Modern nutrition has made it abundantly clear that broilers, like all animals, don't actually require protein. What they need is the amino acids in proteins; hence, the advent of feed-grade pure amino acids such as L-lysine HCl and DL-methionine.
As not all amino acids are required at similar levels, there is an amino acid profile or balance that is established to formulate diets. Complicating matters is the fact that conventional feedstuffs contain a totally different amino acid profile. Mixing different feed ingredients is done to satisfy the requirement for the most limiting (absent) amino acid — the one that conventional ingredients contain the least compared to animal requirements. As it happens, when this amino acid’s requirement (methionine for broilers) is met, then most other amino acids are supplied in excess.
Roughly, to meet requirements for methionine, lysine, threonine and tryptophan (the four most limiting amino acids in maize-soybean meal-based diets), we must provide about 4 percentage points of excess crude protein that would otherwise not be needed. This excess protein, which in the form of amino acids is absorbed, must be excreted at a cost, whereas any part escaping absorption reaches the hindgut where it might cause further trouble.
Excess protein causes three major problems
Amino acids absorbed in excess of actual requirements cannot be stored. Instead they must be deaminated, yielding ammonia as a by-product, which is toxic in the blood stream. Birds capture ammonia in uric acid, which in turn must be excreted. This whole process results in a negative energy balance for the animal — and this is why some human "diet" programs rely on excess lean protein consumption. For broilers, however, this means, less energy for growth and an unfavorable feed efficiency index. In other words, it costs money to get rid of excess protein, even though diets are formulated on a least-cost basis.
We all know that protein, like all nutrients, is never 100 percent digestible. Soybean meal, for example, has a protein digestibility of about 80 percent. This means that roughly 20 percent of ingested protein passes to the hindgut where it can be used by pathogens such as colibacteria, Campylobacter, and Salmonella. End result: pathogenic diarrheas that require expensive veterinary intervention, reduced animal growth and increased mortality.
Every 1 percent of excess dietary protein increases water intake by about 3 percent in broilers.
A minor but significant side effect of excess dietary protein is that of wet litter. This is because uric acid to be excreted requires excess water. It has been estimated that every 1 percent of excess dietary protein increases water intake by about 3 percent in broilers. And, excess water intake will increase litter moisture with its associated problems, including the not so inconsiderable effects on animal welfare in terms of reduced comfort. In addition, wet litter is a contributing factor to leg problems, such as pododermatitis, and breast lesions that diminish carcass value. The disposal of nitrogen-rich litter is a matter that requires little discussion here, but it is a real concern in certain countries.
From science into practice
Having accepted that a low-protein diet with an appropriately balanced amino acid profile is beneficial for broilers, it now remains to examine how this can be implemented into practice. Basically, and discounting the advent of proteases that have been discussed extensively lately, there are three major ways that nutritionists use traditionally to reduce dietary crude protein:
- Trimming excess protein margins. It is no secret that most commercial formulas, except those used by some integrators, contain excess amino acids (and protein) to maximize growth per broiler. These generous safety margins ignore the fact that profitability can and should be maximized not on a per-bird basis, but on a surface-basis. In addition, safety margins are added when generic diets are purchased from local feed mills. This ignores the specific requirements of the particular genetics used in any given farm. In brief, modern nutritionists, along with geneticists, can use a growth modeling software to design a nutritional program that matches actual broiler requirements under real field conditions, minimizing the need for safety margins. It also pays to keep an eye on dietary specifications as nutrient analyses from ingredient quality control schemes come in to ensure broilers are neither over-fed nor under-fed; both sure ways to reduce profitability.
- Increasing protein digestibility. There are several ways to accomplish this, by using appropriate additives — only one of which is a protease. But, certain ingredients are inherently more digestible than others. They are invariably more expensive, and, as such, a balance is required to ensure the desired outcome is achieved. In some cases, ensuring conventional sources of proteins, such as those of vegetable origin, are of the highest possible quality is a good way to maximize protein digestibility. To this end, over-heated and under-heated protein sources should be avoided if possible. Luckily there are easy laboratory tests that can examine such variables, especially in the case of soybean meal.
- Using more feed-grade amino acids. This is no longer in the news, but it is the most widely-recognized method — and easily accomplished — to reduce excess feed protein, without risking lower animal performance. As feed-grade amino acids are priced to compete with conventional protein sources, any benefits should be weighed carefully. Plus, there is always a limit on how much can be added in a diet, and the general rule is that crude protein can be safely reduced by 2 percentage points in most formulas by the use of feed-grade amino acids. This can be taken down to 4 percent or more by a very experienced nutritionist. These rules of thumb are based on the fact that the concept of "limiting amino acids" is not recognized widely outside a small circle of nutritionists with a high-end educational background.