SmaU Ruminant Research, 12 (1993) 45-60 © 1993 Elsevier Science Publishers B.V. All rights reserved. 0921-4488/93/$06.00

45

Comparison of rate of passage, fermentation rate and efficiency of digestion of high fiber diet in

desert Bedouin goats compared to Swiss Saanen Goats

N. Silanikove a, H. Tagar i b and A. Shkoln ik a aDepartment of Zoology, University of Tel-Aviv, Tel-Aviv, Israel

bDepartment of Animal Science, The Hebrew University of Jerusalem, Faculty of Agriculture, Rehovot, Israel

(Accepted 11 January 1993)

ABSTRACT

Fermentation and passage rate in the rumen and along the entire gut were measured in black Be- douin and Swiss Saanen goats fed a high fiber diet (Rhodes grass (Chloris guyana) supplemented with 10% alfalfa hay). The purpose of the study was to clarify the physiological basis that underlies advantages of Bedouin goats over non-desert goats in digesting high fiber based diets. Volume of fluid and rumen digesta particle distribution were similar for both breeds. Ability of Bedouin goats to con- sume amounts of hay equal to those consumed by Saanen goats, despite considerably longer retention time in the rumen, was related to their ability to maintain a higher digestion rate in the rumen. Micro- bial activity in the particulate fraction was higher in Bedouin goats than in Saanen goats, and this was reflected in total rumen fermentation, i.e., in significantly higher volatile fatty acid production rate and significantly higher volatile fatty acid concentrations. The ability of Bedouin goats to maintain a high fermentation rate in the rumen may be partially related to their ability to control rumen pH above levels which cause severe depression in rate of fermentation. The combination of higher fer- mentation rate and slower passage allows maximization of feed intake and digestibility in a given situation, as compared to non-desert ruminants.

Key words: Fermentation; In vitro; Gas production; Volatile fatty acids; Passage rate; Digestion of roughage; Desert goat

I N T R O D U C T I O N

Fermentation rate and retention time of digesta in the rumen are two major factors that determine digestive efficiency of roughage in ruminants (Van Soest, 1982). Increased retention time of digesta often results in an increase in gut fill and hence in reduced voluntary feed intake (Hungate, 1966; Van

Correspondence to: N. Silanikove, Institute of Animal Science, Agriculture Research Organiza- tion, P.O. Box 6, Bet-Dagan 50 250, Israel.

46 N. SILANIKOVE ET AL

Soest, 1982). Feed comminuted by mastication or by mechanical grinding of roughage affects digestive efficiency via enhancement of passage of undig- ested feed particles. Comparative studies have shown that extreme variation exists between breeds, regarding these relationships. Tropical cattle can main- tain a high intake level of high fibrous feeds compared with temperate breeds, and still digest it relatively efficiently (Hungate, 1966; Hunter and Siebert, 1985 ). The voluntary intake of desert-adapted Bedouin goats (BG) was less affected on a high fiber diet, and higher feed intake did not decrease digesti- bility (Silanikove, 1986a). This was not the case in Swiss Saanen (SG) goats. The extent of DM and structural carbohydrate digestibility in BG fed low quality roughage was considerably higher than in other domesticated rumi- nants (Silanikove et al., 1980; Silanikove, 1986a,b; Silanikove and Brosh, 1989).

The present study was conducted to investigate differences between BG and SG in digesta contents and kinetics, fermentation rate in the rumen, and relation of these factors to the efficiency of feed digestion. Factors that might affect the efficiency of fermentation such as diurnal changes in rumen volume and kinetics, pH and volatile fatty acid (VFA) absorption were also compared.

MATERIALS AND METHODS

Animals, feeds and feeding Four BG and four SG were used. The BG were part of a herd purchased

from nomadic Bedouins in the eastern Sinai peninsula and maintained at the Canadian Centre for Ecological Zoology at Tel-Aviv University. Goats were 3 years old, and BW averaged 19.5 kg (SD+2.2 kg) and 49.1 kg (SD_4.4 kg) for BG and SG, respectively. Non-pregnant and non-lactating, healthy adult females were fitted with rumen cannulae ( 15 cm diameter). Goats were maintained indoors in metabolic cages at a temperature of 25°C and 60% relative humidity throughout the experiment, and fed a Rhodes grass ( (Chloris guyana) approx. 90% Rhodes grass and 10% alfalfa of medium quality (Sil- anikove, 1986a)). Feed was given in small excess to enable near ad-lib con- sumption without diet selection once daily at 10:00 h. Refusals ( 1% to 5% of total intake) were collected daily and analyzed. Orts were collected daily prior to feeding. Water was given once daily in the afternoon, removed immedi- ately after drinking to satiation, and the amount consumed was recorded. Goats were allowed 1 month to adjust to the diet. Chemical composition of the hay consumed was (% of DM): 90.1% DM, 11.2% CP, 63.5% cell wall constituents, 37.7% ADF, 6.7% lignin and 7.7% ash (Silanikove, 1986a), and was similar for both breeds.

EFFICIENCY OF DIGESTION IN GOATS 47

Particulate passage rate Estimates of the passage rate of particulate matter in the rumen and along

the entire gut were obtained by using 5~Cr-mordanted particles prepared as follows: parts of the feed were ground and screened, and the particles of 1 to 4 mm length were labelled as 5~Cr-mordant (1.1% Cr with 500 000 cpm/g DM) as described by Brosh et al. ( 1986 ). The marked feed (20 g for BG and 40 g for SG) was introduced into the rumen, through the rumen cannula. Rumen content (20-30 g taken by vacuum through the cannula) sampling and total collection of feces were carried out twice daily during the subse- quent 7 d, at 09: 00 h, prior to feeding and at 18 : 00 h. All samples were kept at - 1 7 ° C until analyzed for 5~Cr, according to Brosh et al. (1986). Mean retention time (MRT) of particulate matter in the rumen and in the entire gut was calculated according to the stochastic method of Faichney ( 1975 ). Rumen and fecal data were used to calculate respective rumen and total gut MRT.

Rumen samples taken prior to feeding and fecal samples taken throughout the day were pooled and subjected to wet screening (Reid et al., 1977 ).

In vitro fermentation rate Rumen contents (500 ml) were sampled from various locations through a

rumen cannula on two consecutive days before feeding (08:00 h). The filtra- tion procedure of Brosh et al. ( 1983 ) was applied to separate fluid from par- ticles. Each rumen sample was divided into three subsamples. Six replications per animal were average and used for breed comparison. Separated fluid or fiber samples were incubated in glass jars with liquid medium and substrate (ground alfalfa hay), providing conditions for maximal fermentation capac- ity during the first hour, and biogas production was measured (Brosh et al., 1983). Media were composed of 20 g rumen fluid (3% DM), or 10 g fiber (29% DM), 100 ml buffer solution (E1-Shazly and Hungate, 1965 ) and 7 g alfalfa hay.

In vivo VFA production, rumen volume and kinetics o f fluid passage VFA production was measured twice in each goat during 24 h from the

dilution kinetics of ~4C-acetate (Gray et al., 1966, 1968; Weston and Hogan, 1968; Van der Walt and Briel, 1976). The following experimental procedures and analytical methods were used: at 06:00 h, 80 #Ci (BG) and 100/tCi (SG) of t4C-acetate were injected into the rumen through a cannula. Two hours were allowed for equilibrium. At 08:00 h 15/tCi of 5~Cr-EDTA were injected into the rumen, followed by additional 11 dosings at intervals of 2 h. Rumen fluids were sampled every 0.5 h during the next 24 h. The samples were treated and analyzed for VFA (acetic, propionic, butyric, valeric and isovaleric) concentration as described by Tagari et al. ( 1977 ). Total 14C-VFA activity was determined by liquid scintillation counter (Packard, Tri Carb)

48 N. SILANIKOVE ET AL.

following centrifugation and addition of 200/zl of rumen fluid with 16 ml of scintillation liquid (Lumagel, Lumac) with precision of 1%, or better. Quenching was corrected using an internal standard procedure and SlCr ac- tivity was measured as described by Silanikove and Tadmor ( 1989 ).

VFA production was calculated by the following equation (Shipley and Clark, 1972 ), which applies to non-steady-state conditions:

Fi=Vi[(CI+Cz)/2)]+(SA~+SA2)/2)]/(tz-t~)/(SAI+SA2)/2 (1)

Where: Fi = VFA production rate at a specific interval i, and V= average ru- men volume at a specific interval; SA1, SA2 = specific activity ( cpm/mM car- bon) at the beginning and at the termination of specific interval; CI, C2 = VFA concentration (raM carbon/l) at the beginning and at the termination of spe- cific interval; tl, t2 = the time at the beginning and termination of a specific interval.

When conditions within the rumen approach steady state, Eqn. 1 reduces to the ordinary form of first-order decay from a single compartment.

Daily (24 h) VFA production was calculated by summation of the VFA produced in each interval. Daily VFA absorption through the rumen was cal- culated by summation of the following equation:

Ai =Fi -NOFi[ (C1 + C2)/2]

where: Ai = VFA absorption at a specific interval, and NOFi = net outflow of fluid from the rumen at a specific interval.

Rumen volume and net outflow of fluid throughout the daily cycle were followed by the repeated dosing procedure of Hyden ( 1961 ). The relevant equations were given by Silanikove and Tadmor ( 1989 ).

Trans-epithelial flux of water from the rumen The kinetic (rate constant, h - l ) of water movement from the rumen to

blood was measured twice in each goat as an index of rumen blood flow (En- gelhardt, 1972; Dobson, 1986 ), using tritiated water as described recently by Dahlborn and Holtenius ( 1990 ) and Holtenius and Dahlborn (1990) in sheep and goats.

A day before the experiment the animals were not watered as usual at 16: 00 h. At 08:00 h on the following morning, 0.5 mCi of tritiated water was intro- duced into the rumen with the drinking water. The amount of water con- sumed was approx, one-third of the rumen volume, allowing rapid and effi- cient equilibration of the dose with the ruminal fluid. Feed was then given as usual. Rumen fluid was sampled every 20 min and blood was sampled through a jugular catheter every 30 min, for 6 h post-dosing. Tritiated water activity in blood plasma and rumen fluid were analyzed as described by Silanikove and Tadmor (1989).

EFFICIENCY OF DIGESTION IN GOATS 49

Statistical analysis Statistical differences between breed means were evaluated by the Stu-

dent's t-test analysis.

R E S U L T S

Rumen volume and kinetics of passage DMD of hay consumed was 68.5% in BG and 61.5% in SG (P<0 .05 ) .

Daily feed intake was equal in both breeds, 73.1 (SD _+ 5.1 ) g / k g °7 5 in BG and 72.9 (SD _+ 4.7) g/kg °75 in SG. The results were similar to those in a trial in which a larger amount of refusals ( 10% ) were allowed (Silanikove, 1986a).

Daily average rumen fluid volume was 20% of body mass in BG and SG (Table 1 ). Average fractional outflow rate of rumen fluid was 0.084/h in BG and 0.099/h in SG, resulting in a 39% greater (P< 0.01 ) fluid outflow from the rumen of SG per unit of metabolic BW. Daily average MRT of fluid was 20% greater (P< 0.05 ) in BG than in SG.

MRT of particulate matter in the rumen was greater (P< 0.05 ) in BG than in SG (Table 1 ). MRT of indigestible particulate matter along the entire gut was also greater (P< 0.05 ) in BG than in SG (Table 1 ).

Distribution of particulate matter in the rumen between small and large particle pools (i.e., smaller and larger than those retained on a 1-mm screen )

TABLE 1

Volume, kinetics of rumen fluids and mean retention time ( M R T ) of particulate matter in the rumen and along the entire gastrointestinal tract in Bedouin and Saanen goats fed medium quality hay (n = 4, mean + SD)

Bedouin Saanen

mean SD mean SD

Fluid volume and kinetics Rumen volume ( 1 ) 4.18 0.4 10.24

ml/kg BW 198 15 197 Rate constant 0.084 a 0.006 0.099 b

fraction of pool /h Outflow

m l / h 370 46 1010 ml /kgBW°-75/h 37.6 a 4.7 52.2 b

M R T (h) 11.9 a 1 9.9 b Passage rate

Rumen M R T (h ) 41.3 a 5.2 31.6 b Post-rumen M R T (h ) 15.3 3.6 13.6 Total tract M R T (h ) 56.4 a 1.5 45.2 b

1.9 16 0.007

187 5.7 0.7

4.7 4.0 1.6

a'bValues within rows with different superscript letters were significantly different between breeds ( P < 0 . 0 5 ) .

50 N. SILANIKOVE ET AL.

was 87% and 13%, respectively, and similar for both breeds. Most particles in feces (97-99%) were smaller than those retained on a 1-mm screen.

In vitro fermentation rate of rumen samples Biogas production (ml /h per g sample) in solid rumen samples was 9.5

(SG) to 14 (BG) times higher than that measured in fluid samples (Table 2 ). Biogas production in solid samples was 45% higher (P< 0.01 ) in BG than in SG, whereas biogas production in fluid samples was about the same in both breeds (Table 2 ).

VFA concentration and production in the rumen VFA production, on a metabolic weight basis, was 20% higher (P< 0.05 )

in BG than in SG (Table 3). However, the ratio between digestible organic matter (DOM) and VFA production was similar in both breeds (0.84 M/ 100 g DOM). VFA concentration in the rumen, calculated from the average of two sets of 48 determinations taken at intervals of 0.5 h during 24 h was, in accordance with the higher production rate, 13% higher (P< 0.05 ) in BG than in SG (Table 4).

TABLE2

In vitro biogas production in fluid and fiber rumen samples of Bedouin and Swiss Saanen goats fed medium quality hay (n = 4, mean + SD )

Fluid Fiber (ml /g fluida/h ) (ml /g DMb/h )

Bedouin 5.4 + 0.8 76.0_+ 3.5 Saanen 5.1 + 0.6 48.4 + 2.9 Statistical differences between breeds (t-test) NS P < 0.01

alncubation medium was composed of 20 g (3% DM) fluid, 100 ml buffer solution and 7 g alfalfa hay. Total gas production in the jar was related to 19.4 ml rumen fluid in the medium. blncubation medium was composed of 10 g (29% DM) fiber, 100 ml buffer solution and 7 g alfalfa hay. Total gas production in the jar was related to 2.9 g DM of particulate matter in the medium.

TABLE 3

Net VFA production in the rumen and its caloric value in Bedouin and Saanen goats fed medium quality hay (n = 4, mean + SD)

Net production

mM (kg BW °-75 day) - l M/100 g DOM"

Caloric value kcal /m

Bedouin 395 + 25 Saanen 330 + 27 Statistical differences (t-test) P < 0.05

0.845+0.15 2 8 0 + 8 0.840+0.20 263+ 10 NS P<0 .10

aDOM = digestible organic matter.

EFFICIENCY OF DIGESTION IN GOATS 51

TABLE 4

Daily average I VFA concentration in the rumen of Bedouin and Swiss Saanen goats fed medium quality hay (n =4, mean + SD)

Molar percentage of total

acetic propionic butyric isovaleric valeric

Total (mM/ l )

Bedouin 69.3* 19.3 a 8.6 a 1.8 a 1.0 a 122 a SD ±5.2 1.2 ±0.5 ±0.3 ±0.1 ±11

Saanen 75.9 16.2 b 6.5 b 1.0 b 0.6 b 108 b SD ±5.4 ±1.3 ±0.4 ±0.3 ±0.1 ±11

IAverage of two sets of 48 determinations taken in intervals of 0.5 h during 24 h. a,brepresent significant (P< 0.05 ) breed differences by t-test. *P<0.10.

I ] FEE;ING , /~ i . I DRINKING . . . . . .

Z

8 ~ 5o -

I I I I I I I I I I I I 08 12 16 20 24 04 08

TIME OF DAY (h)

Fig. 1. Diurnal pattern in total VFA concentrations in Bedouin and Saanen goats fed m e d i u m quality hay. Values marked by asterisks represent significant ( P < 0.05) breed differences by t- test.

In both breeds acetic acid was the main VFA. However, the ratio acetate/ propionate was higher ( P < 0.05 ) in SG than in BG, and the proportions of butyrate, isovalerate and valerate were higher ( P < 0.05 ) in BG than in SG (Table 4). As propionate, C4 and C5 VFA contain more energy per mol than acetate, the amount of energy derived per unit of organic matter digested was also higher in BG than in SG (Table 3).

Diurnal variations in the physico-chemical conditions in the rumen Detailed analysis of VFA production over time suggests three periods:

( 1 ) 1-7 h post-feeding (between 10: 00 and 17: 00 h ), when most of the daily VFA production (50-60%) occurred, especially during the first hour, as judged by VFA concentrations (Fig. 1 ), pH (Fig. 2 ), rumen volume (Fig. 3 )

52 N. SILANIKOVE ET AL.

7.0

-1- 0-6.5

z

6.0

5. = .

I I I I I I I 1 I I I

F E E 0 1 N G D R I N K I N G

I I I t I 1 I I I I I 08 12 16 20 24 04 08

TIME OF DAY (h )

Fig. 2. Diurnal pattern in rumen pH in Bedouin and Saanen goats fed medium quality hay. Values marked by asterisks represent significant (P< 0.05) breed differences by t-test.

J lC bJ :E R

S s IE 221 nr

• I I I I 1 I I I I l I DRINKING

! FEEDING

~ * SE - - ~ SAANEN

* . * BEDOUIN I -

0 I I I I I I I I I I I 08 12 16 20 24 0.~ 08

TIME OF DAY (h)

Fig. 3. Diurnal pattern in rumen fluid volume in Bedouin and Saanen goats fed medium quality hay. Values marked by asterisks are significantly (*P< 0.05; **P< 0.01 ) different from the pre- prandial values by pair t-test.

and rumen fluid outflow rate (Fig. 4) , the conditions in the rumen depart considerably from steady state. (2) 1-7 h post-drinking (between 17: 00 and 24:00 h), which accounted for an additional 15-25% of the daily VFA production. (3) The period of lowest VFA production rate, lasting 10 h (between 24:00 and 10: 00 h) , which accounted for 15-35% of the daily VFA production. The conditions in the rumen during periods 2 and 3, based on the above criteria, unlike period 1 were close to steady state.

No breed differences regarding daily pattern in VFA production and con- centration (Fig. 1 ) or in pattern of rumen volume and kinetics (Figs. 3 and 4) and rumen and plasma osmolality (Fig. 5 ) were noted. VFA concentra-

EFFICIENCY OF DIGESTION IN GOATS 5 3

~-~ 1500 FEEDING~,=.~ *I i i DRIIN KIN i G [ I I I J I SE"~I I.- rr"

O ~_ 500

0

~ ~ ~ "~ BEDOUIN ~'-

] I I I l I J I l I I 08 12 16 20 24 0 4 08

TIME OF" DAY (h )

Fig. 4. Diurnal pattern in rumen fluid outflow rate in Bedouin and Saanen goats fed medium quality hay. Values marked by asterisks are significantly (*P < 0.05; **P< 0.01 ) different from the pre-prandial values by pair t-test.

500

er

U'I ,.,~ ~ 4 0 0 n."

'7- ~ 3 0 0 0

o

I I I I IDRINKiN G i I I I ~ 1 PLASMA RUMEN

• 0 ~ BEDOUIN

2 0 0 I I I I I I I I I I I 0 8 12 16 2 0 24 0 4

TIME OF DAY (h)

Fig. 5. Diurnal pattern in rumen and plasma osmolality in Bedouin and Saanen goats fed me- dium quality hay. Values marked by asterisks are significantly (P< 0.05) different from the pre- prandial values by pair t-test.

tions were highest and conversely pH was lowest in both breeds during period l (Figs. 1 and 2) . However, VFA concentrations in the rumen of SG was lower ( P < 0.05 ) than in BG ( 135 vs. 155 raM/ l ) , and despite this, the rumen pH in SG (5.89) was also lower ( P < 0.05 ) than in BG (6.24) . During period 2, although the VFA concentrations and the pH were lower in SG than in BG, the differences were smaller and statistically not significant ( 120 mM/1 vs. 110 and 6.42 vs. 6 .50) . During period 3 the VFA concentrations were higher ( P < 0.05 ) in BG ( 115 raM/l ) than in SG (95 raM/ l ) , but the pH was similar in both breeds (approx. 6.9 ).

54 N. SILANIKOVE ET AL.

TABLE 5

Kinetics of tritiated water equilibration between the rumen and the systemic fluids and VFA absorp- tion from the rumen in Bedouin and Swiss Saanen goats fed medium quality hay (n = 4, mean + SD )

Tritiated VFA (mol /d ) water rate constant Production Outflow Absorption Absorption/production ( h - ' ) ratio

Bedouin 0.91 +0.1 4.01 +0.3 0.99+0.06 3.02+0.3 0.75+0.03 Saanen 0.43+0.06 6.17+0.5 2.79+0.09 3.38+0.4 0.55+0.05 Breed differences

(t-test) P<0.01 - - - P<0.01

Water flux and VFA absorption The rate of Water movement from the rumen to blood was considerably

faster (P < 0.01 ) in B G than in SG ( Table 5 ). Approx. 75 % o f VFA produced was absorbed through rumen walls in BG compared with 55% (P<0.01) in SG (Table 5).

DISCUSSION

Passage rate It has already been established that the desert BG possesses a higher diges-

tion capacity than goats indigenous to temperate regions, over a wide range of roughage (Silanikove et al., 1980; Silanikove, 1986a,b). The present re- suits indicate that these differences cannot be explained simply by differences in the level of feed intake (approx. 73 g / k g °'75 in both breeds) or by different proportions of rumen volume to BW.

MRT of particulate matter in the rumen was considerably higher in BG as compared with SG, thus allowing a longer exposure to microbial activity in the rumen and hence greater digestibility. However, if the particulate matter is retained longer in the rumen of BG, it is to be expected that these goats would consume less feed relative to metabolic weight than SG (Thornton and Minson, 1973), which was not the case. A possible explanation for this ap- parent contradiction is that the fermentation rate in BG is higher than in SG, so that the overall elimination of organic material from the rumen was about the same in both breeds, resulting in a similar feed consumption. This expla- nation is supported by higher in vitro gas production on particulate matter and whole-rumen in vivo VFA production in BG. The ability to maintain a higher fermentation rate in the rumen is a major reason for the advantage of tropical cattle (Bos indicus) over European cattle (Bos taurus) with respect to digestion of forage with high fiber content (Hungate, 1966; Hunter and Siebert, 1985 ).

EFFICIENCY OF DIGESTION IN GOATS 5 5

In the present experiment only 1.0-3.0% of the particles in feces were re- tained on a screen larger than 1 m m in size. Once the digesta had left the rumen there appears to be little or no further reduction in the size of the par- ticles (Hungate, 1966). It seems that the 'critical particle size' for leaving the rumen that was found in sheep (Poppi et al., 1980) may be similar in goats. This conclusion is consistent with the results of a recent comparison of the size and distribution of particles of digesta in sheep and goats (Fujikura et al., 1990; Domingue et al., 1991 ).

As with other ruminants, the MRT of particulate matter is considerably longer than the MRT of fluid in both breeds and the differences exceed con- siderably the lag time required to reduce the particles to a size allowing their passage from the rumen (Ellis et al., 1979). Selective retention of particulate matter in the rumen is commonly related to the 'bottle-neck' function of the reticulo-omasum orifice (Hofmann, 1989) and to the more effective reten- tion of feed particles with the bulk of the forestomach content and in partic- ular the 'filter bed' effect (Faichney, 1986). Breed differences in kinetics of fluid outflow from the rumen suggest that there are intrinsic breed differences in factors related to passage, most likely in rumen contraction rate. However, the fact that breed differences in MRT of particulate matter (31%) are larger than breed differences in kinetics of fluid passage (20%), also suggests that one of the above-mentioned mechanisms, or both (i.e., 'bottle-neck' and 'fil- ter bed') , is more efficient in BG than in SG. If so, it can be predicted that BG are able to sequestrate post-prandial larger amounts of DM in the rumen than SG. To support this, the ratio between DMI and average rumen volume in BG ( 156 g/l) was 20% higher (P< 0.05 ) than in SG ( 130 g/ l) . Because of the higher fermentation rate in BG, breed differences in DM content in ru- men fluid are expected to disappear toward the end of the feeding cycle, and indeed in samples taken pre-prandial (500 ml samples taken for the in vitro gas production measurements), a similar DM content (100-120 g/l) was found in both breeds.

VFA metabolism Biogas production under conditions enabling maximal fermentation in vi-

tro is a measure of viable microbial activity of the rumen sample (E1-Shazly and Hungate, 1965). Hungate (1966) has shown that most fermentation takes place in the particulate matter, and Forsberg and Lam (1977) and Faichney (1980) found that particle-associated microorganisms make up a major pro- portion (70%) of the total microorganisms in the rumen. It may be concluded that the higher microbial activity in the particulate fraction in BG than in SG is the major factor which contributed to higher VFA production rate and in higher VFA concentrations in the rumen.

The same ratio between VFA produced and organic matter apparently di- gested was found in both breeds. We could not find more data on this subject

56 N. SILANIKOVE ET AL.

on goats. However, in sheep fed a range of roughage of variable quality, a ratio similar to that found in the present experiment with goats (i.e., 0.83 to 0.85 M/100 g DOM) was measured (Bergman et al., 1965; Weston and Hogan, 1968; Van der Walt and Briel, 1976; Van der Walt, 1977). Since similar mi- croorganisms and biochemical pathways are expected to exist in all rumi- nants (Hungate, 1966), such a similarity is not surprising.

Nevertheless, breed differences in VFA proportions indicated that fermen- tation was more efficient in BG than in SG. The lower acetate/propionate ratio and the higher concentration of branched and long chain VFA in BG probably reflected a more efficient microbial nitrogen assimilation and growth (Ishaque et al., 1971; Tagari et al., 1977). Indeed, nitrogen digestion and re- tention under the present dietary conditions were considerably higher in BG than in SG (Silanikove, 1986a). In accordance with the results of Harrison et al. ( 1975 ), the lower acetate/propionate ratio in BG was associated with a lower outflow rate of fluid from the rumen in comparison with SG, which indicates a close association between the fermentation process and the kinet- ics of passage (Ishaque et al., 1971 ).

The positive relationship between rumen tonicity and fluid passage rate, discovered first by Harrison et al. (1975), is probably a major factor con- necting fermentation and passage rate. In parallel to the daily pattern of vari- ations in rumen tonicity, outflow of fluid from the rumen increased by ap- prox. 25% in both breeds following feeding over pre-feeding values. Elevation in rumen tonicity and fluid outflow rate persisted for 7 h following feeding. In parallel, rumen volume increased by 10-15% in both breeds, which is con- sistent with studies in cattle and sheep fed once daily (Wright and Grainger, 1970; Peters et al., 1990). Lower increase ofrumen volume than fluid outflow rate suggests that a sudden large change in rumen volume as a result of the typical increase in saliva secretion at the onset of feeding (Canner and Gro- vum, 1990), was prevented by a parallel increase, roughly of similar magni- tude, in rumen fluid outflow rate. The typical post-feeding increase in rumen volume and decrease in blood plasma volume in animals fed once daily, may have been a result of transepithelial passage of water from blood into the ru- men according to the osmotic gradient (Fig. 5; Canner and Grovum, 1990).

The higher concentration of long and branched VFA in BG may also be related to more efficient phenolic fermentation (Silanikove and Brosh, 1989). Apparent lignin digestibility under present dietary conditions was indeed sig- nificantly higher in BG than in SG (Silanikove, 1986a). It has been shown recently (Silanikove and Brosh, 1989) that apparent lignin digestibility in BG fed on wheat straw was reflected in degradation oflignin to a monomeric level. Phenylpropanoic acid, a product of phenolic acid degradation, has been shown to enhance structural carbohydrate digestion (Hungate and Stack, 1982) by enhancing the association of Ruminococcus albus (a common ru- men bacterium) with particulate matter (Stack and Hungate, 1984 ).

EFFICIENCY OF DIGESTION IN GOATS 5 7

For the entire organism, the advantages to having a low acetate/propionate ratio and high proportions of long and branched VFA are: ( 1 ) propionic acid is a precursor for glucose by gluconeogenesis (Leng, 1970 ), which is essential for diets that are extremely low in metabolizable energy and amino acids, such as desert pastures; (2) the energy content of VFA produced is higher.

Reduced rumen pH has a major impact on fiber digestion, both in vitro and in vivo (Hungate, 1966; Hoover, 1986). Reduction of the pH from 6.8 to approx. 6.0 results in a moderate depression in fiber digestion. A decrease in pH to below 6, as found in SG, causes severe inhibition in fiber digestion. In BG the pH did not drop below 6.2. The capacity of BG to maintain their rumen pH above 6.0 was observed when they were additionally exposed to hot summer environment and to dehydration (Brosh et al., 1988 ). Reduced attachment of bacteria and enzymes to cell walls may be involved in the depression in fiber digestion with decrease in pH (Cheng et al., 1984). The ability of BG to maintain higher activity of microbial populations on partic- ulate matter, in comparison with SG, seems accordingly to be partially related to their ability to regulate their rumen pH within a narrow range. The ability of BG to do so may be partially explained by the higher proportion of VFA absorbed through the rumen wall in comparison with SG. Ash and Dobson ( 1963 ) have shown that VFA are absorbed in exchange with HCO3, which in turn enhances the buffer capacity of rumen fluids. Higher VFA absorption was found to correlate with blood perfusion of the rumen epithelium (Ben- sadoun and Reid, 1962; Dobson and Phillipson, 1965). In accordance, the kinetics of water movement from the rumen to blood, which can serve as an index of blood flow to the rumen epithelium (Engelhardt, 1972; Dobson, 1986), was indeed higher in BG than in SG.

Nevertheless, consumption of a low quality feed such as wheat straw, in- duced a smaller effect on physico-chemical conditions in the rumen, includ- ing in pH (Brosh et al., 1988 ). Breed differences in digestion capacity, how- ever, were even larger (Silanikove, 1986a; Silanikove et al., 1980). The superior urea recycling capacity of BG in comparison with SG and most other domestic ruminants was considered most important, since it reduces nitrogen deficiency in the rumen. Based on the relationship between nitrogen content in the diet and the extent of urea recycling, and on breed differences in the efficiency of urea recycling, it is most likely that the amount of urea that was recycled to the gut was considerably higher in BG (Silanikove et al., 1980). Nitrogen retention and digestion were considerably higher in BG than in SG ( Silanikove, 1986a). Accordingly, the advantage of the former over the latter in digestion capacity is independent to a large extent of the way the animals are being fed.

C O N C L U S I O N S

The main advantage of Bedouin goats over Saanen in digesting high fiber diets may relate to their ability to maintain higher microbial activity in the

58 N. SILANIKOVE ET AL.

particulate matter and hence higher total ruminal fermentation rate. Breed differences in fermentation rate are reflected in the extent and site of absorp- tion of digestion end-products. The ability of Bedouin goats to maintain high fermentation rate in the rumen may be partially related to their ability to prevent reduction in rumen pH to levels which depress fermentation. Earlier studies have shown that desert and tropical breeds of ruminants are also more efficient than temperate breeds in reducing the deficiency of key nutrients, such as nitrogen (Silanikove et al., 1980) and sulfur (Hunter and Siebert, 1985), by their efficient recycling to the gut. Longer retention time of feed particles is a complimentary factor which contributes to the efficiency of digestion. The longer mean retention time of particulate matter in desert goats may be partially related to factors relating to kinetics of passage and partially to their effective retention in the rumen. The combination of higher fermen- tation rate and longer retention time allows maximizing of feed intake and digestibility in a given situation, in comparison with less efficient non-desert ruminants.

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