Effects of Phosphorus and Sulfur Fertilization on Subclover-Grass Pasture Production as Measured by Lamb Gain

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Establishment and maintenance of highly productive subclover (Trifolium subterraneum L.) pastures in northern California are often dependent on fertilization with P and S. Little work has been reported, however, on lamb growth responses to P and S applications on subclover. The objectives of this study were to measure the effect of P and S fertilization of subclover-grass pasture on forage production, botanical composition, average daily gain (ADG), lamb gain per acre (LGA), and sheep grazing days after lambs were removed, and to relate animal weight gains to botanical and chemical composition. Lambs were weaned in late February at approximately 60 d of age, with an average weight of 47 lb, and placed on 1-acre pastures. There were eight treatments: four levels of P (zero to 50 lb/acre), at two levels of S (zero and 88 lb/acre), replicated twice. Lambs were set-stocked with an equal forage allowance (pounds forage per lamb) across treatments. The experiment was repeated for 6 yr. There were significant differences in results between the check and applied P treatments, but not between the three P application rates with or without S. Therefore, the three P treatments were combined, as were the three PS treatments, for comparison with the check and S-only treatments. Fertilization with P and S increased (P < 0.05) ADG, percent subclover, forage N (protein), and decreased (P < 0.05) fiber in the first years of the study. Differences were not apparent later. Multiple regression and pathway analysis of the data indicated that percent subclover in the pasture, and the associated increase in N and decrease in ADF, were the most important factors contributing to ADG. Average 6-yr forage on offer was 1000, 1440, 1470, and 1630 lb/acre (LSD [0.05] = 99), while the LGA values were 250, 380, 410, and 440 lb (LSD [0.05] = 33) for the check, P, S, and PS treatments, respectively. Subsequent to the lamb use there were averages of 129, 275, 288, and 296 additional sheep days per acre grazing (LSD [0.05] = 16) for the respective treatments. Increased forage and subclover production and % N were the most important factors influencing LGA. Under the conditions of this study, grazing early-weaned lambs on subclover pasture fertilized with P and S appeared to be very worthwhile.