The first step to removing switchgrass from the field is the mowing operation. Mowing removes most of the above-ground segment of the plant. Two of the most common mowers used are the sickle-bar mower and disc mower or discbines. Figure 1 shows a disc mower blade. A sickle-bar mower works with reciprocating triangular mower blades that are in the middle of stationary blades. Each movement back and forth will shear any plant stem that is in the middle of these stationary blades [14] . Discbines have several small discs that are mounted on top of a cutter bar and these discs rotate at high speeds. Similar to the sickle-bar the cutter bar of the discbines travel parallel to the ground, this bar is what controls the cutting height [14] .

Farming practices have demonstrated that the most efficient way to harvest switchgrass is with a self-propelled swather equipped with disc header. These kinds of mowers will lay the cut switchgrass into windrows where it may dry in the field. The mowing height of the switchgrass is around 10 to 15 cm above the ground surface. Switchgrass is cut at this height because it allows the windrows to be raised above the soil surface to assist with drying. The biomass in these windrows needs to be dried to a moisture content less than 20% d.b. [12] .

As mentioned previously, the moisture content of the switchgrass should be less than 20% d.b. [12] . The importance of conditioning done during mowing is to accelerate drying. When switchgrass is conditioned, the plant stem is crushed without altering its structure. Conditioning systems fall into either impeller/tine-type or roll-type conditioners [15] . Roll type conditioners consist of two rolls that force the crop between them to break the stems. Flail Type conditioners use metal flails to hit the crop after it is mowed to break the stems. As the crop is fed through the flails it is also rubbed against the conditioning hood which wears away at the stem’s surface allowing moisture to escape more easily [15] .

Switchgrass drying rate can be improved through a combination of intensive conditioning and wide swath drying. This method of roll conditioning crushes and scuffs the stems which disturbs the plants’ cutin layer and flattens the stem nodes. This provides a better way for water vapor to leave the stem and dry faster. Although, wide swath drying decreases field drying time, it requires an additional field operation to rake wide swaths into windrows that can be picked up with commercially available balers [16] .

Switchgrass is normally harvested and baled with commercially available forage equipment, but a few significant items should be considered. From previous field harvesting studies, it was found that self-propelled swathers with a mower and conditioner head are utilized because they are not only efficient, but they can handle different volumes of switchgrass to be harvested [3] [6] [17] .

Once the crop is mowed, conditioned, and windrowed, it is ready for the final step in harvesting, baling and collection. Switchgrass can either be baled or chopped. Large square and round balers are the primary types of balers used in switchgrass harvesting [12] . Figure 2 shows a picture of a large square and round baler. Small square bales are not produced since there is a lack of market for them and biorefineries have trouble handling these bales [6] . A baler consists of a pickup head to pick up the crop and is fed into a compression chamber. The crop is then compressed and tied or net-wrapped to form a bale that can easily be transported. After bales are made, they are collected and transported to a storage location or biorefinery. Harvesting methods that differ to baling may be used in regions where the weather may require the need for wet storage methods.

Round balers typically have lower capital costs than large square balers, but their field capacity tends to be lower because the baler and tractor combination needs to stop moving to wrap and release the bale. Some agricultural machinery companies have released non-stop or continuous round balers that eliminate the need to stop when ejecting a round bale. Additionally, these bales are wrapped with plastic wrap to protect them from precipitation post-harvesting. Some farm

machinery manufacturers developed new equipment continuous round balers, which could wrap and drop the round bale without stopping. Other companies developed large square balers, which could bale constantly without interruption and are estimated to have a lower cost per unit of harvested area [6] . One downside to utilization of balers is the inability to completely collect all biomass that has been mowed, resulting in losses.

Harvesting methods different from baling may be used in areas where climate conditions and existing harvesting systems allow for substitute harvesting scenarios. Wet storage methods have been applied to areas where drying conditions for baling operations are not feasible due to high relative humidity and increased precipitation after harvest. Like baling, a swather harvester is used to mow the switchgrass. After mowing, the switchgrass is chopped utilizing a self-propelled forage harvester. Using a forage chopper to remove biomass crops from a field is currently a fairly uncommon practice; however, sometimes it is still performed in situations where compressed pellets are the end product. Crop can both be chopped while still standing or it can be picked up and chopped after being mowed.

This harvester has a rotary head that blows the material into an adjacent trailer [6] [18] [19] . This harvesting system is a one-pass system, compared to bailing which requires many steps or operations to get the switchgrass off the field [18] [20] [21] .

A disadvantage associated with chopping is if it is chopped at a moisture content that is too low, excessive heating and mold growth can run the risk of self-combustion [18] [19] . A forage harvester is not designed to harvest dry matter meaning that dust has the potential to cause environmental and mass loss issues and the possibility to cause machine fires as observed by [22] . Another disadvantage is the higher equipment and storage structure costs compared to a conventional baling system [6] [23] .

After baling is completed, the bales can be collected or left in the field. Dependent on a producer’s storage availability and capacity, bales can be collected and stacked in-field. These bales either remain at the edge of the field or are transported elsewhere for storage [6] . During bale loading from field to storage, it takes double the time to load round bales on semi-trailers than large square bales [6] [24] . If the switchgrass is large square baled, they are stacked in-field and transported off-field to an indoor storage facility or under-roof storage. This is because, when precipitation occurs, the flat surface of the bale does not shed water and as a result dry matter losses can be substantial [6] [23] . After round bales are baled, they can be left in-field for storage or under-roof storage, like square bales. Round bales may be left in-field because of their sloped surface, water penetration is less likely, especially if they are net wrapped [6] [24] .

Switchgrass can be either harvested in the spring or fall, depending on what time a producer chooses to harvest is dependent upon field conditions, equipment availability, and end-use energy generation [6] [25] . Although it has been found that yield decreases by 20% - 40% if the crop spends the winter in the field, the content of ash and water concentrations reduced. Thus, increasing the energy content of switchgrass for all conversion systems [6] [12] [25] .

When harvest is delayed, the lowered concentration of choice minerals reduces the potential for fusible ash which can result in slagging and fouling or deposits in the boilers used for direct combustion [12] [25] . In this case, switchgrass intended to be used for direct combustion should be harvested in the spring rather than the fall. If the end-use is to be used for ethanol fermentation or gasification systems, a fall harvested crop may be better suited because of their lignocellulose yield [9] [25] .

Improved methods for optimizing switchgrass harvest, storage, and transportation are required to better the financial and net energy economics of switchgrass production. These optimal methods will vary with the type of bioenergy conversion that is to be utilized and aim to reduce the quantity of switchgrass that is left in the field during harvesting.

Recently, the mixed-integer linear programming (MILP) models are being developed and, in some cases, used in conjunction with the Greenhouse gasses, Regulated Emissions, and Energy use in Technologies (GREET) model created by the Argonne National Laboratory, to estimate greenhouse gas emissions based on these model outputs [1] [46] . When these models were developed, field tests were completed, and specific data was recorded so that predictions can be made about how changing specific conditions will alter the outcome of the models. Recent developments in biomass logistics models incorporate the sporadic availability of biomass throughout areas of interest, uncertainties of supply, GIS, emissions from logistics operations, and traffic congestion caused by biomass transportation. Current models focus mainly on economic objectives rather than emissions from logistics activities [47] .

The incorporation of the GREET model with a logistics model [1] has been utilized to calculate the greenhouse gas emissions for the logistics model outputs. This model when used with the GREET model presents an output that determines the energy consumption, costs, and emissions. The logistics model is made up of two sub-models: the harvesting operations and the post-harvesting transportation model. The model can be used for switchgrass and loblolly pine production. This model examines four forms of biomass harvesting methods: whole tree woodchip, clean woodchip, round bale and square bale [1] . Not only does the model produce the biomass collection radius, number of machines required, cost and energy consumption from the inputs, but it also presents the emissions of the entire operation from biomass production to biorefinery.

This model does not consider an agricultural field size or shape, and this may alter the outcome of the model. The model does consider tractors and implements used, bale accumulators, and typical bale sizes for large square (0.9 × 1.2 × 2.4 m) and round bales (1.7 × 2 m). The fuel consumption was estimated by equations from two previous publications: [48] [49] . Turhollow [1] used ASABE Standards EP496.3 [50] [51] when Nebraska Tractor Test Laboratory data was not available.

The integrated biomass supply analysis and logistics model (IBSAL) model is a well-developed model that considers the collection, storage, and transportation of biomass (Sokhansanj et al., 2006). The IBSAL emphasizes weather and the affects it can have on harvest and supply to the biorefinery. It also can predict the date that specific operations will be accomplished. This is based on the equipment available and utilization of weather data.

Some publications use a cost evaluation uses the standards ASAE EP496.3 and ASAE D497.7 [50] [51] and the equations within them to determine how much the harvesting cost is in $/dry Mg. These costs are determined from the equipment use, equipment travel, fuel consumption, labor costs for harvesting operations. These publications usually utilize data from field observations and the material and field capacities from field tests [52] .

Models like the MILP/GRREET [1] and the IBSAL [53] have inputs needed for the model and are listed below in Table 1.

Different harvesting costs for large square and round bale harvesting system for switchgrass are summarized in Table 2. These prices were determined by using a logistics model or an economic evaluation. A cost evaluation uses the ASAE standards [50] [51] and equations in there to determine how much the harvesting cost is in $/dry Mg. The publications that used a logistics model to determine the prices were: [1] [52] [54] - [60] .

There is a large variation in harvesting cost ranging from $4/dry Mg to $32/dry Mg for both large square and round bales. Possible reasoning for this would be because different cost evaluations used or excluded different inputs or factors. One model considers weather conditions and the number of machines [52] , whereas one does not consider either and the number of machines required is an output of the model [1] .

Although there have been many cost evaluations of switchgrass baling harvesting systems, these evaluations fail to consider field shapes when creating a logistics model. Recently, there has been empirical proof that planting perennial grasses like switchgrass on marginal parts of fields can provide sustainability benefits, but can be difficult and costly to harvest [61] . By optimizing or altering field designs that incorporate annual crops (e.g., corn, soybeans, small grains) and switchgrass, has the opportunity to improve a producer’s willingness to participate in switchgrass production [61] . Another possibility to incorporating existing agricultural field shapes into cost and or logistics models is the opportunity for producers to utilize these models as a decision-making tool. For example, those who own these marginal lands may calculate if they should sell the land to be developed or continue to be in agricultural production. Although this may be tedious, the benefit of a user-friendly, decision-making tool could help producers determine the most optimal harvesting scenario dependent on the availability of equipment, field size and shape.

To collect all the necessary information to run a logistics model for switchgrass harvesting, a variety of different data collection methods have been used by researchers. Many aspects of data recording can be done manually using stopwatch and video recording devices. New technologies utilizing Global Positioning Systems (GPS), Controller Area Network Bus (CAN Bus), unmanned aerial vehicles (UAVs or drones), data loggers and other technologies are available to aid in the data collection. Manually collected field harvesting data often is used to verify that the information is collected properly.

During field harvesting data collection, it is important to determine the path of the equipment to and from a field, its path through a field, and where the bales are being dropped during the harvesting operations. By attaching a GPS receiver to the farming equipment (on-board GPS) that is moving through the field, different aspects of the field can be captured. For example, time spent turning, stopping, or idling may be mapped along with bale drop coordinates. The information provided by the GPS data will help to verify field size, bale locations, travel speed, and distance traveled [5] . Additional verification may be done by hand using hand-held GPS devices or other surveying tools to plot various ground control points (GCPs) at field boundaries or bale drop locations.

In addition to on-board GPS receivers, drones and associated mapping software have become more popular for utilization in mapping and surveying in environmental and agricultural applications. One agricultural application of drones is determining a removal system for biomass bales from a stationary position [63] . As the baler moves throughout the field, it drops them in a non-uniform pattern, making the task of bale collection tedious and labor-intensive. The utilization of drones during bale collection presents an opportunity to determine bale location within a field, distances between bales and the best strategy for collecting the bales to reduce collection time.

In a recent study by [63] , researchers developed an UAV-based image system to scan an entire field through utilizing drone photogrammetry, which was later used to determine bale locations in a field. Drone images were processed to explore the best approach for bale collection. That study concluded that the higher flight velocity does not suggest there is higher inaccuracy, since the software used automatically adjusts velocity in accordance with weather conditions. Similarly, a higher flight height improved field coverage capacity [63] . If flight velocity and height do not significantly affect the precision of bale locating, then one could take field pictures at a faster velocity and higher flight elevation, making this action alone quicker to do leaving more time to determine bale location.

The disadvantage of just using a drone for data collection is the following data will not be collected: bale weights, moisture content, and fuel usage. A combination of utilizing a drone and in-field data collection methods may exclude some manual data verification methods like using a tape measure to measure distances between bales. Instead, one may use a drone and a handheld GPS. Most commercial handheld GPS’s have a horizontal accuracy up to 3 m if the unit receives a wide area augmentation system (WAAS) signal, if not the accuracy is 10 m [64] .