Be aware that if you didn't follow the label instructions - more than 10ml per injection site, or if you treated more than once - then you could potentially have excessive residues beyond the 28 day point. Those slaughter withdrawal recommendations are based on proper administration of a single treatment.
For example - I see folks on this board advocating the use of banamine(flunixin), just like it was candy. Approved route of administration is by intravenous injection - how many of y'all are giving it that way? Not many, I'll bet.
Slaughter withdrawal for a single IV dose of banamine is 4 days. If you give it IM (not recommended, nor is it an approved route), a single IM dose extends that recommended slaughter withdrawal period out to at least 30 days; if you gave more than one dose IM...who knows how long you'd need to wait.