Mowing is an essential cultural practice in turfgrass management removing no more than one-third of the total leaf material at any one mowing. The “1/3 rule” is used to avoid scalping and decrease physiological stress from excessive removal of leaf material (Beard, 1973). Physiological response to mowing is both positive and negative. Mowing removes photosynthetic tissues reducing the production of carbohydrates, but the plant responds and compensates with an increase in shoot density (Sheffer et al., 1978; Shepard et al., 1989; Turgeon, 1980). Leaves are consequently produced close to the ground, and in some species, the production of stolons and rhizomes can increase.
Mowing is based on height and frequency. A tall fescue lawn is generally mown once per week at 10-cm height while sports turfgrasses such as golf courses fairways are mown more often (two to three times per week) at heights as low as 1.5 to 2.0 cm (Volterrani and Magni, 2004). In general, ornamental lawns in residential and nonresidential landscapes are mown with rotary-type mowers. Depending on the kind of power supply, ornamental lawn mowers, or simply “lawn mowers,” can be divided in two broad categories: electric mowers and combustion engine mowers. Electric mowers must be connected to an electricity supply (an electric cord or a battery) while combustion engine mowers must be supplied with fuels (usually gasoline). In Italy, the most common mowers are electric mowers with the cord, for very small gardens, and combustion engine mowers. Battery mowers are more innovative but not widespread, probably for higher cost and for the limited surface generally mowed (up to 500 to 1000 m2).
Autonomous mowers are battery-powered machines (no need of electric cord), which perform mowing without requiring an operator. They can be programmed by the user to perform optimal turf maintenance and usually operate every day. Since autonomous mowers typically are programmed to cut every day, the clipping debris is smaller (few millimeters); therefore, it is unnoticeable and easily integrates in the turf to recycle N (Starr and DeRoo, 1981) and other nutrients. Autonomous mowers can provide many perceived advantages such as saving human labor; avoiding exposure to dust, allergens, and potential injury from mowing parts; and reducing pollutants (Hicks and Hall, 2000; Ragonese and Marx, 2015). The first autonomous mower was launched to the market during 1995 by a Swedish company, Husqvarna (Stockholm, Sweden) (MacRae, 2013).
The first autonomous mower was powered by solar energy, but currently all autonomous mowers are powered by batteries. Autonomous lawn mowers are sold mainly in Europe but this market has potential for other geographic areas. Autonomous mowers are mainly employed for home lawns and industrial green areas but also have potential for sports turfs (e.g., football pitches, golf courses). The maximum working area capacity varies from 400 to 5000 m2 for autonomous mowers designed for private or industrial green areas (Honda, 2016; Husqvarna, 2015; Robomow, 2016). Special autonomous mowers may have a capacity ranging from 5000 to 30,000 m2 (Etesia, 2016; Zucchetti, 2016).
The cutting deck of an autonomous mower usually consists of single or multiple cutting discs with “razor-shaped” pivoting blades (Honda, 2016; Husqvarna, 2015) or of single or multiple solid blades with three or four cutting edges (Robomow, 2016; Zucchetti, 2016). The autonomous mowers operate within a boundary wire (usually shallow buried in the soil), which creates an electromagnetic fence (Hicks and Hall, 2000). Autonomous mowers generally move randomly following linear trajectories. In other words, autonomous mowers follow straight lines until they find the boundary wire and then change direction. This kind of pattern can be very effective for lawn areas with many obstacles but leads to mowing overlaps (Ragonese and Marx, 2015). The most recent autonomous mowers designed for large areas have options of a Global Positioning System (GPS) for a “random-assisted” pattern (Husqvarna, 2015) or a differential GPS for systematic trajectories (Zucchetti, 2016). All autonomous mowers automatically return to the charging station when their batteries reach a minimum charge level (Hicks and Hall, 2000). Some autonomous mowers may be equipped with one or two extra wires, or guide wires, which help the autonomous mower to pass through narrow passages in complex areas and to reduce the time required to reach the charging station.
To date, research on autonomous mowers is limited and has primarily focused on mowing efficiency and algorithm development, not impact on turfgrass characteristics. Chang et al. (2015) developed an autonomous electric mower and tested it on an outdoor lawn to evaluate the mowing overlap and the coverage rate. Lu et al. (2014) proposed an algorithm for recognizing uncut lawn to improve the efficiency of autonomous mowers, reaching a recognition ratio between 80% and 90%. Tang and Schiehlen (2014) investigated the motion of a Husqvarna Automower® autonomous mower, derived the equation of the motion and discussed the motion strategy. The autonomous mower mows the grass forward, along straight directions. When detects the boundary or an obstacle brakes, goes backward and rotates with a random degree and starts mowing forward again (Tang and Schiehlen, 2014).
Little is known about turfgrass quality aspects and relative energy consumption of autonomous mowers compared with traditional mowers following the “1/3 rule.” The aim of this research was to compare an autonomous mower with traditional rotary mower on a tall fescue turf fertilized with different N rates. The trial was carried out in order to simulate different N availability that we can find in lawns and to determine turf quality, operative performances, and energy consumption of the two mowing systems.
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