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* Department of Animal and Veterinary Science, University of Idaho, Moscow, 83844-2330; and
and
MTT Agrifood Research Finland, FIN-31600 Jokioinen, Finland
and
Agriculture and Agri-Food Canada Research Centre, Lethbridge, Alberta T1J 4B1
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Abstract |
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G1) models. Inclusion of concentrate in the diet (SC) increased (P < 0.05) apparent total tract digestibility of dietary DM, OM and N. Digestibility of fiber fractions, NDF and ADF, was lower (P < 0.01 and P < 0.05, respectively) for SC compared with AS. The heavy particles had higher (P < 0.01) indigestible NDF and lower (P < 0.01) N concentration than LP. Particles from the HP pool passed from the rumen more rapidly (P < 0.01) than particles from LP (0.044 and 0.019 h-1, respectively). Diet had no effect on particle rate of disappearance or pool size in the rumen. Across diets, pool size of LP was consistently larger (P < 0.05) than that of HP. Diet had no effect on total tract mean retention time (MRT) of LP or HP. Total tract MRT of LP was greater (P < 0.05) than MRT of HP (59.6 vs. 49.0 h, respectively). Results from this study support the hypothesis that functional specific gravity is an important factor determining the rate of outflow and residence time of feed particles within the reticulo-rumen and total digestive tract. Our data indicate that digesta particles with functional specific gravity greater or less than 1.02 have different composition and flow characteristics. Heavier particles contain more indigestible fiber and less N and are likely depleted of substrate available for microbial fermentation, are smaller in size, and have a higher passage rate/shorter retention time in the digestive tract than lighter particles.
Key Words: Dairy Cows • Digestive Tract • Kinetics • Rumen Contents • Specific Gravity
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Introduction |
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). Schettini et al. (1999) reported that particles having greater SG escaped from the rumen at a faster rate than particles with lower SG. Similarly, silage had greater SG (unit specific gravity; Wattiaux et al., 1992b) and shorter retention time in the rumen than hay prepared from the same crop (Nelson and Satter, 1992).
Earlier studies have indicated the importance of particle specific gravity in controlling retention time in the gastrointestinal tract of ruminants (King and Moore, 1957; Campling and Freer, 1962). Most often, however, effect of SG on passage rates was studied with inert particles (Campling and Freer, 1962; Ehle and Stern, 1986; Kaske and Engelhardt, 1990) rather than with feed particles recovered from the digestive tract of the ruminant animal. Luginbuhl et al. (1994) reported faster passage rate for fecal particles compared to masticated forage particles providing evidence for differences in passage kinetics between escapable and non-escapable (without further aging) particles. We hypothesize that functional SG (FSG) is an important factor determining retention time and passage rate of ruminal particles throughout the digestive tract and that particles with different FSG have different nutrient composition and digestibility.
The goal of this study was to determine composition, particle size distribution, and in vivo kinetics of ruminal feed particles having FSG higher or lower than particles found in the omasum and the reticulum of lactating dairy cows.
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Materials and Methods |
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Pre-experimental Phase.
During this phase of the study, FSG of feed particles from the omasum and the reticulum was determined and particles were fractionated by FSG, prior to labeling with rare earth elements.
All four cows were fed a 100% grass silage diet at ad libitum intake (5% orts) for a period of 7 d. Feed was offered at 600 and at 1800.
A total of four omasal (Huhtanen et al., 1997) and reticular samples were taken from each animal on two consecutive days: at 1000 and 1500 each day. Immediately after acquisition, samples were combined for each site resulting in a total of eight composite samples and FSG of omasal and reticular particles was determined. Ruminal fluid from the combined omasal or reticular digesta samples was filtered out by gravity through a 200 µm fabric (Sefar Inc., Ruschlikon, Switzerland). Functional specific gravity of the retained particulate matter was determined in gradient, CO2-saturated NaCl solutions (in distilled water) with SG (at 20°C) of 1.0035, 1.0071, 1.0107, 1.0143, 1.0214, 1.0322, 1.0431, 1.0615, 1.0801, and 1.1028 (Wolf et al., 1989) and by the pycnometer method (Blake and Hartge, 1986; a 200-mL volumetric flask was used instead of a pycnometer). The average (± SE) FSG of omasal particles was 1.032 ± 0.0035 and that of particles recovered from the reticulum was 1.022 ± 0.0073 (P = 0.273). Based on these measurements, the cut-off FSG of feed particles leaving the reticulo-rumen was assumed to be 1.02. A solution of NaCl with this SG was used for separation of ruminal particles.
Approximately 30 kg of whole ruminal contents were removed from two cows fed a 100% grass silage diet and fractionated in 1.02 SG NaCl solution. Whole ruminal contents were taken from the dorsal and ventral rumen and digesta from the reticulum, mixed thoroughly and suspended in the NaCl solution. Ruminal contents represented 10% (wt/vol) of the NaCl solution. The fractionation was performed in large (25-L capacity) plastic bags for 5 min. The NaCl solution was prepared from feed grade salt in warm (39°C) tap water. The concentration of salt in the NaCl solution was 3% (wt/wt). The theoretical density of this solution was 1.021. The actual density (measured by the pycnometer method) was 1.020. To assure gas trapped within the ruminal particles remained associated with the particulate phase, the NaCl solution was saturated with CO2 before fractionation. Particles with FSG greater than 1.02 (heavy particles, HP) sedimented to the bottom of the separatory bag and particles with FSG less than 1.02 (light particles, LP) were recovered from the top of the separatory bag. For all batches of ruminal contents, separation was complete. Both type of particles were filtered by gravity through a 200 µm fabric. Approximately 10 kg (as-fed basis) of HP and 15 kg of LP were harvested (1.25 and 1.86 kg of DM, respectively). Both fractions were washed thoroughly with warm tap water (39°C) through the 200 µm fabric. The filtrate from HP was collected and after settling, filtered through a 38 µm fabric (Sefar Inc.). Particles retained on the 38-µm fabric were combined with the HP fraction. Samples were stored at 2°C until labeled with the markers. Light particles were labeled with Dy and HP were labeled with Er; Moore et al. (1992) found digesta kinetics estimates were identical between Dy- and Er (or Yb)-labeled particles, concluding that these rare-earths could be used simultaneously in studying kinetics of particles in the digestive tract of ruminants.
Particles were soaked in ErCl3 • 6H2O (HP) or DyCl3 • 6H2O (LP; both from Aldrich Chemical Company, Inc., Milwaukee, WI) solutions for 24 h at 39°C and then rinsed repeatedly with tap water through a 38 µm fabric. Following the labeling process, both HP and LP were soaked in acetic acid solution with pH of 4.5 for 1 h at 24°C and rinsed again with tap water. Concentration of marker in the Er- and Dy-solutions was 64.3 g /kg particle DM. The labeled particles were divided into equal portions of 75 and 78 g (HP and LP, respectively) and stored frozen at -20°C until dosed into the rumen; the dose per cow was 75 and 78 g, respectively.
Experimental Phase.
During the experimental phase, the animals were fed 2 diets to ad libitum intake (ensuring 5% orts): 100% grass silage (AS), or 50% grass silage/50% concentrate (SC, Table 1). Silage and concentrate were divided into two equal meals and fed separately at 0600 and at 1800. The amount of refusal was measured daily. Silage samples were collected daily and pooled for each period for determination of chemical composition. The experiment was designed as a cross-over. Following 10 d of adaptation to the diet, before the morning feeding, approximately 10 kg of ruminal contents were removed from the rumen and Co/Li-EDTA (2.5 g of Co/cow; prepared according to Udén et al., 1980) and Dy-labeled LP and Er-labeled HP were mixed with the ruminal contents and returned to the rumen; ruminal contents were further mixed by hand upon introduction of the 10-kg portion.
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Ruminal pool sizes and chemical composition of LP and HP were determined through emptying the rumens on d 6 and 7 following marker administration. On d 6, the rumens were evacuated at 0600 and on d 7 at 1200. The ruminal contents were thoroughly mixed and weighed. Four subsamples of 2 kg each were taken and HP and LP fractions were recovered quantitatively as described above.
Fecal grab samples (200 g each) from the rectum were collected at 4, 6, 8, 10, 12, 16, 20, 24, 28, 32, 36, 40, 42, 48, 54, 60, 66, 72, 78, 84, 90, 98, 108, and 120 h post-dose. The fecal samples were oven-dried at 65°C, ground to pass through a 1-mm sieve, and analyzed for Co, Dy, and Er. Total tract apparent digestibility was determined (Schneider and Flatt, 1975) based on acid-insoluble ash (AIA) concentration measured in feed and feces. Fecal samples were pooled per period and per animal to provide a composite sample of feces.
Wet Sieving and Indigestible NDF Content of LP and HP.
LP and HP particles were isolated from individual ruminal evacuation samples (2 samples per cow per period), composited (to yield 115 and 102 g DM, respectively), and the composited samples were wet-sieved on several mesh sizes (2,500, 1,250, 630, 315, 160, and 80 µm) using an analytical sieve shaker (Retsch AS200 digit, Haan, Germany). Subsamples of 3 g DM were sieved for 10 min with continuous water flow at a rate of 3.5 L/min. The frequency of vibrations was adjusted at 3,000 vibrations/min and vibration height at 2 mm. Filtrate passing through the sieves was collected and subsequently filtered through 38 µm fabric to recover residual particulate matter. Sieving was repeated several times in order to recover a minimum of 2 g of DM on each sieve for subsequent determination of indigestable NDF (INDF). Residues with different particle sizes were incubated in sacco in the rumen for determination of INDF. DM recovered on sieves 2,500 and 1,250 mm was combined and termed >1,250 mm. The in sacco incubation was carried out in 6 µm polyester bags (Sefar Inc.) for 12 d in 3 animals fed 6 kg (air-dry basis) of concentrates (for composition refer to Table 1
) and allowed free access to grass silage (Ahvenjärvi et al. 2001).
In Vitro Migration of Markers.
Samples of labeled LP and HP were incubated with ruminal inoculum for up to 72 h to study marker migration. Labeled samples (5 g, as is basis) were incubated in triplicate with 100 mL ruminal inoculum in 125-mL Erlenmeyer flasks equipped with pressure-releasing valves (Bunsen valves). The inoculum was composed of one part strained ruminal fluid and one part buffer (Goering and Van Soest, 1970). The incubation was carried out for 6, 24, 48 and 72 h at 39°C with continuous agitation. Fermentation was ceased by inserting the incubation flasks in ice and adding of 1% (v/v) formalin. The incubation media was filtered through 38 µm fabric and the filtrate centrifuged at 20,000 x g for 15 min at 4°C. Supernatant and pellets were freeze-dried and analyzed for Dy and Er.
Laboratory Analyses.
Rare earth elements were extracted from fecal and particulate samples according to Combs and Satter (1992). Dy and Er were analyzed by direct current plasma emission spectroscopy, using a Beckman SpectraSpan V spectrometer (Beckman Coulter, Inc., Fullerton, CA) with an Adam9 Analytical Data Manager and a PSM-V autosampler. Cobalt in fecal samples was analyzed as described elsewhere (Hristov et al., 2000). Dry matter of feed ingredients, ruminal digesta, and feces was determined after 18 h in a forced-air oven at 105°C. Silage DM content was corrected for volatile losses according to Huida et al. (1986). For chemical analysis, feed ingredients, ruminal digesta, and feces were dried to a constant weight at 60°C in a forced-air oven, and ground through a 1-mm screen. The DM concentration of air-equilibrated samples was determined after a 16-h incubation at 105°C. Concentration of ash was determined after ignition in a muffle furnace at 600°C for 18 h. Concentration of AIA was determined as described by Anon (1971). Nitrogen concentration in fresh samples of silage, feces, and urine was determined by the Kjeldahl method using CuSO4 as a catalyst. Nitrogen concentration in other feed ingredients and ruminal digesta was determined using a Dumas-type N analyser (Leco FP-428; Leco Corporation, St. Joseph, MI). The NDF and ADF concentration in feed ingredients, digesta, and feces were determined according to Van Soest et al. (1991); 
-amylase (Sigma Chem. Co., St. Louis, MO) and Na2SO3 were used in the NDF analysis. For analysis of Co concentration, ruminal fluid samples were centrifuged at 2,000 x g for 10 min and the supernatant was filtered through Whatman #1 filter paper. Ruminal fluid clarified in this way was then diluted with a solution containing 2.25 M of HNO3 and HCl and 1.9 mg/mL of KCl and Co was measured with inductively coupled plasma spectrometer (Iris Advantage, Thermo Jarrel Ash, MA).
Models and Statistical Analysis.
Fecal marker excretion data were fitted using two-compartment mathematical age-dependent/age-independent (GnG1) models (Pond et al., 1988). Parameters were estimated using the non-linear, least square iterative process of SAS (SAS Inst. Inc., Cary, NC). The time delay (
) was restricted from being shorter than the first appearance of marker in feces. Mean residence time (MRT) in the first compartment (CMRT1) was found as 1/k1 (model G1G1), or n/
(model GnG1; where k1 and are the age-independent or age-dependent outflow rates for the first compartment) and for the second compartment (CMRT2) as 1/k2 (where k2 is the outflow rate for the second, age-independent compartment). Compartmental residence time for the system of two compartments (CMRT) was found as CMRT1 + CMRT2 and the total mean residence time (TMRT) in the digestive tract was calculated as CMRT + . Criteria for best fit were: residual sum of squares (RSS), residuals distribution around the zero line, 95% confidence interval for the estimates, and the R2 values.
Intake, digestibility, particle composition, and ruminal and total tract particle kinetics data were analyzed using ANOVA assuming a cross-over design. Treatment means were separated by pair-wise t-test.
In vitro marker migration data were analyzed through regression. Within the 72-h time course, a quadratic model (Y = ß0 + ß1x + ß2x2 + e) was found to be appropriate to fit the data. The model was fit separately for each marker and evaluated for adequate fit. Subsequently, the estimated models were contrasted using dummy variable regression technique. A weighted regression was used due to heterogeneity of the variance across time.
All data were analyzed using SAS (SAS Inst., Inc., Cary, NC).
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Results |
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). Cows fed SC diet consumed more DM and N (P < 0.05) and OM (P < 0.01) than cows fed the AS diet. Apparent total tract digestibility of DM, OM and N were higher (P < 0.05) with diet SC than diet AS. Digestibility of fiber fractions, NDF and ADF, was decreased (P < 0.01 and P < 0.05, respectively) by inclusion of concentrate in the diet (SC) compared to the all-silage diet (AS).
The chemical composition of HP and LP is presented in Table 2. All samples had comparatively low OM content, most likely due to contamination with NaCl during the fractionation process. Organic matter concentration of HP and LP was lower (P < 0.01) on diet AS compared to diet SC. SC diet had lower (P < 0.001) NDF and ADF content compared to the AS diet. The potentially digestible NDF (as proportion of NDF) was higher (P < 0.01) on AS than on SC. Concentrations of OM, NDF, and INDF were higher (P < 0.01, P < 0.05, and P < 0.01, respectively) and concentration of N was lower (P < 0.01) in HP than in LP. The potentially digestible fraction of NDF was higher (P < 0.05) in LP compared to HP. Concentration of ADF tended to be higher (P < 0.1) in HP than in LP.
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). Particles retained on sieves smaller than 1,250 µm represented 50 and 40% of the total (LP and HP, respectively). A significant proportion of particles were retained on 316 and 160 µm sieves: 13 and 15% (LP) and 15 and 20% (HP), respectively. Particles smaller than 38 µm constituted 2 and 3% of the sample DM (LP and HP, respectively).
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. Particles from HP retained on 38 µm screen had remarkably higher (by 36%) INDF concentration than LP particles retained on the same size sieve. Concentration of INDF in HP was also higher compared to LP for particles retained on screens 80, 160, 315, 630, and >1,250 µm.
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). Predicted regression lines for HP vs. LP did not differ (P > 0.05). In 72 h, 34.0 and 34.7% (LP and HP, respectively) of the total marker recovered was found in the high-speed small particle/microbial matter pellets (Figure 3). A greater proportion of marker was recovered in this fraction for HP than for LP (P < 0.01).
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). Er-labeled particles escaped at a faster rate (P < 0.01) from the HP pool than Dy-labeled particles from the LP pool. The MRT of LP in the rumen was considerably longer (by 30 h) than MRT of HP. Ruminal pool size of HP and LP was not affected (P > 0.05) by diet. The pool size of LP was consistently larger (P < 0.05) than that of HP. However, inclusion of concentrates tended to increase (P < 0.1) total particulate matter pool size.
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). Marker outflow rate in the first compartment was the greatest (P < 0.05) for ruminal fluid (RF). Marker in LP had numerically lower (P = 0.125) rate of outflow compared to HP. Similar to the first compartment, the outflow rate from the second compartment was the greatest (P < 0.05) for RF. LP and HP phases had similar (P > 0.05) k2 values. No differences (P > 0.05) in the mean transit time between labeled particles or ruminal fluid were observed. Retention time in the first compartment (CMRT1) was longer (P < 0.05) for LP and HP than for RF. Retention time of LP in this compartment tended to be longer (P < 0.1) than the retention time of HP. Retention time of LP in the second compartment (CMRT2) was numerically longer (by 9.5 h; P = 0.120) than that of HP and 4.9 times longer (P < 0.05) than CMRT2 of RF. Light particles had the longest (P < 0.05) cumulative MRT (CMRT1 + CMRT2) of all phases; 12.5 h longer (P < 0.05) than HP. The combined residence time of Co (RF) of 11.9 h was the shortest (P < 0.05) of all markers studied. The TMRT in the digestive tract of the cows was 10.6 h longer (P < 0.05) for LP than for HP (59.6 vs. 49.0 h) whereas the TMRT of RF was only 18.4 h.
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). However, with grass silage-based diets, increasing the level of concentrate supplementation has generally not influenced diet digestibility (Huhtanen, 1998). Greater digestibility of SC diet in the present study may be related to a comparatively low digestibility of the grass silage (67.0% OM digestibility) and to the fact that silage was not supplemented with an excessive amount of concentrate. The observed depression in fiber digestion with addition of concentrate to the diet was a result of negative associative effect of feeding concentrate on ruminal fibrolytic activities (Huhtanen and Jaakkola, 1992; Hristov et al., 1999) and fiber digestion in cattle (Robinson et al., 1987; Bines et al., 1988). Higher apparent N digestibility with the SC diet compared to AS can be attributed to higher N concentration of SC. The increase in DM intake was reflected in a considerable increase in milk yield for cows fed diet SC (16 vs. 22 kg/d, AS and SG respectively; P < 0.05).
Specific gravity (or relative density) is the ratio of the density of a substance to that of a standard substance (Encyclopædia Britannica Online, 2002). Feed particles entering the rumen are composed of solid material, gas, and water in different proportions. When density of feed particles is measured on a gas-free sample, it is termed true or intrinsic maximum density (Poncet, 1991). Effective or functional SG (FSG) characterizes particles with their gas spaces intact (Poncet, 1991). In the present experiment, we chose to work with FSG because this characteristic of the feed particles in the reticulo-rumen more truly represents the actual condition of the particles subjected to digestion and passage in the gastrointestinal tract.
In previous research (Wattiaux et al., 1992a), FSG increased with incubation time and was negatively correlated with gas production, suggesting a relationship between available fermentable substrate and particle density. Particle size and SG are interrelated and smaller particles have greater SG than larger particles (Evans et al., 1973; Hooper and Welch, 1985). Siciliano-Jones and Murphy (1991) concluded that FSG of particles in the rumen increased with fermentation and particle size reduction. Consistent with the relationship between particle FSG and gas production, Huhtanen et al. (1993) and Ahvenjärvi et al. (2001) reported decreases in potential NDF digestibility with decreasing particle size. Evans et al. (1973) related particle density and rumination; rumination was triggered when concentration of low-density particles in the rumen was the greatest. Sutherland (1988) suggested that, although particle size correlates with density, density is the primary factor determining the exodus of particles from the rumen. Examination of size distribution of particulate matter entering the duodenum suggests that reduction of particulate matter below the critical size is a prerequisite for passage (Ahvenjärvi et al., 2001) but is not a rate-limiting step because a large fraction of ruminal particles are smaller than critical size. Presumably, passage of small particles is delayed due to entrapment within the particulate mat (Poppi et al., 2001). Consequently, not all small particles can escape from the rumen due to their physical location. In order to escape from the rumen, particles must have the physical characteristics (size and FSG) and be situated in the reticulum where digesta DM is sufficiently low to allow particles to exist in suspension and separate according to physical characteristics. The probability of particle spatial distribution within the rumen may be affected by FSG. In addition, FSG may affect the selective passage of particles from the reticulum through the reticulo-omasal orifice.
Evans et al. (1973), Hooper and Welch (1985), and Siciliano-Jones and Murphy (1991) indicated relationship between size and density of feed particles in the rumen. Evans et al. (1973) reported high correlations (r = 0.95 to 1) between size and density of particles taken from five different sites in the rumen: density increased with decreasing particle size. Similarly, Hooper and Welch (1985) found a gradual decrease in FSG of particles passing through 1, 2, 4, or 6-mm sieves. The distribution by size of HP and LP particles in the present experiment confirms the observations that smaller particle size in the rumen is associated with greater FSG.
There was no difference between the proportions of marker from HP vs. LP migrating to the fluid phase of the in vitro incubation media. Combs et al. (1992) found no difference in the release of Yb or Ce into cell-free ruminal fluid at pH 6.5, but Ce was retained at a greater rate than Yb at pH 2.2. In the present study, a significant proportion of marker was recovered in the small particles/microbial matter fraction of the in vitro incubation media. Combs et al. (1992) also reported a large proportion of Yb or Ce migrated to particles <0.07 mm. A likely explanation is the known high affinity of microorganisms for rare-earth elements (Johnson and Kyker, 1961). Comparatively, a larger proportion of marker was displaced from HP than from LP. Part of the difference between HP and LP may be explained by small (passing through the 38 µm screen) HP particles being recovered in the high-speed pellets.
The higher fiber and indigestible NDF and lower N content of HP compared to LP particles is an indication of less digestible material present in the former fraction of ruminal contents. Hooper and Welch (1985) found a dramatic increase in FSG of grass hay or alfalfa particles with incubation in the rumen of up to 4 h. Wattiaux et al. (1991) also reported an increase in the proportion of higher SG particles and a decrease in the proportion of low-density particles with increasing incubation time in the rumen in sacco, suggesting a lower ruminal degradability of higher SG particles. Similarly, Wattiaux et al. (1992a) observed an increase in FSG of alfalfa hay particles during in vitro digestion. During microbial digestion of particulate matter in the rumen, available substrates are depleted and unavailable substrates such as INDF are concentrated. The increased INDF concentration of HP particles in the present experiment suggests ruminal particles are gradually depleted from digestible matter, a process probably associated with an increase in FSG. Hristov and Broderick (unpublished data from Hristov and Broderick, 1996) observed lower non-ammonia N (NAN) content of ruminal particles with SG greater than 1.2 compared to particles with SG less than 1.2, or ruminal liquor (including particles passing through a 105 µm screen): 0.8, 1.9, and 3.8% (DM basis), respectively. The heavy particles had a remarkably lower concentration of 15N (used as a microbial marker in the study) and despite a comparatively high concentration in the rumen (approximately 15% of ruminal DM) accounted for only 5.7% of the total ruminal microbial NAN. Similarly, HP particles had lower N concentration than LP particles in the present experiment. The process of accumulation of indigestible material in heavier particles could result in less active microbial biofilms, reduced gas production and entrapment, and further increase in particle FSG; detachment, erosion, or sloughing of the biofilms (Characklis, 1990) is a possible explanation of the observed reduction in microbial mass associated with the high FSG ruminal particles.
Previous research suggested ruminal retention times increase with increasing proportion of starch in the diet (Robinson et al., 1987; Poncet, 1991). Other studies, however, found no effect of inclusion of concentrate in all-forage diets or increasing the level of starch in the diet on marker or digesta phases outflow rates or ruminal retention times (Faichney et al., 1997; Reis and Combs, 2000; Yang et al., 2001). The results from the present research also indicated no effect of concentrate on ruminal passage rate or retention time of particles having contrasting FSG. Fractional outflow rate of ruminal fluid, however, was increased with inclusion of concentrate in the diet.
Ehle and Stern (1986) reported particles with lower density were retained longer in the total digestive tract than denser particles. Kaske and Engelhardt (1990) found a negative relationship between density of plastic particles and their MRT in the rumen. A review by Poncet (1991) also indicated increased retention time with decreasing inert particles density. Similarly, Neel et al. (1995) found retention time was greater for particles with 1.1 SG than particles with 1.3 SG. The results from our study confirmed these observations with ruminal digesta particles; particles lighter than FSG 1.02 had longer total tract retention time than particles heavier than FSG 1.02. To our knowledge, this is the first study reporting total tract retention time of particles from ruminal digesta having different FSG. The longer retention time of the lighter particles probably relates to the physical properties of the particles with lower FSG (Poncet, 1991), but may also reflect physiological properties such as substrate availability in parts of the digestive tract harboring microbial activities. Luginbuhl et al. (1994) observed faster passage rate of fecal particles compared to masticated feed particles. Such findings provide direct evidence that freshly ingested feed particles possess properties that delay their escape relative to older, smaller and less digestible particles. The numerically longer residence time in the second, age-independent compartment of LP compared with HP may suggest the former particulate phase is richer in fermentation substrate and requires longer digestion time. The different flow kinetics of LP and HP in this study resulted from differences in the physicochemical characteristics of the two ruminal phases. However, the HP pool had consistently larger proportions of DM in the small particle sizes and higher FSG; thus, FSG and particle size are interrelated and the effects of these variables on passage rate and compartmental retention time cannot be reliably separated. Calculating the compartmental mass from marker kinetics data using the equations of Ellis et al. (1994) suggested that proportionally 0.17 to 0.18 of ruminal DM pool was present in the first, age-dependent compartment. These values are markedly lower than the proportion of LP of total ruminal pool (0.65). The proportion of particles >2,500 µm, which have very limited escape from the rumen in cows given similar diets (Ahvenjärvi et al., 2001; Rinne et al., 2002), was approximately 50% in LP and HP pools. This suggests that distribution of ruminal DM between the two compartments was not reliably estimated from marker kinetics data. Migration of rare earth markers from labeled feed particles to ruminal fluid and microbes (Combs and Satter, 1992) will underestimate the residence time in the first compartment, and thereby the relative contribution of the first compartment to the ruminal DM pool.
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2 Correspondence: P.O. Box 442330 (phone: 208-885-7204; fax: 208-885-6420; E-mail: ahristov{at}uidaho.edu).
Received for publication April 2, 2003. Accepted for publication June 10, 2003.
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dashed line—HP) represent predicted values (YLP = 1.2100 + 0.3900 - 0.0032, R2 = 0.88 and YHP = 1.9922 + 0.2572 - 0.0017, R2 = 0.92; LP and HP, respectively), and error bars indicate SE of the mean predicted values. LP vs. HP, P > 0.05.