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 NO3 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 NH4NO3), 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 NO3 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 NO3 that were related to the amounts of manure and fertilizer N applied. Some decreases in NO3 were measured from fall to spring sampling times, but net losses were minimal where < 60 kg ha-1 NO3-N was present in the fall. Application of manure resulted in similar or slightly lower soil profile NO3 than agronomincally equivalent rates of fertilizer N.