Seasonal variations in chemical composition of pasture

Abstract

As part of a study on seasonal variations in the chemical composition of pastures, Sl, AI, Fe, Zn, Cu, and Mn were determined in monthly samplings at 7 sites in the lower North Island. Total element determinations were made by X-ray fluorescence spectrometry on bulked grass, clover, and (at one site only) 'other species' separates of the pasture herbage. Soil contamination was estimated from the Al content of herbage and soil and was checked by Ti analysis of a limited number of samples, and plant analyses for Si, Zn, Cu, and Mn were corrected accordingly. Corrections for Al and Fe were not attempted because of the uncertainties involved. Results for the uncorrected (contaminated) samples showed means and ranges similar to those reported in previous work. Al and Fe levels in grass and clover separates, and Si in clover, showed similar trends closely paralleling estimated soil contamination levels, with maximum levels during the period of lowest dry matter (OM) production in late winter. Uncorrected Si levels in grass were high throughout the year but corrected values showed a tendency to peak in summer. Of the trace elements, Zn showed high levels in ‘other species’ (herbs) at the one site these were sampled, but grass and clover separates gave variable results with no clear seasonal trends. Low values appeared to be associated with sites or periods of high OM production. Cu levels showed even less seasonal variation, but levels in clover were consistently a little higher than in grass. Mn showed the greatest variability, seasonally and between sites. Similar seasonal trends in Mn levels were observed in grass and clover separates, suggesting a soil effect on plant uptake; Mn in grass, but not in clover, was negatively correlated with topsoil pH. In general, the herbage analyses showed no evidence of a deficiency of Zn, Cu, or Mn.