.82CO2 (mL/(kg BW h)),

RQ = 1.0

(8)

non-lactating goats: HP (J/(kg BW h)) = 24.07CO2 (mL/(kg BW h)),

RQ = 0.87

(9)

Over the base of this previous and necessary research at the laboratory, the CER technique was used to estimate the effects of (a) exposure to low environmental temperatures; (b) grazing (Lachica et al., unpublished data) and (c) stocking rate on the energy expenditure of goats (Lachica et al., 2003). The effect of cold exposure was enhanced by shearing cashmere goats. Heat produced specifically to maintain body temperature originates from fat catabolism and must be regarded as a part of the energy requirements for main-

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tenance. The goats (non-lactating females) were fed at maintenance level (401 kJ ME/kg0.75 per day; Aguilera et al., 1990). It was assumed that cold exposure would likely exert on the animal metabolic rate an effect similar to a soft physical exercise and, consequently, for the estimation of the CO2 production of the unshorn and shorn goats we applied the 13 CO2 recovery values obtained by Prieto et al. (2001) for goats standing at rest (72%) and on intermittent exercise (78%), respectively. The averaged HP attained in the unshorn goat 395 kJ/kg0.75 per day, a value in close agreement with that assumed for the maintenance energy requirement (401 kJ/kg0.75 per day). This fact can be considered as an index of the validity of the method. Shearing caused an average increase in HP of 85%, being therefore a very important factor affecting the energy requirements of the goat. The measurement of the apparent entry rate of CO2 into the body pools using H13 CO3 − as a tracer has been successfully applied to estimate the heat production of goats in grazing conditions. The impact of grazing activity per se and of stocking rate was studied in two different trials. Castrated male goats were implanted one catheter in the parotid duct for saliva sampling and another one in the jugular vein for continuous infusion of NaH13 CO3 . They wore a harness housing an infusion pump and bags containing the NaH13 CO3 infusate and for a continuous saliva sampling. The goats were placed on a paddock of indiangrass (Sorghastrum nutans). Every 10 min from dawn to dusk direct observation of their behaviour took place, classified as (1) standing versus lying and (2) grazing, ruminating, idle, or other activities. Just before sunrise and after sunset, the goats were penned individually in the paddock. 13 CO2 recovery values of 78 and 72% were assumed while the goats were grazing and individually penned in the paddock, respectively. In the restrained goat daily heat production attained 401 kJ/kg0.75 and increased a 43% in the unrestrained goat. The daily distance travelled by the goats on grazing was not higher than 2 km, and therefore could account for no more than 9% of the extra HP. Consequently, most the extra energy expenditure was due to grazing activity per se. The effect of stocking rate was also studied. Four castrated male goats were fitted with a harness and two catheters as has been described earlier. Two paddocks of one hectare each with cheatgrass (Bromus tectorum) as unique pasture source were used. Twenty goats

were grazing in one of them. The four experimental goats were placed in it and once the experimental protocol was completed the experimental animals were moved to the other paddock. The total available ME was 12,475 and 49,368 MJ/ha for the paddocks with low and high stocking rate, respectively. Heat production, estimated by the CER technique, was significantly higher with the greater stocking rate, accounting for 672 and 543 kJ/kg0.75 per day for low and high pasture availability, respectively; this represents increases over maintenance HP (443 kJ/kg0.75 per day; Prieto et al., 1990) of 52 and 23%, respectively, even although the pasture availability in both paddocks was very high. This means that the stoking rate per se may increase the energy requirements of animals in open range, a fact which is in support of the finding of Ca˜nas et al. (2003) stating that the grazing system could maximize the animal product only when the energy availability in the pasture is equal to, or greater than a certain available energy level, fixed at 18,000 MJ/ha. Lower energy availabilities result in increases in the energy dissipation of the system due to stress. Consequently, in order to return to the level of production required, there is a need to introduce exogenous energy, in the form of feed supplementation.

6. General conclusion There are few reports available on the energy requirements of goats. Energy requirements in open range may increase several fold over those assumed for restrained animals. The social concern about maintenance and renovation of natural resources (sustainable development), and about animal welfare is growing up more and more, together with a renaissance of semiand extensive systems of animal production. In this context, more effort on the assessment of the energy requirements of the unrestricted animal has to be done. Further research is needed to develop and improve new and existing techniques for application to animals in open range to accurately assess their energy requirements in free leaving conditions. This research effort appears vital to get some additional profit in livestock production. Data from own research suggest that the use of tabulated values or the extrapolation of theoretical allowances obtained for other animal species may

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result in a great bias when predicting the energy needs of free-ranging goats in specific situations. Consequently, the application of a direct estimation approach should be encouraged whenever an accurate prediction of the additional expenditure of energy under grazing conditions is required with definite advantages over the use of tabulated values.

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