Reading this with great interest - wanted to chime in.
I tested with EA the Pro Elite Grass Advantage last month - Nov 2020.
Two different bags:
Results (ppm):

Iron 1650
Copper 161
Zinc  470

Iron 1550
Copper 162
Zinc 490

The bags list min. guarantees of Iron/Copper/Zinc as 700 / 200/ 600

I asked a rep at Cargill about the (much) higher than expected iron content. 
I was told it was most probably from the Dicalcium Phosphate.  (per their nutritionist).
That the DCP was not digested or absorbed by horses but would show up on tests.  

I have the test results and could email if ECIR wants it in the files. 

Above posts mention "supplemental".   
Does that mean supplements added in feeds - or supplements we buy to add to our feeds?   
Also, Is "machinery iron" thought to be absorbed by horses?    And messes with other minerals?  

Best to you all,   (especially Dr Kellon for her years of hard work /  helping so many horses and people).
Susan and Remy in Houston
(member since 2004 for Halima Two)