Pekins grow at nearly twice the speed of other ducks. Here is the weight chart of my current brood which consists of a Pekin, a Buff Duck and a Khaki Campbell.
You can see they are just getting now to the point where growth is not the priority. Their systems are solidifying and strengthening, feathering and such, but there are some growth breaks now. Looking at the graph, you can see that what is expected of the Pekin is FAR more than the other ducklings and ducklings are already one of the fastest growing babies out there.
I have raised two Pekins so far. Mine have gotten electrolytes, probiotics, and either brewers' yeast in their food (1 tablespoon to 1 cup ratio) or niacin (non-time release and not no-flush) in their water at 150-250mg / gallon (that is a bit more than recommended, but is what the capsules worked out to). My first Pekin had this problem for quite some time and when she would stand her legs would shake a bit - not a lot, but you could tell she was straining. She would lie down between the food and the water and turn her head back and forth between them. I moved the water a couple feet away from the food so she would have to take a couple steps between them. This both forced her to exercise and throttled her food intake. At around 6 lbs, her growth slowed and her legs got stronger. My second Pekin I started out with the food and the water separated by a couple feet. So she had to run back and forth as a little duckling. When she got to 3 lbs or so, she got "lazy" and would lay down as much as possible. She got to where she would very seldom take more than 3 or 4 steps in a row, then lay down again and would lay down for preening everything but her belly, but she never got to where her legs would shake, she was always strong getting up and down, just no endurance. She also sported a somewhat wide legged stance. She is now a little over 6 lbs and can walk fine and is starting to try to scratch her face standing on 1 leg while preening (not successful yet, but close). And her legs are getting closer together. So those are the "facts". Here is my extrapolation which may or may not be accurate. I can think, but I am not a vet and don't have all the answers, just my experience.
A Pekin is still a duck. The fact that we have selectively bread it to be such a large, fast growing duck doesn't change the limitations on muscle and bone development. In the 3-6 lb range, I think they outgrow (and this might vary a bit by who eats the most and who's genetics produce stronger muscles) their legs by a day or two. (Today their legs could lift yesterday's weight, but today's weight is just a bit too much). If they are having a hard time, because they seem a little weak, but it doesn't seem like nerve issues or deformity and they are getting their "extra" nutrients, this may be normal. Allie's (I didn't pay as much attention with Snow) wide legged stance was at max, about 1-1/2 times her shoulder width. This I think is due to two things. 1. A non-perfect surface. She is mostly on towels or carpet. Although these aren't a slick surface, they don't provide as much traction as grassy or rocky terrain. I have observed her legs sliding slowly apart, then she tries to pull them together if they get too far apart. 2. A combination of a soft skeletal system and the previously mentioned weak muscles. Until the birds are close to full grown, their skeletons are somewhat soft (that is why baby wood ducks can bounce and not break when they leap out of their trees) and haven't fully calcified yet. You just look at a 3-6 lb Pekin and you can see they are almost a gelatinous mass. So a wide legged stance itself I don't think is a problem if you can rule out actual splay leg. Splay leg happens to young ducklings or chicks (less than a week old) because of slippery surfaces. The birds' bones are really soft and if the condition is not corrected, they will "grow wrong" and have a life long problem. If your Pekin walked fine until it hit a pound or more, I think it would more likely get a pulled muscle or slipped tendon than splay leg and that would be the same type of problem an adult duck would have. If your duck CAN pull its legs in normally and has full range of motion, and the wide stance is in that 1-1/2 times shoulder width range, I think it may be for stability and compensating for weakness and a flexible rib cage.
Let the Pekin swim a lot. This takes some weight off their legs and allows them more exercise. Motivate the Pekin to walk more. Make it walk 3ft for a pea, then go back the other way for the next pea. I think Pekins are a special case where we have pushed nature to its limit. So I am not telling you to ignore all these symptoms, but rather that if you watch them very carefully to make sure nothing other than a bit of weakness is going on (no slipped tendons or balance issues - although Pekins are somewhat clumsy and prone to plowing through things instead of walking around them) and encourage your Pekin to exercise (with peas / lettuce and swimming) at around 6 lbs, the growth becomes slow enough that the legs can catch up.
I hope this is accurate and I hope it helps. It has been what I have observed so far, but again I have only had two Pekins.
@jessicascakes I have had only one KC and she has been a little spaz since day 1. I have only observed the weak leg issue in my Pekins and in one of my runners who was constipated when we got her. She was very weak for her first week of life. Once she was "flowing" normally, she was running around with the rest of the ducklings in a day or two. Hopefully the Brewers' yeast helps with yours.
@TheFinalGirl Allie's brooder mates stepped all over her when she couldn't get up easily... Now she weighs more than the other two combined... It's STOMPIN' time...
Edited by Tevyes Dad - 4/16/16 at 9:49am