dual purpose duck breed

Less than a year ago i bought 3 dual purpose ducks 2 hens and a drake. today i have about 20+ ducks all offsrpring of those nearly full grown, they are in different gardens at different family members. All because my yard is only about 1/10 of an acre and i dont want to have too much nitrogen in the soil , so i planted some jerusalem artichokes inside the stinging nettles patches. to overcrowd them, that part has worked so far but i was wondering how many of these 6-7lbs ducks i can keep sustainable on 1/10 of an acre . Some extra info they only lay 60 eggs anually , they also never fly since their wings are too small according to the rest of the body ow and the breed is " eend van vorst " no idea what that translates too in english they originated from a city near brussels(belgium). I only keep 4 hens in my garden for now but they cant eat all the pasture and they barely touch their grain when i feed them only once or twice a week.

their main purpose is being a lawn mower . but i dont see the height of the pasture go down with 4 ducks on less than half of my garden give or take 1/25 of an acre is what the live off now

so the question is how many ducks should i keep??

I don't know if you have any of the historical data on your ducks so I am including that info for you.

Origin : The story of this breed started around 1890. Herman Bertrand from Vorst nearby Brussels wanted to create a blue duck with good laying and good meat quality. At first he named his creation after himself and gave it the Latin name ‘Anas bertrandi’ meaning ‘Bertrand’s duck’. Later this duck was named after the community he lived in, Vorst (Forest in French). (So Forest ducks have got nothing to do with woods !) In 1905 the breed was launched with a collection of 30 blue ducks at an international exhibition in the Halfeeuwfeestpaleizen in Brussels. These 30 ducks were beautiful in color with a magnificent small lacing just like Andalusian chickens. In 1924 the breed club sent a collection of 24 high quality Forest ducks to Crystal Palace show in London. After this there were lots of demands for Forest ducks from other countries. At a certain moment the city council of St-Petersburg (Russia) wanted to import 350 hatching eggs. The purpose was to embellish the city parks with the blue ducks and of course also to use them for their utility qualities. Unfortunately the two world wars almost entirely eradicated this once world-famous duck.    

Characteristics : The Forest duck is an easy bird to keep and has several good utility qualities. The ducks lay a nice number of rather large eggs and the ducklings grow up very fast and without any problems.  

Appearance : The Forest is a rather large duck that weighs almost three kilos. It has a deep breast and a slightly upright carriage. The abdomen is very well developed in the females. Very typical is the blue bill in both sexes and all the varieties. Especially in combination with a white plumage this is a rather spectacular sight. The legs should be as dark as possible and preferably also have dark webs.  

Varieties : The most typical but also most striking variety is without any doubt the blue laced. These birds should completely resemble the Andalusian chickens. Unfortunately this quality birds have become very rare these days. Other recognized varieties are blue, pearl gray, pearl gray laced, chocolate, black and white.  

State : Rare. Is kept in all parts of the country and mostly the blue (laced) and black varieties.  The white are very rare and the pearl gray even rarer. The other varieties have probably become extinct. The last few years the interest in this breed has increased, also in other countries. Especially the UK and Germany are interested.

So there is that part, you have some very desirable ducks there.
Now about their feeding habits.  These ducks are going to forage more for slugs, snails and other types of foods more than they are going to eat grass. (Geese are the lawnmowers in the water fowl family).
You should get some very nice eggs from the females (1.5 large chicken eggs = 1 duck egg).  And the meat should also be very nice.

Redhawk

Indeed almost half of the hatchlings are black, i got 3 hatchlings in a special color too, ill post a pic. soon its like a very light brown with silvergreyish wings. Ow and 1 mother hen is chocolate brown.

i'll experiment with how much i will keep in my garden then