Animal agriculture is thought to be a major contributor of ammonia emissions in the USA
Estimates for ammonia emissions by dairy cattle range from below 20 to more than 70 lbs per year.
Both the crude protein content of the ration and milk production level will affect excretion rates.
Ammonia emissions from agriculture have received renewed attention in the USA as a result of the Environmental Protection Agency (EPA) air quality consent agreement. By 2008, EPA is charged with developing ammonia emission factors for various types of animals and farms. Animal agriculture has been estimated to account for 60-90 of the total ammonia emissions in the United States. Figure 1 contains the sources of ammonia in the US. Note that cattle are listed as contributing about 45% of total emissions.
Figure 1. Sources of Ammonia in the US, %
Source: Meisinger and Jokela, 2000 Cornell Nutrition Conference
Animals do not directly excrete ammonia. Excess dietary nitrogen (N) is excreted as urea. Key points relative to ammonia emissions for dairy cattle are:
- 50 – 70% of the total manure N excreted by dairy cattle is in the urine
- 50 -90% of the total urinary N is in the form of urea.
- The fecal portion of the manure contains an enzyme called urease.
- The urease enzyme rapidly converts urea to ammonia.
- pH has an influence on the ammonia loss. Ammonia loss increases at high pH (8 or higher)
- Higher temperatures also increase ammonia loss.
The proportion of N excreted in urine and feces varies with the level of N intake relative to requirements. Figure 2 contains the results of a trial conducted at Cornell using early lactation cows producing about 100 lbs. of milk per day. Daily N intakes were 711, 751 and 878 g/day for diets A, B and C. Note that basically all of the increased N excretion was via the urine in this trial. The potential ammonia emitted by these cows would be directly related to the quantity of urinary N excreted.
Figure 2. Partition of N Excretion in Dairy Cattle
Source: Stone, 1996
How much ammonia do dairy cows emit? There is lot of variation in the emissions estimates in the research literature. This is one of the primary reasons that EPA wants to do some additional monitoring on dairy farms. Current estimates range from less than 20 to more than 70 lbs/cow/year.
Currently, an operation that emits > 100 lbs. of ammonia for any day within a year is supposed to report this release. How many cows would it take to meet this threshold? Obviously, it would depend heavily on the actual emission factor. However, a number of individuals have made estimates. Two examples are:
- Dr. J. Jonker – National Research Council – 120 mature dairy cows plus 120 replacement heifers.
- Dr. Al Rotz – USDA-ARS – 100 to 250 cows plus associated replacement heifers.
A study from Sweden (Frank et.al., Livestock Production Science, 76:171-179, 2002) evaluated the ammonia emissions from dairy cows fed two rations. Dairy cows averaging 65-70 lbs. of milk were fed rations containing either 14 or 19% CP. Ammonia content of the outflow air from a measuring chamber was 3.6 ppm for cows on the 14% CP ration and 10.2 ppm for cows fed the 19% CP ration. This is a 65% reduction for cows fed the lower CP ration. In this trial, there was no difference in milk production due to ration CP level.
Figure 3 contains the daily urinary N excretion for cows fed varying ration CP levels. Milk production of the cows in this trial was 80-85 lbs/day. Note the continuing increase in urinary-N excretion as ration CP levels increased. The higher levels of urinary-N excreted would be predicted to increase ammonia emissions.
Figure 3. Daily Urinary N Excretion, grams
Source: Olmos Colmenero and Broderick, J. Dairy Sci. 86(Supp 1):273, 2003.
What does this mean for ration formulation programs? Basically, it should have minimal impact on the way rations are formulated on a daily basis. The basic message is to formulate dairy rations to minimize the overfeeding a protein to dairy herds.