Manual plucking is highly labour intensive, occupying almost 70% of the total labour force on tea estates and accounting for approximately 40 - 45% of cost of production. The high labour costs and shortages and the cost of production has resulted in tea (Camellia sinensis (L) O. Kuntze) industries in central and southern Africa becoming unprofitable. The solution to this problem has been the full mechanization of shoot harvesting, however, a reduction in yield and quality has been observed with mechanical harvesting. The overall aim of the study was to determine the causes of yield and quality decline in mechanically harvested tea and if higher nitrogen application rates could mitigate against the yield and quality decline. In order to achieve these aims, field experiments were carried out at Tingamira Estate, Chipinge, Zimbabwe and consisted of seven treatments laid out in a 2 x 3 factorial plus added control experiment, in a randomized complete block design replicated three times. Three harvesting methods (hand plucking performed every 10/11 days, hand-held and ride-on machines performed every 14 days) were tested against three fertilizer application rates (265, 300 and 400 kg N ha-1 yr-1), with hand plucking (265 kg N ha-1 yr-1) being the standard.
The study showed that highest yields were produced under hand plucking (HP) across all seasons (43 945 kg green leaf ha-1 yr-1) as compared to hand-held (HHM; 35 114 kg green leaf ha-1 yr-1) and ride-on machines (ROM; 36 268 kg green leaf ha-1 yr-1 (p < 0.05). Continuous mechanical harvesting therefore reduced yield, with yield declining between 17% and 19% compared to hand plucking over the three year pruning cycle. This reduction in yield was associated with a decrease in both the number and mass of desirable shoots in each season. This was largely due to the indiscriminate removal of foliage by the machines, which resulted in the proliferation of immature shoots, with an associated increase in sink strength and competition for available photo-assimilates. In addition, the maintenance layer was depleted in mechanically harvested bushes. This was indicated by reduced fractional interception of photosynthetically active radiation in the top 10 cm in these bushes and reduced photosynthetic rates in these bushes. This suggests that these bushes were also source limited, as compared to hand plucked bushes. Therefore the changes in tea bush architecture, as a result of mechanical harvesting, resulted in changes in sink/source dynamics, which led to a proliferation of immature shoots which competed for limited photo-assimilates.
The decline in yield in mechanically harvested tea has also been associated with a decline in tea quality. In this study hand plucking resulted in a higher % of good leaf quality compared to machine harvesting treatments in the first two seasons, irrespective of N application rate. However, in the third season there were no differences in % good leaf between treatments. Tea tasters' assessments in the third season also showed that there were no significant differences in liquor colour and strength, briskness, brightness and total tea tasters' valuation between hand plucking and mechanical harvesting. However, seasonal differences were observed with higher scores and valuation being observed in the off - season (May 2013), as compared to the main growing period (February 2013). High N-rates tended to reduce made tea density and % fibre content under machine harvesting, but there were no significant differences between treatments. Based on organoleptic evaluation scores and taking hand plucking as a standard for good quality, the harvesting techniques did not show any differences in quality. An analysis of biochemical compounds, important for black tea quality, in tea from February 2013 and May 2013 showed no difference between harvesting techniques and N-application rates. However the dimers and trimers of smaller catechins produced during harvesting initiating field fermentation increased in mechanically harvested bushes and with higher N-application rates, as compared to hand plucking. This could possibly have improved quality in mechanically harvested teas. Based on these results mechanical harvesting can be used as an alternative to hand plucking, as it does not adversely affect black tea quality as previous believed.