From an engineering standpoint, site-specific manure application equipment is very similar to that used for variable rate application (VRA) of liquid fertilizer, but from a management standpoint, variable rate manure (VRM) is quite different from VRA of fertilizer. The goal of this article is to summarize the results of a recent study of the economics of site-specific manure application.
Using the site-specific crop responses to manure, the maximum net present value (NPV) of $607/acre is achieved with the VRM strategy, combined with a variable rate fertilizer (VRF) strategy. The next highest NPV, $605/acre, was achieved by the strategy which applies manure at a field specific optimal rate (WFM), combined with a VRF program, followed by the strategy which applies a uniform rate of manure at the extension recommendation rate of 3,500 gal./acre combined with VRF ($597/acre). Profit maximizing manure rates by zone vary from about 2,500 gal/acre to 8,000 gal/acre. Optimal manure rates are usually lower when some fertilizer is applied to complement the nutrients in manure.
Given the variability of the nutrient content of manure, it is difficult to argue that the VRM-VRF strategy is a better strategy than the WFM strategy combined with the VRF program. The WFM-VRF strategy results in an NPV that is only about $2/acre lower than the VRM-VRF approach. With good agitation and careful calibration, a uniform application of manure is a reasonable goal. With current technology, accurate VRA of manure quantity is possible, but it is difficult to control actual soil nutrients applied. VRF can be used to more economically and accurately manage soil variability.