Switch hair as an indicator of magnesium and copper status of beef cows.

Samples of switch hair, blood, and urine were obtained periodically over 5.5 months from 11 Angus and 13 Angus-Charolais cows grazing either all-grass or grass-legume swards. Liver samples were obtained at the end of the study. Hair growth rate and mineral concentrations in switch hair (magnesium [Mg], copper [Cu]), blood serum (Mg, Cu), urine (Mg), and liver (Cu) were determined. Significant (P less than 0.05) hair-growth rate differences were observed among sampling periods (daily mean = 0.58 +/- 0.01 mm). Angus black-pigmented switch hair contained more (P less than 0.001) Mg than did the light-pigmented Angus-Charolais hair. The effect of season was observed on hair Mg and Cu and on serum Mg (P less than 0.01). Serum and hair Mg concentration correlated in both breed groups after removal of individual cow treatment effects (Angus: r = 0.58, P less than 0.001, n = 64; Angus-Charolais: r = 0.46, P less than 0.001, n = 76). Likewise, urine Mg and hair Mg concentrations correlated (Angus: r = 0.35, P less than 0.05, n = 53; Angus-Charolais: r = 0.26, P less than 0.05, n = 63). Sward type had a pronounced effect on serum and urine Mg concentrations and a slight effect on hair Mg concentrations (P less than 0.10) only during midsummer. Cattle with switch hair Mg values less than 25 to 30 mg (light pigmentation) and 100 to 125 mg (black pigmentation)/kg of dry matter (DM) may be hypomagnesemic.(ABSTRACT TRUNCATED AT 250 WORDS)