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

SUMMARY 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 < 0.05) hair-growth rate differences were observed among sampling periods (daily X = 0.58 ± 0.01 mm), Angus black-pigmented switch hair contained more (P < 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 < 0.01). Serum and hair Mg concentration correlated in both breed groups after removal of individual cow treatment effects (Angus: r = 0.58, P < 0.001, n = 64; Angus-Charolais: r = 0.46, P < 0.001, n = 76). Likewise, urine Mg and hair Mg concentrations correlated (Angus: r = 0.35, P < 0.05, n - 53; Angus-Charolais; r = 0.26, P < 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 < 0.10) only during midsummer, Cattle with switch hair Mg values < 25 to 30 mg (light pigmentation) and 100 to 125 mg (black pigmentation)/kg of dry matter (dm) may be hypomagnesemic. Copper concentrations in the end segment of hair plucked in early spring correlated (r = 0.46, P < 0.05, n = 23) with Cu concentrations in serum obtained in the spring. The corresponding correlation of autumnal hair and serum Cu concentration was 0.54 (P < 0.01; n = 23). Copper concentrations in autumnal hair also correlated with liver Cu concentrations (r = 0.50, P < 0.05, n = 19). Overall mean serum Cu concentrations and individual Cu concentrations (autumnal serum) correlated with liver Cu concentrations (r = 0.48, P < 0.05, n = 20 and r = 0.45, P < 0.05, n = 20, respectively). Six cows with a mean liver Cu concentration of 28.5 ± 5.7 mg/kg of dm, indicating marginal deficiency, had serum and hair Cu concentrations of 0.66 ± 0.04 mg/L and 8.7 ± 0.4 mg/kg of dm, respectively. The end segment of hair (obtained on the previous sampling date) and the corresponding intermediate segment correlated 0.51 for Mg (P < 0.001, n = 89,) and 0.36 for Cu (P < 0.001, n = 79). Over 127 days (using paired-hair segments obtained on different dates), Mg concentrations increased near-linearly with time as a result of environmental contamination. Copper concentration did not increase noticeably until the 127th day.