Bypass protein in ruminants: what Egan's 1981 review tells farmers

Why the question mattered#

The paper sets out to make sense of a fast-growing but scattered literature on what it calls undegraded dietary protein, or bypass protein for short. Egan notes that several protein requirement systems for ruminants had emerged since 1975, built around equations for estimating how much microbial protein and how much undegraded dietary protein actually reaches the small intestine for digestion. The review frames this as more than an academic exercise: it is really an account of how nutritionists tried to work out the amino acid requirements of ruminants and how well pastures, conserved feeds and supplements could meet them.

At the heart of the issue is a simple but consequential fact. Not all the nitrogen an animal eats is treated the same way inside the rumen. Some is broken down by rumen micro-organisms and, if energy and other nutrients are available at the same time, turned into microbial protein. Some resists that breakdown and passes through to the small intestine unchanged. Microbial protein and the surviving dietary protein are then digested together, broadly treated as sharing a common digestibility in most calculation systems, even though the review notes real variation in the estimates reported in the literature.

Working out the balance between these two protein sources, and whether the mix was enough to support growth, wool production or milk yield, became the organising problem for a generation of ruminant nutrition research. Egan's review was written to pull that body of work together and to ask plainly under what circumstances feeding practices that increase the flow of undegraded protein to the abomasum would actually improve animal production.

How the review was built#

This is not a laboratory trial but a synthesis of earlier published work, tracing the subject from the early twentieth century through to around 1980. Egan draws on studies of sheep and cattle conducted across Australia, New Zealand, the United Kingdom and the United States, citing measurements of digesta flow at the duodenum, rumen ammonia concentrations, microbial cell yield, and production responses to protein infused directly past the rumen.

Several strands of evidence are brought together. One concerns direct chemical and physiological measurements of protein hydrolysis and ammonia production in the rumen, work going back to early demonstrations that proteins are broken down there. A second strand follows attempts to measure the amount and composition of digesta flowing out of the rumen and into the intestines, using markers and tracers to separate microbial protein from surviving dietary protein. A third strand reports direct intervention experiments, where researchers infused casein or other protein sources into the duodenum, bypassing the rumen altogether, to see whether animal performance changed.

Because the review spans work using many different animals, diets and measurement techniques across several decades, it does not offer a single data set or consistent experimental design. Its value lies instead in showing where independent lines of evidence from rumen chemistry, digesta measurement and production trials converge on the same conclusion, and where they do not.

What the early rumen studies showed#

The review traces a shift away from the early view, following Schwarz's 1925 proposal, that dietary nitrogen mattered little because most absorbed protein was microbial. Mitchell pointed out weaknesses in Schwarz's evidence, and later work by Johnson and colleagues still concluded that the nature of dietary nitrogen was of relatively minor importance. But Lofgren and colleagues and Hamilton and colleagues, comparing the biological value of different protein sources, suggested the proportions of dietary versus microbial protein actually reaching the host could vary considerably between diets.

A turning point came with MacDonald's work, which established that the extent to which dietary proteins are fermented in the rumen is highly variable. Soluble proteins were shown to ferment rapidly, leading to inefficient use of nitrogen as ammonia was absorbed and lost as urea when it exceeded what the rumen microflora needed. El Shazley demonstrated that amino acids are deaminated in rumen fluid, and Annison confirmed that almost no free amino acid is absorbed directly from the rumen.

Chalmers and Synge then showed, using heat-treated casein, that reducing a protein's fermentability in the rumen could lower ammonia production and improve animal performance compared with untreated casein. Egan describes this as the point where the idea took hold that protecting protein from rumen fermentation could have real nutritional value, setting the stage for later work on bypass protein specifically.

Protein reaching the small intestine#

A separate body of work, reviewed by Egan, focused on directly measuring the protein that reaches the duodenum rather than inferring it from rumen chemistry alone. Early studies by Phillipson, Singleton, and Hogan and Phillipson measured the amount and composition of digesta flowing from the rumen into the intestines. Oyaert and Bouchaert linked average rumen ammonia concentration to the amount of dietary nitrogen lost between mouth and abomasum.

Two further observations sharpened attention on undegraded dietary protein specifically. Reis and Schinkel found that infusing casein directly into the duodenum increased wool growth. Egan and Moir, and Egan separately, observed that duodenal casein infusion increased intake of low-quality roughage, a response the review treats as distinct from any effect of nitrogen acting within the rumen itself.

Alongside this, Bauchop and Elsden's work on microbial cell yield in anaerobic fermentation gave rise to the concept that microbial protein synthesis in the rumen is limited by available energy. Hungate used this idea to argue for research directed at two complementary goals: increasing the proportion of feed converted into microbial bodies in the rumen, and finding ways to shunt protein directly through to the abomasum undigested by microbes.

Linking supply to requirements#

As more measurements of intestinal protein flow became available, Egan describes how researchers began calculating whether the supply of microbial plus undegraded dietary protein was enough to meet animals' estimated requirements for maintenance and production. These calculations, by Hogan, Hogan and Weston, Purser, Armstrong and Annison, Miller, and Egan and Walker among others, relied on averaged, static data and what the review candidly calls best-guess estimates of digestibility and efficiency of use.

Even with that caveat, a consistent picture emerged: microbial protein synthesis alone, driven mainly by the amount of organic matter fermented in the rumen and so tied to digestible or metabolisable energy intake, would often fall short of supporting rapid growth, lactation, or high rates of wool growth in sheep. Chalupa's estimate, cited in the review, suggested a substantial proportion of dietary protein escapes rumen degradation, meaning animals are typically fed on a mixture of microbial and undegraded dietary protein rather than either source alone.

The review is careful to note that total protein reaching the small intestine may in almost all cases be adequate for finishing animals, citing Preston and Young and colleagues, yet production responses to abomasal protein or amino acid supplements still appeared in fast-growing cattle and sheep, in sheep on low energy intakes kept for wool, and in lactating dairy cows. This gap between adequate total supply and observed production responses is presented as the practical justification for pursuing bypass protein as a feeding strategy.

Manipulating bypass protein in ruminants and what followed#

Alongside measurement work, researchers tried to find practical ways of protecting dietary protein from rumen fermentation. The review, drawing on Fergusson's 1975 survey of the topic, lists several approaches: feeding naturally insoluble proteins, reducing the solubility of soluble proteins by heat treatment, formaldehyde binding, or tannin binding, and exploiting the oesophageal groove reflex during sucking to divert milk protein directly to the abomasum in young animals.

These protection methods were then used as experimental tools to test whether more protein reaching the duodenum actually improved production. Egan reports responses recorded in wool growth, nitrogen retention, liveweight gain, voluntary feed intake and milk production across a range of studies. But the review stresses that such responses depend on the basal diet, the animal's genotype, age and physiological state, and whether feed intake itself increased alongside the extra protein supply.

Egan's own earlier work, cited here, proposed that diets providing about 6g of intestinally digested protein per MJDE or more showed no evidence of an intake response on roughage diets, while responses below that level were variable. Semi-purified and high-energy concentrate diets, where intake is less constrained by digesta outflow, offered more scope for a production response, following work by Preston and Willis and by Kempton and Leng.

The review closes by cautioning that a lack of response to bypass protein does not necessarily mean amino acids were not limiting; it could instead reflect over-protection making the protein unavailable even in the intestine, deficiencies in other nutrients, or amino acid imbalance, as Egan and Rogers found with bloodmeal. Predicting when a response will occur, Egan concludes, still depends on a better grasp of amino acid requirements and how amino acids interact with energy-yielding nutrients in the animal.

How much weight this review can carry#

Because this is a synthesis rather than a single trial, its strength lies in showing how independent measurement techniques, rumen chemistry, digesta sampling and direct infusion trials, converged on the same broad conclusion: some dietary protein escapes the rumen, and under the right conditions supplying more of it improves performance. We would treat the specific threshold figure of 6g per MJDE as a useful historical benchmark from the sources Egan cites, rather than a universal value, given how much the review itself emphasises variation between diets, animals and production states.

The review was presented through the RAAN Conference Proceedings, part of the broader Recent Advances in Animal Nutrition community that brought together Australian ruminant nutrition research in this period. Readers wanting the original experimental detail behind any of the production responses described here would need to consult the primary papers Egan cites, since this review summarises rather than replicates them. Other entries in the Livestock Library index that share the word bypass in their titles include a 1984 paper on bypass protein or bypass starch and milk yield in grazing dairy cows, a 1983 abstract on rice as a source of bypass energy, and a 1983 paper describing an assay for bypass protein.

Sources and further reading#

• Trove library search: find a library that holds the paper
• Meat & Livestock Australia: red meat industry research and marketing body
• Australian Wool Innovation: wool industry research and marketing body
• Feedipedia animal feed database: an open-access database of animal feeds

Questions#

— The Livestock Library team