Management of Alfalfa Stubble Defoliators Causing a Complete Suppression of Regrowth

Abstract

Insect defoliators, particularly the alfalfa weevil, Hypera postica (Gyllenhal), and the variegated cutworm, Peridroma saucia (Hubner), have been reported to suppress alfalfa regrowth completely after cutting for up to 2 weeks or more. We investigated the impact of duration of a complete suppression of regrowth on the subsequent yield, quality, and economic return of alfalfa. Complete stubble defoliation for 1 and 3 days had no significant effect on any regrowth parameter. Stubble defoliation for 7 and 11 days delayed plant development and reduced dry-matter and nutrient yields, but increased forage quality as measured by crude protein content and in vitro digestible dry matter. First bloom, however, was delayed several days less than the actual duration of defoliation. Stubble defoliation also reduced dry-matter and nutrient yields at first bloom, but forage quality at first bloom generally was similar in defoliated and nondefoliated plants. Hence, dry-matter production was affected more than plant development and forage quality, with quality being affected primarily by relative delay in plant development. A replacement feed-cost analysis was conducted for alfalfa harvest systems, for which cutting was based either on plant stage or calendar date. Results of this study demonstrate that, depending on commodity price and harvest system, alfalfa can withstand complete stubble defoliation for 2.6–5.7 days before control of defoliators would be justified.