The production of poultry in the state of Iowa is one of the largest in the nation and growth has continued the past few years. Currently Iowa is the number one egg producing state in the USA (USDA, 2005). This increase in production also implies an increase in manure production from poultry sources. The common end use of manure is application for crop production. Concerns exist regarding application at rates higher than needed for crop use, with potential for contamination of water bodies due to excess nutrients. Producers also question the proportion of the total N that should be accounted for as crop-available in the year of application.

In addition to the environmental implications of excess nutrients from manure application, the economic and agronomic aspects of incorrect application rates also need to be addressed. Poultry manures can have more readily crop-available N compared to other manure sources, such as sheep and cattle. For example, Rees and Castle (2002) found that corn had larger plant growth and yield response to poultry manure application. The current ISU Extension publication in Iowa providing suggested first-year crop availability of N from poultry manure indicates use of 65 percent (Killorn and Lorimor, 2003).

On the other hand, research work such as that of Chambers and Richardson (1993) suggest no need for additional commercial N fertilizer following a recent application of poultry manure. Other studies suggest that poultry manures have a rather slow release of plant-available N and in about one year approximately 67 percent of the organic N fraction is mineralized (Bitzer and Sims, 1988).

The large variability observed in nutrient concentrations in manure is well known, with variation depending on source as well as management (Cooper et al, 1984). This creates a need for understanding poultry manure nutrient content and using local sources of poultry manure to study applications with the soils, cropping systems and climatic characteristics of Iowa. Furthermore, a large percentage of N in poultry manure is in the organic fraction (Sims, 1986), therefore more information on the mineralization of this N will help with prediction of N availability. Also, because 20 to 40% of the total N is inorganic N (Bitzer and Sims, 1988), estimates of nitrification and crop recovery of this fraction is needed as well.

The purpose of this study is to estimate under field conditions the supply of plant-available N to corn from several poultry manure sources and timing of application, and to compare corn yield response between poultry manure N and commercial fertilizer.