The solution to poultry waste pollution on Maryland’s lower Eastern Shore may not be dilution.
Water quality in the region has deteriorated in the past two decades as millions of chickens annually produced hundreds of thousands of tons of manure. Excessive amounts of chicken litter applied on fields in the past is blamed for worsening water quality conditions in some of the region’s rivers.
The presumed solution has been to ship the excess manure to other parts of the Eastern Shore where it could reduce the use of commercial fertilizers on cropland.
But a new study suggests that approach, unless carefully managed, may simply spread water quality problems to new areas.
During a four-year study, a University of Maryland scientist used poultry litter instead of chemical fertilizers on a typical crop rotation cycle on two adjacent fields. The manure was incorporated into the soil on one field, but applied to the surface on the other.
The results showed increased nutrient pollution from both fields, but the increase was much higher on the field where the manure was applied to the surface using “no till” methods widely promoted to reduce soil erosion.
The bottom line is that replacing commercial inorganic fertilizers with organic poultry litter could increase the pollution risk to local waterways, especially if farmers don’t incorporate the manure into the ground, said Russell Brinsfield, executive director of the nonprofit Maryland Center for Agro-Ecology, which partially funded the study.
Because of the huge surplus of poultry litter, Brinsfield acknowledged that moving it to new areas may be the only short-term option for dealing with the waste.
“But if you are going to do that, we need to be aware of what the risks are,” he said. “We need to be smarter about where we transport the manure and whether we recommend tilling or not.”
The study raises issues about the degree to which poultry litter—and animal waste in general—can be used as a substitute for commercial fertilizers and still protect water quality.
As is the case on the Eastern Shore, many parts of the Bay watershed have nutrient surpluses largely because of intense animal agriculture. Most large, modern animal farming operations grow only a portion of the feed for their animals, supplementing their needs with grain grown by other farmers.
As a result, the animal farms accumulate more manure than needed on their own fields. That waste, when left to build up on the farm or overapplied on crop lands, is a major source of nutrient pollution.
For many years, many scientists and agency officials have advocated spreading excess animal waste over more cropland as a replacement for chemical fertilizers.
Substituting animal waste for fertilizer has never been easy, though. It is costly to transport, and it is hard to apply tons of manure with the same precision as a few hundred pounds of fertilizer that has the same nutrient value. A growing number of scientists and agency officials have begun questioning whether the redistribution of manure offers a long-term solution to the region’s nutrient problems.
“To me, the writing is on the wall that in addition to being smarter about how we apply the manure, we need to be exploring some alternative uses for manure,” said Brinsfield, a soil scientist and farmer.
The study, which was also funded by the Maryland Grain Producers Utilization Board and Delmarva Poultry Industry Inc., illustrates the problems with substituting manure for inorganic fertilizer.
It was conducted from 1998 through 2002 on adjacent fields which had been monitored since 1984. That provided a baseline for rates of nutrient runoff to surface water and leaching into groundwater when only inorganic nutrients were applied. No poultry manure had previously been used on the fields.
During the test, manure was surface applied on one field using widely promoted no-till methods, and plowed into the soil on the other field. A small control plot on each field got inorganic fertilizer rather than poultry litter.
Both of the fields that got poultry litter had greater nutrient losses compared with past years when inorganic fertilizers were used. Generally, though, the field that was plowed had less surface runoff than the no-till field.
Still, the plowed field lost more nitrogen to groundwater than the no-till field. The reason is probably that the manure had to be plowed into the ground before corn was planted and could take up the nitrogen, said Ken Staver, the scientist from the University of Maryland’s College of Agriculture and Natural Resources Wye Research and Education Center who conducted the study. “You have a high nitrate concentration in the soil well in advance of when the crop actually needs it,” he said.
By contrast, most inorganic fertilizer isn’t applied until the crop actually starts to grow and is ready to absorb the nitrogen, he said.
Such precise application is not possible with manure. The bulky poultry litter had to be applied at a rate of three tons per acre to meet the nitrogen needs of the corn, whereas only a couple of hundred pounds of inorganic fertilizer does the same job. Staver said equipment does not exist to allow farmers to economically apply manure after crops begin growing.
Overall, it was the no-till plot that generally performed worse than the plowed field. Applying poultry litter during no-till corn production increased phosphorus runoff fourfold over the plowed field during the summer months.
Farmers are often advised not to plow their fields, especially if they are on slopes, because tilling soil increases erosion as well as phosphorus losses as the nutrient binds to the eroding soil. But during the study, phosphorus losses related to erosion were minor compared to the levels of dissolved phosphorus that were washed into streams from the unplowed fields where manure was left on the surface.
Staver said the study shows that no-till advice that works for commercial fertilizers does not work when poultry waste is used as an alternate. “No-till has always been promoted as a conservation practice, and it is credited with a lot of phosphorus reductions, but never with the caveat that if you are using animal waste, there could be a problem,” Staver said.
The findings suggest that the Bay Program may have overestimated the nutrient reduction effectiveness of no-till farming in the past for croplands where manure was applied instead of inorganic fertilizer, Staver said.
Also, when poultry manure was not tilled into the ground, an estimated 45 pounds of nitrogen per acre was lost as ammonia—a form of nitrogen—volatilized into the atmosphere. As a result, more fertilizer had to be added to meet the nitrogen needs of corn crops.
Because it was difficult to calculate the amount of nitrogen lost through volatilization, and therefore estimate the additional amount that had to be applied, the potential was created to overapply nitrogen and increase runoff, the study concluded.
The study also found that planting fall cover crops helped to reduce the amount of nitrogen in the groundwater on both the tilled, and untilled, fields. Planting cover crops may be especially important on fields where poultry litter is used, the study said, because the release of some of the nitrogen in manure is delayed, leaving an excess in the soil after crops are harvested in the fall.
The study said the planting of cover crops should be prioritized for farms where poultry litter is applied.
A major concern on both the tilled and untilled fields was the potential for a long-term buildup of phosphorus in the soil because of the ratio of phosphorus to nitrogen in poultry litter. One ton of poultry manure supplies all of the phosphorus needed for a grain harvest in a given year, but at least three tons are needed to meet the nitrogen requirements of corn.
That causes phosphorus to accumulate in the soil over time, increasing the potential for losses in runoff—something that has become a widespread problem on the lower Eastern Shore where poultry litter has been overapplied for years. There was some evidence during the fourth year of the study that phosphorus concentrations in the soil had risen enough to increase the runoff of dissolved phosphorus from the surface.
The research suggested a number of ways to reduce potential negative impacts of poultry litter use.
If it is applied on fields with low erosion potential that are routinely tilled, the increase in nutrient losses will be less than if applied on fields kept in no-till to prevent soil erosion. This could mean that areas with high erosion rates would not be able to receive poultry manure. The aggressive planting of cover crops can reduce nitrogen leakage in the fall. And, restricting poultry litter applications to every few years can minimize the potential for phosphorus levels to build up in the soils.
Brinsfield said the study also shows the importance of research efforts aimed at reducing phosphorus content in poultry litter by changing chicken diets, which has been showing promising results. If phosphorus levels in the manure are reduced, it can be more widely used without leading to a rapid phosphorus buildup in the soil.
Still, such reductions will likely only slow—not eliminate—phosphorus buildup in soils receiving poultry manure. Brinsfield said that means some other uses are needed for poultry litter, so there are practical alternatives to placing it on fields.
“That is critical to retaining the industry on the shore and keeping it economically viable,” Brinsfield said. “If you can’t find alternative uses, then we may see a contraction of the industry.”