PERSISTENCE OF TROPICAL FORAGE GRASSES IN SHADED ENVIRONMENTS

The sowing of improved tropical forages in tree plantations can increa se animal productivity considerably, but planted grasses often fail to persist under grazing, resulting in unstable pastures. An experiment was conducted in Brisbane, Australia during the summer of 1989/90 to i nvestigate whether the failure of tropical grasses to persist in shade when regularly defoliated is related to their growth habit. Two shade -tolerant grasses of contrasting growth habit (prostrate Paspalum wett steinii v. erect Paspalum malacophyllum) were grown in simulated sward s at light levels of 100, 50 and 20% of full sunlight and defoliated e very 2 or 4 weeks. Shading and frequent cutting reduced total dry matt er yield equally in the two grasses. Shading also reduced tiller densi ty. Paspalum wettsteinii persisted poorly under frequent (2-weekly) de foliation in shade. Plant survival rate of this grass was 66% in the 2 -weekly defoliation-20% light transmission treatment, compared with 94 % in P. malacophyllum. The poor survival of P. wettsteinii was related to its high allocation of total yield to roots, low tiller density an d low total nonstructural carbohydrate yield in residual biomass after defoliation. The inability of P. wettsteinii to adjust in shade to fr equent defoliation led to a continual decline in photosynthetic area a nd light interception, resulting in the depletion of energy reserves, increased susceptibility to Rhizoctonia fungal infection and ultimatel y to death. It was concluded that growth habit per se cannot be used a s a simple indicator for selecting tropical grasses for shaded environ ments. Instead, morphophysiological adaptation to defoliation in shade was critical for plant persistence.