Should I be concerned

Hi  I am a first time gosling owner.  I have 5.  One 7 week old chinese gosling, two 5 weeks old Africans and two four week old africans.  They are eating a 20% protein starter that has some scratch mixed in.  They graze for most of the day only going inside to eat feed in the am, maybe in the afternoon and then just before bed.

The smallest of the 4 week olds just doesn't seem to be growing.  It is doing well, eating great.  Today I actually went outside to see it grab ahold of the fluff on the head of one of the 5 weeks old and not let go for a bit.  It has gone after the new chickens (4 month old orpingtons)  I would not be worried except it seems so small.

This is the one I'm worried about.  Not the greatest view but its almost half the size of the other.  These 2 are from the same hatch.  The one on the left is almost as big as the 5 week olds who are almost as big as the 7 week old chinese.  Again it is doing great.  Should I be concerned because of growth rate?

Thanks to all of you.  It is a fiesty little thing so I'm hoping that the size will mean that it is female. 

Celtic-I will look for rolled oats.  The small feed stores around here don't offer much variety.  I can only find unmedicated feed at TSC.  Would the rolled oats be in with horse or cattle feed? 

I looked on TSC website and they have rolled oats the description says

"Producer's Pride® Rolled Oats is nothing but clean, wholesome oats, rolled to crack the hull and improve digestibility. It is designed as an energy source for mature cattle, sheep, goats, and maintenance or lightly-worked horses on good quality forage and a vitamin/mineral supplement. Grains are low in potassium which is good for horses with HYPP needing low-potassium diets. Non-fortified.

Guaranteed Analysis:
Crude Protein (min.) 9.50%, Crude Fat (min.) 4.00%, Crude Fiber (max.) 13.00%, Ruminant meat and bone meal free."

Would this be okay to buy?

edited to add

They also have this

"Producer's Pride® Oats is nothing but clean, wholesome oats for pure grain goodness. It is designed as an energy source for mature, cattle, sheep and goats maintenance or lightly-worked horses on good quality forage and a vitamin/mineral supplement.

• Grains are low in potassium which is good for horses with HYPP needing low-potassium diets
• Non-fortified

Guaranteed Analysis:
Crude Protein (min.) 9.50%, Crude Fat (min.) 4.00%, Crude Fiber (max.) 13.00%, Ruminant meat and bone meal free."