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Forage Quality for Sheep and Chemical Composition Associated with Sulfur Fertilization on a Sulfur Deficient Site¹

Improvements in forage quality and yield have resulted from application of S fertilizer. The purpose of this study was to relate forage composition changes associated with S fertilization to quality, as measured by lamb growth, efficiency of feed eaten by lambs, in vitro digestibility, and blood serum sulfate levels. Five rates of S were applied on a Sutherlin soil series (Ultic Haploxerolls) in each of 2 successive years to two sets of plots, one set seeded to subclover (Trifolium subterraneum L.) (designated "clover-grass"), the other to ryegrass (Lolium multiflorum L.). The ryegrass plots were also fertilized with N, and all plots received applications of P and K. The forage was harvested as hay, ground, pelleted, and fed to lambs in two feeding trials with an average duration of 60 days.

Sulfur concentrations ranged from 0.13 to 0.24% in the clover-grass and from 0.09 to 0.22% in ryegrass. Protein ranged from 10.3 to 13.1% in the clover-grass, and from 6.3 to 8.0% in the ryegrass. The N/S ratios ranged from 8 to 13 for the clover-grass and from 6 to 13 for ryegrass.

Application of S to the soil increased the average daily gains (ADG) of lambs for the two trials, from 141 to 186 g for the clover-grass, and from 32 to 84 g for the ryegrass. Ninety percent of the maximum ADG (denned as a critical value) was obtained at 0.19% S in the clover. A critical value is not given for ryegrass since ADG increased with each increment of S added, including the highest level of S applied. This was likely due to N being more limiting than S in the diet of the lambs. Blood serum sulfate-S in the lambs was more closely related to % S in clover-grass forage (r² = 0.76) than to % S in ryegrass (r² = 0.50). A good relationship (r² = 0.90) was found between serum sulfate-S level and sulfate-S intake for both forages. Blood sulfate was related to growth rate of lambs by a negative exponential function, with estimated critical values of 30 and 39 µg/ml, respectively, for the ryegrass and the clover-grass hay pellets.

An average of 11 kg feed/kg gain was required for sheep on clover-grass with zero S applied. This dropped to nine where S was applied. On ryegrass this ratio was 48 on the zero S treatment, but it dropped to an average of 18 with S applied.

Key Words: Sulfur concentrations • N/S ratios, • Protein • Lamb gains • Blood sulfate • Digestibility • Subterranean clover • (Trifolium subterraneum L.) • Ryegrass (Lolium multiforum L.)

² Agronomist and specialist in animal science, respectively, Univ. of California, Hopland, CA 95449.

³ Professor and staff research associate, Dep. of Land, Air and Water Resources, Univ. of California, Davis, CA 95616.

Received for publication October 16, 1981.