Extractability of Trace Elements in Raw, Heated, and Granulated Poultry Litters.
Gurpal Toor, University of Arkansas, Biological & Agricultural Engineering, 203 Engineering Hall, Fayetteville, AR 72701 and Brian Haggard, USDA-ARS/Bio & Agric. Eng. Dept., 203 Engineering Hall, Fayetteville, AR 72701.
Contamination of soil and water by trace elements is a serious environmental problem for humans and other living organisms. Continuous land application of poultry manure may lead to accumulation of harmful quantities of trace elements such as As, Cd, Cu, Cr, and Zn in soils and can cause toxicity to plants and increase the risk of losses to waters. Trace elements are excreted in manures because of their addition in the animal diets. For example, As is used in poultry feeds as 4-aminophenylarsonic acid (p-ASA) or 3-nitro-4-hydroxyphenylarsonic acid (Roxarsane) to prevent coccidiosis and increase weight gain and improve feed efficiency. Likewise, Cu and Zn are used as growth promoters or biocides in poultry feed and thus are excreted in litter. In addition to directly contributing trace elements in soils, addition of manure can also change soil pH and make metals more available in soils. To manage potential long-term impacts of trace elements in soils, it is important to quantify total and soluble metal inputs to soils from animal manures and identify soils that are most vulnerable to metal loss via surface runoff and leaching. Unfortunately, little information is available on trace element inputs to soils from present day animal manures. The composition of present manures may be different from the past manures. For example, poultry diets now contain many feed additives such as phytase, vitamin D metabolites, which are primarily added to facilitate conversion of phytic acid to inorganic P thereby increasing increasing feed efficiency of phytic acid. During this process, these feed additives may also increase bioavailability of metals because phytic acid is often associated with trace elements. Moreover, the solubility of metals may be changed in post-excreted litters as amendments, such as aluminum sulphate or aluminum chloride, are added to decrease solubility of P in litters. The use of new technology to pellet or granulate poultry litters to increase consistency of particle sizes and reduce moisture content and market it as an alternative fertilizer source may also change trace element contents and solubility. However, no information is available on how the granulation would influence the trace elements solubility in poultry litters. For example, granulation at higher temperature may make some metals more water extractable because of destruction of organic matter. This would, in turn, increase the metal solubility and losses on land application. Our objective, in this study, was to evaluate the influence of granulation on trace metal solubility at various litter to water extraction ratios in raw, ground, heated, and granulated poultry litters. Our results showed that granulation of poultry litters affected the total elemental contents (Al, As, B, Co, Cr, Cu, Mn, Mo, Ni, and Zn) and that the lower contents of these elements in granulated litter than raw litter were because of addition of urea, dicyandiamide, or hydrolyzed feathermeal during litter granulation. The effects of granulation of poultry litter on extractability of metals varied. For example, the extractability (at 1 to 100 litter water extraction ratio) of Al, As, B, Cr, Mo, and Ni was slightly increased (2-10% of total) while extractability of Cu, Fe, Mn, and Zn was decreased (<10% of total) by litter granulation. The extraction of litters with increase in litter to water ratio from 1:10 to 1:250 resulted in increasing contents of Al, As, B, Cr, Cu, Fe, Mn, Mo, Ni, and Zn up to 1:100 extraction ratio, thereafter values were either constant or slightly decreased. At 1 to 100 extraction ratio, B and As were highly extractable metals ranging from 52 to 82% of the total, followed by Ni, Mo, Cu, Cr, and Fe. In contrast, extractability of Zn, Mn, and Al was less than 11% of the total. We conclude that granulation of poultry litter offers an attractive option to reduce trace element inputs in intensive animal production systems and particularly addition of urea and dicyandiamide during litter granulation may result in litters with 10 to 25% less As, Cu, and Zn than raw litters.