Accuracy of the Predicted Fertilizer N Equivalence of Manure Slurry.

Farmers need reliable estimates of N availability from livestock manure to develop nutrient management plans, to estimate the correct manure application rate, and to predict the amount of supplemental fertilizer N that is needed when manure does not fulfill the crop N requirement. We tested the accuracy of University of Minnesota predictions of N availability from dairy and swine manure slurries to corn (Zea mays L.). Field experiments were conducted over 13 site-years in Minnesota on nonirrigated fields with medium- to fine-textured soils. Main plots were manure slurry rate (0, one-half, and full N requirement) and subplots were broadcast urea near planting time the subsequent spring. At each site, manure was applied by direct injection or by surface broadcast and subsequent incorporation within a few weeks, depending on the available equipment, which varied widely. Estimates of fertilizer N equivalence (FNE) of the manure by ear leaf chlorophyll at tasseling were lower than the FNE based on grain yield after physiological maturity. We conclude that midseason ear leaf chlorophyll measurements are not a reliable means of estimating additional fertilizer N requirement for corn under Minnesota conditions. We found that University of Minnesota predictions of FNE, based on grain yield response, were accurate when the manure was injected by knives or sweeps. Predictions of FNE were less accurate from broadcast-incorporated applications, presumably because ammonia losses were affected by local weather conditions before incorporation and the thoroughness of manure incorporation, which is affected by the implement used and the soil conditions during incorporation.