Nutrient Imports to the Cape Fear and
Neuse River Basins in Animal Feeds

Lawrence B. Cahoon
Department of Biological Sciences
UNC Wilmington, Wilmington, NC

Jill A. Mikucki
Program in Environmental Studies
UNC Wilmington, Wilmington, NC

Michael A. Mallin
Center for Marine Science Research
UNC Wilmington, Wilmington, NC

Nutrient loading from agriculture is considered to be a very large fraction of total nutrient loading to North Carolina waters, but the proportion attributable to intensive livestock operations (ILOs) is difficult to ascertain. However, a minimal estimate of the water quality management challenge posed by ILOs can be derived from an estimate of the amount of "new" or imported nutrients in the feeds they use. Here we consider the imports of nitrogen and phosphorus in feeds supporting the principal animal industries in the Cape Fear and Neuse River basins, where ILOs are particularly concentrated.

The Cape Fear River basin is the largest in North Carolina (area = 23,700 km²) and includes portions of 27 counties. The river rises in the north central Piedmont and flows directly to the sea near Cape Fear. Important coastal plain tributaries include the Black and Northeast Cape Fear Rivers. The Neuse River forms in the Piedmont and empties into Pamlico Sound through the Neuse River Estuary. The Neuse River basin (area = 16,040 km²) includes portions of 19 counties. The Neuse River has been designated as Nutrient Sensitive Waters (NC D.E.M., 1996a), whereas the Cape Fear River, owing to its high turbidity, is not considered nutrient sensitive, except in the lower portions of its estuary (NC D.E.M., 1996b). Biological oxygen demand (BOD), primarily from organic loadings, has been identified as a primary water quality concern in the Cape Fear River (NC D.E.M., 1996b).

County animal census data (NC D.A., 1996) show that the two river basins accounted for 71% of the hogs, 66% of the turkeys, 34% of the broiler chickens, 24% of the other chickens (mostly egg layers), and 24% of the total (beef + dairy) cattle in North Carolina in 1995. The Cape Fear River basin is approximately 50% larger than the Neuse River basin, but supported a much larger fraction of animal production, including over half of the hog population in the state. Almost 42% of the state's hog population and 40% of the state's turkey population were concentrated in Sampson and Duplin counties, drained primarily by the Black and Northeast Cape Fear Rivers, respectively.

Estimates of feed grain and soybean imports to the Cape Fear and Neuse River basins in 1995 were calculated by comparing the consumption of these feed crops by the animal populations in each basin with production of these crops in each basin (calculated from data supplied by K. Zering, NC State Univ. and NC D.A., 1996) and assuming conservatively that all feed crops produced within the basins are used as animal feeds. These comparisons show that at least 87.2% of grains and 94.5% of soybeans used as animal feed must be imported to the Cape Fear basin, with import estimates of 60.6% of grains and 73.6% of soybeans for the Neuse basin (Table 1).

Nitrogen and phosphorus imports to the Cape Fear and Neuse River basins in animal feed were calculated using estimates of the proportion of each animal type in each basin, grain and soybean consumption data for each animal type, nitrogen and phosphorus contents of feeds, estimates of supplemental phosphorus in feed mixes, and the import estimates for feed grains and soybeans (Table 1). The nitrogen and phosphorus contents of feeds were calculated from assay data for corn and soybeans, which are typically the bulk of the diet fed to most animals (Luce et al., 1990) and from estimates of inorganic phosphate added to feeds for swine and poultry (Luce et al., 1990; Sell et al., 1994).

Animal feed used in the Cape Fear River basin in 1995 incorporated almost 100,000 metric tons of nitrogen and 33,000 metric tons of phosphorus, of which over 90% was imported from outside the basin, or approximately 92,000 and 32,000 metric tons of nitrogen and phosphorus, respectively (Table 2). Approximately 46% of the nitrogen imported to the basin could be attributed to the swine industry, with 50% attributable to the turkey and broiler chicken industries together. The swine industry accounted for 59% of the phosphorus imported to the Cape Fear River basin in 1995, and the turkey and broiler chicken industries together for 39%.

Animal feed consumption in the Neuse River basin was considerably lower than in the Cape Fear basin, owing to the lower numbers of animals produced there. Animal industries in the Neuse basin consumed approximately 35,000 metric tons of nitrogen and 12,000 tons of phosphorus in 1995 (Table 2). The overall percentages of imported nitrogen and phosphorus were estimated to be lower than in the Cape Fear basin, but represented imports of about 24,400 and 10,400 metric tons of nitrogen and phosphorus, respectively. Hog production accounted for about 51% of the imported nitrogen and 65% of the imported phosphorus in the Neuse River basin.

Comparisons of animal manure nutrient contents with nutrient import values permit an assessment of the amount of imported nutrients potentially contributing to in-basin loadings. Manure and nutrient production were calculated and compared to total 1995 nitrogen and phosphorus consumption in animal feeds in the Cape Fear and Neuse River basins using manure production and manure nutrient content data from Barker and Zublena (1995). Essentially all the nutrients excreted by swine and poultry in the Cape Fear River basin were imported, and a large majority of the nutrients excreted by animals in the Neuse River basin were imported (Table 2). Swine accounted for approximately 58% of the nitrogen and 59% of the manure phosphorus produced in the Cape Fear River basin and approximately 64% of the nitrogen and 63% of the phosphorus generated in animal wastes (not including cattle) in the Neuse River basin in 1995. These disproportionate inputs by swine populations in the two river basins reflect the comparatively low feed conversion efficiency of hogs compared with poultry. Nutrient output in cattle manure was much higher than nutrient consumption by cattle in grain- and soy-based feeds, owing to large nutrient inputs via grazing on natural forages.

Annual nitrogen and phosphorus loads carried by the rivers draining the areas where animal production is particularly concentrated were calculated from data on river flows (J. Bales, pers. comm.) and nutrient concentrations and compared to our estimates of manure nutrient production. Nutrients imported to the Cape Fear basin in animal feeds and excreted as manures were considerably higher than nutrient loads in the Cape Fear River in 1995 (Table 3). Estimates of annual phosphorus loads for the Black and Northeast Cape Fear Rivers, which drain Sampson and Duplin counties, are much smaller than the manure phosphorus generated by animal industries in those counties (Table 3). Nutrients imported to the Neuse basin in animal feeds and excreted as manures are also considerably higher than 1995 nutrient loads in the Neuse River (Table 3).

Comparisons of animal manure phosphorus produced in the two river basins with commercial fertilizer shipments to the basins show that in 1995 animal manures approximately equaled phosphorus imports in commercial fertilizers to the Cape Fear basin and accounted for about 25% of total phosphorus import from these two sources in the Neuse basin (Table 3).

The magnitudes of nitrogen and phosphorus imports and loadings as manures to the Cape Fear and Neuse River basins pose a significant management challenge. Loadings of nitrogen have received much attention recently, particularly with reference to nitrogen stimulation of algal blooms in North Carolina coastal waters. Assessments of aerial transport of ammonia nitrogen released from animal wastes and land-applied manures suggest very large loadings to these basins (NC D.A.Q., 1997). However, the relatively high proportion of phosphorus in animal wastes (atomic N:P ratios from 1.24 to 1.81) and lack of an atmospheric export mechanism suggest that phosphorus accumulation will pose substantial water quality threats, especially stimulation of nitrogen-fixing cyanobacterial blooms, in the future if not managed properly.

Acknowledgements: We thank J. Bales (U.S. Geological Survey), B. McGee and D. Holsinger (N. C. Division of Water Quality), Dr. K. Zering (Department of Agricultural and Resource Economics at NC State University), Dr. M. Williams (Animal and Poultry Waste Management Center at NC State University), R. Murphy (NC Dept. of Agriculture's Agricultural Statistics Division), the Cape Fear River Program, and the UNC Water Resources Research Institute (research grant #5-36634 to M.A. Mallin and L.B. Cahoon) for advice, assistance and support for this work. This is Contribution #122 of the UNC Wilmington Center for Marine Science Research.

Table 1. Feed grain and soybean production and consumption by animals (1,000 metric tons in the Cape Fear and Neuse River basins in 1995.

Table 2. Nitrogen (N) and phosphorus (P) consumption, import, and excretion (1,000 metric tons) in manures by animals in the Cape Fear and Neuse River basins in 1995.

Table 3. Comparisons of annual river nutrient loads, manure
nutrient outputs, and commercial fertilizer (assuming P @ 10%
total weight) usage in the basins in 1995 (metric tons).

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