Question

My 10-year-old Mustang mare weighs about 1,100 lb (500 kg). I use her for pleasure riding. She’s kept outside most of the time except for four or five hours a day in the stall. In addition to pasture, she receives 1.75 lb (0.8 kg) of soaked beet pulp and about 1 lb (0.45 kg) of soaked soybean meal to provide a medium to feed her supplements. On an as-fed dry matter (DM) basis, the pasture tested 307 ppm iron. I stumbled across an article on the Internet that stated horses only absorb 15% of the iron they consume. Based on what I’ve told you, can you tell me if my mare has enough iron in her diet?

Answer

In all my years of evaluating equine diets, I have yet to see one that was deficient in iron. The consumption of grass and the bits of soil that are on the grass usually provides more than adequate iron.

The iron requirement for a horse this size, age, and workload is 400 mg per day. Your grass provides 307 mg/kg DM of forage. Your horse is probably eating about the equivalent of 2% of her body weight in pasture on a dry matter basis, so about 10 kg per day. In the 10 kg of grass, there would be 3,070 mg of iron (307 mg/kg x 10 kg = 3,070 mg). Even if only 15% is absorbed, the current diet would still meet the requirement (3,070 mg x 0.15 = 460.5 mg).

To me, what seems to be lacking in this diet is the other trace minerals like selenium, copper, and zinc. You mention feeding the beet pulp and soybean meal as a medium for supplements, but you do not mention what supplements.  I would recommend a general vitamin and mineral supplement like Micro-Max  or Gold Pellet (available in Australasia) from KERx, which would supply the trace minerals that are lacking, if you are not already feeding one.