Nitrogen Fertilizer and Dairy Manure Effects on Corn Yield and Soil Nitrate

Manure from livestock is an important source of N for crop production in many areas, but efficient management of manure is critical to improve the economics of manure use and to minimize the impact on water quality. A field study was conducted on an Enosburg fine sandy loam (sandy over loamy, mixed, nonacid, mesic Mollic Haplaquent) in northwestern Vermont to evaluate the effect of dairy‐manure and N‐fertilizer application on corn ( Zea mays L.) yields and soil profile NO 3 in a silage production system. Treatments consisted of a factorial arrangement of manure (0 and 9 Mg ha ‐1 , dry‐matter basis), N rate (56 and 112 kg ha ‐1 as NH 4 NO 3 ), and time of N application (planting or six‐leaf stage), as well as 0 and 168 kg N ha ‐1 rate at planting (with and without manure). Yields and N uptake were increased by N fertilizer and by manure. Without manure, grain and silage yields were increased by fertilizer N to the 112 kg ha ‐1 rate in all years; with manure, N fertilizer did not increase yields significantly. Time of application had little or no effect on yield. Plant uptake of N followed a similar pattern but with somewhat more pronounced effects. A presidedress NO 3 soil test reflected N availability, as indicated by relative yields. Manure application rates were equivalent, in terms of yield response, to 73 to 122 kg fertilizer N ha ‐1 in individual years, which represented 27 to 44% of the total manure N in the year of application. Sampling of the 1.5‐m soil profile before planting and after harvest showed increases in soil NO 3 that were related to the amounts of manure and fertilizer N applied. Some decreases in NO 3 were measured from fall to spring sampling times, but net losses were minimal where <60 kg ha ‐1 NO 3 ‐N was present in the fall. Application of manure resulted in similar or slightly lower soil profile NO 3 than agronomincally equivalent rates of fertilizer N.