Effects of defoliatingStipa tenuisandPiptochaetium napostaenseat different phenological stages: root growth

During 1991–1993, root growth after a single annual defoliation at different phenological stages (vegetative or during early, late or post-internode elongation) to 5–7 cm stubble height was determined on field-grown plants ofStipa tenuisandPiptochaetium napostaense, two important forage grasses in the semi-arid, temperate rangelands of Central Argentina. Nondefoliated plants served as controls. Root growth was estimated by root length density measurements obtained through destructive harvesting, and by using a root periscope to monitor changes in the number of roots intercepting glass tubes buried in the soil. The root periscope allowed registration of the angle of each root interception, which made it possible to measure the root system dynamics (root appearance and disappearance). The effects of defoliatingS. tenuisandP. napostaenseat different phenological stages on root growth varied with the methodology used. Root growth of both species was not reduced by defoliation when it was estimated from root length density determinations. However, it was lower (p< 0•05) on plants which were defoliated late compared to those not defoliated during the growing season when it was estimated using the root periscope. Lack of agreement in the results obtained by the two methods could be associated with various problems inherent in the use of the root periscope. This instrument was, however, useful in providing a rapid evaluation of root system dynamics (through root appearance and disappearance) in both species. A greater cumulative root appearance and disappearance, for example, occurred after plants were defoliated early rather than late in the growing season. This may have contributed, at least in part, to the overcompensation in forage production which was observed on plants defoliated at the vegetative stage during 2 consecutive years of study.