Fall Harvest Management of Alfalfa. II. The Implications of Photosynthesis, Respiration, and Taproot Nonstructural Carbohydrate Accumulation on Fall Harvest Management

Abstract

AbstractAlfalfa (Medicago saliva L.) harvests may be delayed by rainfall, environmental conditions that retard growth, or unfavorable weather for hay curing. Fall harvests may be delayed until dates that may affect winter survival. Harvests made between 20 September and 20 October have generally been considered detrimental to alfalfa stands in geographical areas such as western Virginia, presumably because regrowth depletes reserves needed for winter survival and spring growth. The objective of this study was to determine if photosynthesis following late harvests can help offset losses of total nonstructural carbohydrates (TNC) in taproots and allow more flexible fall harvest management than is currently recommended. ‘Arc’ alfalfa was managed to achieve 50 d of growth prior to final fall harvests made 16 September or 2, 16, or 30 October for 2 yr. The soil was a mixture of Landisburg silt loam (fine‐loamy, mixed, mesic Typic Fragiudult) and Greendale silt loam (fine‐loamy, siliceous, mesic Fluventic Dystrochrept). Gross photosynthesis, plant respiration, and TNC were measured at 2‐wk intervals in the fall following each final fall harvest. In a parallel study, three levels of shade were imposed following each fall harvest date to evaluate the influence of photosynthetic rate on TNC levels and plant persistence. Fall regrowth and plant maintenance were not dependent on root TNC accumulation, and TNC levels ranged from 234 to 295 g kg−1 by 15 December. Photosynthesis offset respiration losses for regrowth and maintenance during the fall. High photosynthetic rates (1.0 to 19.9 μmol CO2, m−2 s−1), compared to respiration (0.1 to 4.2 μmol CO2, m−2 s−1), on a land‐area basis were possible due to temperatures occurring within the optimum range for photosynthesis of alfalfa during 68% of the daylight hours from September through November. Slow regrowth, low respiration rates, and relatively high photosynthetic rates resulting from cool temperatures limited TNC losses following fall harvests. This favorable environmental period lasts much longer in Virginia than at northern latitudes, where most fall harvest management research has been conducted, and allowed sufficient recovery of alfalfa harvested with 50 d of growth in the fall.