Nylon bag technique shows how feed degradability shifts with diet

Why a simple bag test mattered for rumen nutrition#

By 1980, nutritionists recognised that ruminant diets often needed supplements containing protein that passed through the rumen without being broken down there, sometimes called bypass protein, so that more amino acids reached the small intestine intact. Kempton notes this need directly, citing earlier work by Kempton and colleagues on the value of rumen non-degraded protein for production.

That created a practical problem: how do you grade commercial supplements by how much of their protein and dry matter actually gets degraded in the rumen, as opposed to passing through undigested? The nylon bag technique, already used for decades by the time of this paper, offered a relatively direct answer. Samples enclosed in porous bags are exposed to the rumen's microbial population and fluid, then pulled out at intervals so researchers can measure how much material has disappeared.

The paper positions itself as a review and refinement of this method rather than a single new experiment. It draws on a mix of published studies and unpublished observations from the same laboratory to explain what influences the results, how to calculate degradability properly, and how the technique has been applied to real feeds and grazed pasture.

How the nylon bag technique works in practice#

The core procedure is straightforward. Nylon bags containing a known 5g weight of test material are tied onto nylon string and lowered into the rumen of a cannulated sheep or cow. Bags are pulled out at set times over 24 to 72 hours depending on the feed being tested, rinsed under tap water until the rinse runs clear, then oven dried at 70 degrees for 24 hours. The loss of dry matter or protein weight compared with the original sample gives the disappearance value at each time point.

Practical details matter. Rumen cannulae in sheep need to be roughly 40mm across to make inserting and removing bags manageable, and up to six bags can usually be fitted into a sheep's rumen at one time, with more possible in cattle. Strings are given 20 to 25cm of free length inside the rumen; the paper notes they tend to tangle, and covering them with thin plastic tubing helps.

Several technical choices turn out not to matter much. Pore size of the bag material had no measurable effect on dry matter disappearance in one cited study, nor did grinding versus chopping lucerne in the author's own unpublished comparison, where both forms gave a 13 hour half time for dry matter loss. Washing method and duration were likewise found to have little bearing on the variability of results in other cited work. What does matter, repeatedly, is the diet fed to the host animal and the species or form of the test feed, discussed in detail below.

The basal diet changes the answer substantially#

The single biggest source of variation identified in the paper is not the bag or the washing regime but what the test animal itself is eating. In one unpublished comparison, the half time for loss of dry matter from alkali-treated rice hulls was considerably shorter in sheep fed chopped lucerne chaff than in sheep fed liquid molasses plus 100g of wheaten chaff.

A similar pattern turned up in cattle. Orskov and De B Hovell found dry matter disappearance of chopped hay was 18% lower after 40 hours incubation in the rumens of Zebu cattle fed chopped sugar cane compared with those fed pangola hay. In sheep, protein from various plant meals disappeared faster when the host animals were on a grass-based diet than on a whole barley diet, a difference Kempton attributes mainly to faster rumen turnover on the grass diet.

Not every feed responded the same way to diet, though. Fishmeal protein disappearance was not different between sheep fed whole barley or dried grass diets in work by Ganev and colleagues, suggesting animal-derived protein sources can behave differently from plant meals under this kind of dietary contrast. The practical message the paper draws from all this is that supplements should be tested in animals eating the same basal diet the supplement is actually meant to go with, and that between-animal variation can be managed by using at least three replicate animals.

Correcting for particles that simply wash out of the rumen#

A plain reading of disappearance curves tends to overstate true degradation, because the standard calculation assumes everything that vanishes from the bag has been fermented by microbes. In reality, some undigested particles of a suitable size can leave the rumen in digesta flow without ever being broken down chemically. Orskov and McDonald addressed this by treating soya-bean meal with sodium dichromate to make it chemically inert, then tracking chromium disappearance from rumen contents after a single dose to estimate the fractional rate at which undigested material actually leaves the rumen.

Working this outflow rate into the degradability calculation produced a markedly different picture for soya-bean meal protein. The simple equation predicted potential degradability approaching 100% at infinite time, while the corrected equation that accounts for rumen outflow gave a lower figure described in the paper's own table of results. Kempton concludes that without factoring in this clearance of undegraded residues, potential degradability after 24 hours of incubation can be overestimated by at least 20%.

The paper suggests that for most practical purposes, measuring liquid turnover in the rumen using chromium EDTA as a marker that is not absorbed can stand in for a full particle turnover measurement, making the corrected model workable without requiring the more elaborate chromium-treatment procedure for every feed tested.

Breaking disappearance curves into separate pools#

Plotting how much sample stays behind in the bag against time on a semi-logarithmic scale lets researchers separate feed components that degrade at different speeds. For the soya-bean meal example in the paper, 80% of the protein sat in a pool degraded at a measurable rate with a half time of 8 hours, while the remaining 20% was immediately soluble in rumen fluid, together suggesting the material was essentially completely degraded in the rumen.

Some feeds do not fit a single straight line on this plot, indicating more than one degradable fraction. Rolled barley dry matter, analysed from data by Mehrez and Orskov, showed two distinguishable pools with different rates of breakdown, separated using a technique the paper calls peeling, where the slower, final exponential component is subtracted from the earlier data to reveal the faster pool underneath.

When the same multi-pool analysis was applied to soya-bean meal protein disappearance corrected for rumen outflow, it resolved into three fractions: 40% degraded relatively slowly with a half time of 55 hours, 35% degraded more rapidly with a half time of 5.5 hours, and 25% was immediately soluble. This is a more detailed picture than the single-pool estimate gives, and it illustrates why the paper treats the uncorrected calculation as a simplification rather than a reliable final answer.

Using bag results to predict whole-feed digestibility#

Beyond characterising supplements, the paper reports an attempt to use the nylon bag method as a stand-in for whole-animal digestibility testing. Several low quality roughages were treated with various alkalis, then either incubated in nylon bags in sheep fed lucerne chaff or tested in vitro by the method of Tilley and Terry. Comparing in vitro digestibility against how quickly dry matter disappeared from bags showed materials with potential digestibility above 40% had a half time under 50 hours.

That relationship was then applied in the field. Samples of oesophageal extrusa, material collected directly from the mouths of grazing lambs, were taken from animals on either lucerne or phalaris and fescue pastures, then placed back into the rumens of the same animals that had grazed them. Because the first bags in this trial were not pulled until after 19 hours, only the slower, terminal pool of dry matter could be analysed.

Those preliminary results suggested a noticeably larger pool of slowly degraded dry matter in the grass diet, at 40%, compared with 28% on the legume diet. The paper itself describes these as preliminary observations rather than settled figures, a caution that seems sensible given only one late sampling point was available to characterise the slower pool in each pasture type.

What the method can and cannot tell a nutritionist#

Kempton is careful to flag where the technique's weaknesses lie alongside its strengths. Diet of the cannulated animal and the identity of the test material both produce real, sometimes large, shifts in measured degradability, while bag porosity, sample particle size for fibrous feeds, and washing technique are comparatively minor sources of noise. Grains and protein meals are an exception to the particle size point, since grinding genuinely speeds degradation for those feed types.

One finding worth flagging for anyone using degraded protein residues to estimate microbial contamination: the total protein pool size calculated for rolled barley exceeded 100%, which the paper attributes to microbial protein contaminating the undigested residue left in the bag, despite only trace amounts of a microbial marker compound being detected in that particular sample. This illustrates a genuine limitation of the nylon bag technique when applied to protein rather than dry matter.

We would treat the pasture extrusa comparisons in this paper as a useful pointer toward testing supplements under conditions matched to their intended basal diet, rather than as a precise ranking of feed values, given how much the diet of the test animal alone was shown to move the results. The appendix included in the paper sets out a recommended protocol with 6 bags per animal and at least 3 animals, which reads as a practical attempt to standardise a method the author's own data show is otherwise quite sensitive to local conditions. Related work from the same author on grazed pastures and supplementary feeding, including a paper on grass and legume pasture degradation in grazing lambs and one on weaner cattle grazing dormant tropical pastures, sits alongside this paper in the RAAN Conference Proceedings collection.

Sources and further reading#

• Trove library search: find a library that holds the paper
• Meat & Livestock Australia: red meat industry research and marketing body
• Integrity Systems (NLIS and LPA): which runs NLIS, LPA and the National Vendor Declaration
• Feedipedia animal feed database: an open-access database of animal feeds

Questions#

— The Livestock Library team