Seasonal variations in chemical composition of pasture

Abstract

Seasonal variations in pasture herbage for total nitrogen (N_t), total sulphur (S_t), the ntrogen/sulphur ratio ( (N/S)_t), non-protein nitrogen (N_np), total water-soluble carbohydrates (TWSC), and the N_t/TWSC ratio, were studied by monthly samplings at seven sites in the lower North Island. Trends in N_t were similar in grass and clover components of the pasture, although clover levels were generally higher than grass levels. Highest concentrations occurred in late autumn to early spring, and lowest in summer, although the summer minimum was much more evident in grass than in clover. Monthly fluctuations were similar in grasses and clovers, suggesting possible random effects from urinary return of grazing sheep. Seasonal variations in S_t were not pronounced, but levels in grasses were generally higher than those in the corresponding clovers, reflecting the greater tendency of grasses to accumulate sulphate-sulphur where the soil sulphur supply is not limiting. However, (N/S)_t ratios ranged from 7 to 16 in grasses, and from 14 to 21 in clovers, suggesting that the sulphur supply may have been barely adequate at some sites. N_nP levels were determined only in mixed herbages, and tended to follow N_t levels. N_up comprised 16–20% of Nt, with little variation between sites. TWSC levels varied considerably between sites, and from month to month, which was not unexpected for a constituent known to show wide diurnal and seasonal fluctuations. The mean levels showed some indication of a minimum in late summer to autumn, and a maximum in winter. The N_t/TWSC ratio was frequently above the reported danger level of 0.3 (for spring grass pastures in the semi-arid ranges of western U.S.A.), but its significance as an indicator of a grass tetany hazard under New Zealand conditions is not known.