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4.5 Mass Balance Calculations

4.5.1 Theory of the Mass Balance Calculations Approach

A proper application rate is the principal manure management practice affecting the potential contamination of water resources by manure nutrients, and in fact has very little to do with manure management technology per se. It is difficult to determine the effectiveness of manure management measures or whether they are necessary. To know the latter, one would have to actually follow the nutrient chain from each household to the final uptake of the nutrient by some source. Furthermore, to ensure that the actual uptake was occurring, one would have to do specific measurements of the disposal of the nutrients and the uptake. As noted above, this is beyond the ability of the project to address.

However, it is possible to estimate the potential of the externality in terms of the ability of a farm to utilize all nutrients on farm that it produces from the household survey conducted. If the manure produced exceeds the potential for on-farm use, then one needs to either (1) sell the manure, (2) transport the manure to an area where there is enough land for application, or (3) utilize a processing technology to transform the manure to a product amenable to long-distance transport (or eliminates the need for transportation entirely). This may also help us understand why some farms may be spending more money on investing on manure mitigation technology than others, and it may also help to understand differences across size of operations, particularly if large farms have limited land to dispose of manure.

To see whether a farmer has the ability to utilize all manure on his own farm, the farm balance of manure nutrients relative to the farm's potential to utilize the nutrients through crop production is calculated based on the household surveys for each country. Four different livestock population types (broilers, layers, swine, and dairy cows) were included in the analysis when data was available. From these numbers, the amount of nutrients in the manure is estimated in terms of organic nitrogen (N) and phosphate (P2O5). These two nutrients were chosen because they are the nutrients for which regulations are primarily written, assuming that there is any regulation at all. The amount of chemical fertilizer applied per land unit was also included, when available, to compute the mass balance of nutrients applied to the land. Crop assimilation capacity was estimated to determine whether a crop could assimilate all the nutrients produced on farm. Next, the amount of manure sold off-farm, if any, was subtracted.

4.5.2 Animal Unit Calculations

Nutrient values from livestock were calculated based on animal units, the most commonly used metric to calculate nutrient levels. Animal unit conversions are used because it is recognized that the amount of manure produced from a cow is not equivalent to the amount produced by one broiler or one swine. This method is used to equate excretion across species. Table 4.1 illustrate that one animal unit is equivalent to one cattle, five pigs, or 250 broilers or layers, a calculation based on averages used by the US Extension Service. Since the level of nutrients may also differ by species based on what is eaten, the amount of nutrients for each species also differs. For instance, while 250 chickens produce 298 pounds of nitrogen and 209 pounds of P2O5, it only takes nearly one cow to produce the equivalent.

Table 4.1 Animal unit conversions


Animal Unit

Amount of N (lbs)

Amount of P2O5 (lbs)

















Source: Kellog, 2000.

Different countries have different conversions and limits. For instance, according to the European Community Directive, the number of manure-producing animals per hectare of land (for up to 16 weeks) is limited to 2 dairy cows, 4 ground stock/beef cattle, 16 fattening pigs, 5 sows with piglets, 100 turkeys or ducks, 133 laying hens, or 285 ground hens. This is equivalent to a limit of 170 kg per ha per yr of total nitrogen (including that deposited while grazing) in zones deemed vulnerable to nitrate leaching (Williams, 1992). It is expected that the above conversion factors will be lower for many of the developing countries, since the amount of nitrogen and phosphate excreted in animal manure depends on diets, species, and age of animal (Faassen and van Dijk, 1987).

4.5.3 Total Nutrient Production

From animal unit estimations, the total nutrient deposition from livestock for each household is estimated where the total nutrient deposited by household h was the sum of the nutrient produced by animal units of livestock type l in household h. If data on commercial fertilizer use were available, these were added to the calculations to derive total nutrient use on farm, which would include both organic and inorganic nutrients using the following formula:



l = Livestock category

n = Nutrient type

h = Household

= Total nutrient n deposited by household

AUlh = Animal units of livestock type l in household h

= Form of nutrient n applied as commercial fertilizer by household h

= Amount of nutrient n produced per animal unit of livestock type l

4.5.4 Estimation of Crop Uptake

The capacity of these nutrients at the household level is estimated assuming that all the available land was planted with a crop that would uptake the nutrients. This is done to determine whether a household would have the potential to utilize all the nutrients produced given their current number of animals.[22]

The capacity for each household to use the nutrients produced by livestock operations is computed as the area or cropland available to the household multiplied by the nutrient uptake by the crops planted on the land. To determine this, we calculated the potential for either corn (for Brazil) or rice (for the Philippines, India, and Thailand) to take up these nutrients under the assumption that all the available cropland was planted with either corn or rice. In our calculations we assumed that the average yield of corn on this permanent agriculture land would be 125 bu/acre. Using such assumptions, it is estimated that 125 lbs. (140 MT) of N and 50 lbs. (56 MT) of P2O5 would be removed per acre. It should be noted that the actual nitrogen requirements for corn production vary between 75 and 150 lbs. per acre without irrigation and as much as 250 lb. per acre with irrigated lands. We assumed that the nitrogen uptake for rice production is 100 kg per hectare and the phosphorous uptake is 32 kg per hectare. The actual figure depends on the ability of the soil to utilize nutrients; tropical soils require far more nutrients than other types.

4.5.5 Mass Balance

In order to determine the nutrient balance on the farm, the difference of manure nutrient production and consumption is calculated. The mass balance (MB) for each nutrient of interest (nitrogen and phosphorous) is expressed by the following equation:



Ah = area of cropland on household h

= Absorptive capacity for nutrient n per unit of land

= Total nutrient up take by commodity (corn or rice)

The result is indicative of a household's potential assimilative capacity of nutrients based on the current number of animals on their property. A positive mass balance would imply that there is sufficient land to assimilate the nutrients produced, while a negative mass balance suggests that there is not enough land to absorb them. The results are also interpretable in physical terms. A negative mass balance of -10 metric tons for nitrogen, for example, implies that the farmer is loading nitrogen at the net rate of 10 metric tons per hectare, creating a surplus that will cause problems.

4.5.6 Manure Sold Off-Farm

Although manure is a potentially valuable fertilizer and soil conditioner, areas with concentrated livestock production may not have adequate cropland for nutrient utilization stemming from by-products on livestock. Therefore, exporting nutrients from concentrated areas to surrounding areas may be both environmentally and economically beneficial. Markets for manure seem to exist in all countries, primarily for poultry manure, and to some degree for dry swine manure and dairy manure in some countries. The manure that is sold in these countries tends to be sold in the dry form and is rarely treated. Occasionally the poultry and swine manure is used directly as a fish feed. Other uses of manure reported were for flooring in buildings and mushroom farming. There appears to be limited markets for liquid swine manure, as there appeared to be considerable costs to disposing of this bulky liquid manure. Dairy manure for the most part was not collected when animals were raised in pastures; in areas where they were not, such as India, it was collected and sold. Though there may be a market for manure, the market for unprocessed manure may be seasonal, as crops only need fertilizer at certain periods of their growing cycle during sometimes of the year.

[22] This estimation inevitably overestimates what can be absorbed. Unfortunately, it is necessary to use this estimate, as most of the surveys are not detailed enough to delineate crop acreage from building area.

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