Grazing impacts on water quality

by Tamara Scully
Grazing is environmentally beneficial. It has been shown to increase water infiltration and decrease soil erosion; to enhance the water-holding capacity of the soil; to support the sequestration of nutrients; and to promote soil health and biodiversity. However, grazing has been seen as a contributor to water pollution.
In a 2017 Environmental Protection Agency (EPA) study, agriculture was shown to be a contributor in 22 percent of the impaired streams and rivers, and eight percent of the impaired ponds or lakes. A 2007 study by the United States Geological Survey (USGS) determined that pasture and rangelands contributed to 37 percent of the phosphorous found in the Gulf of Mexico. Grazing can have detrimental effects on water bodies. Stream bank erosion, nutrient runoff, and pathogens from animal waste are all primary concerns
Jim Russell, Professor Emeritus, Department of Animal Science, Iowa State, presented research findings in a recent webinar, as a part of Penn State’s “Dairy Grazing Management Guide Webinar Series.”
“There isn’t a major problem associated with grazing,” Russell said. “On the other hand, we do know that in some cases, pollutants from rangeland and pasture can be considerable. While “all common water pollutants are caused by natural processes,” grazing can accelerate pollutant loading.
Primary influences on water pollution include the location, the intensity and the duration of grazing. The proximity and concentration of cattle near the water body, and the stocking rate in cow days per foot of stream, impact the level of water pollution contributed from pasture grazing.
The key to proper grazing is to optimize forage nutrition while minimizing water pollutants. Prolonged grazing, resulting in bare ground and concentrated manure deposits, will cause water quality concerns. Studies by Mat Haan, Penn State Extension Educator, who studied under Russell, showed that a 10-acre pasture, continually grazed by 25 cows, contributed to double the concentrations of nitrogen downstream in the winter and spring, and a ten-fold increase in escherichia coli, sediment and phosphorous downstream in the spring and fall.
“If we can reduce the concentration of animals by a water source, we can really control, pretty well, any problems with pollution associated with grazing,” Russell said.
Cows and water bodies
When cows graze, they have four objectives which guide their behaviors, Russell said. Cows want to maximize their nutrient intake while limiting their time foraging. Cows want to minimize energy expenditures, reducing the need to move and to thermoregulate. The cows also need to meet their nutrient and water requirements, and will seek protection from predators and pests.
Riparian areas can attract cows because they offer shade. Forages in riparian areas can provide quality nutrition, and the water body can provide drinking water as well as offer a means of cooling. Insects may be greater in a riparian area, and if cows have to walk too far to get to the area, it won’t be as attractive to them.
In Eastern region pastures, the size and shape of the pasture has been found to be a primary factor in pollution concerns. Smaller pastures require that the cows spend more time near the stream. In larger pastures, there are other alternatives.
In an Iowa study involving five farms with pasture access to streams or ponds, researcher found that cows were not behaving the same across the farms. On one farm, there was low probability that the cows would be near the water, no matter the air temperature. On two farms, the likelihood that the cows would access the water area increased as the temperature increased. On the two remaining farms, there was a high probability that the cows would be near the water, no matter the temperature, and would be regularly found within the riparian zone more frequently than at the other farms.
“If the cow doesn’t have a choice but to be near a stream, it’s going to be near a stream,” Russell said, indicating that pasture size and the distance to the water body made the difference as to whether or not the cows sought out the riparian zone. “Pasture shape and size really needs to be considered.”
Grazing strategies
Haan conducted studies, using GPS monitors, on the frequency in which cows would be found near the water in relation to grazing management techniques. A continuous stocking situation with unrestricted access to a water body will result in cows accessing riparian areas — within 100 feet of the water body — more often than in the same sized pasture under the same grazing management strategy, but where access to the water body was restricted to a stabilized crossing area.
“By limiting access to those crossings, we cut down on the percentage of time that cows were in the stream,” which was the expected result, Russell said.
Further studies have shown that rotational grazing, rather than continual stocking, can reduce water pollutants while allowing access to valuable riparian area forages. In these studies, the riparian areas were grazed for less than four consecutive days, or until the forages were 10 centimeters in height, in a rotational system.
“If you were to control the access of the cows to these riparian paddocks, that would allow you to access that forage, while limiting the problems with water quality,” Russell said.
Another factor which effects on water quality is the availability of off-stream water. Off-stream water is a primary strategy, used by Western ranchers, to divert cows from water bodies. Haan’s research has shown that in the East, providing off-stream water in a continuous stocking system with either restricted or unrestricted access to the water body, reduces the amount of time that cows spend in the riparian areas. While other studies have had different results, the size of the pasture seems to be the underlying factor causing the discrepancy, Russell said.
“Pasture size had a major factor in terms of enticing cows to be near the steam,” he said.
And, off-stream water had no effect on cows accessing the water body in either a restricted or unrestricted scheme, if the temperature humidity index (THI) was above the cows’ heat stress threshold of 72.
Providing shade away from the riparian areas can reduce the time cows spend near the water body. In large pastures, where cows could access non-riparian shade, “cows were just as happy to go up into the hills, and lay in the shade and catch the breeze…if they had the opportunity,” Russell said.
Supplementing cows near a water source is negatively correlated with water quality. Manure and urine nutrient runoff increases, and feed waste nutrients add to the problem.
Grazing isn’t the primary contributor to water quality issues, although grazing strategies that result in bare ground, those that concentrate animals so that manure is visible, or the use of small pastures which keep animals near riparian areas, are all detrimental.
Precipitation levels, freeze and thaw activity, soil types and textures, topography, along with the water course flow, morphology, slope and vegetation coverage all play a role in water pollution levels, and do so whether land is grazed or not.
“It’s much more complicated when we talk about pollutants in pasture systems than simply talking about animal distribution,” Russell said. “Grazing management can be superseded by other effects.”

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