Passage Kinetics Research Articles

Nutrient demand interacts with legume maturity to affect rumen pool sizes in dairy cows

Effects of legume maturity on dry matter intake (DMI), milk production, ruminal fermentation and pool sizes, and digestion and passage kinetics, and the relationship of these effects with preliminary DMI (pDMI) were evaluated using 16 ruminally and duodenally cannulated Holstein cows in a crossover design with a 14-d preliminary period and two 17-d treatment periods. During the preliminary period, the pDMI of individual cows ranged from 22.9 to 30.0kg/d (mean=25.9kg/d) and the 3.5% fat-corrected milk yield ranged from 34.1 to 68.2kg/d (mean=43.7kg/d). Experimental treatments were diets containing alfalfa silage harvested either a) early-cut, less mature (EC) or b) late-cut, more mature (LC) as the sole forage. Early- and late-cut alfalfa contained 40.8 and 53.1% neutral detergent fiber (NDF) and 23.7 and 18.1% crude protein, respectively. Forage:concentrate ratios were 53:47 and 42:58 for EC and LC, respectively; both diets contained approximately 22% forage NDF and 27% total NDF. Preliminary DMI, an index of nutrient demand, was determined during the last 4d of the preliminary period when cows were fed a common diet and used as a covariate. Main effects of alfalfa maturity and their interaction with pDMI were tested by ANOVA. Alfalfa maturity and its interaction with pDMI did not affect milk yield but EC increased DMI compared with LC; thus, EC had lower efficiency of milk production than LC. The EC diet decreased milk fat concentration more per kilogram of pDMI increase than the LC diet, but milk fat yield was not affected. The lower concentration and faster passage rate of indigestible NDF for EC resulted in lower rumen pools of indigestible NDF, total NDF, and dry matter than did LC, which EC increased at a slower rate than did LC as pDMI increased. The EC diet decreased starch intake and increased ruminal pH compared with the LC diet. The rate of ruminal starch digestion was related to level of intake, but this did not affect ruminal or postruminal starch digestion. Total-tract digestibility of NDF, organic matter, and dry matter was higher for EC than LC. Microbial efficiency tended to be related to pDMI and the response differed by treatment. When alfalfa silage was the only source of forage in the diet, cows supplemented with additional concentrate to account for decreased protein and increased fiber concentrations associated with LC produced similar fat-corrected milk yields with greater efficiency than cows fed EC.

Nutrient demand interacts with grass particle length to affect digestion responses and chewing activity in dairy cows

Effects of grass particle length on dry matter intake (DMI), milk production, ruminal fermentation and pool sizes, digestion and passage kinetics, and chewing activity and the relationship of these effects with preliminary DMI (pDMI) were evaluated using 15 ruminally and duodenally cannulated Holstein cows in a crossover design with a 14-d preliminary period and two 18-d treatment periods. During the preliminary period, pDMI of individual cows ranged from 22.6 to 29.8kg/d (mean=25.8kg/d) and 3.5% fat-corrected milk yield ranged from 29.2 to 56.9kg/d (mean=41.9kg/d). Experimental treatments were diets containing orchardgrass silage chopped to either (a) 19-mm (long) or (b) 10-mm (short) theoretical length of cut as the sole forage. Grass silages contained approximately 46% neutral detergent fiber (NDF); diets contained 50% forage, 23% forage NDF, and 28% total NDF. Preliminary DMI, an index of nutrient demand, was determined during the last 4 d of the preliminary period when cows were fed a common diet and used as a covariate. Main effects of grass particle length and their interaction with pDMI were tested by ANOVA. Grass particle length and its interaction with pDMI did not affect milk yield, milk composition, or rumen pH. Long particle length tended to decrease DMI compared with short particle length, which might have been limited by rumen fill or chewing time, or both. Passage rates of feed fractions did not differ between long and short particle lengths and were not related to level of intake. As pDMI increased, long particles decreased ruminal digestion rate of potentially digestible NDF at a faster rate than short particles. As a result, long particles decreased or tended to decrease rates of ruminal turnover for NDF, organic matter, and dry matter and increased their rumen pools compared with short particles for cows with high pDMI. Long particles increased eating time, which affected cows with high intake to the greatest extent, and total chewing time compared with short particles. As intake increased, ruminal digestion (kg/d) and digestibility (%) of starch decreased, rumen pool size of starch increased, and postruminal digestion and digestibility of starch increased quadratically. When grass silage was the only source of forage in the diet, increasing chop length from 10 to 19mm tended to decrease DMI but did not negatively affect productivity of cows, which were fed adequate fiber.

A model for evaluating beef cattle rations considering effects of ruminal fiber mass

A mathematical model based on Cornell Net Carbohydrate and Protein System (CNCPS) was developed and adapted in order to evaluate beef cattle rations at tropical climate conditions. The presented system differs from CNCPS in the modeling of insoluble particles' digestion and passage kinetics, which enabled the estimation of fiber mass in rumen and its effects on animal performance. The equations used to estimate metabolizable protein and net energy requirements for gain, net energy requirement for maintenance and total efficiency of metabolizable energy utilization were obtained from scientific articles published in Brazil. The parameters of the regression equations in these papers were estimated using data from Bos indicus purebred and crossbred animals reared under tropical conditions. The model was evaluated by using a 368-piece of information database originally published on 11 Doctoral theses, 14 Master's dissertations and four scientific articles. Outputs of the model can be considered adequate.

Relationships between chewing behavior, digestibility, and digesta passage kinetics in steers fed oat hay at restricted and ad libitum intakes1

The objective of this study was to evaluate the relationships between chewing behavior, digestibility, and digesta passage kinetics in steers fed oat hay at restricted and ad libitum intakes. Four Hereford steers, with an initial average BW of 136 kg, were used in an experiment conducted as a balanced 4 × 4 Latin square with 4 treatments (levels of intake) and 4 periods. Animals were fed lopsided oat hay (Avena strigosa Schreb.) at 4 levels of intake (as a percentage of BW): 1.5, 2.0, 2.5, and ad libitum. Digestibility, chewing behavior, and digesta passage kinetic measurements were recorded during the experimental period. Chewing rates during eating and ruminating [(chews•min(-1))/g of DMI•kg(-1) of BW•d(-1)] decreased (P = 0.018 and P = 0.032, respectively) with increased DMI (g•kg(-1) of BW•d(-1)), whereas total chewing and total time spent on each chewing activity increased. Calculated total energy expended by the chewing activity was 4.2, 4.4, 5.2, and 5.3% of ME intake for DMI of 1.5, 2.0, and 2.5% of BW and ad libitum, respectively, indicating that adjustments in animal chewing behavior may be a mechanism of reducing energy expenditure when forages are fed at restricted intake. Hay digestibility decreased (P < 0.001) with increased DMI (r = -0.865). Digesta mean retention time (h) was strongly correlated with DMI (r = -0.868) and OM digestibility (r = 0.844). At reduced intake, hay digestibility was enhanced (P < 0.001) by extending digesta retention time and by increasing chewing efficiency, highlighting the relationship between chewing behavior and the digestive process. Fractional outflow rate of particulate matter from the reticulorumen (k(1)) was positively correlated with total chews, emphasizing that the decrease in particle size caused by chewing facilitates particle flow through the digestive tract. Increased hay intake also increased (P < 0.001) k(1), whereas passage rate of the liquid phase, transit time, and rumen fill were not affected (P > 0.05). The latter was correlated with rumen volume (r = 0.803). In conclusion, the results of this study indicate that animals fed at restricted intake increased chewing rate when eating and ruminating, which, along with a longer digesta retention time, contributed to enhance feed digestibility.

Digesta retention time in roe deer Capreolus capreolus, as measured with cerium-, lanthanum- and chromium-mordanted fibre

Studies on particle retention time in ruminants are commonly conducted utilizing chromium (Cr) and/or a lanthanide bound to feeds. Both types of markers have different chemical properties which potentially bias estimates of digesta retention—their combined use is therefore a topic of ongoing discussion. In order to evaluate the suitability of two lanthanides for studies assessing the passage kinetics of different-sized particles, we measured the mean retention time in roe deer with cerium (Ce) and lanthanum (La) labelled to the same particle length and compared it to Cr-mordanted fibre of the same size. We expected a simultaneous excretion of Ce- and La-labelled fibre, but a delayed excretion of Cr-mordanted fibre compared to the other markers. In this study, the mean retention times of Ce- and La-labelled fibre did not differ significantly, and Cr-mordanted fibre was retained on average 5 h longer than Ce and La. Despite the limitation of the small sample size, Ce and La showed the same excretion pattern and can therefore be recommended as mordants in studies assessing passage kinetics of different particle sizes.

The effect of harvesting strategy of grass silage on digestion and nutrient supply in dairy cows

This study examined the effects of primary growth (PG) and regrowth (RG) timothy-meadow fescue silages harvested at 2 stages of growth on feed intake, cell wall digestion and ruminal passage kinetics in lactating dairy cows. Four dairy cows equipped with rumen cannulas were used in a study designed as a 4 × 4 Latin square with 21-d periods. The experimental silages were offered ad libitum with 8 kg/d of concentrate. Ruminal digestion and passage kinetics were assessed by the rumen evacuation technique. Silages of PG were on average more digestible than RG silages. The concentration of neutral detergent fiber (NDF) and indigestible NDF (iNDF) increased and the concentration of digestible organic matter in dry matter (DM) of silages decreased with advancing maturity in PG and RG. Cows consumed more feed DM, energy, and protein and produced more milk when fed PG diets rather than RG diets. Delaying the harvest decreased DM intake and milk production in PG and RG. There were no differences between PG and RG in rumen pH, ammonia N, or total volatile fatty acid concentrations. The intake of N, omasal canal flow of total nonammonia N and microbial N, excretion of N in feces, and ruminal true digestibility of N were higher for PG than for RG diets. The efficiency of microbial N synthesis was not different between PG and RG. Intake and omasal canal flow of organic matter, NDF, and potentially digestible NDF (pdNDF) were higher in PG than in RG. Whole-diet digestibility of organic matter, NDF, or pdNDF in the rumen or in the total tract was not different between PG and RG despite the higher digestibility of PG silages measured in sheep. Rumen pool sizes of crude protein and iNDF were lower for PG diets, whereas the pool size of pdNDF was higher for PG diets than for RG diets. The rate of passage of iNDF was higher for PG diets than for RG diets, with no difference between them in rate of digestion or passage of pdNDF. The lower milk production in cows fed regrowth grass silages compared with primary growth silages could be attributed to the lower silage DM intake potential. Chemical composition of the silages, rumen fill, digestion and passage kinetics of NDF, or the ratio of protein to energy in absorbed nutrients could not explain the differences in DM intake between silages made from primary and regrowth grass.

Brown midrib corn silage and Tifton 85 bermudagrass in rations for early-lactation cows

Forty Holstein cows were used in an 8-wk randomized trial to evaluate the effects of feeding combinations of forages with improved fiber digestibility on performance during early lactation. Treatments were arranged as a 2 × 2 factorial to include silage from normal (NCS) or brown midrib (BMR) corn silage with or without 10% Tifton 85 bermudagrass hay (T85). In a simultaneous digestion trial, degradation and passage kinetics and ruminal fermentation parameters were evaluated in a 4 × 4 Latin square design trial using late-lactation Holstein cows fitted with ruminal cannulas. Dry matter intake (DMI) and neutral detergent fiber (NDF) intake were greater with BMR than with NCS; however, milk yield and composition were similar among corn silage types. Inclusion of T85 reduced milk yield but supported higher milk fat percentage, resulting in similar yields of energy-corrected milk. Blood glucose concentrations were higher for BMR compared with NCS, and inclusion of T85 increased blood urea N concentrations. Treatments did not alter liquid or solid phase passage rates or rumen turnover. Corn silage type did not affect ruminal pH or volatile fatty acid concentrations, but inclusion of T85 increased pH and molar proportion of acetate but decreased butyrate. Molar proportions of propionate were greater for NCS and T85 compared with BMR and T85, resulting in an interaction. Results of this trial indicate that combinations of forages with improved fiber digestibility can be used to support intake and performance of cows during early lactation.

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.

Effects of feeding grass or red clover silage cut at two maturity stages in dairy cows. 2. Dry matter intake and cell wall digestion kinetics

This study examined the effects of red clover or grass silages cut at 2 stages of growth on feed intake, cell wall digestion, and ruminal passage kinetics in lactating dairy cows. Five dairy cows equipped with rumen cannulas were used in a study designed as a 5×5 Latin square with 21-d periods. Diets consisted of early-cut and late-cut grass and red clover silages and a mixture of late-cut grass and early-cut red clover silages offered ad libitum. All diets were supplemented with 9 kg/d of concentrate. Ruminal digestion and passage kinetics were assessed by the rumen evacuation technique. Apparent total-tract digestibility was determined by total fecal collection. The silage dry matter intake was highest when the mixed forage diet was fed and lowest with the early-cut red clover diet. Delaying the harvest tended to decrease DMI of grass and increase that of red clover. The intake of neutral detergent fiber (NDF) and potentially digestible NDF (pdNDF) was lower but the intake of indigestible NDF (iNDF) was higher for red clover diets than for grass diets. The rumen pool size of iNDF and the ratio of iNDF to pdNDF in the rumen contents were larger, and pool sizes of NDF and pdNDF were smaller for red clover than for grass silage diets. Outflow of iNDF and the ratio of iNDF to pdNDF in digesta entering the omasal canal were larger, and the outflow of pdNDF was smaller for red clover than for grass silage diets. The digestion rate (kd) of pdNDF was faster for red clover diets than for grass silage diets. Delaying the harvest decreased kd for grass but increased it for red clover silage diets. Observed differences in fiber characteristics of red clover and grass silages were reflected in ruminal digestion and passage kinetics of these forages. The low intake of early-cut red clover silage could not be explained by silage digestibility, fermentation quality, or rumen fill, but was most likely related to nutritionally suboptimal composition because inclusion of moderate quality grass silage improved silage intake. Increasing the maturity of ensiled red clover does not seem to affect silage dry matter intake as consistently as that of grasses.

Effect of wheat forage maturity and preservation method on forage chemical composition and performance of growing calves fed mixed diets12

Three 2.4-ha wheat (Triticum aestivum L.) fields were used to test the effects of maturity at harvest (boot vs. dough) and preservation method (hay vs. silage) on forage yield, chemical composition, and animal performance when fed in mixed diets. Forages were incorporated into 4 diets in a 2 x 2 factorial arrangement of treatments with hominy feed, soybean hulls, and cottonseed meal as the primary concentrate ingredients. In Exp. 1 diets contained 20% wheat forage (DM basis) and were fed to 96 beef calves (n = 48 steers and 48 heifers; initial BW 229 +/- 6.0 kg) in 12 mixed-sex pens. In Exp. 2 diets contained 40% wheat forage (DM basis) and were fed to beef steers (n = 48; initial BW 198 +/- 6.8 kg) in 12 pens. These diets were also individually fed to 32 calves (Exp. 1, n = 16, BW = 187 +/- 9.4 kg; Exp. 2, n = 16 calves, BW = 160 +/- 8.2 kg) to determine DM and NDF digestibility and gastrointestinal tract passage kinetics. Advanced maturity increased (P < 0.01) DM yield, decreased (P < 0.01) CP concentrations, and tended (P = 0.10) to increase nonfiber carbohydrate concentrations, but did not affect (P >or= 0.22) NDF, ADF, or TDN concentrations. Maturity at harvest, preservation method, or their interaction did not affect (P >or= 0.15) ADG when wheat forage was fed as 20 or 40% of the diet. When calves were fed the 40% wheat forage diets, maturity at harvest did not affect (P >or= 0.27) DMI or G:F. Calves fed 40% hay diets consumed more (P = 0.04) feed DM as a percentage of BW than calves fed silage diets, but tended (P = 0.09) to be less efficient. With 20 or 40% wheat forage diets, there were no differences (P >or= 0.13) in passage rate, ruminal retention time, or fecal output due to maturity or preservation method. Digestibility of DM tended (P = 0.07) to be greater for silage than hay diets when fed in 20% wheat forage diets. Dry matter and NDF digestibility of 40% boot-stage wheat forage diets were greater (P < 0.01) than diets containing forage harvested in dough stage. Forty percent hay diets also tended (P = 0.07) to have greater DM digestibility, and NDF digestibility was greater (P < 0.01) compared with silage diets. Although differences in performance were not noted in the present experiments, increased maturity at harvest and preservation as silage can cause differences in DMI and digestibility of DM and NDF in diets containing 40% wheat forage.

Investigations on the stratification of forestomach contents in ruminants: an ultrasonographic approach

The degree of stratification in the reticulorumen contents is considered a major explanatory factor for other morphological and physiological differences between ruminant feeding types. However, information on reticulorumen (RR) contents is usually gathered from dead animals or indirect such as passage kinetics. We hypothesized that, although the contents of the gastrointestinal tract can usually not be evaluated by ultrasonography, the three typical layers of stratified RR contents (gas dome, fibre mat, fluid) can be demonstrated by this technique. In three domestic cows, the gas dome in the cows’ rumens could be demonstrated sonographically by reverberation lines running in parallel to the line demarcating the rumen mucosa, as is typical for the sonographic image of large, gas-filled spaces. More ventrally, the area behind the rumen mucosa showed typical pattern indicating gaseous inclusions, corresponding to the fibre mat inside the rumen. Further ventrally, in two of the animals, the area behind the demarcation line appeared dark without reverberation lines, as is typical for large fluid-filled spaces. When the technique was applied to a captive, habituated, browse-fed moose, no gas dome could be demonstrated, supporting the hypothesis that the reticulorumen contents of browsers are less stratified. The results of this study indicate that sonography represents a useful tool for the demonstration of RR contents stratification in live animals.

Avaliação nutricional da grama-estrela cv. Africana para vacas leiteiras em condições de pastejo

Objetivou-se avaliar nutricionalmente a grama-estrela (Cynodon nlemfuensis) cv. Africana utilizada sob pastejo rotacionado por vacas leiteiras. Dez vacas mestiças foram manejadas em 2 ha de grama-estrela e divididas em 11 piquetes/ha. O período de pastejo foi de três dias e os 30 subseqüentes foram destinados à recuperação da pastagem. Durante o período experimental, os animais foram ordenhados duas vezes ao dia e receberam suplementação com 2 kg de concentrado. Amostras representativas do pasto ingeridas (extrusa) foram colhidas para determinação de sua composição nutricional. O consumo de matéria seca (MS) pelos animais foi estimado utilizando-se cromo e a MS indigestível como indicadores externo e interno. O desempenho individual das vacas foi avaliado pela produção de leite diária e pela pesagem dos animais. A dinâmica da matéria alimentar foi estimada com base nas técnicas in vitro gravimétricas, de produção cumulativa de gases da fermentação microbiana e da estimativa da cinética de passagem das fases sólida e líquida. A quantidade de energia líquida total (ELt), em MJ/dia, atendeu à demanda energética exigida pelos animais. Os valores de proteína metabolizável (PM) preditos corresponderam ao suprimento de 91% da PM exigida por esses animais. As predições das exigências em macrominerais atenderam apenas 75% do Ca exigido, mas atenderam às exigências dos demais macrominerais. A grama-estrela atende à demanda energética nutricional de mantença e produção de 11,7 kg de leite por dia. Nas circunstâncias estudadas, é necessário suplementar nutrientes que complementem a PM e Ca não atendidos completamente. O teor e as características cinéticas da fibra não causam repleção ruminal e restrição sobre o consumo de vacas leiteiras em pastejo.

Towards Non Contrast Enhanced Myocardial Perfusion Imaging Using Blood Oxygenation Weighted Fast Spin-Echo at 3.0 T

Purpose: In current clinical myocardial perfusion MRI first passage kinetics of a contrast agent bolus are assessed to detect perfusion deficits. Major limitations of these techniques are the need for 1 or 2 R-R interval temporal resolution plus limiting signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR), all confining the realizable in-plane resolution to 3–4mm.

Effect of fibre content in the diet on the mean retention time in different segments of the digestive tract in growing pigs

Development of mechanistic models of digestion is a means to account for interactions between different nutrients, and between nutrients and the animal. Although some digestion models for pigs have been proposed, quantitative information concerning the underlying processes is extremely limiting. The aim of this study was to determine the effect of dietary fibre on the transit of the solid and liquid phases of digesta in different segments of the gastro-intestinal tract (GIT) in growing pigs. Six barrows (initial BW 30 kg and fitted with two simple T-cannulas at the proximal duodenum and distal ileum) were used in a double 3×3 Latin square design. Pigs were offered diets differing in total dietary fibre content (17, 22 and 27%) at 4 h intervals. A single meal marked with YbO2 and Cr-EDTA was used to determine the passage kinetics of respectively the solid and liquid phases of digesta in different segments of the GIT. The mean retention time (MRT) averaged 1 h from the mouth to the proximal duodenum (stomach), 4 h from the proximal duodenum to the distal ileum (small intestine), and 35 h from the distal ileum to the faecal excretion (large intestine). Increase in dietary fibre content had no effect on MRT in the stomach, tended to decrease MRT of solid phase in the small intestine (P=0.07), and decreased MRT of both phases in the large intestine (43 to 29 h, P=0.06). For all diets, transit of the liquid phase in the large intestine was 4–8 h less than transit of the solid phase. © 2007 Elsevier B.V. All rights reserved.

Models for estimating digesta passage kinetics in the gastrointestinal tract of the horse

Fecal samples were collected to evaluate mathematical models to describe the kinetics of digesta passage in the segments of the equine gastrointestinal tract and to compare the passage kinetics of hay and oats. Four Norwegian Cold-blooded trotters (cecally cannulated, approximately 500 kg of BW) were fed Cr-mordanted hay and Yb-marked oats with their morning meal. The meal consisted of 2 kg of hay and 1 kg of oats processed as ground, pelleted, extruded, or micronized. Each horse was fed each type of oats on different days of collection, after a 5-d adaptation period, in a 4 x 4 Latin square design. Fecal samples were collected 5, 6, 7, 8, 9, 10, 11, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 48, and 52 h after administration of the marker dose. The samples were analyzed for Cr and Yb, and values were plotted using 1- and 2-compartment nonlinear passage models and an algebraic model. The 1-compartment G4 model and the 2-compartment G4G1 model showed an equally good fit to the observed excretion curves, based on low mean square error and SE. The excretion curves for hay (Cr) and oats (Yb) showed a striking similarity, and there seemed to be no difference in retention time between hay and oats in the horse. The mixing compartments in the horse are believed to be the cecum or both the cecum and the right ventral and dorsal segments of the colon, but further research in this area is needed to make a final conclusion.

Digestible NDF is selectively retained in the rumen of dairy cows compared to indigestible NDF

Data was compiled from four experiments (60 observations) with fistulated dairy cows. Different forages were fed ad libitum as the only feed or supplemented with a fixed amount of concentrate. Three rumen evacuations were performed on different days, and fibre kinetics was determined based on average rumen pools and average daily flows. Mean retention time in the rumen (MRT) of indigestible NDF (INDF) varied from 35 to 92 h. MRT determined using the rumen evacuation technique has traditionally been based on first-order kinetics and a one-compartment model, and overestimates MRT of DNDF, but not of INDF. A new method is presented for correcting MRT of DNDF for the simultaneous digestion and the selective retention in a more biologically reasonable model with two successive rumen compartments. MRT of DNDF using the traditional rumen evacuation approach is merely a function of fractional rate of passage, whereas MRT in the new equation also depends upon fractional rate of digestion and distribution of MRT between rumen compartments. The distribution of MRT between rumen compartments was obtained from interpretation of ytterbium marker excretion curves. Use of uncorrected rumen evacuation data for calculation of MRT for DNDF overestimated MRT with on average 52 h, equal to a 64% overestimation compared to corrected MRT of DNDF. The degree of overestimation increases with increasing fractional rate of degradation, decreasing fractional rate of passage, and as the distribution of MRT between compartments approaches 0.5:0.5. The overestimation was higher for highly digestible grass silages, grass hay, lucerne hay, and pea silage, compared to low digestible grass silage, maize silage, and whole crop barley silage. This was probably due to differences in INDF:NDF ratio between different parts in the plant. Corrected MRT of DNDF varied from 52 h for supplemented maize silage to 116 h for unsupplemented grass hay, and was on average 19 h higher than MRT for INDF, proving selective rumen retention, probably because INDF and DNDF are heterogeneously distributed between different particle size fractions and between different parts of the plant, differing in chemical composition and digestion and passage kinetics (leaves, steams or seeds).

Cell wall digestion and passage kinetics estimated by marker and in situ methods or by rumen evacuations in cattle fed hay 2 or 18 times daily

This study was conducted to determine the effects of timing rumen evacuations on kinetic parameter estimates of wall digestion and passage. Another objective was to estimate the rate constants of rumen neutral detergent fibre (NDF) kinetics in cattle fed grass hay using rumen evacuations, and to compare the results to those obtained by conventional methods ( in situ incubation, marker kinetics). Four young cattle were used in a switch-back design and fed a grass hay diet either twice daily at 7.00 and 19.00 h or 18 times per day starting at 7.00 h. Rumen evacuations were made just before morning feeding (7.00 h) and 11.00 and 15.00 h. The ingested feed, rumen and duodenal digesta and faeces were divided into five particle size fractions by wet sieving. Particulate matter retained on the sieves were analysed for NDF which was further subdivided into digestible (DNDF) and indigestible NDF (INDF) by 12-day in situ incubation. The rate constants were derived from average rumen pool sizes by pooling the particle fractions into large, medium and small particles (three-pool) or into large and small particles (two-pool). Feeding frequency had no effect on ruminal or total digestibility, average rumen pool size or kinetic parameters. The results showed that kinetic parameters, estimated from a single evacuation, can be severely biased due to under- or overestimation of rumen pool size. The rate of particle breakdown based on rumen INDF kinetics was slower than the passage rate of small particles (0.033 h −1 versus 0.047 h −1). Digestion rate of medium particles was faster than that of large or small particles. The passage rate of both DNDF and INDF increased with decreasing particle size and it was slower for DNDF than for INDF irrespective of particle size indicating selective retention. Digestion rate of DNDF estimated by the in situ incubation was lower than that derived from rumen evacuation. Model calculations suggested that the values based on the in situ incubations were underestimated. Ytterbium labelled hay underestimated the residence time in ruminal compartments compared with INDF estimated by rumen evacuation technique (42.5 h versus 49.8 h). However, the passage rates of small particles estimated from the marker kinetics were similar to those derived from rumen evacuation.

Effects of Replacing Grass Silage with Barley Silage in Dairy Cow Diets

This study examined the effects of gradually replacing grass silage with whole-crop barley silage on feed intake, ruminal and total tract digestibility, and milk yield in lactating dairy cows. Four dairy cows in early lactation, equipped with rumen cannulas, were fed 4 diets over four 21-d periods. The diets consisted of 4 forage mixtures of grass silage and whole-crop barley silage supplemented with 8.9kg/d of concentrates [dry matter (DM) basis]. The proportion of barley silage in the forage was adjusted to 0, 0.20, 0.40, and 0.60kg/kg of DM. Ruminal nutrient metabolism was measured on the basis of digesta flow entering the omasal canal. Ammonia concentrations and volatile fatty acid profiles were determined in the rumen fluid. Ruminal digestion and passage kinetics were assessed by the rumen evacuation technique. Replacement of grass silage with barley silage had no effect on DM, digestible organic matter, or neutral detergent fiber (NDF) intake, but starch intake increased, whereas nitrogen and digestible NDF (dNDF) intake decreased. Increases in the proportion of barley silage linearly decreased milk yield, and the molar proportion of acetate in the rumen, and increased that of propionate, butyrate, and valerate. Decreases in milk yield due to inclusion of barley silage were attributed to decreases in diet digestibility and nutrient supply to the animal. Barley silage linearly decreased organic matter digestibility in the total tract and NDF and dNDF digestibility in the rumen and the total tract, and decreased nonammonia N flow entering the omasal canal. No significant differences between diets were noted in the digestion rate of dNDF or passage rate of indigestible NDF from the rumen. Decreases in organic matter and NDF digestibility were attributed to the higher indigestible NDF concentration of barley silage compared with that of grass silage and to the smaller pool size of dNDF in the rumen.

Evaluation of methods for estimating starch digestibility and digestion kinetics in ruminants

The objective of the paper is to review literature concerning the methods used to estimate starch digestibility in different segments of the digestive tract of ruminants and the kinetics of starch digestion. Starch digestibility and site of starch digestion have a profound effect on the amount and profile of substrates absorbed from the digestive tract. The prediction of starch digestibility in the rumen is laborious and also technically difficult because of unrepresentative sampling of digesta from duodenal cannulae. The double-marker technique does not solve the problems related to unrepresentative digesta sampling because any of the particulate markers are not intimately associated with particles rich in starch. Furthermore, physical separation of particles rich in starch and fibre, which is a requisite in applying multiple marker techniques, would be difficult. However, despite problems in the determination of starch flow, the double-marker technique is likely to improve the accuracy of the flow measurements of other components. Mechanistic dynamic models describing digestion and absorption of substrates require reliable estimates of the digestion kinetics of feed components. The in situ technique has been extensively evaluated and used in estimating the digestion rate of starch. Initial particle loss, often described as soluble starch, is the major problem of the technique. Various assumptions have been used for the behaviour of this fraction in the kinetic models. It is also possible that the secondary particle loss during the incubation would overestimate the rate of digestion. Both the patterns of duodenal marker excretion curves and duodenal starch flow strongly suggest that the passage of particles rich in starch do not follow the first-order passage kinetics. This means that the models used to estimate ruminal starch digestibility from the kinetic parameters are incomplete. The published data suggest that the in situ technique underestimates ruminal starch digestibility for slowly degradable starch sources such as maize and overestimates it for rapidly degradable starch sources such as barley. An advantage of the in vitro methods is that the estimates of starch disappearance are not biased by particle loss. In vitro gas production technique may also be used for estimation of the rate of starch digestion for cereal grains by applying the curve subtraction method; i.e. gas production from the cell wall fraction is subtracted from the total gas production curve. Starch represents the major proportion of cell solubles in cereal grains and even a greater proportion of gas production potential. A requisite of the in vitro techniques in estimating the rate of starch digestion is that the system itself is not limiting.

Influence of passage model and caecal cannulation on estimated passage kinetics of roughage and concentrate in the gastrointestinal tract of horses

The objectives of this study were to evaluate the use of non-linear passage models to estimate total tract retention time and hindgut fractional passage rates of hay and concentrate, and to investigate the effect of caecal cannulation on passage parameters in horses. Animals were four geldings of Norwegian Cold-blooded Trotter fed a hay and concentrate diet at a dry matter intake of 1.7% of body weight. Marker studies were conducted one month before and three and 15 months after caecal cannulation using Cr-mordanted hay and Yb-labelled concentrate as markers. Faecal marker excretion curves were fitted either to one-compartment models with gamma time dependency (Gn) or to two-compartment models (GnG1), with gamma time dependency in the first compartment (Gn) and time independency in the second one (G1). Lowest error mean square was observed for the G4 and G4G1 models. Caecal cannulation increased total tract mean retention time (TMRT). No differences were observed between hay and concentrate in TMRT or fractional passage rates. It is concluded that using time dependent passage models will improve our understanding of passage kinetics in horses and that there is only minor differences between hay and concentrate in hindgut retention time.