Enzyme activities of rumen particles and feed samples incubated in situ with differing types of cloth

Three ruminally cannulated non-lactating dairy cows were used to investigate the effects of six different bag cloth types with pore size (microns): free surface area (%) ratios of 200:45, 41:33, 16:5, 10:2, 6:5 and 1:2 respectively on the disappearance of grass silage DM and neutral-detergent fibre (NDF), and on particle-associated carboxymethylcellulase (EC 3.2.1.4; CMCase) and xylanase (EC 3.2.1.8) activities extracted from feed residues. Another objective was to compare microbial activity inside the bags and in rumen ingesta. Rumen incubation periods were 3, 6, 12, 24, 48 and 96 h. DM and NDF disappearance and particle-associated enzyme activities were greatly reduced with the smaller pore size and/or open surface area. Re-analysing some of the data as a 2 x 2 factorial (pore size x free surface area) indicated that, generally, free surface area rather than pore size affected the disappearance of feed components and particle-associated enzyme activities. Enzyme activities were highly correlated with NDF disappearance at 6-48 h of incubation. Cumulative area under CMCase and xylanase activity curves explained 0.79 and 0.88 of the variation in NDF disappearance when different cloth type and 6-48 h incubation data were combined. Weighted mean enzyme activities inside the bags were less than 0.35 those in rumen ingesta. The highest activity values inside the bags (24 or 48 h) were less than 0.50 those found in rumen ingesta. The lower microbial activity inside the bags explains the slower rates of NDF digestion reported with in situ techniques than with rumen evacuation techniques. The general assumption of similar microbial activity inside the bags and in rumen ingesta is not justified by the present results, and caution must be taken in interpreting in situ results quantitatively for feed evaluation systems.

Ruminal and total plant cell-wall digestibility estimated by a combined in situ method utilizing mathematical models

Three ruminally and duodenally cannulated non-lactating Finnish Ayrshire cows were used to investigate ruminal and intestinal digestion of cell-wall carbohydrates by a combined in situ method. Five grasses cut at 10 d intervals were incubated in the rumen for 0, 6, 12, 24, 48, 72 and 96 h, and the undegraded residues were exposed to intestinal digestion. With advancing maturity of grass both the rate and extent of cell-wall digestion decreased. At early stages of growth the decreases were faster for the rate of digestion and at late stages of growth for the extent of digestion. Applying a passage rate of 0.02/h in one compartmental rumen model resulted in digestibility values markedly lower than typically observed in vivo. However, applying a rumen model incorporating a selective retention of particles and time-dependent release of particles from the non-escapable pool resulted in much higher digestibility values. Recovery of lignin after 96 h ruminal incubation with a subsequent mobile-bag incubation was very low (from 244 to 460 mg/g). Intestinal disappearance of neutral-detergent fibre (NDF) and hemicellulose decreased with advancing maturity of grass and with increasing length of preceding ruminal incubation period, i.e. with decreasing potential digestibility of the material. Disappearance of hemicellulose was much greater than that of cellulose for intact grasses but the difference diminished with increasing length of preceding rumen incubation period. On average, 195 mg/g of potentially digestible NDF disappeared from the mobile bags in the intestines. The post-ruminal digestion as a proportion of the total NDF digestibility varied between 0.034 and 0.058. Despite methodological problems both in ruminal in situ and intestinal mobile bag techniques, these methods can be used to investigate ruminal and intestinal cell-wall digestion and to partition cell-wall digestibility between ruminal and post-ruminal digestion providing that appropriate rumen models are used.

The role of the large intestine in post-ruminal digestion of feeds as measured by the mobile-bag method in cattle

To study the importance of site of recovery and other factors related to mobile-bag (MB) digestion values, two consecutive experiments in which diets were applied in a 3 x 3 Latin square design, were carried out with cannulated growing heifers. In Expt 1, several types of experimental feed were exposed to intestinal digestion in mobile bags made of two cloth types and filled with intact or rumen-undegradable (RUD) feed material to be recovered either from the ileum (IB) or faeces (FB). In Expt 2, mean retention time (MRT) of Yb-labelled digesta particles within the intestine and in vivo digestibility of diets were measured. With vegetable concentrates, FB resulted generally in slight overestimation of small-intestinal dry matter and N digestion, while with meat-and-bone meal no difference between FB and IB was found. The respective N digestibility of RUD late-cut silage was clearly underestimated as measured from FB. The disappearance of neutral-detergent fibre (NDF) of all feeds under test was higher from FB than from IB. It was not possible to isolate the influence of the large intestine on the MB values by changing bag cloth type. Irrespective of the longer retention time of bags and longer MRT of Yb in the intestine on a low as compared with a high level of feeding, only NDF disappearance of feeds increased due to lower feeding level. Altering the diet type to increase large-intestinal fermentation, as indicated in vivo, usually had no effect on the MB values. It is concluded that the site of collection of bags does not practically affect small-intestinal digestion values of feed N, unless the feed is rich in fibre.

Use of a combined synthetic fibre bag method to estimate the true total tract digestion of organic matter and nitrogen of hay and grass silage in cows

A 4 x 4 Latin square experiment was designed to determine and compare the total tract digestion of organic matter (OM) and nitrogen of hay and grass silage by combining the ruminal and intestinal digestion estimates obtained by the rumen bag and mobile bag methods. The diets were composed of forage alone or 30% forage and 70% barley. The rumen degradation (RD) was calculated using the degradation constants a, b, and c obtained to the forages, according to the equation RD = a+bc/(c+k). Intestinal digestion of escape feeds was estimated using the feed residues that resisted rumen degradation during 16 hours. For comparative purposes, the total tract digestion was also calculated based on the actual rumen degradation of forages during 16 hours rumen incubation. The effect of dietary interaction on the digestion of forages was assessed. Rumen degradation values obtained by 16 hours incubation did not provide sufficient basis to estimate reliably the total tract digestion by the combined bag method. Using calculated RD total tract digestion of OM of hay and grass silage were rather close to conventional in vivo digestion coefficient of respective feeds similar in chemical composition, when barley was not included in the diet. With hay, but not with grass silage, barley always markedly reduced the rumen degradation and total digestion of OM. Barley supplementation always increased the proportion of feed digested in the intestine. Nitrogen digestion was always higher with grass silage than with hay, and the combined bag method produced N digestion values 17.6-22.0%-units higher than those in the feed table. Barley supplementation did not affect the N digestion of the forages.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of supplementary energy and protein feeding on the true digestion of grass-silage organic matter, cell walls and nitrogen estimated by the combined synthetic-fibre-bag method in cows

Four ruminally and duodenally cannulated non-lactating Finnish Ayrshire cows were used in a balanced 4 × 4 Latin square to study the effect of different concentrate supplements on the true partial and total-tract digestion (TTD) of grass silage, estimated by using the combined rumen-bag-intestinal-bag method. The cows were fed, at maintenance level, grass silage alone or supplemented with good-quality ground barley, ground barley and rapeseed meal, or ground barley and soybean meal. The determination of the proportion of grass silage degraded in the rumen (RD) was based on disappearance of feeds from nylon bags during the rumen incubation as a function of time, using the outflow rate of k = 0.0625. The intestinal digestion (ID) was estimated by the mobile-bag method with the residues that resisted degradation during the 16-h rumen incubation. Combination of these two was calculated to provide the TTD. Concentrate supplementation always caused a clear and consistent decline in rumen degradation and TTD of organic matter (OM), neutral detergent fibre (NDF) and Kjeldahl-N of grass silage but had no real influence on its ID. The type of concentrate, however, had only little effect. The average TTD of NDF was 16% lower than that of OM, but TTD of N was always very much higher than the respective value for OM. The results indicate that concentrate supplementation decreases the total-tract digestion of OM, cell walls and nitrogen of grass silage owing to impaired ruminal degradation. The combined bag method appears a convenient tool to provide digestion coefficients close to the true feed digestion of the individual feeds. Key words: Grass silage, nylon bag, mobile bag, combined bag, ruminal degradation, intestinal digestion, true digestion

The effect of differing types of cloth and of contamination by non-feed nitrogen on intestinal digestion estimates using porous synthetic-fibre bags in a cow

The influence of free surface area and pore size of a synthetic-fibre bag, and flow of intestinal substances into the bag, on the intestinal digestion estimates by the nylon-bag method of dry matter (DM), neutral detergent fibre (NDF), nitrogen, NDF-N and feed 15N was studied using a non-lactating cow fed on hay supplemented with barley and oats at maintenance level. The bags containing 15N-labelled ground ryegrass (Lolium perenne), barley, barley straw or rapeseed (Brassica napus) straw were introduced into the duodenum through a T-shaped cannula and collected from the faeces. Also, the disappearance of N from the bag was related to the true intestinal N digestion by comparing it with the net loss between duodenum and faeces of 15N in ryegrass introduced into the duodenum in an aqueous suspension. It was noted that the bag cloth often significantly affected both disappearance values and the subsequent chemical composition of the residues. Re-analysing part of the data as a 2 x 2 factorial (free surface x pore size) suggested that free surface area was often more important than pore size as a determinant of both disappearance values and chemical composition. Lower apparent (Kjeldahl N) than true (15N) feed N disappearance from the bag suggested a notable proportion of non-feed N in the residues, especially with fibrous feeds with low N. With ryegrass, 15N net loss within the intestine was lower than 15N disappearance from the bags. It was concluded that disappearance of Kjeldahl N is an underestimate of feed N disappearance from bags, but may possibly be an overestimate of the true intestinal digestion of feed N.