Summary
Pricklypear Opuntia spp. occurs over about 28% of rangeland (10.3 million ha) in Texas (southern USA) and is sometimes considered invasive. The recommended herbicide control practice in the Rolling Plains and Edwards Plateau (western Texas) was aerial spraying with a 1:l mixture of 2,4,5-T and picloram applied at 0.56 kg/ha in late spring-early summer. However, this was not always effective.
Experiments were undertaken at two sites (8 km northwest of San Angelo, 10 km northeast of Coleman), to determine if efficacy of the herbicide mixture could be improved by doubling the application rate or by spraying at night.
The 2,4,5-T and picloram mixture was applied at 0.56 and 1.12 kg/ha to a dense stand of Lindheimer pricklypear O.lindheimeri andEdwards pricklypear O.edwardsii at one site (San Angelo), and a hybridO.lindheimeri X (O.edwardii - O.phaeacantha var. major) at the other (Coleman).
Applications were made in the morning (between 09:00-11:30) and near midnight (22:30-02:00) in December 1981, June 1982, August 1982 and October 1982, to 12.2 x 30.5 m plots (3 replicates of each treatment and an untreated control).
Live pricklypear was recorded along two, permanent 32.6 m transects in each plot the day of application, and annually for 3 years after treatment.
After 3 years, live pricklypear cover on untreated plots had increased by 47% at San Angelo and by 38% at Coleman. The higher application rate killed significantly more Lindheimer and Edwards pricklypear compared to the (recommended) low rate following December, June and August applications (cover reductions of 80-98% vs. 58-95%; note: approx. values read off original graphs).
At the hybrid site (Coleman), only following December application was the higher dose significantly more effective (c.55% reduction vs.15% increase); the kill rates achieved (45-85% cover reduction) were not deemed sufficient to justify the added treatment cost or to satisfy control objectives of most land managers. It should be noted that, whilst the hybrids appeared more resistant to herbicide treatments, soil at Colemand had 13% higher clay and 1.6% higher organic matter contents than at San Angelo; efficacy of picloram (a soil-active herbicide) decreases as soil clay and organic matter contents increase.
Night treatments killed significantly more pricklypear than daytime treatments only following June application (averages of both application rates: San Angelo - 85% vs. 50% reduction in live cover; Coleman - 50% decrease vs. 5% increase).
At both sites, pricklypear was most susceptible to applications in August and October, and least susceptible in June.