Influence of mechanical and chemical weed control on growth and survival of tree plantings in semiarid regions

Abstract

Weed control strategies were evaluated for Prosopis alba and Leucaena leucocephala in south Texas for site preparation prior to transplanting, during initial establishment of transplanted seedlings, and for mature plantings. In site preparation, glyphosate has been used prior to deep tillage to eliminate perennial grasses such as Cynodon dactylon, Sorghum halepense, and Cenchrus ciliaris. Moderately expensive, preemergence row crop herbicides that have short residuals (45–90 days) but low toxicity to seedlings have been examined for use at transplant. These herbicides included oryzalin, oxyfluorfen, napropamide, atrazine, linuron, metolachlor, and alachlor. The most effective herbicide was oryzalin at 2.8 kg ha −1, which gave a 30% increase in survival and a 300–400% increase in biomass for Prosopis alba and Leucaena leucocephala over non-cultivated, non-herbicide treated control plots. Tree growth following preemergence/transplant herbicide treatment was compared to mechanical weed control using a single row sweep cultivator. In 1983 and 1984 cultivation alone doubled the biomass productivity of Prosopis and Leucaena, while use of herbicides increased productivity only 30% over the cultivation-alone treatment. Long residual (180 day), inexpensive herbicides that are not particularly safe for use at transplant were examined at two rates on 2.5 y old Prosopis alba trees. These herbicides included DCPA, diuron, norflurazon, simazine, bromacil and bromacil/diuron combinations. Bromacil was phytotoxic at the lowest rate (0.25 kg ha −1), and simazine was phytotoxic at 4.5 kg ha −1. Diuron provided the most cost effective weed control where perennial grasses were not present. The more costly norflurazon controlled problem perennial grasses Cynodon dactylon and Sorghum halepense.