Development of herbicide control options for Prosopis velutina as part of an integrated control strategy

Prosopis spp. are prolific seeders with estimates of seed production reaching from 630,000 to 980,000 seeds per mature tree per annum (Felker 1979; Harding 1988). Prosopis velutina shrubs have been recorded to produce up to 5,000 seeds per year (Glendening and Paulsen

1955). The plant has the ability to establish on disturbed or bare soil and can develop into very dense infestations. The plant often grows on scalded areas devoid of other vegetation and out-competes native herbage for moisture and light.

The plant is armed with long axillary spines; it branches from the base and has long arching stems. Its leaves are not commonly eaten by sheep and bovine breeds. Consequently, it forms dense stands that restrict the movement of stock, particularly around watering points, and this interferes with mustering.

There are many options for control of mesquite with herbicide application being integral in a management suite of mesquite control initiatives. Other control methods, which limit its distribution and spread, include mechanical removal, grazing management, competitive pasture

establishment, biological control, and a burning regime where adequate fuel is available.

The thesis discusses merits of the Prosopis species. Commonly known as mesquite or prickly bush, the Australian pest plant is endemic in the southern United States. The description then turns to how the weed was introduced into Queensland at both Quilpie and Hughenden in the north of the state. Some detail is given to its distribution and the effects on primary producers and the environment, and on the cost of control initiatives conducted through the Queensland Department of Natural Resources and Mines. 

Scientific findings from the research process are considerable given the base knowledge when the mesquite research project was revived in 1989. Initially, a research review of available literature was conducted revealing a large pool of knowledge from the United States of America. Their research findings on foliar applied herbicides have resulted in a high utilization of tank mixes principally containing triclopyr + clopyralid formulations sometimes with hydrocarbon additives. A more comprehensive summary of field research findings for Queensland is compiled as part of this thesis (Chapter 8) and presents some herbicide control options and constraints that may qualify their performance. Basal stem research in North America has produced many adopted recommendations. There have been numerous experiments on controlling mesquite with herbicides with many referred to in Control of Mesquite in the USA (Chapter 1). 

Chapter 2 presents a comparative analysis conducted to determine the different effects of applied herbicides to targeted young mesquite when applied in early summer compared to application in autumn. Formulations of fluroxypyr as well as metsulfuron methyl did not prove

efficacious in this trial. For the first time glyphosate proved to be a valuable herbicide in controlling mesquite when applied in autumn. Triclopyr + picloram and triclopyr alone proved to be more effective following wet conditions when applied in the early summer compared to the autumn application. However, treated plants growing adjacent to continuous paddock ponding were able to recover. Clopyralid exhibited high efficacy in both early summer and autumn treatment applications. 

Following poor control of Prosopis velutina at Quilpie by treatments recommended for control

of Prosopis pallida, a potted plant pilot trial was conducted to determine if the two species responded similarly (Chapter 3). The results indicated that Prosopis velutina was ineffectively controlled by herbicide application, at standard rates, compared to the control of Prosopis pallida. The follow-on replicated trial detail in Chapter 4 indicates a similar response for both

species. The effects of the treatments indicated differing susceptibility between the two species,

with metsulfuron, fluroxypyr, 2,4-D + picloram and triclopyr + picloram demonstrating the most pronounced differences compared to the glyphosate formulations here negligible difference occurred. A re -application of all herbicides to the surviving plants, and to a control

group, indicated that susceptibility can decrease when a follow-up application is in autumn and the time since initial application is short. This trend was particularly noticeable for P. velutina where previous sub -lethal damage prevented effective herbicide action. 

More fieldwork was conducted following this shade -house work as a prolific growth phase occurred in Quilpie mesquite in the early summer of 1999 (Chapter 5). Four glyphosate, and four triclopyr tank mixes and a control set out in each of three habitats made up the 27 plots evaluated in this aerial herbicide experiment. The triclopyr + picloram formulations at 5 and 7 L ha -1 with the addition of paraffinic oil (582 g L-1) and non-ionic surfactants (208 g L-1) produced the most efficacious and uniform results although triclopyr treatment with the addition of 1000 g L-1 alcohol alkoxylate produced similar results. However, the long-term effect was compromised because of the lack of a residual component in this formulation. 

A more manageable result was obtained when larger older plants were foliar overall sprayed using truck -mounted high pressure equipment (Chapter 6). A total of 33 treatments were assessed after each of four assessments of various herbicide mixtures. A blocking factor was plant density. Glyphosate tank formulations were consistently more efficient than other treatments, at all densities, except in combination with metsulfuron methyl. In the low density plots flumetsulam 0.10 g L-1 + glyphosate 3.60 g L-1 tank mix performed significantly better

than other treatments between the second and third applications and the third and fourth applications. Treatments containing clopyralid also showed high efficacy.  

A further experiment using the most common method of control (basal stem application technology) was conducted using dieseline as the herbicide carrier in March 1995 (Chapter 7).

The higher strength treatments with dicamba and triclopyr formulations showed the most activity irrespective of plant size (> 1.5 m or < 1.5 m) or water availability with the higher strength of each formulation, dichloromethoxybenzoic amine @ 10 g a.i. L-1 and triclopyr butoxyethyl ester 10 g a.i. L-1, being the most efficacious and not showing significant differenc from each other. 

In conclusion a three-year cycle planner was developed incorporating best control strategies researched within this thesis (Chapter 8). Suggestions were also put forward covering grazing management to enhance the effect of control strategies.