Water quality and the grazing animal

†Department of Animal and Dairy Science, University of Georgia, Tifton 31793

ABSTRACT Grazing animals and pasture production can affect water quality both positively and negatively. Good management practices for forage production protect the soil surface from erosion compared with conventionally produced crops. Grazing animals and pasture production can negatively affect water quality through erosion and sediment transport into surface waters, through nutrients from urine and feces dropped by the animals and fertility practices associated with production of high-quality pasture, and through pathogens from the wastes. Erosion and sediment transport is primarily associated with high-density stocking and/or poor forage stands. The two nutrients of primary concern relating to animal production are N and P. Nitrogen is of concern because high concentrations in drinking water in the NO₃ form cause methemoglobinemia (blue baby disease), whereas other forms of N (primarily nitrite, NO₂) are considered to be potentially carcinogenic. Phosphorus in the PO₄ form is of concern because it causes eutrophication of surface water bodies. The effect of grazing animals on soil and water quality must be evaluated at both the field and watershed scales. Such evaluation must account for both direct input of animal wastes from the grazing animal and also applications of inorganic fertilizers to produce quality pastures. Watershed-scale studies have primarily used the approach of nutrient loadings per land area and nutrient removals as livestock harvests. A number of studies have measured nutrient loads in surface runoff from grazed land and compared loads with other land uses, including row crop agriculture and forestry. Concentrations in discharge have been regressed against standard grazing animal units per land area. Watersheds with concentrated livestock populations have been shown to discharge as much as 5 to 10 times more nutrients than watersheds in cropland or forestry. The other major water quality concern with grazing animals is pathogens, which may move from the wastes into surface water bodies or ground water. Major surface water quality problems associated with pathogens have been associated with grazing animals, particularly when they are not fenced out from streams and farm ponds. This paper presents an overview of water quality issues relating to grazing animals.

Key Words: Forages, Manure, Nitrogen, Pathogens, Phosphorus, Sediment

© 2004, by the American Society of Animal Science. All rights reserved.

J. Anim. Sci. 2004. 82(E. Suppl.):E255-E263

Implications

Forage production and grazing animal systems can both positively and negatively affect water quality. Compared with cropland, forage systems protect the soil surface from erosion, and, if fertilizer and animal waste inputs are low to moderate, both surface and ground water quality under grazed areas may be better than that under cropped areas. The water quality contaminants of concern from grazing systems are sediment (erosion), N, P, pathogens, and organic matter. Grazing animals negatively affect water quality when the number of animals exceeds the carrying capacity of the land (at both the pasture and watershed scales). Forage production may have negative effects on water quality when fertilizer plus animal waste inputs exceed crop nutrient needs, or when forage quality is poor and soil erosion can occur. Grazing animal systems should be managed to include adequate land area for animal numbers at the field and landscape scale, fencing animals out of streams and lakes, and use of riparian buffer systems to assimilate sediment, nutrients, and pathogens from grazing animals.