Hi Mary,
The Vit C in FF is not high and there are supplements ECIR recommends for specific situations that includes Vit C. so I would not be concerned with it. However, depending on what your friend's horse is consuming regarding bagged feeds, etc. there are some issues with FF . IF this horse is only receiving hay and this supplement, then it will be very deficient in selenium.  As you know, our area of Michigan has very low selenium in the soil. FF has no added selenium. You would need to know what else the horse is eating to determine if it is receiving adequate selenium. Also, hays in this area are typically low in copper, zinc, possibly high in manganese,  FF is low in copper and zinc. The analysis on the bag is reported as ppm. An 85 gram serving provides 92 mgs copper, 249 mgs zinc, and 1.2 mgs.iodine if I did my conversions correctly. That may not be enough copper and zinc to overcome high iron. Iodine might be too low.  Depends on what else horse is eating.
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Dawn Wagstaff and Tipperary   

Saline, MI  2003

Tipperary Case History

Juniper Case history: https://ecir.groups.io/g/CaseHistory/files/Dawn%20and%20Juniper/Case%20history%20Juniper.pdf .