Prosopis velutina

Hand clearance was the first method used to deal with Prosopis as a weed in the Americas. Work teams were sent into invaded pastures, all trees were felled and all seedlings and stumps uprooted. Although very effective and obtaining a harvest of firewood and possibly fence posts and poles, the operation was too labour intensive and expensive for the land owner to consider carrying it out on anything but a small scale. Hand clearing remains practical only for small land holdings of high value. Young seedlings are sensitive to fire but older trees become increasingly protected by thick bark as they mature and will resprout rapidly after fire. However, fire can be used successfully as a management tool for preventing the re-establishment of young Prosopis seedlings whilst also improving forage production, and is now seen as the most cost-effective control method for use in Texas, USA (Teague et al., 2001). Fire has been used in conjunction with other methods in the development of integrated eradication programmes in Australia. For example, spraying with herbicides produces dead wood that will ignite and support a sustained fire with more likelihood of killing the remaining trees. Marked differences were noted in the germination of seed following passage through different animals (Mooney et al., 1977);seed germination was 82% with horses, 69% with cattle, but only 25% with sheep. Replacing free ranging cattle with other livestock, particularly sheep and pigs, possibly in conjunction with other control methods, could drastically reduce the spread of Prosopis species.
Mechanical Control
Mechanical site clearance involves tractor operations developed for removing trees, in which roots are severed below ground level to ensure tree kill. These operations include root ploughing and chaining, which are often the most effective mechanical means, using a mouldboard plough pulled behind a Caterpillar tractor, or a heavy chain pulled between two machines. For root ploughing, large trees must first be felled by hand, but this treatment can be used to remove stumps up to 50 cm in diameter without difficulty and has a treatment life of 20 years or more (Jacoby and Ansley, 1991). Other advantages are that only a single pass is required. Whole site cultivation is effected leading to improved soil water conservation, and there is a chance to reseed with improved forage species. However, this method is one of the most expensive control treatments and is recommended only on deep soils that have a high potential for subsequent increased forage production (Jacoby and Ansley, 1991). The soil should be neither too wet nor too dry for effective root ploughing. Chaining involves pulling a heavy chain between two slow-moving Caterpillar tractors, with the effect of pulling over and uprooting larger trees. A second pass in the opposite direction ensures that roots on all sides are severed to ease tree removal (Jacoby and Ansley, 1991). Soil moisture is important, with soil that is dry on the surface but moist below giving the optimal conditions. If the soil is too dry, the tree stem breaks leading to coppicing, if too wet, the soil and understorey is damaged (Jacoby and Ansley, 1991). Smaller, unbroken trees have to be removed by other means. While this is an expensive treatment, it is effective where there are many mature trees. It is most widely used following herbicide application to remove dead standing trees. Clearance with a biomass harvester produces wood chips that can be sold for energy production offsetting the operational costs (Felker et al., 1999).
Chemical Control
Chemical treatments involve the use of herbicides to kill trees, with the most effective being stem or aerial applications of systemic herbicides. Effectiveness is dependent upon chemical uptake, which in Prosopis is limited by the thick bark, woody stems and small leaves with a protective waxy outer layer. The formulation and application of chemicals for trees of mixed ages and sizes within a stand is difficult. Many herbicides and herbicide mixtures have been tested. Until the banning of its use in the 1980s, 2,4,5-T was the herbicide of choice in the USA (Jacoby and Ansley, 1991) and Australia (Csurhes, 1996). Although 2,4-D provided excellent suppression of top growth, few trees were actually killed and such chemical treatments had to be applied periodically to ensure that forage yields were maintained. Infested sites often needed spraying every 5-7 years. The most effective chemical for high tree kill of P. glandulosa in the USA is clopyralid, but dicamba, picloram and triclopyr have also been successfully used, either alone or in combination (Jacoby and Ansley, 1991).