Waterfowl have a tendency to put on extra fat if given the opportunity. A reasonable amount of fat on a duck or goose in the wild is acceptable as it helps insulate them in cold water. It is also less dense than bone or muscle so they float easier in the water with extra fat. But just as with most animals, a little fat is good but too much fat is bad.
Some breeds of ducks and geese are naturally lean, and some gain weight more easily. For ducks, the Runners rarely carry extra fat. Pekins are at the opposite end of the spectrum and can easily be too fat. Khaki Campbells are closer to Runners in this regard but Rouen, Buff, Blue Swedish and Cayuga are closer to a Pekin.
An extremely overweight Embden goose. Notice her abdomen between her legs.
There is less variation in geese. But of all geese, the Chinese are the least likely to gain extra weight but the larger Embden and Toulouse are the most likely to be overweight. Other breeds are in between the lighter Chinese and heavier Embden and Toulouse.
What is the problem with excess weight? The bird's life is probably shortened and may make it more uncomfortable due to the extra weight they must carry. Remember that the legs and feet of waterfowl are not terribly strong. Add an extra 20-30% in body weight and those feet and legs will develop problems sooner due to the extra weight they are carrying.
A flock of fit Embden geese. Compare their profile with the goose above.
Farms that have breeding birds must be extra careful in controlling the weight in their breeder birds. Extra weight in a breeding duck or goose can greatly reduce egg production and fertility.
Our commercial Pekin ducks grow extremely fast and can weigh 7.5 pounds or more in seven weeks. However, the breeder birds that produce these fast growing birds must be kept on a diet starting at two weeks of age. Due to this restricted feed, a female Pekin breeder should only weigh about 7.5 pounds at 23 weeks of age. But due to this reduced feed, she will live longer and lay more eggs with increased fertility.
So if you do want to control your birds' weight, how do you do it?
1) Only feed them a certain amount once a day.
Pekin - .33-.45 lbs per day per bird with the higher amount if they are in full egg production
Runners - ..25-.35/day/bird
Other duck breeds - .3-.40 lbs/day/bird
Heavy geese .45-.6 lbs/day/bird
Chinese .35-.5 lbs/day/bird
Other goose breeds .4-.55 lbs/day/bird
2) Only allow them access to their feed a limited amount of time each day
Pekin - 2-8 hours/day (8 hours is for breeders in high egg production)
Runners - no limit needed
Other duck breeds - 3-12 hours/day
Heavy geese - 2-12 hours/day
Chinese - no limit needed
Other goose breeds - 4-24 hours/day
Birds that must forage for much of their feed are less likely to be obese. Some people use a less dense feed (fewer calories per pound of feed) but this usually does not control the weight in a duck or goose as they have the ability to simply eat more.
If you have ducks and geese simply for pleasure, it is not critical you control their weight. But if you are keeping them commercially, it is critical you monitor their weight for increased performance and improved longevity.