Digesta retention patterns of solute and different-sized particles in camelids compared with ruminants and other foregut fermenters

Teeth and the gastrointestinal tract in mammals: when 1 + 1 = 3

Both teeth and the digestive tract show adaptations that are commonly interpreted in the context of trophic guilds-faunivory, herbivory and omnivory. Teeth prepare food for the digestive tract, and dental evolution focuses on increasing durability and functionality; in particular, size reduction of plant particles is an important preparation for microbial fermentative digestion. In narratives of digestive adaptations, microbes are typically considered as service providers, facilitating digestion. That the majority of 'herbivorous' (and possibly 'omnivorous') mammals display adaptations to maximize microbes' use as prey-by harvesting the microbes multiplying in their guts-is less emphasized and not reflected in trophic labels. Harvesting of microbes occurs either via coprophagy after separation from indigestible material by a separation mechanism in the hindgut, or from a forestomach by a 'washing mechanism' that selectively removes fines, including microbes, to the lower digestive tract. The evolution of this washing mechanism as part of the microbe farming niche opened the opportunity for the evolution of another mechanism that links teeth and guts in an innovative way-the sorting and cleaning of not-yet-sufficiently-size-reduced food that is then re-submitted to repeated mastication (rumination), leading to unprecedented chewing and digestive efficiency. This article is part of the theme issue 'Food processing and nutritional assimilation in animals'.

Fluid and particle retention in a small New World and a small Old World cervid, the southern pudu (Pudu puda) and Reeves's muntjac (Muntiacus reevesi)

Ruminants differ in the pattern how small particles and liquids pass through their gastrointestinal tract, and in particular their reticulorumen (RR). Based on that they may be classified into 'moose-type' and 'cattle-type' species (smaller and larger differences between particle and liquid passage, respectively). The ratio between the retention of particles and fluids is called the 'selectivity factor' (SF) and is a species-specific characteristic, studied in tragulids, giraffids and bovids, but not in many cervid species. Recently, it has been suggested that a high SF might also serve to wash digesta clean of external abrasives prior to regurgitation for rumination. In this study, we measured SF and passage kinetics (using a liquid marker and markers of different particle size, fed with the diet) in a capreoline deer, the southern pudu (Pudu puda, n = 5, 10.3 ± 2.9 kg, kept at two zoos) and a cervine deer, the Reeves's muntjac (Muntiacus reevesi, n = 6, 11.0 ± 1.7 kg, kept at a research facility). The relative daily dry matter intake (38 ± 3 g/kg0.75 for pudu and 76 ± 5 g/kg0.75 for muntjac) was higher, and the mean retention times (MRT) correspondingly shorter (e.g., MRT small particles in the total digestive tract 39 ± 8 h for pudu and 15 ± 2 h for muntjac), in the muntjac. The SF for small particles/liquid in the reticulorumen were, however, similar for both species, at 1.47 ± 0.21 for pudu and 1.66 ± 0.20 for muntjac, indicating a 'moose-type' physiology for both, irrespective of their different phylogenetic origin. To date, SF recorded in bovids attain distinctively higher values than the few reported for cervids. This situation reflects the degree of hypsodonty (tooth crown height) attained by these taxa, which is higher in bovids than in cervids. Together, constraints in hypsodonty as well as SF might limit cervids to more mesic habitats without distinct loads of external abrasives (such as dust or grit) on their food. In both species, some animals showed the typical ruminant pattern of a longer MRT for large than for small particle markers, but in some animals, this difference was not evident. This may be due to variable degrees of marker chewing during ingestion.

A pilot investigation on the effect of induced saliva flow on digestive parameters in sheep, and a comparison with cattle

Sheep with a relatively low methane yield were observed to have shorter fluid and particle mean retention times (MRT). Because the application of pilocarpine, a saliva stimulant, was successful in reducing retention times in ruminants in previous studies, we applied this substance to sheep, expecting a reduction in MRT and methane yield. Three non-pregnant sheep (74 ± 10 kg) were fed a hay-only diet in a 3 × 3 Latin square design with oral doses of 0, 2.5 and 5 mg pilocarpine/kg body weight and day. Measurements included feed and water intake, MRT of liquid and particulate phases in the reticulorumen (RR) and total gastrointestinal tract (GIT), ruminal microbial yield (via urinary purine bases and metabolic faecal nitrogen), total tract methane emission, apparent nutrient digestibility and rumen fluid parameters. Data were investigated for linear and quadratic effects using orthogonal polynomial contrasts. The MRT of liquid and small particles in the RR and total GIT, and the short-chain fatty acid concentration in rumen fluid, linearly declined with increasing pilocarpine dosage, while no quadratic relationship was detected. Intake of feed DM and water, apparent nutrient digestibility, methane yield and microbial yield were not affected by pilocarpine. When combining the sheep data with that of a similar experiment in cattle, we found that the MRT of the liquid phase was positively associated with estimated NDF digestibility and with methane production per digested NDF, but was not associated with microbial yield or the ratio of acetate to propionate. The ratio between MRT of the particulate and the liquid phase was smaller for sheep than that for cattle, and was not affected by treatment. Differences in this ratio might explain why species reacted differently to the saliva-inducing agent, which might help to explain the discrepancy between species in the effect of induced saliva flow on digestive parameters.

Equid nutritional physiology and behavior: an evolutionary perspective

Like other members of the even-toed ungulates (the perissodactyls), equids once had a higher species diversity in the fossil record than they have today. This is generally explained in comparison to the enormous diversity of bovid ruminants. Theories on putative competitive disadvantages of equids include the use of a single toe as opposed to two toes per leg, the lack of a specific brain cooling (and hence water-saving) mechanism, longer gestation periods that delay reproductive output, and in particular digestive physiology. To date, there is no empirical support for the theory that equids fare better on low-quality forage than ruminants. In contrast to the traditional juxtaposition of hindgut and foregut fermenters, we suggest that it is more insightful to sketch the evolution of equid and ruminant digestive physiology as a case of convergence: both evolved a particularly high chewing efficacy in their respective groups, which facilitates comparatively high feed and hence energy intakes. But because the ruminant system, less based on tooth anatomy but more on a forestomach sorting mechanism, is more effective, equids depend more on high feed intakes than ruminants and may well be more susceptible to feed shortages. Arguably, the most under-emphasized characteristic of equids may be that in contrast to many other herbivores including ruminants and coprophageous hindgut fermenters, equids do not use the microbial biomass growing in their gastrointestinal tract. Equids display behavioral and morphophysiological adaptations to high feed intakes, and their cranial anatomy that facilitates the cropping of forage while performing grinding chewing at the same time might be unique. Rather than looking for explanations how equids are better adapted to their present niches than other organisms, considering them remnants of a different morphophysiological solution may be more appropriate.

Is there a difference in ruminal fermentation control between cattle and sheep? A meta-analytical test of a hypothesis on differential particle and fluid retention

Ruminant species differ in digestive physiology. The species-specific ratio of mean retention time of particles and fluid (MRT_particle/MRT_fluid) in the reticulorumen has been interpreted as controlling ruminal fermentation: a higher ratio indicates of a more distinct 'washing' of particulate digesta by liquid. This should increase the harvest of microbes from the reticulorumen, and keep the microbiome in a state of more intense growth; at the same time, this should increase the metabolic losses of faecal nitrogen of microbial origin, leading to lower values for the apparent digestibility of crude protein (aD CP). A systematic difference has been hypothesized between cattle (higher ratio) and sheep (lower ratio), with a lower MRT_fluid in cattle due to a higher saliva production. Here, we test these hypotheses in a meta-analysis, using only studies that investigated cattle and sheep simultaneously. The datasets included 12 studies on MRT (of which 11 contained information on feed intake), yielding 102 (or 89) individual data; and 26 studies on protein digestibility (of which 18 contained information on intake), yielding 349 individual data. Cattle had a higher MRT_particle/MRT_fluid (2.1) than sheep (1.7), mainly due to longer MRT_particle; only if body mass was included in the model, MRT_fluid was significantly shorter in cattle in the larger MRT dataset (and tended to be shorter in the slightly smaller dataset). Cattle had a significantly lower aD CP than sheep, while there was no such difference in overall (dry or organic matter) digestibility. The dataset confirms a shift in fermentation strategy towards microbial production in cattle. While this has been suggested for ruminants in general, cattle appear particularly far on an evolutionary trajectory of maximizing microbial yield from the forestomach. The application of more specific digestive physiology data (like endogenous losses) gained from sheep to cattle should be done bearing these differences in mind.

No size-dependent net particle retention in the hindgut of horses

Sieve analyses of hindgut contents of horses as well as observations in horses where plastic markers had been applied to a caecal cannula suggested that there may be a discrimination by particle size in the passage or retention of digesta. Here, we performed a similar experiment with five caecum-cannulated horses (562 ± 31 kg) fed a constant amount (6.81 kg dry matter/day) of grass hay. Passage markers representing the liquid (Co-EDTA) as well as the particulate digesta phase (Yb-undefined; Cr mordanted fibre 1-2 mm; Ce-mordanted fibre 8 mm) were given as a pulse-dose into the cannula to measure their mean retention times (MRT). The MRTs were compared by repeated-measurements analysis of variance. The MRT in the hindgut was 22.2 ± 2.4 h for Co, 25.0 ± 3.4 h for Yb, 26.2 ± 1.6 h for Cr and 26.3 ± 1.5 h for Ce. Whereas differences between the particle marker MRTs were not significant (p_adj. > 0.05), significant differences were observed between the solute marker Co and each of the particle markers Cr and Ce (p_adj. < 0.009). The results confirm the well-known significant, albeit small, difference in MRT in horses between the fluid and the particle digesta phase, and corroborate another recent study that used a combination of whole, marked hay and individual marker analysis in different particle size fractions of the faeces, which also did not detect a selective retention of any particle size class.

Reticular contraction frequency and ruminal gas dome development in goats do not differ between grass and browse diets

In investigations of differences between ruminant species feeding on browse or grass, it is often unclear whether observed differences are animal- or forage-specific. Ruminant species have been classified as 'moose-type', with little rumen content stratification, or 'cattle-type' with a distinct rumen contents stratification, including a gas layer. To which extent putative differences in forestomach motility are involved in these patterns is unknown. Using sonography, we investigated the frequency of reticular contractions and the stratification of rumen contents in goats fed exclusively on grass hay (n = 6) or dried browse (n = 5) directly after feeding, and after another 6 and 12 h with no access to feed. The frequency of reticular contractions decreased from immediately after feeding (1.8 ± 0.3 min^-1 ) to 6 h afterwards (1.2 ± 0.2 min^-1 ) and then remained constant, with no difference between diets. A gas dome became more visible over time, but neither its incidence nor its extent differed between diets. The results are in accord with classifying goats as 'cattle-type' in terms of their digestive physiology, and they add to a growing body of evidence that differences in digestive physiology between ruminant species are more due to species characteristics than different kinds of ingested forages.

A pilot study on dietary and faecal calcium/phosphorus ratios in different types of captive ruminating herbivores

Quantitative differences in calcium and phosphorus metabolism between domestic species exist and can be visualised using data on calcium and phosphorus intake and faecal excretion. The parameter for analysing the results was defined as Δ = dietary calcium/phosphorus (Ca/P) ratio – faecal Ca/P ratio. In previous studies, hindgut fermenters had significantly higher Δ values than ruminants (sheep, cattle, goats), which was explained by the high calcium digestibilities in hindgut fermenters in contrast to highly efficient phosphorus recycling in ruminants. The first hypothesis of the present study was that different types of ruminants (grazer, browser, intermediate feeder) would show differences in Δ as a proxy for quantitative calcium and phosphorus metabolism. The second hypothesis was that camelids as functional, but not taxonomic ruminants would show Δ values similar to ruminants. We used herbivorous zoo animals (17 species, hindgut and foregut fermenters), which were kept on their regular diet without variation for 1 week. Then, faecal samples were obtained from the individual animals. Feed items and faecal samples were analysed for calcium and phosphorus, and dietary and faecal Ca/P ratios as well as Δ were calculated. A comparison of the species groups (one‐way ANOVA on ranks, p < 0.05) showed that zoo hindgut fermenters had significantly higher Δ values (0.67 ± 0.34) than camelids and zoo ruminants (–1.07 ± 0.35 and –1.87 ± 1.56). There was no significant difference between camelids, grazers (–1.49 ± 1.31), browsers (–1.63 ± 0.88) and intermediate feeders (–2.11 ± 1.76). The ruminant species from this study had markedly lower Δ than domestic ruminants from literature data. Especially intermediate feeders had low Δ, possibly due to more efficient phosphorus recycling than the domestic ruminants.

Preliminary evidence for a forestomach washing mechanism in llamas (Lama glama)

Dust and grit are ingested by herbivores in their natural habitats along with the plants that represent their selected diet. Among the functions of the rumen, a washing of ingesta from adhering dust and grit has recently been demonstrated. The putative consequence is a less strenuous wear on ruminant teeth by external abrasives during rumination. The same function should theoretically apply to camelids, but has not been investigated so far. We fed six llamas (Lama glama) a diet of grass hay and a lucerne-based pelleted food in which fine sand had been included at about 8% of ingredients, for ad libitum consumption for 6 weeks. Subsequently, animals were slaughtered and content of the different sections of the gastrointestinal tract was sampled for the analysis of dry matter (DM), total ash, and acid detergent insoluble ash (ADIA, a measure for silica). Additionally, two of the animals were subjected to whole-body computer tomography (CT) after death in the natural sternal resting position. No clinical problems or macroscopic changes in the faeces were observed during the experimental period. The results indicate an accumulation of ADIA in the C3 compartment of the stomach complex, in particular in the posterior portion that is the equivalent of the abomasum in ruminants. By contrast, contents of the C1, from which material is recruited for regurgitation and rumination, were depleted of ADIA, indicating that the contents had largely been washed free of sand. The washing effect is an unavoidable side effect of the flotation- and sedimentation-based sorting mechanisms in the ruminant and the camelid forestomachs. In theory, this should allow ruminants and camelids to live in similar habitats as nonruminant herbivores at lower degrees of hypsodonty.

Increasing feed intake in domestic goats (Capra hircus): Measured effects on chewing intensity are probably driven by escape of few, large particles from the forestomach

On the one hand, oral processing - mastication - is considered a relatively inflexible component of mammalian feed acquisition that constrains instantaneous intake rates. On the other hand, experimental data shows that the level of feed intake affects faecal particle size and hence net chewing efficiency in ruminants, with larger particles occurring in the faeces at higher intakes. Here, we report the effect of an increased feed intake during maintenance (L1), late (200% of L1) and peak lactation (300% of L1) of a consistent diet (hay:concentrates 50:50) in eight domestic goats on various measures of digestive physiology including faecal mean particle size (MPS). Increasing intake led to an increased gut fill, a reduction in digesta retention times, and an increase in faecal MPS (from 0.57 to 0.72 mm). However, this was an effect of the large particle fraction (>2 mm) being disproportionately excreted at higher intakes; if MPS was assessed on the basis of particles below the typical escape threshold (≤1 mm), there was no difference between intake levels. These findings suggest that the effect of intake on the calculated net chewing efficiency in ruminants may rather be an effect of increased large particle escape from the forestomach than a reduced chewing intensity per bolus during ingestion or rumination.

Digesta passage in common eland (Taurotragus oryx) on a monocot or a dicot diet

The way that fluids and particles move through the forestomach of a ruminant is species-specific, and can be used to classify ruminants according to their digestive physiology into 'moose-types' (with little difference in fluid and small particle passage) and 'cattle-types' (where fluids move through the forestomach much faster than small particles). So far, 'moose-types' appear limited to a dietary niche of browsing, whereas 'cattle-types' are particularly prominent in the intermediate and grazing diet niches. However, some species, including members of the spiral-horned antelopes (the Tragelaphini), have a 'cattle-type' physiology but a browse-dominated diet niche. Eland (Taurotragus oryx), the largest member of the Tragelaphini, are strict browsers in the wild but have been considered intermediate feeders in the past, and can seemingly be maintained on grass diets. We quantified food intake, mean retention time (MRT) in the gastrointestinal tract and the reticulorumen (RR) of a solute, a small and a large particle marker, and diet digestibility in six eland each fed a monocot (grass hay) and a dicot (lucerne silage) forage. Food intake and digestibility was lower on the diet with higher fibre content (grass hay), with corresponding longer MRT. At the higher intakes on lucerne, the difference in MRT between small and large particles was larger, indicating a greater reliance on particle sorting and clearance under this condition of potentially limiting gut capacity. Regardless of diet or intake, the ratio of small particle and solute MRT in the RR was constant and small, at a quotient of 1.54, classifying the eland as a typical 'moose-type' ruminant. This finding is consistent with previous literature reports on low faecal metabolic nitrogen and high apparent protein digestibility in eland. Given the relative ease at which eland can be maintained under farm husbandry conditions, they appear ideal model ruminants to study the effects of differences in rumen physiology compared to cattle.

Digestive anatomy, physiology, resting metabolism and methane production of captive maras (Dolichotis patagonum)

The digestive physiology of maras (Dolichotis patagonum) has not been investigated in detail. Maras have a particular limb anatomy facilitating a unique cursoriality among rodents. This may also have led to additional adaptations such as a reduced volume of the gastrointestinal tract. We performed macroanatomical measurements of, and determined mean particle size along, the digestive tract of 10 semi-free-ranging animals (7.04 ± 1.05 kg). Additionally, we measured CH₄ emission in five captive animals (7.67 ± 0.98 kg) fed a diet of pelleted lucerne, and measured food intake, digestibility, and digesta mean retention time (MRT) of a solute and three particle markers (fed at <2, 10 and 20 mm particle size). The digestive tract contents represented 11.1 ± 1.4% of body mass, similar to other mammals and rodents, and there was slight indication of selective small particle retention in the caecum. Secondary peaks in marker elimination patterns suggested the possibility of caecotrophy. The MRTs were 15.4 h for the solute and 13.6 h, 13.3 h and 13.3 h for the three particle markers, respectively. At a dry matter intake of 61 ± 12 g kg body mass^-0.75 d^-1, the maras digested organic matter and neutral detergent fibre to 48 ± 8% and 34 ± 10%, respectively, which is in the lower range of results from horses fed on a diet with a similar fibre content. The respiratory quotient (CO₂/O₂) was 0.93 ± 0.03, the resting metabolic rate 346 ± 35 kJ kg body mass^-0.75 d^-1, and CH₄ emissions averaged at 3.85 ± 0.47 L d^-1 and 14.5 ± 5.2 L per kg dry matter intake; this at a CH₄/CO₂ ratio of 0.042 ± 0.004. Thus, the methane yield was of a magnitude expected for a hypothetical ruminant of this body mass. The results are consistent with the general understanding of hystricomorph rodent digestive physiology, including caecotrophy, but do not indicate a reduction of digestive capacity to support cursoriality. These results, and those obtained from other hystricomorph rodents, suggest that CH₄ production may be more prominent in rodents than previously thought.

Physical characteristics of gastrointestinal content of llama (Lama glama)

Changes in digesta dry matter (DM) and mean digesta particle size (MPS) along the gastrointestinal tract are well known in ruminants, but not in camelids. We collected digesta from the dorsal (d) and ventral (v) first forestomach compartment (C1), the second forestomach compartment (C2), three proximal segments and the subsequent glandular part of the third compartment (C3A-D), the caecum and the faeces twelve llamas (Lama glama). DM analysis indicates the presence of digesta stratification in the C1, the presence of fluid in the C2 to facilitate the sorting function of this compartment, the fluid-absorbing function of the proximal parts of the C3, the secretion of enzymes and digestive acids in the C3D, and the water-resorbing function of the lower intestinal tract. These findings illustrate the functional resemblance between the gastrointestinal tract of camelids and cattle-like ruminants (C1 equivalent to the rumen with stratified contents, C2 to the reticulum, C3A/B/C to the omasum and C3D to the abomasum). MPS analysis revealed a progressive reduction in MPS from the C1 to the distal C3. This gradual transition is different from the clear-cut threshold in ruminants between the reticulum and the omasum and had so far only been described in dromedaries (Camelus dromedarius). These findings indicate that regardless of the convergent property of rumination and resemblance of general mechanisms involved in contents stratification and particle sorting, differences between ruminants and camelids exist that could be interpreted as a more efficient functionality of the ruminant forestomach.

Are carnivore digestive separation mechanisms revealed on structure-rich diets?: Faecal inconsistency in dogs (Canis familiaris) fed day old chicks

Pronounced variations in faecal consistency have been described anecdotally for some carnivore species fed a structure-rich diet. Typically two faecal consistencies are distinguished, namely hard and firm versus liquid and viscous faeces. It is possible that a separation mechanism is operating in the carnivore digestive tract, as in many herbivore species. Six beagle dogs were fed two experimental diets in a cross-over design of 7 days. Test diets consisted of chunked day old chicks differing only in particle size (fine = 7.8 mm vs coarse = 13 mm) in order to vary dietary structure. Digestive retention time was measured using titanium oxide (TiO2) as marker. The total faecal output was scored for consistency and faecal fermentation profiles were evaluated through faecal short-chain fatty acid (SCFA) and ammonia (NH3) analyses. A total of 181 faecal samples were collected. Dietary particle size did not affect faecal consistency, fermentative end products nor mean retention time (MRT). However, a faecal consistency dichotomy was observed with firm faeces (score 2-2.5) and soft faeces (score 4-4.5) being the most frequently occurring consistencies in an almost alternating pattern in every single dog. Firm and soft faeces differed distinctively in fermentative profiles. Although the structure difference between diets did not affect the faecal dichotomy, feeding whole prey provoked the occurrence of the latter which raises suspicion of a digestive separation mechanism in the canine digestive tract. Further faecal characterisation is however required in order to unravel the underlying mechanism.

Comparative selective retention of particle size classes in the gastrointestinal tract of ponies and goats

There is a discrepancy in the literature on potential digesta separation mechanisms in horses, with both a selective retention of fine and of large particles postulated in different publications. To assess the net effect of such mechanisms, we fed ponies on a hay-only diet a pulse dose of whole (unchopped) marked hay together with a solute marker, collected faeces on a regular basis, measured marker concentrations in whole faeces and in their large (2.0-16 mm), medium (0.5-1.0 mm) and small (0.063-0.25 mm) particle fraction, and calculated the corresponding mean retention times (MRTs). For comparison, the same experiment was performed in goats. In goats, as expected, MRT_solute (35 hr) was significantly shorter than MRT_particle (51 hr); only a very small fraction of particle marker was excreted as large particles (2%); and the MRT of these large particles was significantly shorter than that of small particles (with a relevant difference of 8.6 hr), indicating that those few large particles that escape the rumen do so mostly soon after ingestion. In ponies, MRT_solute (24 hr) did not differ from MRT_particle (24 hr); a higher fraction of particle marker was excreted as large particles (5%); and the MRT of these large particles was longer than that of small particles (but with a non-relevant difference of less than 1 hr). These results indicate that no relevant net separation of digesta phases occurs in horses and that selective particle retention mechanisms in the large intestine are unlikely to represent important characteristics of the horse's digestive physiology.

Ingestive mastication in horses resembles rumination but not ingestive mastication in cattle and camels

Horses achieve a higher degree of particle size reduction through ingestive mastication than functional ruminants. We characterized mastication using chew-monitoring halters (RumiWatch) in six domestic horses, cattle, and Bactrian camels each. All animals were offered grass hay of the same batch for 15 min. In cattle and camels, measurements were continued after eating until rumination was observed. Except for one horse, 96% of the horses' ingestive mastication data were identified as "rumination" by the proprietary RumiWatch algorithm, whereas ingestion and rumination by cattle and camels were mostly classified correctly. There were no systematic differences between cattle and camels. In cattle and camels, ingestive mastication was less regular than rumination, indicated by significantly higher standard deviations of chewing peak intervals, peak heights, and peak breadths in intraindividual comparisons. The average standard deviations of these measures were lower in horses than in cattle and camel ingestive mastication, indicating a more consistent chewing pattern in horses. Horse values were similar to those of rumination mastication, suggesting equally regular chewing motions. Regular, rhythmic chewing represents a common feature of horses and functional ruminants, but the less uniform ingestive mastication in functional ruminants represents a deviating pattern, the adaptive value of which remains unclear. In particular, it does not appear to promote a higher ingestion rate. A potential cause may be the avoidance of high tooth wear rates by delaying a more regular, systematic mastication until ingesta has been softened and the grit has been washed off in the forestomach.

Influence of ruminal methane on digesta retention and digestive physiology in non-lactating dairy cattle

Enteric methane (CH4) production is a side-effect of herbivore digestion, but it is unknown whether CH4 itself influences digestive physiology. We investigated the effect of adding CH4 to, or reducing it in, the reticulorumen (RR) in a 4×4 Latin square experiment with rumen-fistulated, non-lactating cows, with four treatments: (i) control, (ii) insufflation of CH4 (iCH4), (iii) N via rumen fistula, (iv) reduction of CH4 via administration of bromochloromethane (BCM). DM intake (DMI), apparent total tract digestibility, digesta mean retention times (MRT), rumen motility and chewing activity, spot breath CH4 emission (CH4exhal, litre/kg DMI) as well as CH4 dissolved in rumen fluid (CH4RRf, µg/ml) were measured. Data were analysed using mixed models, including treatment (or, alternatively, CH4exhal or CH4RRf) and DMI relative to body mass0·85 (rDMI) as covariates. rDMI was the lowest on the BCM treatment. CH4exhal was highest for iCH4 and lowest for BCM treatments, whereas only BCM affected (reduced) CH4RRf. After adjusting for rDMI, CH4RRf had a negative association with MRT in the gastrointestinal tract but not in the RR, and negative associations with fibre digestibility and measures of rumination activity. Adjusting for rDMI, CH4exhal had additionally a negative association with particle MRT in the RR and a positive association with rumen motility. Thus, higher rumen levels of CH4 (CH4exhal or CH4RRf) were associated with shorter MRT and increased motility. These findings are tentatively interpreted as a feedback mechanism in the ruminant digestive tract that aims at mitigating CH4 losses by shortening MRT at higher CH4.

Comparison of gut fill in sheep (Ovis aries) measured by intake, digestibility, and digesta retention compared with measurements at harvest

Gut capacity is an important factor in digestive physiology and is often measured as dry matter fill (DMF) following dissection, which prevents repeated measures in the same animal. It was proposed to calculate DMF from food intake, digestibility, and gut mean retention time (MRT), but empirical tests of this are few. We calculated DMF from intake, digestibility, and the MRT of small-particle (1 mm) and large-particle (20 mm) markers in 20 sheep (Ovis aries L., 1758) fed at different intake levels and compared results with DMF at dissection at the end of the feeding trial. MRT for smaller particles was significantly shorter than for larger particles (34.4 ± 6.1 vs. 42.5 ± 7.6 h, respectively). Correspondingly, DMF calculated from smaller particles (0.98 ± 0.27 kg) was significantly lower than DMF calculated from larger particles (1.20 ± 0.30 kg). The latter was not significantly different from DMF measured at dissection (1.18 ± 0.34 kg). These results suggest that DMF can be estimated from measures of digestive physiology. The choice of particle marker to determine MRT is crucial for the accuracy of the proxy. In ruminants, where small particles are consistently eliminated faster than larger particles, considerations of marker particle size are particularly important.