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.
