The determination of digesta particle size in some herbivores

Methods for determining and describing digesta and faecal particle size were studied. Higher mean particle size values were demonstrated for wet sieving as compared to dry sieving. Equines had larger faecal particle size than ruminants and rabbits. Particle size decreased with body weight in rumen-reticulum and facese for the ruminants. The fibre particle size was the same in rabbit hard and soft faeces and similar to sheep and goat faeces.The excretion pattern of Cr-mordanted fibre in sheep was shown to be influenced by the fraction that was labelled in the rumen. Retention time in the rumen and whole tract increased with increasing particle size.In rabbits, <75 μ or > 300 μ Cr-mordanted fibre particles gave similar marker retention times.     

Effects of dietary fibre on digesta passage, nutrient digestibility, and gastrointestinal tract morphology in the granivorous Mongolian gerbil (Meriones unguiculatus)

To investigate digestive tract performance in Mongolian gerbils (Meriones unguiculatus), food intake and digestibility, digesta passage rate, and gastrointestinal tract morphology were measured in captive animals fed low- or high-fibre diets. We used two markers (Co-ethylene diamine tetra-acetic acid for solutes and Cr-mordanted cell walls for particles) to measure differential passage rates of digesta fractions in order to test for the presence of a colonic separation mechanism (CSM). Although dry-matter intakes on the high-fibre diet did not differ from those on the low-fibre diet, digestibilities of dry matter, neutral-detergent fibre, acid-detergent fibre, crude protein, and crude fat were all significantly lower on the high-fibre diet. Gross energy intake on the high-fibre diet also did not differ from that on the low-fibre diet, but energy lost in faeces was much higher than on the low-fibre diet; thus, energy digestibility and digestible energy intake were significantly lower on the high-fibre diet. The lengths and dry-tissue masses of all segments of the gastrointestinal tract tended to enlarge in response to increased dietary fibre, but only the total tract contents, contents of the small intestine, and length and dry-tissue mass of the caecum increased significantly. The mean retention time (MRT) of the particle marker was significantly greater than that of the solute marker on the low-fibre but not the high-fibre diet; the solute/particle differential retention ratio was 0.62 on the low-fibre diet and 0.90 on the high-fibre diet. Thus, there was no evidence for selective retention of the solute marker on either diet. The MRT of the particle marker was significantly lower on the high-fibre diet and in the same direction as the MRT of the solute marker. These results suggest that the granivorous Mongolian gerbil has no CSM but can adjust its digestive tract capacity to accommodate greater quantities of low-quality food.     

No distinct difference in the excretion of large particles of varying size in a wild ruminant,the banteng (<Emphasis Type="Italic">Bos javanicus</Emphasis>)

The forestomach of ruminants and camelids does not only allow a differential excretion of fluids and small particles but also a differential excretion of small and large particles. The question whether larger particles of different size classes are also retained for different time periods, or whether simply a particle-size threshold exists above which all particles of a size higher than this threshold are retained in an undifferentiated manner, has not been addressed so far. We determined the mean retention time (MRT) of different-sized large particles (10 and 20 mm) in three banteng (Bos javanicus) on two forage only diets, grass and grass hay. We used cerium-mordanted fibre (10 mm) and lanthanum-mordanted fibre (20 mm) as particle markers, mixed in the food. Average total tract MRT for large and very large particles at the grass diet was 58 and 56 h and at the grass hay diet 66 and 64 h, respectively. Very large particles moved slightly faster than large particles through the gut of the banteng. Three interpretations are possible: Very large particles are resubmitted to rumination sooner than large particles; ingestive mastication of the particle markers could have reduced the difference in the size of the particle markers; alternatively, particle retention may be governed by a threshold, above which all particles of a size higher than this threshold are retained in an undifferentiated manner. In order to test these possibilities, experiments with fistulated animals would have to be performed.     

Intake, ingesta retention, particle size distribution and digestibility in the hippopotamidae

Although several aspects of the digestive physiology of the hippopotamidae-non-ruminating foregut fermenters-have been described, ingesta kinetics and passage characteristics of these species are not well understood. The most outstanding feature of the hippo digestive physiology reported so far is the very long mean ingesta retention times (MRTs) measured by Foose [Foose, T., 1982. Trophic strategies of ruminant versus nonruminant ungulates. PhD dissertation, University of Chicago, Chicago.]. Since those data had been investigated with animals without water access, we intended to measure MRT in hippos which were allowed to enter water pools during the night. MRT parameters as well as dry matter (DM) digestibility were determined in four common (Hippopotamus amphibius) and four pygmy hippos (Hexaprotodon liberiensis) on two different diets each using cobalt ethylendiamintetraacetate (Co-EDTA) as a fluid, chromium (Cr)-mordanted fibre (<2 mm) as a particle and acid detergent lignin (ADL) as an internal digestibility marker. Four of the animals additionally received cerium (Ce)-mordanted fibres (2-10 mm) as particle markers. Total MRTs for fluids and particles ranged between 20-35 and 48-106 h in the common and between 13-39 and 32-107 h in the pygmy hippos. The difference between fluid and particle retention was greater than usually reported in ruminants. Excretion patterns of the markers differed from those usually observed in ruminants but resembled those reported for macropods (kangaroos), indicating a plug-flow reactor-like physiology in the hippo forestomach (FRST). This finding complements other described similarities between the macropod and the hippo forestomach. The measurements of larger particle retention profiles suggest that in the hippo, larger particles might be excreted either faster or at the same rate as smaller particles, indicating a general difference between ruminants and hippos with respect to differential particle retention. The digestive physiology of hippos is characterised by a generally low food intake, long ingesta retention times and dry matter digestibilities lower than reported in ruminants. Moderate digestibilities in spite of long retention times might be the result of the generally high average ingesta particle size in hippos. The comparatively easy management of pygmy hippos, together with the significant correlations between food intake, MRT and digestibility in the pygmy hippos of this study, recommends this species for further studies on the interplay of these parameters in herbivore digestive physiology.     

Demonstrating coprophagy with passage markers? The example of the plains viscacha (Lagostomus maximus)

Coprophagy, or the ingestion of a certain fraction of the faeces, is a well-known strategy of small herbivores. However, the question of whether a particular species actually uses this cryptic strategy or not is not easily answered experimentally. In this study we introduce the use of ingesta passage markers as a tool to verify coprophagy in species where individual observation might be difficult. In two captive adult plains viscachas (Lagostomus maximus), we demonstrate recurrent excretion peaks for both a fluid (cobald-EDTA) and a particle (chromium-mordanted fibre) marker, the most parsimonious explanation for which is faeces re-ingestion. In addition, a literature review of graphical presentations of passage marker excretion revealed such recurrent marker peaks in a large number of studies; however, these observations were rarely explicitly contributed to a coprophagic digestion strategy. The widespread use of semi-logarithmic plots or cumulative marker excretion curves in displaying passage studies makes recurrent marker excretion peaks less evident. We conclude that passage markers offer a comparatively easy way of assessing the incidence of coprophagy. Due to the reported absence of coprophagy in rabbits, a species well-known for using this strategy, on high-protein, low-fibre diets, it is recommended that investigations of the occurrence of coprophagy should be made with animals fed challenging, high-fibre diets.     

Differential passage of fluids and different-sized particles in fistulated oxen (Bos primigenius f. taurus), muskoxen (Ovibos moschatus), reindeer (Rangifer tarandus) and moose (Alces alces): Rumen particle size discrimination is independent from contents stratification

Ruminant species differ in the degree that their rumen contents are stratified but are similar insofar that only very fine particles are passed from the forestomach to the lower digestive tract. We investigated the passage kinetics of fluid and particle markers (2, 10 and 20 mm) in fistulated cattle (Bos primigenius f. taurus), muskoxen (Ovibos moschatus), reindeer (Rangifer tarandus) and moose (Alces alces) on different diets. The distribution of dry matter in the rumen and the viscosity of rumen fluids suggested that the rumen contents were more stratified in muskoxen than moose. Correspondingly, as in previous studies, the species differed in the ratio of mean retention times of small particles to fluids in the reticulorumen, which was highest in cattle (2.03) and muskoxen (1.97–1.98), intermediate in reindeer (1.70) and lowest in moose (0.98–1.29). However, the ratio of large to small particle retention did not differ between the species, indicating similarity in the efficiency of the particle sorting mechanism. Passage kinetics of the two largest particle classes did not differ, indicating that particle retention is not a continuous function of particle size but rather threshold-dependent. Overall, the results suggest that fluid flow through the forestomach differs between ruminant species. A lower relative fluid passage, such as in moose, might limit species to a browse-based dietary niche, whereas a higher relative fluid passage broadens the dietary niche options and facilitates the inclusion of, or specialization on, grass. The function of fluid flow in the ruminant forestomach should be further investigated.     

Fluid and particle retention in captive okapi (Okapia johnstoni)

Retention time of food in the digestive tract is among the key variables that describe the digestive strategy of a herbivore. Mean retention time (MRT) was measured on 4 captive specimens of the okapi, a strictly browsing ruminant. Retention time was quantified on different diets, using Co-EDTA (fluid phase) and Cr-mordanted fibres (1–2 mm) (particle phase) as pulse-fed markers. Average food intake was 55–65 g DM/(kg BW^0.75*d). Fecal excretion of the markers was quantified over 10 days. Different models to calculate retention time and passage rate in the gastrointestinal tract (GIT) and the reticulorumen (RR) were applied. Average MRT_particleGIT was quantified to be 47 ± 8 h and MRT_fluidGIT 36 ± 5 h. Concerning estimation of retention times in the reticulorumen, MRT_particleRR was quantified to be 27 ± 7 h, while MRT_fluidRR was 17 ± 4 h. The quotients MRT_particle/MRT_fluid were quantified to be 1.3 ± 0.1 for the GIT and 1.6 ± 0.2 for the RR. Compared to data established with comparable markers, the okapi has low coefficients of MRT_particle/MRT_fluid. A less well developed retention mechanism for fibres compared to species like cattle or sheep can be explained by a comparatively high fermentation rate and low digestibility of the natural food of the okapi—browse—in comparison to grass.     

Digestive performance and selective digesta retention in the long-nosed bandicoot,Perameles nasuta,a small omnivorous marsupial

Bandicoots are opportunistic omnivores that feed on invertebrates, fungi and both epigeal and hypogeal plant parts. We examined the performance of the digestive tract of the long-nosed bandicoot (Perameles nasuta) in terms of intake and total digestibility, patterns of excretion of inert digesta markers, and likely sites of digesta retention, on two diets designed to mimic part of their natural plant and insect diets. On the insect diet (mealworm larvae), bandicoots virtually maintained body mass at a digestible energy intake of 511 kJ · kg^-0.75 · day^-1 and were in strongly positive nitrogen balance. In contrast, on the plant diet (shredded sweet potato), bandicoots ate only one-third as much digestible energy, lost 7% body mass, and were in negative nitrogen balance. Mean retention times of two particle markers on the plant diet (27.5 and 27.0 h) were more than double those on the insect diet (12.4 and 11.2 h), and on both diets the mean retention time of the fluid digesta marker was greater than those of the particle markers, indicating consistent selective retention of fluid digesta in the gut. It was seen radiographically than in mealwormfed bandicoots major sites of digesta retention were the distal colon and rectum, whereas in the sweet potato-fed animals the caecum and proximal colon were principal sites. It was concluded that retention of plant material in the caecum and proximal colon (the main sites of microbial digestion) and the preferential retention of fluid digesta (together with bacteria and small feed particles) in the caecum were important factors in the ability of bandicoots to switch between insect and plant foods, depending on relative availabilities, and thus to exploit nutritionally unpredictable environments.Abbreviations ADF acid-detergent fibre - bm body mass - Co-ED-TA cobalt-ethylenediaminetetra-acetic acid - CWC cell wall constituents - DE digestable energy - dm dry matter - EUN endogenous urinary nitrogen - ICP inductively-coupled plasma atomic emission spectroscopy - MFN metabolic faecal nitrogen - MRT mean retention time - NDF neutral-detergent fibre - ww wet weight     

Retention of solutes and different-sized particles in the digestive tract of the ostrich (Struthio camelus massaicus), and a comparison with mammals and reptiles

Ostriches (Struthio camelus) achieve digesta retention times, digesta particle size reduction and digestibilities equal to similar-sized herbivorous mammals, in contrast to some other avian herbivores. The sequence of digestive processes in their gastrointestinal tract, however, is still unexplored. Using two groups of four ostriches (mean body mass 75.1 ± 17.3 kg) kept on fresh alfalfa, we tested the effect of two intake levels (17 and 42 g dry matter kg(-0.75)d(-1)) on the mean retention time (MRT) of a solute and three different-sized (2, 10, 20 mm) particle markers, mean faecal particle size (MPS), and digestibility. Intake level did not affect MRT, but MPS (0.74 vs. 1.52 mm) and dry matter digestibility (81 vs. 78%). The solute marker (MRT 22-26 h) was excreted faster than the particle markers; there was no difference in the MRT of 10 and 20 mm particles (MRT 28-32 h), but 2mm particles were retained longer (MRT 39-40 h). Because the solute marker was not selectively retained, and wet-sieving of gut contents of slaughtered animals did not indicate smaller particles in the caeca, the long MRT of small particles is interpreted as intermittent excretion from the gizzard, potentially due to entrapment in small grit. The marker excretion pattern also showed intermittent peaks for all markers in five of the animals, which indicates non-continuous outflow from the gizzard. When adding our data to literature data on avian herbivores, a dichotomy is evident, with ostrich and hoatzin (Opisthocomus hoazin) displaying long MRTs, high digestibilities, and gut capacities similar to mammalian herbivores, and other avian herbivores such as grouse, geese or emus with shorter MRTs, lower fibre digestibilities and lower gut capacities. In the available data for all avian herbivores where food intake and MRTs were measured, this dichotomy and food intake level, but not body mass, was related to MRT, adding to the evidence that body mass itself may not be sole major determinant of digestive physiology. The most striking difference between mammalian and avian herbivores from the literature is the fundamentally lower methane production measured in the very few studies in birds including ostriches, which appears to be at the level of reptiles, in spite of general food intake levels of a magnitude as in mammals. Further studies in ostriches and other avian herbivores are required to understand the differences in digestive mechanisms between avian and mammalian herbivores.     

Selective digesta retention and coprophagy in Brandt's vole (Microtus brandti)

Digestive performance, gut morphology and rate of digesta passage were measured in Brandt's voles (Microtus brandti) offered pelleted diets of low (25% neutral-detergent fibre) and high (38%) fibre content. Digestion coefficients of dry matter, crude fat, crude protein, energy and fibre were all significantly lower on the higher fibre diet. Although not significantly higher, dry matter intakes were more than maintained when extra cellulose was included in the diet, so that intakes of digestible energy were only 22% lower on the higher fibre diet. Total length and total gut tissue weight increased significantly, and the length and tissue weight of the caecum, proximal colon, and distal colon were significantly greater on the higher fibre diet as well. Total tract mean retention time (MRT) of a solute marker (Co-EDTA) was significantly greater than that of a particle marker (Cr-cell walls) on the lower fibre diet, and in the same direction on the higher fibre diet. The ratio of solute to particle MRTs (the solute/particle differential retention ratio) was 1.45 on the lower fibre diet and 1.19 on the higher fibre diet. There were no significant differences in marker MRTs between diets. Examination of marker concentrations in the stomach, small intestine, caecum and colon of voles killed at 0.5-h intervals after a pulse dose of Co-EDTA indicated that the marker was recycled to the stomach by coprophagy. Thus, as in other microtine rodents, an increase in gut capacity, selective digesta retention and recycling of digesta via coprophagy enables Brandt's voles to utilise diets of higher fibre content than may be expected for such a small (45 g) mammalian herbivore.     

Excretion patterns of fluid and different sized particle passage markers in banteng (Bos javanicus) and pygmy hippopotamus (Hexaprotodon liberiensis): two functionally different foregut fermenters

Processing of ingesta particles plays a crucial role in the digestive physiology of herbivores. In the ruminant forestomach different sized particles are stratified into a small and a large particle fraction and only the latter is regurgitated and remasticated to smaller, easier-to-digest particles. In contrast, it has been suggested that in non-ruminating foregut fermenters, such as hippopotamuses, larger particles should be selectively excreted since they tend to be digested at a slower rate and hence can be considered intake-limiting bulk. In our study we determined the mean retention time (MRT) of fluids and different sized particles (2 mm and 10 mm) in six pygmy hippos (Hexaprotodon liberiensis) and six banteng (Bos javanicus) on a diet of fresh grass at two intake levels. We used cobalt ethylendiamintetraacetate (Co-EDTA) as fluid and chromium (Cr)-mordanted fibre (2 mm) and cerium (Ce)-mordanted fibre (10 mm) as particle markers, mixed in the food. Average total tract MRT for fluid, small and large particles at the high intake level was 32, 76 and 73 h in pygmy hippos and 25, 56 and 60 h in banteng, and at the low intake level 39, 109, and 105 h in pygmy hippos and 22, 51 and 58 h in banteng, respectively. In accordance with the prediction, large particles moved faster than, or as fast as the small particles, through the gut of pygmy hippos. In contrast, large particles were excreted slower than the small particles in the ruminant of this study, the banteng. Pygmy hippos had longer retention times than the banteng, which probably compensate for the less efficient particle size reduction. Although the results were not as distinct as expected, most likely due to the fact that ingestive mastication of the larger particle marker could not be prevented, they confirm our hypothesis of a functional difference in selective particle retention between ruminating and non-ruminating foregut fermenters.     

Magnesium absorption as influenced by the rumen passage kinetics in lactating dairy cows fed modified levels of fibre and protein

The potassium sensitive magnesium absorption through the rumen wall may be influenced by additional dietary properties, such as diet type, forage type or forage to concentrate ratio. These properties are likely associated to rumen passage kinetics modified by dietary fibre content. The study aimed to assess the effects of rumen passage kinetics on apparent Mg absorption and retention in lactating dairy cows fed modified levels of fibre. Six lactating Red-Holstein and Holstein cows, including four fitted with ruminal cannulas were randomly assigned to a 3 × 3 cross-over design. The experimental diets consisted of early harvested low NDF (341 g NDF/kg DM) and late harvested high NDF (572 g NDF/kg DM) grass silage (80% DM) and of concentrates (20% of DM). As the low-fibre diet was excessive in protein, a third high-fibre diet was formulated to be balanced in digestible protein with the low-fibre diet to avoid any eventual confounding effects of NDF and protein excess. All diets were formulated to contain iso-Ca, -P, -Mg, -K and -Na. Passage kinetics of solid and liquid phase of rumen digesta were evaluated using ruminal marker disappearance profiles. Cows fed the low-fibre diet had compared to the other diets, an up to 40% lower solid and 26% lower liquid phase volume of rumen digesta and a 10% numerically higher fractional rumen liquid passage rate. Rumen pH lost 0.6 units and Mg concentration in the rumen liquid phase tripled when cows were fed the low-fibre diet. Faecal Mg excretion was up to 14% higher in cows fed the low-fibre diet and Mg absorbability was 12% compared to up to 19% in other diets. Urinary Mg excretion in cows fed the low-fibre diet was half of the ones in the other treatments, but Mg retention was not affected. Dietary protein excess neither affected rumen passage kinetics nor Mg absorption and retention. Absorption of Mg was correlated with rumen liquid volume which both decreased with decreasing daily NDF intake (NDFi, 11.8 ± 2.4 l/kg NDFi). Consequently, daily Mg absorption decreased by 1.32 ± 0.28 g/kg decreasing NDFi. To conclude, in addition to the known antagonistic effect of dietary K, the present data indicate that Mg absorption was dependent from NDFi which modified rumen liquid volume, but was independent of dietary protein excess likely associated to low NDF herbages.     

Fluid and particle retention in the digestive tract of the addax antelope (Addax nasomaculatus)--adaptations of a grazing desert ruminant.

Retention time of food in the digestive tract is a major aspect describing the digestive physiology of herbivores. Differences in feed retention times have been described for different ruminant feeding types. In this study, a dominantly grazing desert ruminant, the addax (Addax nasomaculatus), was investigated in this respect. Eight animals with a body weight (BW) of 87+/-5.3 kg on an ad libitum grass hay (Chloris gayana) diet were available. Co-EDTA and Cr-mordanted fibers (<2 mm) were used as pulse-dose markers. Mean retention time (MRT) in the digestive tract was calculated from faecal marker excretion. Average daily intake of the addax was found to be 1.7 kg dry matter (DM) or 60+/-8.3 g DM/kg BW(0.75). The MRT of fluid and particles in the reticulo-rumen (MRT(fluid)RR and MRT(particle)RR) were quantified to be 20+/-5.8 and 42+/-7.0 h respectively. When compared to literature data, MRT(fluid)RR was significantly longer than in cattle species, and MRT(particle)RR was significantly longer than in 11 taxa of all feeding types. The ratio of MRT(particle)RR/MRT(fluid)RR (2.3+/-0.5) was found to be within the range described for grazing ruminants. The long retention times found in the addax can be interpreted as an adaptation to a diet including a high proportion of slow fermenting grasses, while the long retention time of the fluid phase can be interpreted as a consequence of water saving mechanisms of the desert-adapted addax with a potentially low water turnover and capacious water storing rumen.     

Fluid and particle retention in the digestive tract of the addax antelope (Addax nasomaculatus)--adaptations of a grazing desert ruminant

Retention time of food in the digestive tract is a major aspect describing the digestive physiology of herbivores. Differences in feed retention times have been described for different ruminant feeding types. In this study, a dominantly grazing desert ruminant, the addax (Addax nasomaculatus), was investigated in this respect. Eight animals with a body weight (BW) of 87+/-5.3 kg on an ad libitum grass hay (Chloris gayana) diet were available. Co-EDTA and Cr-mordanted fibers (<2 mm) were used as pulse-dose markers. Mean retention time (MRT) in the digestive tract was calculated from faecal marker excretion. Average daily intake of the addax was found to be 1.7 kg dry matter (DM) or 60+/-8.3 g DM/kg BW(0.75). The MRT of fluid and particles in the reticulo-rumen (MRT(fluid)RR and MRT(particle)RR) were quantified to be 20+/-5.8 and 42+/-7.0 h respectively. When compared to literature data, MRT(fluid)RR was significantly longer than in cattle species, and MRT(particle)RR was significantly longer than in 11 taxa of all feeding types. The ratio of MRT(particle)RR/MRT(fluid)RR (2.3+/-0.5) was found to be within the range described for grazing ruminants. The long retention times found in the addax can be interpreted as an adaptation to a diet including a high proportion of slow fermenting grasses, while the long retention time of the fluid phase can be interpreted as a consequence of water saving mechanisms of the desert-adapted addax with a potentially low water turnover and capacious water storing rumen.     

Soil Ingestion Estimates for Children in Anaconda Using Trace Element Concentrations in Different Particle Size Fractions

This investigation assessed the effect of soil particle size on soil ingestion estimates of children residing at a superfund site. Earlier research indicated that wide intertracer variability in soil ingestion estimates are based on soil concentrations with a soil particle size of 0 to 2?µm was markedly reduced when the estimates were based on soil tracer concentrations for a soil particle size of 0-250?µm. The reduced intertracer variation was principally attributed to changes in soil concentrations of only three of the soil tracers (i.e., Ce, La, Nd) which became concentrated in the finer particle size by approximately 2.5 to 4.0-fold. It was hypothesized that the intertracer agreement in soil ingestion estimates may continue to improve if the estimates are based on concentrations of tracers at finer particle sizes assuming that children ingest finer particles and that the above three tracers would continue to be further concentrated in the finer sized soil particles. The principal findings indicate: 1. The soil concentrations of Al, Si, and Ti do not increase at the two finer particle size ranges measured. 2. The soil concentrations of Ce, La, and Nd increased by a factor 2.5 to 4.0 in the 100 to 250?µm particle size range when compared with the 0 to 2?µm particle size range. No further substantial increase in concentration was observed in the 53 to 100 |jm particle size range. 3. The soil ingestion estimates are consistently and markedly changed only between the estimates based in 0 to 2?µm and 100 to 250?µm for Ce, La, and Nd. These changes reduced the intertracer variability in estimating soil ingestion, suggesting that the children eat finer soil particle sizes. 4. Because the particle sizes for all tracers (except Zr) were only modestly affected at the 53 to 100?µm range, it was not possible to confidently resolve the particle size of soil ingested by the children. 5. Residual intertracer variability in soil ingestion estimates based on Ce, La, Nd are likely to be significantly affected by non-food, non-soil sources of these tracers (i.e., source error). 6. Soil ingestion estimates of this study will be more reliable when derived from the finer-sized particles.     

Solute and particle retention in the digestive tract of the Phillip's dikdik (Madoqua saltiana phillipsi), a very small browsing ruminant: biological and methodological implications

Morphological characteristics of the forestomach, as well as reports of a natural diet that mostly excludes monocots, suggest that dikdiks (Madoqua spp.), among smallest extant ruminants, should have a 'moose-type' forestomach physiology characterised by a low degree of selective particle retention. We tested this assumption in a series of feeding experiments with 12 adult Phillip's dikdiks (Madoqua saltiana phillipsi) on three different intake levels per animal, using cobalt-EDTA as a solute marker and a 'conventional' chromium-mordanted fibre (<2 mm; mean particle size 0.63 mm) marker for the particle phase. Body mass had no influence on retention measurements, whereas food intake level clearly had. Drinking water intake was not related to the retention of the solute marker. In contrast to our expectations, the particle marker was retained distinctively longer than the solute marker. Comparisons with results in larger ruminants and with faecal particle sizes measured in dikdiks suggested that in these small animals, the chosen particle marker was above the critical size threshold, above which particle delay in the forestomach is not only due to selective particle retention (as compared to fluids), but additionally due to the ruminal particle sorting mechanism that retains particles above this threshold longer than particles below this threshold. A second study with a similar marker of a lower mean particle size (0.17 mm, which is below the faecal particle size reported for dikdiks) resulted in particle and fluid retention patterns similar to those documented in other 'moose-type' ruminants. Nevertheless, even this smaller particle marker yielded retention times that were longer than those predicted by allometric equations based on quarter-power scaling, providing further support for observations that small ruminants generally achieve longer retention times and higher digestive efficiencies than expected based on their body size.     

Digesta retention and fibre digestion in brushtail possums, ringtail possums and rabbits

1. Mean retention times (MRTs) of fluid (marked with Co-EDTA), fine particles (mordanted with Yb) and large particles (mordanted with Cr) were measured in brushtail possums (Trichosurus vulpecula), ringtail possums (Pseudocheirus peregrinus) and laboratory rabbits fed semipurified diets. 2. In brushtail possums there were no significant differences in MRT among the three digesta markers. 3. In ringtail possums MRTs of the fluid and fine particle markers were approximately twice that of the large particle marker, indicative of selective retention of both fluid and fine particles in the caecum. 4. In the rabbit MRT of fine particles was also greater than that of large particles, again indicative of selective retention of fine particles in the caecum. 5. Fibre digestibility was greater in the rabbits than in the ringtail possums, and greater for neutral-detergent fibre (including agar) but less for acid-detergent fibre in the rabbits than in the brushtails. Differences in fibre digestibility between brushtails and rabbits were explained by differences in patterns of digesta flow. However, the higher digestibilities of fibre in the rabbits than in the ringtail possums could not be explained on a similar basis.     

More efficient mastication allows increasing intake without compromising digestibility or necessitating a larger gut: Comparative feeding trials in banteng (Bos javanicus) and pygmy hippopotamus (Hexaprotodon liberiensis)

The digestion of plant material in mammalian herbivores basically depends on the chemical and structural composition of the diet, the mean particle size to which the forage is processed, and the ingesta retention time. These different factors can be influenced by the animal, and they can presumably compensate for each other. The pygmy hippopotamus, a non-ruminating foregut fermenter, has longer mean retention times than ruminants; however hippos do not achieve higher (fibre) digestibilities on comparable diets, which could be due to ineffective mastication. We performed feeding trials with six pygmy hippos (Hexaprotodon liberiensis) and six banteng cattle (Bos javanicus) on a grass diet. As predicted, both species achieved similar dry matter, organic matter, crude protein and gross energy digestibilities. However, neutral and acid detergent fibre digestibility was lower in pygmy hippos. Apparently, in these species, fibre digestibility was more influenced by particle size, which was larger in pygmy hippos compared to banteng, than by retention time. In spite of their higher relative food intake, the banteng in this study did not have greater relative gut fills than the hippos. Ruminants traditionally appear intake-limited when compared to equids, because feed particles above a certain size cannot leave the rumen. But when compared to nonruminating foregut fermenters, rumination seems to free foregut fermenters from an intrinsic food intake limitation. The higher energy intakes and metabolic rates in wild cattle compared to hippos could have life-history consequences, such as a higher relative reproductive rate.     

Digestive strategies and food choice in mantled howler monkeys Alouatta palliata mexicana: bases of their dietary flexibility

Mantled howler monkeys (Alouatta palliata) occupy a wide variety of tropical habitats and are the most folivorous of New World primates. However, their diet may include fruits, buds, petioles, and flowers, as well as leaves, suggesting they must cope with variations in the nutrient composition of their food. We studied the physiological basis of the dietary flexibility of these monkeys by comparing food choice, digestive performance and patterns of digesta flow in six adults, fed diets of either leaves or a mixture of fruit and leaves. Although monkeys ate similar amounts of the two diets, they ingested more digestible protein when offered the leaf diet, on which they lost body mass, but they ingested much more soluble sugars when offered fruit and leaves on which they gained mass. Digestibilities of dry matter, fat, energy and fibre did not differ between diets, but those of crude protein, soluble sugars and minerals were higher on the fruit–leaf diet. Mean retention times in the gut of solute (Co-EDTA) and particulate markers (Cr-mordanted cell walls) did not differ between diets, but on both diets the monkeys retained the particulate marker (mean retention time ca 55 h) for longer than they did the solute marker (MRT ca 50 h). A lack of selective retention of solutes and small particles in the gastro-intestinal tract of howler monkeys probably restricts them to mixed diets but their digestive strategy is sufficiently flexible to allow them to feed on a diet of leaves when fruit is unavailable.     

Effects of a dietary organic acid mixture and of dietary fibre levels on ileal and faecal nutrient apparent digestibility, bacterial nitrogen flow, microbial metabolite concentrations and rate of passage in the digestive tract of pigs

Six 34-kg barrows were fitted with a post-valve T-caecum cannula and assigned to six dietary treatments according to a 6 × 5 change-over design to study how a mixture of formic acid, sorbate, and benzoate (0 or 8.4 g/kg feed) influences apparent ileal and faecal digestibility coefficients, bacterial nitrogen (N) flow, microbial metabolite concentrations, and passage rate in pigs fed isoenergetic diets with medium, high, or very high fibre content (neutral-detergent fibre (NDF): 199, 224, and 248 g/kg dry matter, respectively). These barley and soya-bean meal based diets contained 0, 75, and 150 g/kg barley fibre (NDF: 577 g/kg) and 0, 8, and 16 g/kg rapeseed oil, respectively. The dietary organic acid mixture improved the apparent ileal digestibility of 14 of the 17 amino acids analysed (P < 0.05). Increasing levels of dietary fibre linearly decreased the apparent ileal digestibility of six of the 17 amino acids analysed (P < 0.05). Ileal flows of bacterial N and amino acids as assessed on the basis of purine flow were decreased by the dietary organic acid mixture (P < 0.05) but were not affected by dietary fibre level (P>0.05). As assessed on the basis of diaminopimelic acid flow, bacterial N flow was increased by both the dietary organic acid mixture and increased dietary fibre levels (P < 0.05). The dietary organic acid mixture reduced the concentration of lactic acid and increased that of acetic acid in ileal digesta (P < 0.05), while dietary fibre levels had a quadratic effect on concentrations of acetic, propionic, and butyric acid (P < 0.05). The mean retention time of Co (solute marker) and Yb (particle marker) in the large intestine decreased in a linear manner by increasing dietary fibre levels (P < 0.05) but was not affected by the dietary organic acid mixture (P>0.05). The results show that a dietary organic acid mixture has a positive effect on the apparent ileal digestibility of most amino acids irrespective of dietary fibre levels. This could be at least partly related to changes in bacterial N flow in the ileum. However, different bacterial markers showed opposite effects on bacterial N flow, which makes it questionable to use a constant bacterial marker / bacterial N ratio to estimate bacterial N flow. Increasing levels of dietary fibre had negative effects on the apparent ileal amino acid digestibilities and shortened the mean retention time of digesta in the large intestine.