Dr. Bill Weiss, Extension Dairy Specialist, The Ohio State University
Dairy cows consume substantial amounts of nitrogen (otherwise known as crude protein or CP) each day. If cows are fed diets that approximately meet their requirements for protein, about 33% of the N consumed will be secreted in milk and retained by the cow (for example, by a growing fetus) and 67% will be excreted in urine and feces. The proportion of dietary N used for productive purposes (milk secretion and body retention) is reduced when cows are overfed CP. At the Ohio Agricultural Research and Development Center (OARDC), we have accumulated a large set of manure excretion and manure nutrient excretion data collected from lactating cows (see article on factors affecting manure production). In that dataset, average N intake was 564 g/day (equal to 7.8 lb of CP). Of the N consumed, on average, 4% (22 g) was apparently retained by the cow, 28% (158 g) was secreted in milk, 32% (181 g) was excreted in urine, and 36% (203 g) was excreted in feces.
Cows excrete very little ammonia via feces, urine, and respiration; however, manure is a very significant source of ammonia on most farms. Of the N excreted via urine by a typical lactating dairy cow, 65 to 75% is in the form of urea. Urea can be broken down to ammonia and carbon dioxide by an enzyme called urease. Urease is produced by numerous bacteria that live in soil and feces. The rate at which ammonia is produced under field conditions is primarily a function of:
1. The amount of urea in the system (greater amounts = greater ammonia production),
2. Degree of mixing of urine (source of urea) and feces (source of urease) (greater mixing = greater ammonia production), and
3. Temperature (higher temperatures = greater ammonia production).
Under laboratory conditions where urine and feces are well-mixed and held at temperatures ranging from approximately 70 to 80oF, 40 to 50% of the N in manure can be converted to ammonia within 48 hours. Based on the average N excretion data from our experiments (384 g of manure N/day per cow), approximately 200 g of ammonia (not ammonia-N) could be produced from dairy cow manure within 48 hours of being excreted. The actual amount produced under field conditions will likely be less because of differences in mixing and temperature between laboratory and barn conditions.
Ammonia that is produced from manure is not necessarily volatilized into the atmosphere. The quantity that is volatilized is a function of a multitude of factors, including the amount of ammonia in the manure slurry, surface area of the slurry, temperature, air flow across the slurry, and pH of the slurry. The pH of the slurry determines how much of the ammonia that is present is in the potentially volatile form. Depending on the pH of the solution, ammonia exists as either NH3 (ammonia) or NH4+ (ammonium); NH3 is volatile. As the pH of a solution increases, the relative proportion of NH3 increases and ammonia volatilization increases. In a solution that has a pH of 9.4, approximately 50% of the ammonia is NH3 and 50% is NH4+. The pH of fresh dairy cow manure averages about 8.2 (substantial variation exists depending on diet). At a pH of 8.2, approximately 6% of the ammonia will be NH3 (the volatile form) and 94% will be NH4+, but at a pH of 8.6, 15% of the ammonia will be NH3. Although we can calculate with reasonable accuracy the amount of NH3 in manure that can be volatilized, we cannot calculate accurately the amount of NH3 that is actually volatilized because of the complexity of the system. However, the following general relationships are correct:
1. Increased surface area increases ammonia volatilization (a thin film of manure will result in greater ammonia emissions than a deep pile of manure),
2. Increased air movement above the slurry will increase ammonia volatilization,
3. Increased temperature increases ammonia volatilization, and
4. Reducing the pH of the slurry will reduce ammonia volatilization (although we can formulate diets that will result in significantly reduced manure pH, this is not recommended currently as a means of reducing ammonia emissions because of metabolic effects on the animal).
By far, the easiest and most effective means of reducing ammonia volatilization is to simply feed diets that provide adequate, but not excessive, amounts of protein. Numerous studies have shown that if protein fractions are balanced correctly, diets with 14 to 15% CP will support in excess of 70 lb/day of milk. In a recent experiment that we conducted, the excretion of manure N was 90 g/day per cow (a 20% reduction) lower when Holstein cows were fed a diet with 14% CP compared with a diet containing 17.5% CP. Milk production (averaged 80 lb/day) and milk protein yield were not affected by treatment.