Calculating protein and energy requirements of laying hens by the gram

Why percentage protein falls short#

The paper opens by setting out a problem familiar to anyone who has read a feed label: rations for laying hens and broiler breeders are usually judged by their percentage protein content. Bornstein argues this is convenient but misleading, because the proportion of protein a bird needs depends heavily on how much feed it eats, and feed intake itself is driven mainly by the energy density of the ration and the surrounding temperature, with body weight a distant third factor and egg output mattering even less to intake.

Expressing requirements as grams of protein per hen per day is described as an improvement, but still incomplete. It ignores two things that any practical nutritionist would want to account for: how well a given hen is producing, in terms of rate of lay and egg size, and how closely the pattern of amino acids in the feed protein matches the pattern actually needed for making an egg. The closer those two patterns match, the less total protein has to be fed.

Earlier researchers, including Fisher, Scott and colleagues, and Combs, had tried to solve this with partition equations that added up separate requirements for body maintenance, body weight gain and egg production. The paper reports that this approach consistently underestimated real requirements, a gap usually blamed on the hen being inefficient at converting feed protein into egg protein. Bornstein's paper sets out to explain that inefficiency rather than simply accept it as unavoidable.

A different explanation for how hens budget protein#

The key idea, borrowed and extended from Moran and Chiah, is that egg albumen is secreted by the oviduct far too quickly for all of it to come directly from the gut or the lining of the oviduct itself. Egg yolk forms continuously, but the paper describes white and membrane formation as a brief, concentrated event lasting only a few hours, in which close to four grams of protein are laid down. Given typical feed consumption during that window and allowing for around 85% absorption of nitrogen compounds, the paper judges it unlikely that protein synthesis at anywhere near that pace could draw directly on newly digested amino acids.

The alternative explanation offered is that much of the amino acid supply for egg white and shell membrane comes from breakdown of the hen's own tissue protein, drawing on a shared pool of free amino acids in the blood plasma that is topped up by both digestion and tissue turnover. Work by Harms and colleagues on protein storage in laying hens is cited as supporting evidence for this idea.

This matters because tissue protein and albumen protein are not identical in composition. Table 1 in the paper shows that albumen carries higher levels of several amino acids than tissue protein does, including a noticeably richer supply of sulphur amino acids. Because of this mismatch, the paper calculates that two units of tissue protein are needed to produce one unit of albumen protein. In this account, it is the shuffling of amino acids between body stores and egg protein, not some general wastefulness in digestion, that explains why hens appear inefficient converters of feed protein to egg protein.

Building and testing Models A and B#

Working from the original 1973 paper by Hurwitz and Bornstein, two calculation models were developed, both treating the hen as being in a steady state for maintenance, body weight change and yolk formation, and both assuming that any tissue protein broken down for egg white synthesis matches the amino acid make-up of chicken tissue shown in Table 1. The models differ in how much of the egg white they assume comes from this tissue-breakdown route.

Model A assumes that the entire amino acid supply for albumen and membrane formation is drawn from tissue breakdown. Model B instead assumes that one fraction of egg white, the ovalbumins, is made on an ongoing basis and held in storage in the magnum, so only the remaining glycoprotein and membrane fractions need amino acids pulled from tissue. Because shell membrane protein carries a much higher concentration of sulphur amino acids than tissue protein does, this distinction has a real effect on the calculated requirement.

From these assumptions, the paper works out a set of simplified formulae covering the essential amino acids, expressed in milligrams per hen per day. These require estimates of egg mass, body weight, body weight change and daily feed intake, and feed into a least-cost computer ration formulation in which protein content is allowed to vary freely rather than being fixed. The resulting rations are reported to have given worthwhile protein savings under the feed ingredient conditions available in Israel at the time.

Results from seven layer trials and four broiler breeder trials#

The models were tested in seven experiments with egg-type layers, covering Leghorns and crossbred hens kept in cages and on litter, mostly at peak production and on one occasion later in the laying cycle. Across these trials, Model A tended to overestimate requirements, while diets built on Model B produced an egg mass that matched the intended target closely, which the paper treats as good evidence that Model B captures the most limiting amino acids reasonably well.

In the first experiment described in detail, hens on the Model B diet had a significantly lower rate of production than those on Model A or on the control diet, yet in terms of total egg mass the Model B birds hit their planned target almost exactly, while Model A birds exceeded it. Feed intake for Model B hens was a little above the prediction, and their body weight was slightly above the predicted figure too. The diets were deliberately set below the genetic potential of the birds so the test would be a meaningful check of the models rather than simply confirming that well-fed hens perform well.

The same approach was then applied in four broiler breeder experiments, again with success according to the paper. One such trial, summarised in Table 5, involved White Rock broiler breeders fed without restriction to avoid any confounding from limited intake. Over a 10-week period of peak production, with a rate of lay of 74% and an egg weight of 66 g, giving an egg mass of 49 g per hen per day, the hens on the Model B diet consumed only 21.9 g of protein daily, containing 870 mg of lysine and 890 mg of sulphur amino acids. The paper notes that the same equations worked for both the light Leghorn-type hen and the much heavier White Rock broiler breeder, which it takes as a sign that the underlying principles are sound.

From amino acids to energy requirements of laying hens in practice#

Once daily amino acid needs can be pinned down in milligrams per hen per day, the paper argues, feed restriction in layers and especially broiler breeders can be put on a more rational footing. Restricting feed intake is really a tool for restricting energy intake, since birds fed without limit, broiler breeders in particular, tend to eat more energy than they need. The problem with blunt feed restriction is that it can also cut back every other nutrient at the same time. The paper's position is that as feed restriction gets more severe, the concentration of every nutrient apart from energy needs to rise accordingly, so that birds are still fed to requirement for amino acids even while energy intake is being held down.

Minimum energy requirements, the paper notes, can be worked out experimentally or from partition equations, and it points to reviews by Leeson and colleagues, Gleaves and colleagues, and Ivy and Gleaves for that literature, alongside empirical work by Auckland and Fulton on layers and by Chaney and Fuller on broiler breeders as examples of the experimental route.

Two of the paper's own experiments with broiler breeders ran across periods of peak performance in litter pens fitted with windows, during Israeli winter conditions, with night temperatures averaging around 10°C and daytime maximums near 20°C. Each trial compared three or four graded levels of feed restriction against a control fed without limit, using four pens of 42 White Rock layers per treatment, with all birds receiving the same daily amino acid intake regardless of energy level.

What the energy trials found#

Based on these two experiments, the paper suggests that White Rock hens around 3.5 kg in weight, producing 65 g eggs at rates described as 75% and 73%, could have their energy intake cut to figures the paper gives in kilocalories of metabolisable energy per hen per day, with only a modest reduction in egg size. The paper treats this outcome as a benefit rather than a drawback for broiler breeders well into their laying period, when eggs can otherwise grow too large to use efficiently as hatching eggs. These restricted intakes worked out, in the paper's own terms, to roughly 86% and 79% of what control hens fed without limit were eating.

For comparison, Chaney and Fuller had reported that a 20% cut in energy intake reduced both rate of lay and egg size significantly during cold winter months, but had no effect during summer. The paper also applies an equation from Combs, which works out daily energy needs from temperature, body weight, daily weight change and egg mass, to its own data assuming an ambient temperature of 60°F. That calculation gives a result the paper describes as agreeing well with its own empirical figure.

Taken together, the paper reads this agreement as support for using careful energy restriction in broiler breeders as a practical way of curbing excessive egg size later in the laying period, provided amino acid intake is kept at the calculated requirement throughout. It is one of several papers collected in the RAAN Conference Proceedings within this index, part of a broader record of work shared through the Recent Advances in Animal Nutrition community.

How much weight this evidence carries#

The trials described are modest in scale, run at a single research station in Israel with specific hen strains, housing types and feed ingredients, and the energy work in particular covers only two seasons of broiler breeder data. We think the agreement between the empirical restriction levels and the Combs equation is a genuinely useful cross-check, but the overall case for the models rests more on internal consistency, egg mass matching the intended target, than on independent replication elsewhere, so the figures are best treated as a demonstrated approach rather than fixed numbers to apply unchanged to any flock.

Even with that caution, the paper's central contribution is conceptual as much as numerical: it reframes the question of how much protein a laying hen needs around the biology of egg white formation and tissue amino acid turnover, rather than around a fixed percentage figure in the ration. That reframing, and the practical formulae it produced for calculating requirements per hen per day, is what the paper offers readers working through the protein and energy needs of their own flocks.

The same author also has a separate paper catalogued in this index, titled the development of the pre-laying pullet and young hen (1977), found at the pre-laying pullet and young hen record. Other catalogue entries in this index sharing words with this paper's title are listed at the Schizochytrium DHA record and the nutrition and cannibalism record.

Sources and further reading#

• Trove library search: find a library that holds the paper
• Feedipedia animal feed database: an open-access database of animal feeds
• Poultry Hub Australia: poultry science information
• AgriFutures Australia: which funds chicken meat and other rural industry research

Questions#

— The Livestock Library team