The agricultural machinery cost software, MACHSEL (Kletke and Sestak 1991) is used to estimate machinery cost. MACHSEL uses the American Society of Agricultural and Biological Engineers standards to calculate ownership costs including depreciation, interest on average investment, insurance, and taxes, and operating costs including fuel, oil, lubricants, and repairs (AAEA 2000; ASABE 2010a, b). Harvest field operations are weather dependent. Hwang et al. (2009) used historical weather data to estimate probability distributions for the number of days per month that mowing operations and baling operations can be conducted. Weather requirements for baling are more stringent than requirements for mowing since baling switchgrass biomass with excessive moisture may result in molding and heating and in some extreme cases spontaneous combustion. The length of time required for mowed switchgrass to dry to levels required for safe baling depends on the moisture content when cut and the weather, and it differs across month and county (Hwang 2007). Estimates of field work days such as those produced by Hwang et al. (2009) may be incorporated into MACHSEL to verify that the selected machinery has the capacity to complete the required field operations during the available time frame.

Cost estimates for the windrower, assuming that it would be used on weather-favorable days from July through March, are included in Table 8. Estimates are provided for three yield levels, 4.48, 8.97, and 13.45 dry Mg/ ha. The estimated cost are $12.28, $24.56, and $36.82/ha for yields of 4.48, 8.97, and 13.45 dry Mg/ha, respectively. By this measure, the estimated cost of the windrowing operation is $2.74/Mg. These estimates follow from the assumption that for different yields, machine speed may be adjusted to maintain an efficient level of biomass throughput. For the enterprise budget reported in Table 5, the estimated cost to hire a custom operator to windrow is $33.56/ha.

Table 9 includes cost and capacity estimates for the raking-baling- stacking unit that consists of three 7.3 m wheel rakes, three 40 kW tractors, three balers, three 147 kW tractors, a bale transporter stacker, and seven laborers. The rakes are available to turn the biomass when warranted to aid drying and to merge two or more windrows, depending on yield, to form windrows that enable efficient baling. One field transporter stacker

has sufficient capacity to collect and stack bales produced by three balers. As shown in Table 9, the machines are budgeted based on throughput capacity. For the relatively low yield of 4.48 Mg/ha, the rakes could be used to merge windrows, and the speed of the balers could be adjusted to meet the designed baler capacity. For higher yielding fields, the rakes may be used, if necessary, to turn the material to enhance drying. The coordinated system is designed to enable the balers to operate near capacity on those days when the biomass is suitable for baling. The throughput capacity of the baler is defined in terms of biomass volume and not hectares.

Given that the system can be managed to maintain a relatively constant throughput capacity, the estimated cost to rake, bale, and stack is approximately $13/Mg for yields ranging from 4.48 to 13.45 Mg/ha (Table 9). The custom rate for baling used in Table 5 that is based on survey data is $17.25/635 kg bale ($27.59/Mg). By this measure, the custom rate for baling as reported by Doye and Sahs (2012) is more than 210 percent of the estimated cost to rake, bale, and stack. This difference is the result of several factors. First, most of the custom rate estimates are for the cost of baling hay for use as a livestock feed, an operation for which timeliness to achieve quality hay is critical. Timeliness is not expected to be as critical for biomass. Second, the window for hay harvest is relatively narrow, which restricts the annual land area over which machine fixed costs can be allocated. Switchgrass harvest in the U. S. Southern Plains is expected to extend from July through March. The harvest machine fixed costs can be spread over substantially more biomass volume when baling switchgrass for biomass

aA raking-baling-stacking harvest unit consists of three 7.3 m wheel rakes, three 40 kW tractors; three balers, three 147 kW tractors; a bale transporter stacker; and seven laborers. bPrice of diesel fuel is budgeted at $0.79/L.

rather than forage for hay. Third, during the nine month harvest window, the machines are budgeted to be operating during all weather favorable days, whereas conventional forage harvesting operations have a much narrower harvest window. By these measures and for these reasons, the cost to harvest switchgrass could be substantially lower than the cost to harvest hay. But, this finding depends critically on the assumption that the switchgrass for biomass harvest window could extend over nine months.