Individual Intake of Free-Choice Mineral Mix by Grazing Beef Cows May Be Less Than Typical Formulation Assumptions and Form of Selenium in Mineral Mix Affects Blood Se Concentrations of Cows and Their Suckling Calves

The objectives of this study were to determine (1) the individual ad libitum intake of mineral mix by beef cows managed under a year-long, fall-calving, forage-based production regimen and (2) if Se form in the mineral mix affected the blood Se concentrations of cows and suckling calves. Twenty-four late-gestation (6 to 8 months) Angus-cross cows (2.7?±?0.8 years; body weight [BW]?=?585?±?58 kg) were blocked by BW and randomly assigned (n?=?8) to a mineral supplement treatment (TRT) containing 35 ppm Se as either inorganic (ISe; sodium selenite), organic (OSe; Sel-Plex®), or a 1:1 combination of ISe/OSe (MIX). Cows commonly grazed a 10.1-ha predominately tall fescue pasture and had individual ad libitum access to TRT using in-pasture Calan gates. Cows calved from August to November and calves had common ad libitum access to creep feed and a mineral supplement that lacked Se. Cow jugular blood was taken at 28-day intervals (13 periods) and calf blood was taken with cows from birth through weaning. Individual cow mineral mix (mean?=?54.0?±?7.0 g/day, range?=?97.3 to 27.9?±?7.4 g/day) and Se (mean?=?1.82?±?0.25 mg/day, range?=?3.31 to 0.95?±?0.25 mg/day) intakes were affected by period (P?P?>?0.30). Cow blood Se (0.109 to 0.229?±?0.01 μg/mL) was affected (P?P?