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I have seen a couple of videos on youtube where people are raising duckweed and freezing it into blocks. The fish really seem to like it. If it's frozen is it safe as a fish food? I don't want it to take over my grow beds.

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I'm also stronger on fish and water (aquatic ecologist) than plants, but have research duckweed a lot for a project that I am busy with.  I have a lot of faith in its future but I think, as with algae, knowing its potential and managing production at those levels are still a bit removed in time.

Terri Mikkola said:
Kobus, I don't look at your comments as being negative AP is a learning process and I'm here to learn. I have a fairly good understanding of the water quality aspect of AP, but plants and fish are another story. I usually kill house plants, which one reason why I like AP, I won't forget to water them!
On a side not.  My Ducks love duckweed.

There are two aspects of my work with the duckweed system that is starting to make it very attractive to me.  Teething problems aside, it runs in a narrow, stable band at the moment and the yields are already looking good.  Considering that I am in winter, with sub-optimal temperatures, things can only get better.  I am trying to create a situation where I think the Lemna will not be able to cope any more, so I have introduced what I believe is an over stocked fish level.  As soon as the ammonia level left in the fish tank hits 2 mg/L, I know the filter has hit max absorption potential.  We are not there yet and the food input is steadily rising.

 

The data in the tables below come from a time period (3 months) where food input averaged 10 g per day.  In UVI and media-based terms, you need 60 - 100 g of food input per day to keep 1 square meters of aquaponic bed happy (that is their data, not sure what the LD stocking data says).  At present, 7 square meters of duckweed is humming along on the 10 g per day food input.  I recently replaced the entire duckweed stock.  After a year, I figured the original stock was looking tired.  11 days ago, I topped the filter up with duckweed, but it was only sitting at araound 70% of capacity.  In 11 winter days, it has filled the filter up plus produced 1 kg wet mass of harvest.

 

 

Although the system is sensitive (each arrow represents something I did to the system), it is steady.

 

 

The nice thing here is to see how my yields, even in estimated dry mass, far exceeds my inputs.  Will keep eveyone in touch with how things progress when the water heats up.

Hi Kobus,

Duckweed seems to have natural steroids built in to them! If this is the input/output comparison during low ebb winter temps, I'll really be interested to see what happens in higher temps. Do you have data on wet/dry mass protein for this particular duckweed in your system? Do you think the input sys. water( Tap or rain) will influence growth/protein values here?

Harold - at present, the focus is on producing the fertilizer in large enough quantities for nutrient analysis, but I would love to be able to break the testing down into components - testing the sludge and the duckweed seperately would be very useful.  I am amazed at the growth.  You clear some out the one afternoon and by the next day, it looks like it is all grown back.  That on a sniff of food.

 

The rain water is useful, as it lets me know exactly what the plants absorb.  The project can get side-tracked too easily, but then in the end, I would love to add trace elements to experiment with nutrient loads and plant chemical make-up.  I'm positive that you can make up a custom mix, add it to the system and a week or two later, your duckweed will start resembling that.  I think this is a magic plant with far more applications than what we give it credit for at this stage.  We will have to see about protein, as it can be anywhere between 6.8 to 45%, but I honestly think that when the ammonia is this low, you need to expect protein on the lower scale.  Concentrated ammonia is easy to get a hold of though, thus I can predict that if you make a custom mix (say you want to be making a tilapia diet) of chemicals, dissolve it in the water and park the duckweed in it, you will eventually succeed in making a duckweed that is an excellent fish food supplement.

Harold Sukhbir said:

Hi Kobus,

Duckweed seems to have natural steroids built in to them! If this is the input/output comparison during low ebb winter temps, I'll really be interested to see what happens in higher temps. Do you have data on wet/dry mass protein for this particular duckweed in your system? Do you think the input sys. water( Tap or rain) will influence growth/protein values here?

Hi Kobus,

(say you want to be making a tilapia diet) of chemicals, dissolve it in the water and park the duckweed in it, you will eventually succeed in making a duckweed that is an excellent fish food supplement.

 

I think you're really going somewhere here, when we try "designing" a more suitable supplement fish feed. I see no reason, if we tailor nutrient inputs properly in the growing of duckweed, its possible to approach a more complete fish food, may even become the sole food! Among the few challenges of home grown food will be the sourcing of amino acids, and even that can be done with the growing of selected algae. In aquaculture we have gotten used to the idea of pelletized aquatic life as our only route to fish farming and believe this is the only option and it is this idea that makes us 100% dependent on a very limited and dwindling marine resource. This is a very serious problem left to the very few who would dare to look!

In nature, Tilapia reach good size on stuff such as duckweed.  Only thing is that they are not pressured to do it in 168 days.  Once you stop trying to be a commercial grower on a tight timeline, all kind of options open up.  Duckweed will scavenge for almost any dissolved element in the water, including metals (unfortunately).  We are looking for all the critical plant and animal growth factors in our fish food / AP water.  Thus, if we are going to feed or fish lots of duckweed, we may as well soup up the duckweed by growing it and spirulina together in a pre-prepared water body.  Take a look at this breakdown - this is all the stuff that duckweed could absorb in a natural environment

Variation of content of elements,  % of dry weight

Ag
Al
As
B
Ba
Br
C
Ca
Cd
Ce
Cl
Co
Cr
Cs
Cu
F
Fe
Ga
H
Hg
J
K
La
Li

0.3 — 50 x10-6
0.000—11.4
0.2—23.5 x10-3
0.02—3.25
0.03—0.11
0.25—0.65 x10-2
30.5—43.7
0.18—4.5
<0.1x10-4 — 6.7
0.2 x 10-3
0.08—4.29
0.9 x 10-4 —1.1
0.3—17.8 x 10-3
0.4—50 x 10-3
0.2 X 10-3 - 3.2 
0.2 X 10-3
0.007—3.2
0.9 X 10-4
5.4
0.04—18 x 10-4
0.4—25 x l0-4
0.03—7.0
0.9 x 10-4
0.8—6 x 10-3

Mg
Mn
Mo
N
Na
Nb
Ni
P
Pb
Pr
Ra
Rb
S
Sb
Se
Si
Sn
Sr
Ti
V
Y
Zn
Zr

0.04—2.8
0.003—6.4
0.2—0.4 x10-3
0.8—7.8
0.03—1.3
0.2 x 10-3
0.7 x 10-4 — 0.2
0.03—2.8
0.2x10-4 — 0.02
0.4 x 10-4
traces
0.0054
0.33—7.0
0.0015—0.012
0.0018—0.012 
0.41-5.35 
0.2 - 3.6 x 10-2 
0.008 - 0.11 
0.0018 - 0.32
0.3—10 x 10-3
0.4 x 10-4
0.004—0.14
0.9 x 10-4

 

Now tell me why we cannot get away from fish meal in the end?  Three to four ingredients: duckweed, spirulina, earthworms and BSFL.  There is no problem without a solution, only variable degrees of need for finding the solution. 

Harold Sukhbir said:

Hi Kobus,

(say you want to be making a tilapia diet) of chemicals, dissolve it in the water and park the duckweed in it, you will eventually succeed in making a duckweed that is an excellent fish food supplement.

 

I think you're really going somewhere here, when we try "designing" a more suitable supplement fish feed. I see no reason, if we tailor nutrient inputs properly in the growing of duckweed, its possible to approach a more complete fish food, may even become the sole food! Among the few challenges of home grown food will be the sourcing of amino acids, and even that can be done with the growing of selected algae. In aquaculture we have gotten used to the idea of pelletized aquatic life as our only route to fish farming and believe this is the only option and it is this idea that makes us 100% dependent on a very limited and dwindling marine resource. This is a very serious problem left to the very few who would dare to look!

I believe it is only a matter of time before they will greatly reduce or even remove the need for wild caught fish meal in most fish feeds.  They have already experimented with growing trout on a fish feed where the fish meal with replaced by an algae.

Hi Kobus.

So from the chart duckweed "eat's" everything. Aquaponics at this point is really dependent Aquaponics and for this to change we need a different approach. Some people argue that it's too time consuming  as well as a "diversion" from main stream AP to develop home grown fish feed. For sustainable AP, surely we will have to include the worm bins the algae set up the BSF bins and duckweed beds as intrinsic interdependent parts of complete AP for backyard and commercial farming. These systems are going to be developed by us for us as we are going to find very little interest from anyone else! So that makes the work you do now extremely important. May be this is a whole "nother" topic altogether!

Would probably be worth starting another thread perhaps over in the feeding group to explore how to grow one's own foarage for the fish the way people do such things for their free range flocks or the way farm ponds are fertilized to encourage fish growth.

Hi TC,

I'm with you on this! Would you like to start one? Kobus is doing a lot of work here and it will be good if we get some serious minded folks involved. We have work ahead, this is not really a time to take it easy, I for one am very interested in the area of algae growing.

TCLynx said:

Would probably be worth starting another thread perhaps over in the feeding group to explore how to grow one's own foarage for the fish the way people do such things for their free range flocks or the way farm ponds are fertilized to encourage fish growth.

Go for it Harold, was your idea.

Here I recommend adding it to the

Feeding Group Discussions

 

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