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Variation in Regrowth and Its Implications for Multiple Harvest of Guayule¹

To distribute the high cost of stand establishment across several years of production, guayule (Parthenium argentatum A. Gray) is harvested after a minimum of 4 yr by digging entire plants. An alternative is multiple harvest of the shrub, cut at ground level, at an optimal interval as long as an economic yield is obtainable. for this, guayule cultivars with nearly 100% survival after each harvest are required. The objectives of this study were to compare guayule cultivars and selections in the filed for their regrowth ability after harvest, and to determine the relative contributions of under- and aboveground parts to the overall rubber yield of the plant. In a completely randomized design with three replications, 84-week-old dormant, 96-week-old actively growing, and 132-week-old dormant plants of 11 cultivars and selections were cut at ground level in February and May 1985, and February 1986. Harvest date was found to be a critical factor: an average of 49% of the plants harvested in February regrew, while only 1.4% of those harvested in May regrew. Marked differences between lines were observed in the February harvests. Approximately 97% of selection C2501 plants produced vigorous regrowth, but only 5.5% of C2551 plants regrew. Regrowth of the other entries lay within these extremes. Five of the entries were compared for the contributions of the under- and aboveground parts to the overall yield. The underground part produced 6.0 to 13.5% of the total rubber yield. If plants were harvested at 5 and 10 cm above the soil surface, 3.4 to 8.9% and 9.4 to 22.5%, respectively, of the aboveground yield was not recovered. This study identified selections that meet the demands of multiple harvest.

Key Words: Parthenium argentatum A. Gray • Natural rubber • Rubber yield • Aboveground yield • Underground yield

² Assistant research agronomist, Dep. of Agronomy and Range Science, Univ. of California, Davis, and professor of genetics, Dep. of Botany and Plant Sciences, Univ. of California, Riverside, respectively.

Received for publication March 21, 1986.