Effects of grazing animals on the plant availability of sulphur fertilisers in grazed pastures

Abstract

Most field trials conducted in New Zealand for assessing pasture plant sulphur (S) requirements use the mowing and clipping return technique without the inclusion of grazing animals. A rotational grazing experiment and a field plot trial were conducted on a Templeton silt loam soil (Udic Ustochrept) in Canterbury using radioactive ³⁵S-labelled fertilisers to examine the effects of grazed sheep on the plant availability and cycling of applied S fertilisers. An additional plot trial was conducted to determine leaching losses of ³⁵S-labelled urine applied to pastures. The results obtained showed that maximum ³⁵S concentration in pastures treated with elemental S occurred 3 months earlier in the grazed trials compared with that of ungrazed plots. This increased herbage S was also reflected in the wool ³⁵S of ewes which grazed the pastures. In addition, the amount of residual elemental S remaining in the soil after 3 months of its application was significantly lower in grazed than in ungrazed pastures. These results suggest that grazing animals enhanced the plant availability of elemental S probably because of the effects of animal trampling and the return of animal faeces. It was also found that about 36-44% of the applied ³⁵S fertilisers and up to 68% of the applied urinary ³⁵5 were not recovered in the soil within one growing season in the field. Thus grazing animals enhanced not only the availability of elemental S, but were also responsible for substantial losses of S from pastures through animal excreta. The present data suggest that field trial results obtained from mowing plot trials without the inclusion of grazing animals are unlikely to assess adequately the plant availability of elemental S.