Ruminal passage rate and digestibility of fiber from dairy cows consuming diets containing alfalfa and orchardgrass hays with different concentrations of undegradable neutral detergent fiber (uNDF).

This study aimed to evaluate the effect of feeding diets with different fractions of undegraded NDF (uNDF) and potentially degradable (pdNDF) on ruminal NDF degradation and passage kinetics of lactating dairy cows. Six rumen-cannulated (533 ± 43 kg BW and 122 ± 15 DIM) and 6 non-cannulated (558 ± 62 kg BW and 126 ± 16 DIM) primiparous Holstein dairy cows were randomly assigned to 1 of 2 experimental diets in a crossover design with 2 28-d periods. The experimental diets were formulated to include either alfalfa hay (ALFA) or orchardgrass hay (ORCH) in addition to corn silage. Rations were formulated to contain 30% NDF (DM basis), where the concentrate, corn silage, and each of the hays provided one third of the dietary NDF. The marker dilution technique was used to measure the passage rate utilizing a pulse dose of marked corn silage fiber. On d 17 and 24 of each period, ruminal contents were evacuated to determine ruminal pool size. Following the return of the ruminal contents containing the pulse dose of marked corn silage to the rumen, ruminal grab samples were collected at 0, 3, 6, 9, 12, 24, 36, 48, 60, and 72 h. Samples from each time point were separated into solids and liquid, and the solids were analyzed for NDF, uNDF, and marker concentration. Alfalfa hay had a higher concentration of CP (16.4 vs. 10.7%) and a lower concentration of NDF (38.0 vs. 63.2) than orchardgrass hay. Alfalfa hay had a greater concentration of uNDF than orchardgrass hay (36.5 vs. 32.8% uNDF; NDF basis). Cows consuming the ALFA diet had similar milk yield (39.1 kg/d) and similar milk fat and protein concentrations (3.72% fat and 3.24% protein, respectively) than cows consuming the ORCH diet. Cows consuming the ALFA diet consumed more DM (26.7 vs. 24.6 kg/d) and uNDF (2.7 vs. 2.3 kg/d), than cows consuming the ORCH diet. Cows consuming the ALFA diet digested more NDF and pdNDF than cows consuming the ORCH diet (3.3 vs. 2.8 kg/d). Even though cows consuming the ALFA diet had a smaller pool size of NDF than cows consuming the ORCH diet (5.4 vs. 6.7 kg), the pool size of uNDF did not differ between groups (2.4 kg). Cows consuming the ALFA diet had a faster rate of passage of uNDF than cows consuming the ORCH diet (5.02 vs. 4.03%/h). This translated into a shorter mean retention time of uNDF for cows consuming the ALFA diet relative to cows consuming the ORCH diet (21.0 vs. 26.2 h). In conclusion, cows consuming diets containing alfalfa hay had a faster ruminal passage rate and a shorter mean retention time of uNDF than cows consuming diets containing orchardgrass hay, and this occurred despite the greater concentrations of dietary uNDF in the alfalfa-based diet. These findings suggest that the kinetics of ruminal digestion and passage influence NDF degradation in ways beyond uNDF concentration or forage quality.

Ration particle size has different effects on digestive but not production parameters in higher-yielding (Holstein) compared to lower-yielding (Girolando) cows.

The aim of the study was to evaluate the effect of total mixed ration particle size (length) and breed of cow on intake dynamics, animal performance and CH4 emissions, comparing high yielding Holstein and low yielding Girolando cows. The experimental design was 2 × 2 Latin Square arranged as a crossover factorial scheme with two diets (short particle size, SPS and long particle size, LPS) and the two breed compositions. The design comprised two periods of 26 d each, where all data collection was performed at cow level. No influence of the particle size occurred for the passage rate, neutral detergent fiber digestibility, performance and milk composition, methane emissions or ruminal fermentation parameters. Girolando cows had greater dry matter intake (DMI) when fed SPS, while Holsteins had the same (P < 0.05). Girolando cows had lower dry matter digestibility when fed LPS compared to SPS, while Holsteins had the opposite effect (P < 0.05). Also, the digestibility of crude protein and non-fibrous carbohydrates decreased in Girolando cows fed LPS, but not in Holsteins (P < 0.05). Girolando cows reduced DMI by 10.6% when fed LPS diet (P < 0.05). Girolando had an increased eating rate (+24 g of DM/min; P < 0.05) compared to Holstein cows, but Holstein cows had a lower CH4 intensity (by 29.7%: P < 0.05). Girolando cows increased the dry matter intake when fed a diet with short particle size, while the same did not happen in Holsteins. Dry matter digestibility increased in Holsteins when fed long particle size, while the opposite was observed in Girolando cows. Nutrient digestibility was reduced in Girolando cows when fed short particle size. Particle size did not influence eating time, eating rate, feed trough visits, visits with intake, milk yield and composition regardless of the breed. Reducing particle size increased CH4 intensity in both breeds.

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 (MRTparticle/MRTfluid) 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 MRTfluid 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 MRTparticle/MRTfluid (2.1) than sheep (1.7), mainly due to longer MRTparticle; only if body mass was included in the model, MRTfluid 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.

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.

Effect of induced saliva flow on fluid retention time, ruminal microbial yield and methane emission in cattle.

Both in vitro and animal studies indicated that a higher dilution rate is related to a more efficient microbial synthesis and a lower methane (CH4 ) yield. The latter could be a consequence of the former, as an increase in microbial cell synthesis offers an alternative hydrogen sink competing with methanogenesis. To test this assumption in live animals, we applied a saliva stimulant, pilocarpine, to modify liquid flow rate in cattle. Four non-lactating cows (750 ± 71 kg) were fed forage only (restricted to constant intake) in a 4 × 4 Latin square design with oral doses of 0, 1, 2.5 and 5mg pilocarpine/kg body weight and day. We quantified feed and water intake, ruminal and total tract mean retention time (MRT) of solute and particle markers, ruminal microbial yield (via urinary purine bases or metabolic faecal nitrogen), CH4 emission, digestibility, chewing behaviour, reticular motility and rumen fluid parameters. The effect of induced saliva flow was evident by visibly increased salivation and water intake. Increasing the pilocarpine dosages resulted in a linearly decreased MRT of fluid and small particles (p < 0.001 and p< 0.05, respectively) and methane yield as related to digested DM (p < 0.05), the latter at a magnitude of 5%. No effect of treatment was found on ruminal microbial yield estimated via purine derivates. Metabolic faecal N as an indicator of microbial growth linearly correlated with pilocarpine dosages (p < 0.05). No significant relationship was found between pilocarpine dosages and large particle MRT, nutrient digestibility, ruminal pH and short-chain fatty acids. In conclusion, different from some in vitro studies, there was little indication of a reciprocal effect of CH4 and microbial biomass production in cows fed a forage-only diet.

The effects of pelleted dried distillers grains and solubles fed with different forage concentrations on rumen fermentation, feeding behavior, and milk production of lactating dairy cows.

The physical form of feeds can influence dairy cow chewing behavior, rumen characteristics, and ruminal passage rate. Changing particle size of feeds is usually done through grinding or chopping forages, but pelleting feed ingredients also changes particle size. Our objective was to determine if pelleted dried distillers grains and solubles (DDGS) affected the feeding value for lactating dairy cattle. Seven lactating Jersey cows that were each fitted with a ruminal cannula averaging (± standard deviation) 56 ± 10.3 d in milk and 462 ± 75.3 kg were used in a crossover design. The treatments contained 15% DDGS in either meal or pelleted form with 45% or 55% forage on a dry matter basis. The forages were alfalfa hay, corn silage, and wheat straw. The factorial treatment arrangement was meal DDGS and low forage (mDDGS-LF), pelleted DDGS and low forage (pDDGS-LF), meal DDGS and high forage (mDDGS-HF), and pelleted DDGS and high forage (pDDGS-HF). Dry matter intake and energy-corrected milk were both unaffected by treatment averaging 19.8 ± 2.10 kg/d and 33.9 ± 1.02 kg/d, respectively. Fat yield was unaffected averaging 1.7 ± 0.13 kg/d, but protein yield was affected by the interaction of forage and DDGS. Protein yield was similar for both low forage treatments but was increased by when pDDGS was fed in the high forage treatment (1.05 vs. 0.99 ± 0.035 kg/d). When forage concentration was increased, starch digestibility increased by 1.9 percentage units, crude protein digestibility tended to increase 1.1 percentage units, and residual organic matter digestibility decreased 3.4 percentage units. Pelleting DDGS increased digestibility of neutral detergent fiber (NDF) digestibility (49.2 vs. 47.5 ± 1.85%) and gross energy (68.2 vs. 67.1 ± 1.18%). Increasing forage increased ruminal pH (5.85 to 5.94 ± 0.052). Passage rate slowed from 2.84 to 2.65 ± 0.205 %/h when feeding HF compared with LF. Rumination time increased from 417 to 454 ± 49.4 min with increasing forage concentration but was unaffected by the form of DDGS or the interaction of forage and DDGS. Eating time increased with pDDGS (235 vs. 209 ± 19.8 min), which may be a result of increased feed sorting behavior. Pelleting DDGS increased preference for particles retained on the 8-mm sieve and decreased preference for particles on the 1.18-mm sieve and in the pan (<1.18 mm). Results confirm that increasing forage concentration increases ruminal pH, rumination time, and slows passage rate, but contrary to our hypothesis increasing forage concentration did not increase NDF digestibility. Results also suggest that pelleted DDGS do not appear to affect milk production, ruminal characteristics, or passage rate, but pelleted DDGS may increase sorting behavior of lactating Jersey cows and increase NDF and gross energy digestibility.

Conditions stimulating neutral detergent fiber degradation by dosing branched-chain volatile fatty acids. III: Relation with solid passage rate and pH on prokaryotic fatty acid profile and community in continuous culture.

Our objectives were to evaluate potential interactions in culture conditions that influence how exogenously dosed branched-chain VFA (BCVFA) would be recovered as elongated fatty acids (FA) or would affect bacterial populations. A 2 × 2 × 2 factorial arrangement of treatments evaluated 3 factors: (1) without versus with BCVFA (0 vs. 2 mmol/d each of isobutyrate, isovalerate, and 2-methylbutyrate; each dose was partially substituted with 13C-enriched tracers before and during the collection period); (2) high versus low pH (ranging diurnally from 6.3 to 6.8 vs. 5.7 to 6.2); and (3) low versus high particulate-phase passage rate (kp; 2.5 vs. 5.0%/h) in continuous cultures administered a 50:50 forage:concentrate diet twice daily. Samples of effluent were collected and composited before harvesting bacteria from which FA and DNA were extracted. Profiles and enrichments of FA in bacteria were evaluated by gas chromatography and isotope-ratio mass spectrometry. The 13C enrichment in bacterial FA was calculated as percentage recovery of dosed 13C-labeled BCVFA. Dosing BCVFA increased the even-chain iso-FA, preventing the reduced concentration at higher kp and potentially as a physiological response to decreased pH. However, decreasing pH decreased recovery of 13C in these even-chain FA, suggesting greater reliance on isobutyrate produced from degradation of dietary valine. The iso-FA were decreased, whereas anteiso-FA and 16:0 increased with decreasing pH. Thus, 2-methylbutyrate still appeared to be important as a precursor for anteiso-FA to counter the increased rigidity of bacterial membranes that had more saturated straight-chain FA when pH decreased. Provision of BCVFA stimulated the relative sequence abundance of Fibrobacter and Treponema, both of which require isobutyrate and 2-methylbutyrate. Numerous bacterial community members were shifted by low pH, including increased Prevotella and genera within the phylum Proteobacteria, at the expense of members within phylum Firmicutes. Because of relatively few interactions with pH and kp, supplementation of BCVFA can stimulate neutral detergent fiber degradability via key fibrolytic bacteria across a range of conditions. Decreasing pH shifted bacterial populations and their FA composition, suggesting that further research is needed to distinguish pH from dietary changes.

Predicting ruminally undegraded and microbial protein flows from the rumen.

The objectives of the present work were (1) to identify the cause of the linear bias in predictions of rumen-undegradable protein (RUP) content of feeds, and devise methods to remove the bias from prediction equations, and (2) to further explore the impact of rumen-degradable protein (RDP) on microbial N (MiN) outflow from the rumen. The kinetic model used by NRC (2001), which is based on protein fractionation and rates of degradation (Kd) and passage (Kp), displays considerable slope bias (-0.30 kg/kg), indicating parameter or structural problems. Regressing Kp by feed class and a static adjustment factor for the in situ-derived Kd on observed RUP flows completely resolved the slope bias problem, and the model performed significantly better than models using unadjusted Kd and marker-based Kp. The Kd adjustment was 3.82%/h, which represents approximately a 50% increase in rates of degradation over the in situ values, indicating that in situ analyses severely underestimate true rates of protein degradation. The Kp for concentrate-derived protein was 5.83%/h, which was slightly less than the marker-predicted rate of 6.69%/h. However, the derived forage protein rate was 0.49%/h, which was considerably less than the marker-based rate of 5.07%/h. Compartmental analysis of data from a single study corroborated the regression analysis, indicating that a 25% reduction in the overall passage rate and an 87% increase in the rate of degradation were required to align ruminal N pool sizes and the extent of protein degradation with the observed data. Therefore, one must conclude that both the in situ-derived degradation rates and the marker-based particle passage rates are biased relative to protein passage and cannot be used directly to predict RUP outflow from the rumen. The effects of RDP supply on microbial nitrogen (MiN) flow were apparent when intakes of individual nutrients were offered but not when DM intake and individual nutrient concentrations were offered, due to collinearity problems. Microbial N flow from the rumen was found to be linearly related to ruminally degraded starch, ruminally degraded neutral detergent fiber (NDF), RDP, and forage NDF intakes; and quadratically related to residual OM intake. More complicated models containing 2- and 3-way interactions among nutrients were also supported by the data. Independent MiN responses to RDP, ruminally degraded starch, and ruminally degraded NDF aligned with the expected responses to each of those nutrients. Nonlinear representations of MiN were found to be inferior to the linear models. Despite using unbiased predictions of RUP and MiN as drivers of AA flows, predictions of Arg, His, Ile, and Lys flow exhibited linear slope bias relative to the observed data, indicating that representations of the AA composition of the proteins may be biased or the observed data are biased. This is an improvement over the NRC (2001) predictions, where bias adjustments were required for all of the essential AA. Despite the bias for 4 AA flows, the revised prediction system was a substantial improvement over the prior work.

Evaluation of source of corn silage and trace minerals on rumen characteristics and passage rate of Holstein cows.

The effects of source of corn silage and trace mineral on rumen fermentation, turnover, and particle passage rates were evaluated with 8 ruminally cannulated Holstein cows averaging 83 (standard error = 5) days in milk in a replicated 4 × 4 Latin square design with a 2 × 2 factorial arrangement of treatments and 28-d periods. The diets consisted (dry basis) of 55% conventional (CON) or brown midrib-3 (BM3) corn silage, 2% chopped wheat straw, and 43% grain mix with either sulfate (STM) or hydroxy (HTM) source of Cu, Zn, and Mn trace minerals. The targeted supplemental amount of Cu, Zn, and Mn was 194, 1,657, and 687 mg/d, respectively. The dietary treatments were (1) CON-STM, (2) CON-HTM, (3) BM3-STM, and (4) BM3-HTM. Dietary nutrient composition of BM3 diets averaged 32.1% amylase neutral detergent fiber on an organic matter basis (aNDFom) and 6.9% undigested neutral detergent fiber at 240 h of in vitro fermentation (uNDF240om; % of dry matter), and CON diets averaged 36.2% aNDFom and 8.6% uNDF240om (% of dry matter). Data were summarized by period and analyzed as a replicated Latin square design with fixed model effects for corn silage, trace mineral, corn silage and trace mineral interaction, period within replicated square, and replicated square using the MIXED procedure of SAS (version 9.4, SAS Institute Inc., Cary, NC). Cow within replicate was a random effect. Daily mean, standard deviation, minimum, and maximum for rumen pH were unaffected by corn silage or trace mineral source. Cows fed the CON diets had greater rumen acetate percentage than cows fed the BM3 diets (65.7 vs. 64.7 molar %). In contrast, cows fed the BM3 diets had greater rumen propionate percentage than cows fed the CON diets (21.4 vs. 20.4 molar %). Total volatile fatty acid concentration was lower for cows fed STM versus HTM in BM3 diets, but not for the cows fed the CON diets. Cows fed the BM3 diets had faster turnover rate and shorter turnover time for uNDF240om than cows fed the CON diets (3.12 vs. 2.86%/h and 33.3 vs. 36.5 h, respectively). Cows fed the BM3 diets had a faster passage rate of small and medium corn silage neutral detergent fiber particles than cows fed the CON diets (5.73 vs. 5.37%/h and 4.74 vs. 4.31%/h, respectively). We observed a corn silage by source of trace mineral interaction on organic matter and uNDF240om rumen pool size and organic matter turnover. Overall, source of corn silage had a pronounced influence on rumen dynamics presumably related to greater in vitro neutral detergent fiber digestibility and lower uNDF240om content of BM3 corn silage that allowed for faster turnover of indigestible neutral detergent fiber and greater passage rate of corn silage particles. In contrast, the source of trace mineral had much less significant effects on rumen fermentation, turnover, and particle passage rates. Corn silage-based diets intended to enhance rumen fiber fermentation, turnover, and passage are more affected by source and digestibility of neutral detergent fiber than source of dietary trace minerals.

Digesta passage and nutrient digestibility in Boran steers at low feed intake levels.

The present study evaluated the effects of energetic undernutrition on liquid and solid digesta passage and on nutrient digestibility as well as their interdependencies. Using a 4 x 4 Latin square design, 12 growing Boran steers (183 ± 15.2 kg live weight) were allocated to four levels of metabolizable energy (ME) supply fixed at 100, 80, 60 and 40% of individual maintenance energy requirements (MER) during four experimental periods. Each period comprised three weeks of adaptation, two weeks of data collection and two weeks of recovery. Diets MER80, MER60 and MER40 only consisted of Rhodes grass hay (RGH), whereas diet MER100 contained (as fed) 83% RGH, 8% cotton seed meal and 9% sugarcane molasses. Feed intake differed between treatments (p < .001) and ranged from 40 ± 0.6 g dry matter (DM) per kg of metabolic weight (kg0.75 ) in MER40 to 81 ± 1.3 g DM in MER100. Digestibility of neutral and acid detergent fibre (NDF, ADF) was highest at MER80, whereas rumen retention time of liquid and solid digesta was longest at MER40. The correlation of rumen retention time of liquid and solid digesta with the digestibility of proximate diet components was weak but positive, whereas the correlation of liquid and solid rumen retention time with quantitative feed and nutrient intake was strong (p < .01) and negative. Our results suggest that tropical cattle are able to buffer a moderate energy deficit by prolonging rumen retention time of digesta and hence improve diet digestibility. Conversely, a severe energy deficit cannot be buffered by digestive adaptation mechanisms and will inevitably lead to productivity losses.

Technical note: Infusion, sampling, and vacuum-assisted collection devices for use in ruminally cannulated calves.

Calves can be ruminally cannulated at young ages, but equipment size limitations preclude use of an infusion and sampling device in these small animals. Likewise, a procedure to easily evacuate rumen contents in young calves has not been described. Overcoming these technical complications related to assessment of ruminal passage kinetics, nutrient digestion, and volatile fatty acid absorption would aid in future studies advancing our knowledge of dairy calf nutrition. The first objective was to design and fabricate 2 devices (one device for infusion and sampling, and another for vacuum-assisted collection) suitable for use in young ruminally cannulated dairy calves. The second objective was to test the utility of these tools when performing procedures commonly used in ruminant nutrition research. A single weaned 62-d-old ruminally cannulated calf was used to evaluate the ability to infuse a solution of LiCoEDTA and sample rumen contents through the cannula cap over a period of 2 h to assess the rumen liquid passage rate (procedure 1). The device was capable of infusing the LiCoEDTA and sampling the rumen fluid, as evidenced by the presence of elevated Co concentrations in the sampled rumen fluid. Using the fluid samples obtained, liquid passage rate within the calf was estimated to be 40.2% of ruminal fluid/h. The second procedure tested the vacuum-assisted collection device and consisted of evacuating and weighing the rumen contents, which is considered a key preparatory step in washed reticulorumen technique experiments that aim to measure nutrient absorption. In agreement with existing literature, evacuated rumen contents represented approximately 4% of the calf's body weight. In conclusion, custom-built devices for infusion, sampling, and vacuum-assisted collection were efficacious when tested in a 62-d-old ruminally cannulated calf fed a diet of 100% texturized starter (18% crude protein, as-fed). Fellow scientists may employ and further modify these techniques to suit their needs when assessing passage kinetics, nutrient digestion, and volatile fatty acid absorption in calves.

The particulate passage rate, nutrient composition and fermentation characteristics across gastrointestinal tracts in lactating dairy cows fed three different forage source diets.

This study was conducted to investigate the particulate passage rate, nutrient characteristics and fermentation parameters across the gastrointestinal tract (GIT) in lactating dairy cows fed cereal straws in comparison with alfalfa hay. Eighteen multiparous Holstein cows were randomly assigned to one of three experimental diets consisting of 55% concentrate, 15% corn silage and 30% different forage sources as follows (% of dry matter [DM]): (i) 23% alfalfa hay and 7% Chinese wild rye hay (AH); (ii) 30% corn stover (CS); and (iii) 30% rice straw (RS). The Cr-mordanted corn silage-neutral detergent fibre was used to estimate the passage flow at week 14. After 14-week feeding, the animals were slaughtered to collect the gastrointestinal digesta. Dietary forage sources had little effect on the fractional passage rates in the rumen (range from 5.05 to 6.25%/hr) or hindgut (range from 4.49 to 5.24%/hr). Total volatile fatty acid (VFA) concentration in the caecum was highest, followed by the rumen, colon and rectum, and the lowest in the abomasum and duodenum, indicating that the large intestines, especially caecum, are the important positions for carbohydrate degradation. Greater proportion of propionate and butyrate and lower acetate were found in the AH compared to CS or RS in colon, but higher acetate in abomasum was found in the cows fed CS or RS compared to AH. In conclusion, cereal straw diets did not change the particulate passage rate in the rumen and hindgut which might be mainly due to the similar DM intake among these three diets. Different forage source diets significantly changed VFA proportion in the abomasum and colon, indicating the existence of different digestion or absorption rates in these tracts among the experimental diets.

Association of aqueous hydrogen concentration with methane production in continuous cultures modulated to vary pH and solids passage rate.

The objective of this study was to evaluate the effects of altering pH and solids passage rate (kp) on concentration of aqueous H2 [H2(aq)], CH4 production, volatile fatty acids (VFA) production, and fiber digestibility in a continuous culture fermentation system. The present study was conducted as a 2 × 2 factorial treatment arrangement in a Latin square design using continuous culture fermentors (n = 4). Our continuous culture system was converted to a closed system to measure CH4 and H2 emission while measuring H2(aq) concentration and VFA production for complete stoichiometric assessment of fermentation pattern. Treatments were control pH (CpH; ranging from 6.3 to 6.9) or low pH (LpH; 5.8 to 6.4) factorialized with solids kp that was adjusted to be either low (Lkp; 2.5%/h) or high (Hkp; 5.0%/h); liquid dilution was maintained at 7.0%/h. Fermentors were fed once daily (40 g of dry matter; 50:50 concentrate:forage diet). Four periods lasted 10 d each, with 3 d of sample collection. The main effect of LpH increased nonammonia nitrogen flow, and both LpH and Hkp increased nonammonia nonbacterial N flow. We observed a tendency for Hkp to increase bacterial N flow per unit of nonstructural carbohydrates and neutral detergent fiber degraded. The main effect of LpH decreased H2(aq) by 4.33 µM compared with CpH. The main effect of LpH decreased CH4 production rate from 5 to 9 h postfeeding, and Hkp decreased CH4 production rate from 3 to 9 h postfeeding. We found no effect of LpH on daily CH4 production or CH4 produced per gram of neutral detergent fiber degraded, but Hkp decreased daily CH4 production by 33.2%. Both the main effects of LpH and Hkp decreased acetate molar percentage compared with CpH and Lkp, respectively. The main effect of both LpH and Hkp increased propionate molar percentage, decreasing acetate-to-propionate ratio from 2.62 to 2.34. We noted no treatment effects on butyrate molar percentage or total VFA production. The results indicate increasing kp and decreasing pH decreased acetate-to-propionate ratio, but only increasing kp decreased CH4 production; lack of differences for LpH might be a result of compensatory methanogenesis during the second half of the day postfeeding.

Between-cow variation in digestion and rumen fermentation variables associated with methane production.

A meta-analysis based on an individual-cow data set was conducted to investigate the effects of between-cow variation and related animal variables on predicted CH4 emissions from dairy cows. Data were taken from 40 change-over studies consisting of a total of 637 cow/period observations. Animal production and rumen fermentation characteristics were measured for 154 diets in 40 studies; diet digestibility was measured for 135 diets in 34 studies, and ruminal digestion kinetics was measured for 56 diets in 15 studies. The experimental diets were based on grass silage, with cereal grains or by-products as energy supplements, and soybean or canola meal as protein supplements. Average forage:concentrate ratio across all diets on a dry matter basis was 59:41. Methane production was predicted from apparently fermented substrate using stoichiometric principles. Data were analyzed by mixed-model regression using diet and period within experiment as random effects, thereby allowing the effect of experiment, diet, and period to be excluded. Dry matter intake and milk yield were more repeatable experimental measures than rumen fermentation, nutrient outflow, diet digestibility, or estimated CH4 yield. Between-cow coefficient of variation (CV) was 0.010 for stoichiometric CH4 per mol of volatile fatty acids and 0.067 for predicted CH4 yield (CH4/dry matter intake). Organic matter digestibility (OMD) also displayed little between-cow variation (CV = 0.013), indicating that between-cow variation in diet digestibility and rumen fermentation pattern do not markedly contribute to between cow-variation in CH4 yield. Digesta passage rate was much more variable (CV = 0.08) between cows than OMD or rumen fermentation pattern. Increased digesta passage rate is associated with improved energetic efficiency of microbial N synthesis, which partitions fermented substrate from volatile fatty acids and gases to microbial cells that are more reduced than fermented carbohydrates. Positive relationships were observed between CH4 per mol of volatile fatty acids versus OMD and rumen ammonia N concentration versus OMD; and negative relationships between the efficiency of microbial N synthesis versus OMD and digesta passage rate versus OMD, suggesting that the effects of these variables on CH4 yield were additive. It can be concluded that variations in OMD and efficiency in microbial N synthesis resulting from variations in digesta passage contribute more to between-animal variation in CH4 emissions than rumen fermentation pattern.

Effects of fibre digestibility and level of roughage on performance and rumen fermentation of finishing beef cattle.

The objective was to evaluate effects of neutral detergent fibre (NDF) digestibility and level of fresh sugarcane on intake, body fatness, carcass characteristics, and rumen kinetics and fermentation of beef cattle. Forty-eight Nellore young bulls were used in a complete randomized block design with a 2 × 2 factorial arrangement of treatments. Eight rumen-cannulated Nellore steers were used in a replicated 4 × 4 Latin square design. Two sugarcane genotypes divergent for stalk NDF digestibility (NDFD) were used. Experimental diets were formulated with 20 or 40% of sugarcane on a dry matter (DM) basis. High-NDFD genotype associated with the lower level of roughage in the diet promoted greater DM intake, resulting in greater body gain. Sugarcane with high-NDFD increased final body weight, hot carcass weight, and back-fat thickness. Animals receiving the genotype with high NDFD had greater rump-fat thickness only with 40% sugarcane in the diet. Animals receiving the low-NDFD genotype at 20% of the diet had lower NDF passage rate. Rumen pH was greater for diets with greater NDF content. There was greater proportion of butyrate in the rumen of animals receiving diets with greater NDF content. In conclusion, high-NDFD sugarcane increased final body and carcass weight, HCW, and fat thickness. When associated with lower inclusion of roughage in the diet, it can also increase DM intake and body weight gain of beef cattle.

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.

Solute and particle retention in a small grazing antelope, the blackbuck (Antilope cervicapra).

Digesta retention patterns have been suggested to play a major role in ruminant diversification. Two major digestion types have been proposed, termed 'cattle-type' and 'moose-type', that broadly correspond to the feeding categories of grazers and intermediate feeders on the one, and browsers on the other hand. We measured and calculated the mean retention time (MRT) of a solute and a particle (<2 mm) marker in the gastrointestinal tract (GIT) and the reticulorumen (RR) of a small grazer, the Indian blackbuck (Antilope cervicapra, n=5, body mass of 26±4 kg) and an intermediate feeder, the nilgai (Boselaphus tragocamelus, n=5, body mass of 168±21 kg). MRT(solute) and MRT(particle) were 29±4.1 h and 60±6.6 h in blackbuck and 28±2.5 h and 54±8.9 h in the nilgai for the GIT, and 14±1.7 h, 45±5.0 h, 19±2.0 h and 45±8.4 h for the RR, respectively. With a selectivity factor (SF, the ratio of MRT(particle) to MRT(solute)) in the RR of 3.2±0.28 for blackbuck and 2.3±0.36 for nilgai, both species are clearly in the category of 'cattle-type' ruminants. In particular, the high SFRR of blackbuck, in spite of its small body size, is remarkable, and leads to specific predictions on the RR anatomy of this species (such as a particularly large omasum), which can be tested in further studies. The adaptive value of a high SFRR is mainly considered as an increase in microbial productivity in the RR.

Effects of solid feed level and roughage-to-concentrate ratio on ruminal drinking and passage kinetics of milk replacer, concentrates, and roughage in veal calves.

Effects of solid feed (SF) level and roughage-to-concentrate (R:C) ratio on ruminal drinking and passage kinetics of milk replacer, concentrate, and roughage were studied in veal calves. In total, 80 male Holstein-Friesian calves (45±0.2kg of body weight) were divided over 16 pens (5 calves per pen). Pens were randomly assigned to either a low (LSF) or a high (HSF) SF level and to 1 of 2 R:C ratios: 20:80 or 50:50 on a dry matter (DM) basis. Roughage was composed of 50% corn silage and 50% chopped wheat straw on a DM basis. At 27 wk of age, measurements were conducted in 32 calves. During the measurement period, SF intake was 1.2kg of DM/d for LSF and 3.0kg of DM/d for HSF, and milk replacer intake averaged 2.3kg of DM/d for LSF and 1.3kg of DM/d for HSF. To estimate passage kinetics of milk replacer, concentrate, and straw, indigestible markers (CoEDTA, hexatriacontane C36, Cr-neutral detergent fiber) were supplied with the feed as a single dose 4, 24, and 48h before assessment of their quantitative recovery in the rumen, abomasum, small intestine, and large intestine. Rumen Co recovery averaged 20% of the last milk replacer meal. Recoveries of Co remained largely unaffected by SF level and R:C ratio. The R:C ratio did not affect rumen recovery of C36 or Cr. Rumen fractional passage rate of concentrate was estimated from recovery of C36 in the rumen and increased from 3.3%/h for LSF to 4.9%/h for HSF. Rumen fractional passage rate of straw was estimated from Cr recovery in the rumen and increased from 1.3%/h for LSF to 1.7%/h for HSF. An increase in SF level was accompanied by an increase in fresh and dry rumen contents. In HSF calves, pH decreased and VFA concentrations increased with increasing concentrate proportion, indicating increased fermentation. The ratio between Cr and C36 was similar in the small and large intestine, indicating that passage of concentrate and straw is mainly determined by rumen and abomasum emptying. In conclusion, increasing SF level introduces large variation in passage kinetics of dietary components, predominantly in the rumen compartment. The SF level, rather than the R:C ratio, influences rumen recovery of concentrate and roughage. Our data provide insight in passage kinetics of milk (Co representing the milk replacer) and SF (Cr and C36 representing roughage and concentrate, respectively) and may contribute to the development of feed evaluation models for calves fed milk and SF.

Short communication: Comparison of 3 solid digesta passage markers in dairy cows.

This study investigated the usefulness of acid-detergent fiber-bound (15)N [acid detergent insoluble (ADI)-(15)N] as a solid digesta passage marker in dairy cows compared with chromium (Cr) and ytterbium (Yb) (as labeled fiber or forage, respectively). Intrinsically (ADI-(15)N) or extrinsically (Cr, Yb) labeled alfalfa hay was pulse-dosed intraruminally to 7 lactating dairy cows. Following marker administration, spot fecal samples were collected for up to 72 h for marker analyses. Urine and milk samples were also collected and analyzed for Yb and Cr. Fecal marker excretion data were processed using 2-compartment mathematical age-dependent/age-independent (Gn→G1) models. The rate of passage of the marker in the first, age-dependent compartment tended to be slower for Yb compared with Cr and ADI-(15)N, which resulted in a trend for longer mean retention time (MRT) in this compartment when Yb was used as a marker (19.0 h) compared with Cr and ADI-(15)N (14.5 and 13.9h, respectively). The rate constant of marker disappearance for the second or age-independent compartment tended to be greater for Yb compared with Cr and ADI-(15)N, which led to a shorter MRT of Yb in this compartment (15.6) versus ADI-(15)N (32.1) and Cr (24.8h). The cumulative MRT was greater for ADI-(15)N versus Cr and Yb (46.0, 39.3, and 34.4h, respectively). Total MRT of marker tended to be greater for ADI-(15)N than for Yb (46.6 vs. 36.6h, respectively). Urine and milk analyses data suggested no measurable losses of Yb along the digestive tract, but about 0.79% of Cr dosed intraruminally was secreted or excreted in milk and urine in the 48-h period following marker administration. Collectively, this study confirmed previous observations that ADI-(15)N can be used reliably as a solid digesta passage marker for ruminants, producing pre-duodenal and total-tract retention times similar to that of Cr-labeled fiber. Retention time in the age-independent compartment was underestimated when Yb was used as a marker, emphasizing the need to process forages to isolate fiber before labeling with Yb.

Stable isotope-labelled feed nutrients to assess nutrient-specific feed passage kinetics in ruminants.

Knowledge of digesta passage kinetics in ruminants is essential to predict nutrient supply to the animal in relation to optimal animal performance, environmental pollution and animal health. Fractional passage rates (FPR) of feed are widely used in modern feed evaluation systems and mechanistic rumen models, but data on nutrient-specific FPR are scarce. Such models generally rely on conventional external marker techniques, which do not always describe digesta passage kinetics in a satisfactory manner. Here the use of stable isotope-labelled dietary nutrients as a promising novel tool to assess nutrient-specific passage kinetics is discussed. Some major limitations of this technique include a potential marker migration, a poor isotope distribution in the labelled feed and a differential disappearance rate of isotopes upon microbial fermentation in non-steady state conditions. Such limitations can often be circumvented by using intrinsically stable isotope-labelled plant material. Data are limited but indicate that external particulate markers overestimate rumen FPR of plant fibre compared with the internal stable isotope markers. Stable isotopes undergo the same digestive mechanism as the labelled feed components and are thus of particular interest to specifically measure passage kinetics of digestible dietary nutrients.