Long-Term Study on the Impact of Manure on Winter Wheat Grain Yields.

Manures contain several essential crop nutrients that contribute to plants health and increase crop yields, which makes manure a valuable resource to farmers. The Magruder Plot is a long-term continuous winter wheat experiment located at Stillwater, OK that have both manure and commercial fertilizer treatment structures dating back to 1930. Cattle manure from feedlot were applied every four years as total nitrogen (N) source at 124 kg N ha^-1 (1931 to 1967) and 269 kg N ha^-1 (1968 to 2014). Fertilizer applications of commercial based N, phosphorus (P), and potassium (K), as well as lime, was applied annually prior to planting. Nitrogen was applied at 37 kg N ha^-1 between 1930 and 1967, at 67 kg N ha^-1 from 1968 until 2004, and at 67 kg ha^-1 since 2005. Phosphorus was applied at 15 kg P₂O₅ ha^-1 and potassium at 29 kg K₂O ha^-1 since 1931. Lime was applied twice (1929 and 1954) when the pH dropped below 5.5. Applications of manure applied at 269 kg N ha^-1 contributed nearly 157 kg N ha^-1 which is equivalent to a nitrogen use efficiency of 58%. Average manure N availability was 18, 17, 13, and 11% for one, two, three, and four years after application (YAA). Overall, mineralization rate one YAA ranged between 25-30% for five of the 12 years, 20% or higher for six out of the 12 years (50% of the observations) and 15% or higher for nine out of 12 years (75% of the observations). At two YAA, minimum mineralization was always less than 20%.