Warm-season grasses reach their peak of production about a month later than cool-season grasses. Although warm-season grasses produce less yield, their virtue is to provide superior midsummer grazing when cool-season grasses are semi-dormant. Both types can be stockpiled during late summer and fall to provide maintenance energy for livestock during the winter months.

The forage of warm-season (C4) grasses normally contains less protein than is found in cool-season (C3) grasses. This might be expected because warm-season grasses are seldom fertilized with supplemental nitrogen. However, to achieve yield goals with cool-season grasses, they are often fertilized with some form of nitrogen. This increases the protein content of the grass as nitrogen accounts for 16 per cent of the mass of a protein molecule. Nitrogen that is not incorporated into proteins is temporarily stored in various forms: free amino acids, nitrates, amides, and amines, broadly classed as non-protein nitrogen (NPN). In chemical analyses of feed stuffs, these forms of nitrogen are commonly considered as being as nutritious as true proteins. This may not hold true if the NPN level is too high.

It is reported that the protein in C4 grasses is used more efficiently by ruminant livestock. In other words, a higher percentage of the protein in C4 grasses is retained in the carcass and less is voided via the kidneys as urea. It is well established that cattle reach a higher degree of finish on C4 range grasses than on more lush C3 grasses when grass represents the sole ration. How does this relate to NPN?

Research suggests that reduced efficiency in protein utilization in C3 grasses might be due to excessive levels of NPN in plant cells. These substances, being quite labile, are rapidly deaminated by enyzmes (chiefly urease) present n the rumen microflora. The ammonium radical (NH4) thus released can cause stress similar to the type which often occurs when feeding excessive amounts of urea to livestock. The nature of this stress is well documented.

There is evidence that livestock may suffer illnesses from lush pastures because the rapid release of ammonium N from the labile nitrogenonous substances in grasses.

High levels of soil nitrogen lead to rapid uptake of this element by plant roots. Some of it may be stored as NPN. If rumen microflora fails to incorporate the liberated ammonium N into microbial protein (this being the normal function of rumen bacteria), a significant portion may be absorbed through the rumen wall into the portal vein leading to the liver. Additionally, in a worse case scenario, some of the nitrogen-rich material may pass into the secum where it is degraded by bacteria rather than by the enzyme urease. This is known as intestinal putrefaction, (proteins rot or putrify, whereas carbohydrates ferment). The ammonium N released via putrification is absorbed directly into the portal blood system leading to the liver. The liver is challenged to convert the the nitrogen in the portal blood system to urea so that will enter the general blood stream which nourishes the brain, kidneys, muscles, and other organs.

If the liver malfunctions, or its capacity to filter the ammonium nitrogen is exceeded, this toxic ammonium radical eventually reaches the brain via the general circulation and causes various forms of livestock disorders, broadly classed as ammonia intoxication.

The above interpretation suggests that cool-season grasses should not be heavily fertilized with nitrogen. At best, nitrogen should be applied in split applications. As an extra precaution to maintain low levels of NPN in cool-season grasses, maintain ample levels of phosphorus and potassium in the soil. Potassium serves a catalytic function in protein synthesis thereby lowering the level of NPN.

What about pasture supplements? Energy-rich supplements can be offered as additional insurance against NPN stress. For example, when urea is added to livestock rations, it is essential to supply grain or molasses to stimulate growth of rumen microflora, thereby creating a demand for the ammonium nitrogen released in the rumen. Additionally, livestock relish a mineral supplement which contains clay. The cation exchange properties (buffering capacity) of clay minerals promotes the absorption (and possible fixation) of ammonium ions.

Reference: Bessman, S.P.. "Ammonia Metablolism in Animals: Symposium on Inorganic Nitrogen Metabolism" 1956 McElroy and Glass (eds) The Johns Hopkins Press