Techniques to assess mineral availability in forages and mineral sources for beef cattle

A 4 yr study was conducted to determine the mineral element concentration of smooth bromegrass and mixed warm-season grasses. Liver biopsies were used during production cycles to determine trace element status. Ruminally fistulated cows were used to determine diet selection and estimate in situ mineral disappearance of clipped grass. Mineral elemental concentrations in warm season grass did not change with maturity. Copper and Fe concentrations were higher in smooth bromegrass leaves than stems. Increased maturity decreased in situ disappearance for Zn and Cu in smooth bromegrass and warm season grass. Cows did not select a diet different in trace element concentration than clipped samples. Results indicate smooth bromegrass and warm-season grass at this location can maintain trace element status of the beef cow in the absence of trace element supplementation. Two studies were conducted to compare elemental concentrations in the liver biopsy specimen with the same site removed at death of the animal. Five beef cows were liver biopsied and slaughtered, liver recovered, and cut into 20 sections. An area within 8 cm caudal and cranial to the portal vein was different for Cu, Zn, Mn, Fe, Mg, and Ca than the remainder of the liver. Liver biopsy mineral element concentrations were lower than whole liver concentrations of Cu, Zn, Mn, Mg, and P, and greater for Fe, Ca, and Na. Eight lambs were liver biopsied and euthanized by lethal injection or a simulated slaughter within 10 min. No difference was detected between the biopsy and the biopsy site post-slaughter, and Mg and P were higher in the biopsy of necropsy lambs. In beef heifers with chronic indwelling catheters, portal Cu concentrations were greater for heifers fed 200 mg CuSO$\sb4$ than 400 mg CuSO$\sb4$ and arterial Cu concentrations were greater than 200 mg Cu-proteinate. More of the 400 mg of Cu from CuSO$\sb4$ was absorbed than 200 mg Cu from Cu-proteinate, and slightly more of the Cu-proteinate was absorbed than same level of CuSO$\sb4.$ In a second study, increasing dietary protein from 5.7% to 11% increased absorption of Cu from CuSO$\sb4.