Eddie P. Millhollon and Darinda R. Dans

Poultry production is the largest animal agricultural industry in Louisiana with a gross farm value in 2009 of $450.8 million and an industry value-added total of $432.8 million, ranking it second to forestry in total income production from statewide agricultural commodities. Approximately 2,000 poultry houses in Louisiana collectively produce nearly 200,000 tons of poultry litter annually. Poultry litter consists of both poultry manure and the bedding material – usually sawdust, wood chips or rice hulls. Although litter is  considered a waste byproduct, it contains beneficial plant nutrients – including nitrogen, phosphorus and potassium – and, therefore, can be used as an organic fertilizer.

The fertilizer nutrient content of poultry litter varies depending on the source, moisture and bedding material content. A recent survey of the nutrient content of litter from several poultry houses in Arkansas showed that, on average, each ton of litter on a dry weight basis contained 36 pounds of nitrogen, 40 pounds of phosphorus and 42 pounds of potassium. When applied to soil, it is also a source of organic matter and can improve soil physical and chemical properties that contribute to buffering, water-holding capacity and erosion resistance.

Historically, litter has primarily been applied to pasture and hay meadows adjacent to poultry houses and only occasionally to row crops. Because the manufacture of nitrogen fertilizer is energy-intensive, there is a close relationship between world oil and natural gas prices and nitrogen fertilizer prices. Therefore, as oil and natural gas prices escalated in 2009, so did nitrogen fertilizer prices. This caused row crop producers to seek alternative, less-expensive sources of nutrients for their crops, with poultry litter being a logical choice.

Poultry litter is an excellent alternative source of plant nutrients if it is used correctly. Studies in Arkansas show the amounts of phosphorus and potassium in each ton of litter are more than the amount of nitrogen, which is fairly typical. If poultry litter is applied based on the nitrogen requirements of a crop, the amount of phosphorus applied can exceed crop needs. After the plant has taken up all it needs and the soil has absorbed its capacity, the phosphorus that remains in the soil is susceptible to loss in runoff, potentially winding up in nearby water bodies and contributing to algal growth and eutrophication – or nutrient enrichment.

In 1998 and 1999, researchers at the LSU AgCenter’s Red River Research Station in northwest Louisiana near Shreveport conducted a study to determine the benefits of using poultry litter as an alternate nutrient source for cotton production and to assess the environmental impact of this practice on soil phosphorus accumulation. Three rates of poultry litter – zero, 2 and 4 tons per acre – have been applied annually since the beginning of the study. The study also was designed to compare the influence of conventional and conservation tillage practices on runoff water quality, but only the conservation tillage results will be discussed here.

Each year, poultry litter is obtained from a local poultry producer and analyzed for nutrient and moisture content. It is applied to replicated plots of 12 40-inch rows 445 feet long and then incorporated into the soil. Cotton is planted within one to two days following litter application. Each spring soil samples from each plot are collected and analyzed for chemical properties, including phosphorus content.

Comparing cotton yield in response to applications of zero, 2, and 4 tons of poultry litter per acre applied annually since 1998 shows that, as poultry litter application rates increased, so did cotton yield (Figure 1). Increasing rates of poultry litter also increased soil organic matter, indicating a positive correlation between cotton yield and soil organic matter (Figure 1).

Comparing soil phosphorus content at depths of zero to 6 inches in 1999 to 2008 shows the influence long-term use of poultry litter had on soil phosphorus content (Figure 2). For Red River alluvial soils, the optimum soil phosphorus range for cotton is from 40 to 70 ppm. In 1999, the year following the beginning of this study and the initial application of poultry litter, soil phosphorus increased from 69 ppm with no addition of litter to 98 ppm following application of two tons per acre and 107 ppm following four tons per acre.

Soil samples taken in 2008 showed that phosphorus levels continued to increase in response to annual applications of two and four tons of litter per acre. From 1999 to 2008, annual applications of two tons of litter per acre resulted in a 149 percent increase in soil phosphorus. Soil phosphorus doubled from 1999 to 2008 in response to annual applications of four tons of litter per acre. Although cotton plants remove soil phosphorus as they grow, harvest only removes lint and seed from the field, leaving the majority of phosphorus in the remaining plant material that is eventually returned to the soil. This is why there was a decline in soil phosphorus from 1999 to 2008 in plots where no additional phosphorus was added (Figure 2).

One way to manage excess soil phosphorus that results from using poultry litter is to alternate cotton with a crop that is effective in moving phosphorus from the soil into plant tissue and then removing that plant tissue from the field.

Rotation crops such as corn have been investigated, but corn is only partially effective because only the grain is removed, leaving much of the phosphorus returned to the soil in remaining plant tissue. Annual ryegrass grown during the winter and harvested and baled for hay is a better choice because most of the above-ground plant tissue is removed along with much of the phosphorus that plants have extracted from the soil over the winter.

To investigate the effectiveness of annual ryegrass in removing excess soil phosphorus, the study was modified in 2009 to focus on investigating management practices that would allow the continued beneficial use of litter as an organic fertilizer but avoid the continued rise in soil phosphorus and resultant environmental impact. Following cotton harvest in 2008, annual ryegrass was planted in plots that had previously received poultry litter applications. In spring 2009, annual ryegrass was harvested and weighed. The tissue samples were collected, and the ryegrass was baled and removed from the field. Ryegrass tissue samples were analyzed for phosphorus content, and soil samples were collected to see what changes had occurred in soil phosphorus.

Figure 3 shows the results of growing annual ryegrass over the winter of 2008-2009 on soil phosphorus content changes and the phosphorus-accumulating ability of annual ryegrass. Annual ryegrass accumulated soil phosphorus linearly in response to increasing soil phosphorus from two to four tons of litter applied per acre. The result was a decline in soil phosphorus in plots that have received poultry litter since 1998. Although soil phosphorus levels following long-term application of poultry litter are still well above optimum levels for cotton, results demonstrate how a rotation of a summer cotton crop with winter annual ryegrass can manage phosphorus accumulation after using poultry litter as an organic source of plant nutrients.

This study will be continued to determine the extent to which annual ryegrass can be used as a management practice with poultry litter to reduce phosphorus in the soil and to avoid a negative environmental impact from excessive phosphorus in runoff water.

Eddie P. Millhollon, Associate Professor, and Darinda R. Dans, Research Associate, Red River Research Station, Bossier City, La.

(This article was published in the spring 2010 issue of Louisiana Agriculture.)