Dr. Casey L. Bradley
Animal Nutrition Expert

I always say I am a poop watcher because a diet can meet every specification on paper while the animals themselves tell a completely different story. Lowering crude protein and relying on synthetic amino acids offers clear benefits, but true performance depends on what happens between intake and manure. By looking closely at protein quality, peptide absorption, and nonessential amino acid balance, we can move beyond simple least-cost calculations to build diets that truly support animal biology.

I am a poop watcher.

When I walk into a barn, I pay attention to manure consistency, air quality, the animals’ eyes and overall appearance, and the proportion of animals falling behind. Sometimes the environment simply feels off. The problem may be subtle long before it becomes obvious in the final performance numbers.

Those observations do not diagnose a protein problem. They tell me to investigate. I combine the real situation with theory, then formulation. A diet can meet every specification on paper while the animal tells a different story.

That is how I approach the debate over high versus low crude protein in swine and poultry. The question is how much usable nutrition we deliver, and whether the diet supports the biology of the animals eating it.

THE BENEFITS AND LIMITS OF LOWERING PROTEIN

Reducing crude protein has real value. In weaned pigs, appropriately formulated lower-protein diets can reduce the supply of undigested protein available for intestinal fermentation and help reduce post-weaning diarrhea. Experimental work has demonstrated reductions in diarrhea and fecal ammonia nitrogen, including under an enterotoxigenic Escherichia coli challenge [1].

Lowering excess dietary nitrogen can also reduce nitrogen excretion and ammonia emissions from pig manure [2]. This matters for sustainability, but the implications reach into the barn and beyond it. Air quality affects animals and the people caring for them. Nitrogen management continues through manure storage and land application. Odor deserves attention too, although ammonia is only one component of the odor coming from a livestock facility.

Higher protein can provide a broader amino acid supply and some protection against deficiencies. However, supplying more nitrogen than the animal can use increases the burden of disposing of that excess. Equally, lowering protein too far can undermine performance despite an apparently balanced formulation.

Neither a high nor a low crude protein percentage establishes quality. Crude protein is an estimate based on nitrogen content. It cannot tell us whether the amino acids are digestible, appropriately balanced, or available when the animal needs them.

NONESSENTIAL DOES NOT MEAN UNIMPORTANT

Feed-grade amino acids have made precision formulation possible. They allow us to meet essential amino acid targets while reducing reliance on intact protein sources. I consider them important tools.

My concern is what disappears when we remove those protein sources. We may carefully replace the limiting essential amino acids while giving insufficient attention to nonessential amino acids and the nitrogen and metabolic precursors required to synthesize them.

The animal’s ability to make an amino acid does not mean its capacity is unlimited. Synthesis also draws on substrates and energy. Under rapid growth or physiological stress, that distinction matters. If essential amino acids must contribute nitrogen to other compounds, their supply must support more than direct incorporation into body protein.

Broiler research illustrates the limitation. Hofmann and colleagues found that growth could decline in very low-protein diets even when essential amino acids and glycine equivalents were adequate; supplementation with certain other nonessential amino acids partially improved performance [3]. A spreadsheet showing acceptable essential amino acid ratios therefore deserves further biological scrutiny.

QUALITY INCLUDES EVERYTHING ACCOMPANYING THE PROTEIN

When assessing an ingredient, I ask what comes with its protein. Antinutritional factors, fiber, processing conditions, and variability can influence what the animal receives. An ingredient suitable for an older animal may be a poor choice for a newly weaned pig with a developing digestive system.

Digestibility must be considered alongside energy supply. Digestible lysine relative to metabolizable or net energy, as appropriate for the species and formulation system, remains important. So does the balance of the other amino acids. These relationships should reflect actual intake and the animal’s stage of production.

My early work with cheese products helped shape this perspective. We often discuss dairy ingredients in nursery diets primarily through lactose. I believe that can undervalue their milk proteins and the role of digestible protein during the transition from sow’s milk. Research on cheese coproducts supports evaluating their amino acid and energy contributions, while recognizing that products differ [4].

In my view, pressure to cheapen nursery diets has sometimes removed specialty ingredients faster than we have understood how to replace their contribution. The developing gut still needs support. Historically, high zinc and copper inclusion in US swine production has also shaped feeding programs. I worry that reliance on those tools can delay a closer examination of protein quality and gut function.

PEPTIDES AND THE ROUTE TO ABSORPTION

Digestibility values are essential, but absorption involves more than a coefficient. Free amino acids use several transport systems, some shared among amino acids. Competition and differences in absorption timing deserve attention as we increase reliance on free amino acids. Their inclusion alone, however, does not demonstrate that transport has become limiting.

Di- and tripeptides have another route through the intestinal peptide transporter PepT1. Work characterizing the pig transporter confirmed its ability to transport these small peptides [5]. Once absorbed, many are hydrolyzed within intestinal cells, contributing to the amino acid supply.

This gives us a reason to consider the balance of intact proteins, peptides, and supplemental amino acids. It does not mean peptides create extra amino acids or automatically permit lower requirements. Peptide size, composition, inclusion level, and the surrounding diet matter. Peptides serving as nutrient carriers should also be distinguished from specific peptides investigated for biological activity.

My work with Peptiva, a peptide-based product from Vitech, reinforced the importance of the complete feeding program. I remain willing to discuss products I helped research and believe in, while recognizing that the evidence is specific to the formulation tested.

In a University of Arkansas study I coauthored, published as a Journal of Animal Science meeting abstract, 288 weaned pigs received diets evaluating peptides with pharmaceutical zinc or organic acids. During the first two phases, the combination of 0.25% peptide, 0.1% sodium butyrate, and 0.5% benzoic acid with standard zinc produced similar gain and body weight to the 0.25% peptide treatment with pharmaceutical zinc. The combination also had the highest reported gain-to-feed ratio and nitrogen digestibility [6]. These results reinforce my approach of optimizing protein sources, supplemental amino acids, and additives together. There is no single silver bullet.

Complementary work by Poudel and colleagues evaluated Peptiva with mannan oligosaccharides, with or without protease. They reported improvements in some growth and feed-efficiency measures during treatment, but responses varied across feeding periods, including poorer feed efficiency in previously supplemented groups during the subsequent common-diet period [7]. Together, these studies encourage us to evaluate combinations within a defined feeding program and follow outcomes beyond the supplementation period.

EXCESS LEUCINE DESERVES ATTENTION

Leucine shows why meeting minimum requirements is only part of formulation. Corn and some corn coproducts can supply substantial leucine relative to valine and isoleucine. These branched-chain amino acids share catabolic pathways, and excess leucine can stimulate breakdown of the others.

Research from Hans Stein’s group demonstrated impaired feed intake and feed efficiency with increasing dietary leucine, along with changes in amino acid metabolism [8]. Broiler research likewise shows that leucine responses depend on its relationship with valine and isoleucine, with consequences for growth and feed conversion [9].

The practical concern is the complete ingredient combination. A feedstuff can meet a purchasing specification and still push the diet toward an amino acid imbalance. Simply adding more of every branched-chain amino acid is not a universal solution.

IMPROVING WHAT HAPPENS BETWEEN INTAKE AND MANURE

Enzymes and acidification belong in this conversation. Protease acts directly on protein. Phytase and xylanase target other dietary components but can also improve amino acid utilization under suitable conditions. Broiler experiments have demonstrated benefits from enzyme combinations, although responses depend on the diet and enzyme program [10]. Matrix credits should reflect that evidence and avoid counting overlapping benefits twice.

Acidification can support conditions for endogenous digestion, including gastric pepsin activity. Organic acids and butyrate also warrant consideration for effects beyond digestion. University of Arkansas and University of Illinois research that I coauthored found improved nursery performance with selected combinations of benzoic acid and sodium butyrate, alongside changes in fecal microbial composition [11].

What excites me is the space between intake and the poop: intestinal function, microbial interactions, nutrient transport, and how absorbed nutrients are allocated within the animal. Innovation should help us understand and improve those processes.

We must also formulate for variation. Ingredient averages and population averages can conceal animals whose intake or digestive capacity falls short. Look at the percentage of light animals alongside mean gain and feed conversion. Investigate manure and air quality while checking health, water, ventilation, and management.

My goal is best cost, not least cost. That means judging savings against growth, uniformity, health, and nutrient losses. Formulation software helps us calculate a diet. Watching the animals helps us determine whether it works.