The effectiveness of trace mineral supplementation isn't only dependent on concentration in the diet but also the bioavailability of the trace mineral once it reaches the animal's digestive tract. North Carolina State University's Jerry Spears, in a review of the bioavailability of certain trace minerals in feeds, says selenium (Se) in feeds for ruminants is more bioavailable than inorganic Se from selenite. A portion of the zinc (Zn), copper (Cu) and manganese (Mn) in plants is present as various complexes or "chelates."

"A sizable portion (20% or more) of the Zn, Cu, and Mn in forage is associated with the plant cell wall," Spears says. "A prerequisite for trace-mineral absorption is release of the mineral from feeds in a soluble form in the digestive tract."

Several studies show more than 50% and 70% of the Zn and Cu, respectively, in dried forages is rendered soluble in the rumen. Research with grass silage indicates more than 90% of the total Zn and Cu present is released in the rumen.

Another study found similar absorption of Zn in calves from radioactive ⁶⁵Zn in calves from radioactive labeled ⁶⁵Zn in ZnCl or from corn forage where labeled Zn was incorporated during plant growth. However, retention of labeled Zn at 7 days post-dosing was higher in calves fed Zn labeled corn forage compared with ZnCl.
-- Clint Peck