Seasonal variations in chemical composition of pasture

Abstract

Seasonal variations in pasture herbage for total N (Nt), total S (St), the N/S ratio ((N/S)t), non-protein N (Nnp), total water-soluble carbohydrates (TWSC) and the Nt/TWSC ratio, were studied by monthly samplings at 7 sites in the lower North Island, New Zealand. Trends in Nt 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 St were not pronounced, but levels in grasses were generally higher than those in the corresponding clovers, reflecting the greater tendency of grasses to accumulate sulfate-S where the soil S supply is not limiting. (N/S)t ratios ranged from 7-16 in grasses, and from 14-21 in clovers, suggesting that the S supply may have been barely adequate at some sites. Nnp levels were determined only in mixed herbages, and tended to follow Nt levels. Nnp 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 Nt/TWSC ratio was frequently above the reported danger level of 0.3 (for spring grass pastures in the semi-arid ranges of western USA), but its significance as an indicator of a grass tetany hazard under New Zealand conditions is not known.