When conditions are ideal, in terms of water temperature, pH, incident light and nutrient concentrations [duckweed] competes in terms of biomass production with the most vigorous photosynthetic terrestrial plants, doubling their biomass in between 16 hours and 2 days, depending on conditions. In natural or farming conditions, however, the growth rate is altered by crowding, nutrient supply, light incidence and both air and water temperature in addition to harvesting by natural predators (fish, ducks, crustaceans and humans). In addition to the above limiting factors there also appears to be a senescence and rejuvenation cycle which is also apparent in Azolla.
Vegetative growth in Lemna minor exhibits cycles of senescence and rejuvenation under constant nutrient availability and consistent climatic conditions (Ashbey & Wangermann, 1949). Fronds of Lemna have a definite life span, during which, a set number of daughter fronds are produced; each of these daughter fronds is of smaller mass than the one preceding it and its life span is reduced. The size reduction is due to a change in cell numbers. Late daughter fronds also produce fewer daughters than early daughters.
The phenomena of cyclical senescence and rejuvenation may cause considerable errors of interpretation in studies that examine, for example, the response of a few plants to differing nutrient sources over short time periods. In practice this cycle may be responsible for the need to restock many production units after a few weeks of harvesting.
The literature contains a great deal of information on the potential growth rates of duckweeds. In many early studies the growth rates were measured under controlled conditions for short periods of time. In the absence of large scale field data obtained over 12 month periods these data have been used to estimate potential production rates. The data needs to be treated with reservations as the data in Figures 4 and 5 point to serious problems in doing growth trials with duckweed under laboratory conditions.
The results in Table 4 are from research with near optimum conditions for duckweed growth. Landolt and Kandeler (1987) concluded that under such conditions 73 ton of dry matter are possibly produced per hectare per year or 20g DM/m2/d. Results up to 180 ton DM/ha/y have been recorded. Under less than optimum conditions it is more realistic to target between 5 and 20ton DM/ha/y (Table 5).
In practice the yields of duckweed often depend on the skill of the farmer in solving the problem of how to balance the mineral requirements of duckweeds and to identify with time the need for continuing and varying mineral supplementation. Waters that are high in P and K and trace element need minimal but repeated inputs of an ammonia source, keeping ammonia at around the 60mg N/l when growth and protein accretion is greatest.