|Abstract or Summary
- Pastures consisting of mixtures of subclover (Trifolium
subterraneum) and grasses have responded to sulfur fertilization
on many sites in Douglas County, Oregon. The objectives of this
study were to examine specific changes in forage quality which occur
as sulfur is applied in excess of the amount required for maximum
yield of dry matter.
Plant samples and yield data were obtained from field plots
treated with 0, 10, 20, 40, 80 and 160 pounds of sulfur per acre in
the form of gypsum. Samples were then examined for species
composition, total nitrogen, total sulfur and in vitro digestibility.
Dry matter yields were not significantly increased by sulfur
application. However, the percentage of clover in the forage changed
significantly. The amount of clover increased from 42% in the check
plot to 81% when 20 pounds of sulfur per acre was applied. As the
sulfur rate increased up to 160 pounds per acre, the percentage of subclover declined to 65%. Subclover has a higher requirement for
sulfur than the grasses. This is reflected by the sharp increase in
clover with the application of 20 pounds of sulfur per acre. At higher
rates of sulfur application, the companion grasses became competitive
with the clover, apparently due to the addition of nitrogen to the
plant community through biological fixation.
The increase in nitrogen and sulfur content with increasing
sulfur fertilization was highly significant for both the grass and
The increase in the nitrogen content of the grass from 1. 2%
in the check plot to 1.8% at the rate of 160 pounds of sulfur is
attributed to underground transfer of nitrogen from the clover to
the grass. The sulfur content increased at a more rapid rate than
did the nitrogen content which resulted in a narrowing of the nitrogen
to sulfur ratio. The nitrogen to sulfur ratio narrowed from 14:1 to
9:1 in the grass, from 22:1 to 13:1 in the clover and from 18:1 to
12:1 in the forage as sulfur application was increased from 0 to 160
pounds per acre.
Average digestibility as measured with the in vitro technique
was 36 and 49% respectively for grass and clover. Digestibility of
the forage increased significantly with sulfur applications, while the
digestibility of the grass or clover measured separately was not
changed. In summary, sulfur fertilization influences the quality of
subclover-grass forage largely through changes in species composition,
nitrogen content, and by narrowing the nitrogen to sulfur
ratio. The increase in forage digestibility is due primarily to the
change in species composition and nitrogen content.