| | Automatic sampler located at the end of a 12-row cotton plot. Samplers collect runoff from conventional and conservation tillage plots receiving poultry litter for determination of water quality effects. |
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| | Figure 2. Seed and lint cotton yield following poultry litter (PL) best management practices. Bars represent the mean ± the standard error of the mean. |
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| | Figure 3. Changes in nitrate (top) and phosphate (bottom) levels of water samples collected as runoff from each treatment plots. Symbols represent the mean ± the standard error of the mean. |
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Much of the land where cotton is grown in Louisiana has been used for cotton production for decades. This has left the soil deficient in both nutrients and organic matter. Some of these deficiencies could potentially be corrected by supplementing these soils with organic waste from Louisiana’s poultry industry. Approximately 2000 poultry houses in Louisiana collectively produce 200,000 tons of poultry litter annually. Poultry litter has been traditionally applied to pastureland, a practice that has been beneficial and environmentally sound. However, because many of these pastures have become nutrient-saturated, alternative disposal locations, such as agricultural fields, are being investigated.
Litter produced by Louisiana’s poultry industry includes manure and bedding material. The nutrient content of the litter varies depending upon the type of bedding material and feed used. Generally, though, each ton of poultry litter contains about 50 to 60 pounds of nitrogen (N), 50 pounds of phosphorus (P) and 40 pounds of potassium (K). The amount of nitrogen from litter that will become available for cotton growth depends upon several factors including soil pH, temperature and moisture. However, it is generally assumed that about 60 percent (30 to 36 pounds per ton) will become available. Therefore, at least two tons of litter are required to supply 60 pounds of nitrogen.
To determine the potential benefits from the use of poultry litter for cotton production in Louisiana and the subsequent impact on water quality, a study was initiated in 1998 at the LSU AgCenter’s Red River Research Station near Bossier City, La. The study compared applications of poultry litter and inorganic fertilizer in conventional and conservation tillage systems. For purposes of this study, conventional tillage involved incorporating shredded cotton stalks, followed by deep tillage in the fall. Rows were bedded approximately three weeks before planting. The conservation tillage system involved delayed seedbed preparation and deep tillage following the shredding of cotton stalks in the fall, but crop residue and volunteer vegetation remained on the surface until the rows were bedded about three weeks before planting. The goal was to encourage native winter cover and maintain at least 30 percent ground cover from harvest until three weeks before planting. This original study, referred to as Phase I, was continued as Phase II in 2006. The results of Phase II are reported here.
The study was conducted on a Caplis very fine sandy loam soil. The site consisted of approximately eight acres that were precision sloped at 2 inches per 100 feet with laser land-grading equipment. Twelve-row plots (0.25 acre) were replicated four times. Treatments consisted of conventionally tilled and conservation plots that received either 60 pounds of nitrogen or two tons of poultry litter per acre, and a conservation plot that received four tons of poultry litter per acre.
Poultry litter was applied in the spring with a tractor-mounted spreader that was calibrated to apply a rate of two tons per acre. For this study, the two-ton rates were applied with a single pass and the four-ton rate with two passes. Analysis of the poultry litter indicated that the application of 2 tons per acre was equivalent to 71 pounds of nitrogen per acre, 35 pounds of phosphorous per acre, and 56 pounds of potassium per acre. After litter application, all plots were disked to incorporate winter vegetation and litter and bedded to form 40-inch rows.
Cultural practices for cotton production recommended by the LSU AgCenter were followed. Plots that were fertilized received nitrogen (32 percent solution at 60 pounds of nitrogen per acre) placed in the top of the row with a knife applicator. Rows in all plots were shaped with a row conditioner to provide a seedbed uniformly raised to about 4 inches. A cotton variety recommended by the LSU AgCenter was planted at the rate of 10 pounds of seed per acre. A pre-emerge herbicide, a seed-protecting fungicide and insecticide were applied at planting.
Poultry Litter Provides Yields Comparable to Inorganic Fertilizer
Comparing the various treatments for their influence on cotton yield (Figure 1) indicated no significant differences among the different treatments. Two tons of poultry litter per acre with conservation tillage produced the highest seed cotton and lint yield, followed by this rate of poultry litter in a conventional tillage system. Conventional tillage and 60 pounds of nitrogen per acre, considered a standard practice for cotton production, resulted in the lowest seed cotton and lint yield. All plots that received poultry litter as a soil supplement produced higher cotton yields than plots that received inorganic nitrogen fertilizer. Results indicate that two tons of poultry litter per acre appears to be the optimum rate because application of four tons of poultry litter actually decreased yield.
Water Quality
There were significant differences in nitrates in samples collected from runoff. Averaged over all collection dates, runoff collected from conventionally tilled plots supplemented with 60 lbs N/A had the greatest nitrate levels, which were significantly greater than plots where conservation tillage practices were employed followed by the addition of 60 lbs. N/A prior to planting cotton, which had the lowest levels (Figure 2). Nitrate levels in runoff from all plots were less than 1.0 PPM and therefore well below the MCL (Maximum Contaminant Level) of 10 PPM established by EPA.
There were occasions when runoff from plots where 4 tons of poultry litter per acre was applied and conservation tillage practices used contained significantly greater phosphate levels compared to the other management practices (Figure 2). Although the EPA has not established an MCL for phosphate, an MCL of 10 PPM has been suggested. Runoff from all treatment plots were well below this level.
This study demonstrates that poultry litter can effectively be used as a source of nutrients for cotton production in Louisiana with the added benefits of improvements in soil chemistry and organic matter. It also demonstrates the benefits of conservation tillage, particularly when poultry litter is used as a soil amendment.