By: Casey L. Bradley, Ph.D. - March 18th, 2026; President and Founder of Animistic
Protein nutrition in pigs is often reduced to a few familiar metrics: crude protein, digestible lysine, soybean meal inclusion, and amino acid ratios. While those measurements remain essential, they do not fully explain feed efficiency in pigs or how efficiently pigs use dietary nitrogen. A more complete technical view must include nitrogen balance, nitrogen retention, and protein digestibility.
That was one of the most important messages emphasized in a recent presentation by Dr. Hans Stein of the University of Illinois. As Stein noted, “nitrogen retention has steadily increased in the pigs we have over the last 60 years.” He also highlighted that “nitrogen excretion is much lower from the pigs we have today than what we had earlier.” Together, those observations point to a major shift in how modern pigs utilize protein and how nutritionists should interpret older nutrient data.
Nitrogen balance is a way of measuring how effectively pigs use dietary protein. In simple terms, it compares:
From that, nutritionists can estimate several key values, including apparent total tract digestibility of nitrogen, absorbed nitrogen, retained nitrogen, and biological value.
This matters because protein digestibility is only the first step. A pig may digest and absorb amino acids efficiently, but the true question is whether those amino acids are retained for lean tissue growth or lost through excretion.
Modern swine systems are increasingly evaluated not only on growth performance, but also on nutrient efficiency, feed efficiency in pigs, and environmental stewardship. Nitrogen retention sits at the center of that conversation.
According to Stein, using outdated values to estimate nitrogen losses can create a distorted picture of modern pork production. In his presentation, he stated: “If you use old data for that, you’re going to overestimate the carbon footprint from one pig.”
That is a critical point. If modern pigs retain more nitrogen and excrete less—especially less urinary nitrogen—then older equations may overstate manure nitrogen losses, ammonia emissions, and overall environmental impact.
Urinary nitrogen is especially important because it is the fraction most closely associated with ammonia volatilization. A reduction in urinary nitrogen excretion is not just a nutritional improvement; it is also relevant to how the industry evaluates sustainability and nutrient management.
Digestibility remains foundational to pig nutrition. If protein is not digested and amino acids are not absorbed, they cannot be used for growth. But digestibility alone does not define biological efficiency or feed efficiency in pigs.
The modern pig is influenced by a full production system that includes:
This systems view is important because improvements in nitrogen retention should not be attributed to only one factor. Genetics likely contributed, but so did better formulation, broader use of crystalline amino acids, improved feed enzymes, better processing, and more consistent ingredient quality.
As a result, modern pigs cannot always be accurately described using assumptions built from older populations, older feeds, or older production systems.
One of the most technically interesting points from Stein’s presentation was the response to low-protein diets.
When crude protein is reduced and amino acid balance is improved, pigs often retain a greater percentage of dietary nitrogen. In other words, nitrogen use becomes more efficient. But Stein also highlighted a complication: lower-protein diets may reduce nitrogen retention when measured as grams per day, even when the diet appears properly balanced in essential amino acids.
That distinction matters.
A diet can look better from an efficiency standpoint while still supporting less absolute protein deposition per day. For commercial producers and nutritionists, that means low-protein formulation must be evaluated carefully. The goal is not simply to reduce nitrogen excretion. The goal is to reduce nitrogen losses without sacrificing lean gain, growth performance, or economic return.
Stein directly acknowledged that this remains an open technical question, saying: “Why is it that [crystalline amino acids] reduce nitrogen retention? I don’t really understand it.” That honesty is important. It reminds the industry that even well-established nutritional strategies still require refinement, especially when biological efficiency and growth outcomes do not align perfectly.
Nitrogen retention also influences how the industry should think about feed energy systems and ingredient valuation.
If pigs are retaining more nitrogen than they did when older equations were developed, then the energetic value of the protein fraction may be underestimated in some current systems. This is especially relevant for ingredients such as soybean meal.
Stein emphasized this broader implication in his presentation, arguing that improved nitrogen retention in modern pigs means nutritionists may need updated net energy prediction equations. Put simply, if protein is being used more efficiently today, then some traditional assumptions about the energy penalty of protein may no longer be fully accurate.
That does not make older systems useless. It means they may need recalibration to reflect the biology of the modern pig.
For swine nutritionists, nitrogen balance is more than a research concept. It directly affects:
The biggest takeaway is that protein should not be viewed as a single number in a matrix. It should be understood as part of a biological system. Crude protein, digestible amino acids, retention efficiency, and nitrogen excretion are all connected.
Modern pigs are different. Modern feed systems are different. And the data used to evaluate them should reflect that reality.
A technical review of nitrogen balance and protein digestibility in pigs leads to a clear conclusion: modern swine nutrition must move beyond static protein values and place greater emphasis on how nitrogen is actually used by the animal.
As Stein stated, “We really need to use data from recent experiments” when estimating nitrogen excretion and environmental outputs in pigs. That principle applies more broadly to formulation itself. The more accurately the industry can measure retained versus excreted nitrogen, the better it can design feeding programs that improve pig performance, enhance feed efficiency in pigs, and reduce unnecessary nutrient losses, and support more precise and responsible production.
Technical summary based on: Stein et al., 2026. Abstract 99; Proceedings of the Midwest ASAS Annual Meetings.
