.... No easy, simple convenient way to measure the excess food / manure waste and excess plant growing nutrients when using original 32% protein vs higher 40%+ protein feed...
It is easy to calculate. BG are 80.5% water what remains is proportioned nutrient properties. Analyses I have read put protein% of the dry weight around 72% and phosphorus between 2.5% and 3%. So if you know how much it grew you can make an estimate of what dry weight of the gain is protein and what is phosphorus.
If the FCR of a 42% feed is 1.5 then 1.5 lbs of feed gains 1 lb of wet BG of which .195 lbs is the dry matter of the fish. The rest is water. If 72% of the .195 lbs is protein then .1404 lbs of the BG's 1 lb gain is dry matter protein. To gain this .1404 lbs of dry matter protein ... the BG ate 1.5 *.42 = .63 lbs of dry matter protein. So the BG eats 4.5 times the weight of protein that it gains. All the rest is waste. The protein assimilation efficiency at 1.5 FCR is roughly 22.2% the consumption. If the FCR of a 42% feed is 1.8 it is only 18.6% efficient. To really understand the efficiency on that first pass .. one must clear wastes and not allow them to generate food or be consumed again, only then will the direct single pass FCR be understood. I recall Kenny sharing that in clean water tanks with water exchange his best FCR with YP was around 2.2. But YP may be more dry matter dense than BG so that conversion factor could be better than just the number might otherwise suggest.
We can do a similar estimate of the assimilation of phosphorus. The .195 lbs of BG dry matter gain will contain 3% phosphorus or .00585 lbs and the BG will consume 1.5 *.01 = .015 lbs of phosphorus in a 1% phosphorus feed. So assimilation efficiency is around 33% for a feed that converts at 1.5 to 1 and is 1% phosphorus.
So the point is we don't have catch and measure the pooh in order to understand how much waste there is. We only need to know what goes in the fish... and to know what stays in the fish ... to understand what exits the fish.
