Graduate Thesis Or Dissertation
 

Some effects of sulfur fertilization on the nutrient value of subclover-grass forage

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  • 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 clover. 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.
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