Evaluation and mowing optimization of a robotic lawnmower in a Japanese pear orchard

概要

Introduction:
Commercial robotic lawnmowers have recently gained popularity in orchards. The use of such machines is beneficial because they need very little human labor and creates less pollution. Besides, they prevent humans from coming into contact with dust, allergens, and noise, etc. Commercial autonomous mowers have the potential to move between the trees and under the canopies. These mowers are programmed to operate every day; thus, the grass clippings are very small (a few millimeters) and can be left in place. These clippings are easily integrated into the soil, which leads to a higher turf quality, lower weed percentage, lower need for nitrogen fertilization, and reduction in the amount of thatch produced. However, autonomous mowers have been mainly designed for the lawn. The weed management in the orchard and lawn is not similar. Orchard weed management can be a big challenge for small robotic lawnmowers. However, very little information has been found about the working ability of robotic lawnmowers in orchards. Thus, the objective of this research is (1) to study the working ability of a robotic lawnmower in a Japanese pear orchard (2) to find field challenges of a robotic lawnmower in the orchard (3) to optimize the mowing frequency.

Methods:
We installed a robotic lawnmower and studied the working ability, field challenges, economics, work optimization of a robotic lawnmower in a Japanese pear orchard (1318 m2) at the center for international field agriculture research & education, Ibaraki University, Japan, in 2017–2021. The mowing ability of the robotic lawnmower was compared with a gasoline-powered riding mower in two different pear orchards. We also compared energy consumption, energy cost, co2 emissions, human workload (in terms of the increased ratio of the heart rate) of the robotic lawnmower with a riding mower, brush cutter, and walking mower, etc. Moreover, we compared the total mowing cost (annual ownership, repair and maintenance, energy, oil, and labor) of the robotic lawnmower with other mowers. Besides, we have pointed out potential threats, that can negatively affect the working ability of the mower, such as, fallen fruits, tree hitting, weed physiology, etc. We also proposed mowing schedules, which can reduce unnecessary wearing and maximize working performance. The optimization of the mowing schedule was prepared by the detection of threshold weed cutting height and cutting frequency.

Results:
The average weed height was lower in the robotic lawnmower-managed plots (35–87 mm) than that in the riding mower-managed plots (approximately 15–281 mm) between flowering and harvesting (March to October). The robotic lawnmower was superior to the riding mower, brush cutter, and walking mower in terms of energy consumption (6 vs. 13, 56, and 18 kWh 10 a−1 month−1), energy costs (133 vs. 206, 1122, and 277 JPY 10 a−1 month−1), CO2 emissions (3 vs. 3.4, 14, and 4.5 kg 10 a−1 month−1), and labor requirements (28 vs. 40, 604, and 62 min 10 a−1 month−1). Because the robotic lawnmower can be used without manual operation, it reduces the human workload in comparison with operating a riding mower, brush cutter, and walking mower (increased ratio of heart rate: 1.2, 1.5, and 1.4 respectively). The economic analysis reveals that the robotic lawnmower performs better than other conventional mowers in a small orchard (0.33 ha). However, for a medium-sized (0.66 ha) and large (1 ha) orchard, the robotic lawnmower is less cost-effective than riding a mower and walking mower. We observed that the robotic lawnmower experiences higher acceleration while running over small pears (33 ± 8 mm dia.) during fruit thinning periods, which can stop blade mobility. During pear harvesting, fallen fruits (80 ± 12 mm dia.) strike the blade and become stuck within the chassis of the robotic lawnmower; consequently, the machine stops frequently. Our analysis indicates that the robotic lawnmower should be operated when the field average weed height reaches 20 cm. Using this threshold height, we propose three mowing schedules: To maintain weed height at 6‒10, 6‒15, and 6‒20 cm tall, the mower should, on average, be operated for 32, 14, and 10 h week‒1 between the 13th of March and the 28th of October, between the 26th of March and the 3rd of November, and between the 5th of April and the 13th of October, respectively.

Conclusion:
It is expected, the proposed schedules would maximize the mowing efficiency in the orchard. The existing robotic lawnmower performed weed control well and showed promise for profitability in our research field. We believe that, if field challenges like fallen fruit and tree striking problems can be properly addressed, the robotic lawnmower could be successfully used in many small orchards.