Alfalfa Development, Dry Matter Accumulation, and Partitioning After Surrogate Insect Defoliation of Stubble 1

Stubble defoliation by insects may completely suppress alfalfa, Medicago sativa L., regrowth for several days to 2 weeks. A 2‐year study was conducted to investigate the stubble defoliation for various durations after the first cutting on rates of growth, development, and dry matter partitioning of ‘Valor’ alfalfa. Stubble was defoliated by hand every second day for 0, 1, 3, 7, and 11 days following cutting, Damage was designed to mimic insect injury sufficient to cause complete delays of regrowth. Dry matter was produced slowly for the first 3 days of regrowth. Herbage production quadrupled from day 3 to day 7 and remained high after day 7. Regrowth delays of I and 3 days did not significantly delay plant development or significantly alter rates of growth and partitioning. Delays of 7 and 11 days retarded subsequent plant development and suppressed the crop growth rate (CGR) of subsequent regrowth. An l 1‐day delay reduced CGR by 18.4%, with nearly all this reduction caused by a reduction in growth of leaf‐support structures. Leaf weight and leaf area growth rates were not significantly affected by regrowth delays. Consequently, plants delayed for more than 3 days, produced more leaf area per unit of dry weight through the allocation of greater leaf weight per unit of total weight and greater leaf area per unit of leaf weight. A delay of 11 days resulted in increases of 34.5%, 16.3%, and 15.9% for leaf area ratio, leaf weight ratio, and specific leaf area, respectively. These increases resulted from the production of larger but thinner leaves, which enhanced leaf area more than leaf weight. The results support the hypothesis that leaves have priority for current photosynthesis during the partitioning of photosynthates in alfalfa resulting in the maintenance of leaf growth rates at the expense of support‐structure growth rates.