If the umbilicus has broken where it should - about an inch or so from the abdominal wall of the foal - and there is no fluid leaking from it, then all you need to do is dip it 3 or 4 times a day with a dilute solution of chlorhexidine. "Nolvasan" is probably available from your feed store, and should be diluted to ½% chlorhexidine (read the outside of the bottle to see what the undiluted percentage is). Once the umbilicus is sealed, you can cease dipping - usually after 3 or 4 days.
If there is fluid leaking from the umbilicus you should get your vet to look at it, as you may have a case of patent urachus, where it has failed to seal off and is allowing passage of urine to continue. This can lead to a variety of complications, and needs to be treated.
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