DARRELL N. UECKERT, Texas Agricultural Experiment Station, Texas A&M University Agricultural Research & Extension Center, San Angelo, TX 76901.
Abstract: Pricklypears (Opuntia spp.) have many adaptations for survival in arid and semiarid environments and they occupy millions of acres of Texas rangeland used for livestock and wildlife production. Their status as a nuisance versus a valuable plant varies among resource areas and among ranchers, depending upon the frequency and severity of drought, local customs, the type of livestock enterprise, pricklypear abundance, knowledge of pricklypear’s importance to wildlife, and the importance of wildlife in the ranch enterprise. Technology is available to effectively manipulate the abundance and distribution of pricklypear to optimize the productivity of rangeland for livestock production, wildlife production, or both livestock and wildlife production. Pricklypear control can positively or negatively impact wildlife habitat, depending upon the control method used, size of the treated area, pattern of application, efficacy of the treatment, other plants available in the habitat, and the habitat requirements of the wildlife species of interest. Pricklypear’s propensity to reproduce vegetatively, its rapid growth rate, the availability of various growth forms, and its tenacity make it an ideal candidate as a plant material for restoration of wildlife habitats in areas where it is not sufficiently abundant.
Pricklypear (Opuntia spp.) and other plants in the Cactaceae family are of New World origin and occur from northern Canada to the Strait of Magellan. Their greatest development in both diversity and numbers is within the Tropic of Cancer in Mexico and the Tropic of Capricorn in South America (Benson 1982). There are about 1,000 Cactaceae species from Canada to southern South America (Correll and Johnston 1970). These plants have been introduced into many other parts of the world and are now cosmopolitan. Pricklypear occurs on about 30.7 million acres of the rangelands inhabited by wildlife and livestock in the western two-thirds of Texas (Soil Conservation Service 1985), and it is often a dominant component of the vegetation. Moderate-to- dense stands (11 to 100% canopy cover) occur on about 1.9 million acres.
Other Cactaceae also common on Texas rangelands include tasajillo (Opuntia leptocaulis) (also called pencil cholla, desert Christmas cactus, turkey pear, and tesajo) and tree cholla (Opuntia imbricata) (also called walkingstick cholla, cardensia, and coyonostle). Tasajillo occurs on about 11.7 million acres of the rangelands in Texas while tree cholla occurs on about 4.3 million acres (Soil Conservation Service 1985). Tasajillo and cholla commonly are associated with pricklypear.
Pricklypear’s status as a “weed” versus a valuable plant varies among the various regions of the State, and among landowners, depending upon their management objectives (Lundgren et al. 1981). It’s status as a “problem” species is most prevalent in the Edwards Plateau, Rolling Plains, and Cross Timbers resource areas, and especially within selected areas within these regions. Almost 50% of the ranchers in the Edwards Plateau experience moderate-to-serious livestock health problems due to pricklypear.
The prevalent health problem is in sheep and goats flocks. The small spines (glochids) on pricklypear fruits cause ulceration and bacterial infection in the lips, tongue, palate (a syndrome referred to as “pearmouth”), and throughout the gastrointestinal tracts of these small ruminants (Merrill et al. 1980). Rumen impaction by the small, indigestible seeds of pricklypear is also a common problem in sheep and goats. Pearmouth has also been diagnosed in white-tailed deer during droughts in the Edwards Plateau, as has mortality of cattle caused by peritonitis resulting from pricklypear glochids penetrating the abomasum (John Reagor, Texas Veterinary Medical Diagnostic Laboratory, personal communication). These problems in white-tailed deer and cattle have not been observed in the Rio Grande Plains of South Texas.
Dense stands of pricklypear interfere with forage utilization, livestock movement and handling, and compete with herbaceous forage plants for space, light, water and nutrients (Dodd 1940). Pricklypear is controlled by about 16% of the producers in Texas. It is considered to have positive values for livestock production in 60% of the counties and to have positive values for wildlife production in 80% of the counties of the State according to a 1981 survey (Lundgren et al. 1981). Pricklypear is highly regarded as an emergency fodder for cattle during winter and droughts and as an important wildlife food plant in the South Texas mixed brush region.
Morphological characteristics of pricklypear
The Cactaceae family is characterized by the following morphological characteristics: a) possession of modified axillary buds called areoles that can give rise to large spines, small spines (glochids), flowers, new stems, or roots; b) absence or reduction of leaves in most genera; c) succulent stems; d) thick, waxy cuticle; e) presence of nitrogen-containing, betacyanin pigments, which give the reddish-purple color to many of the fruits; f) shallow, wide-spreading roots; g) presence of mucilage-producing cells, which secrete a polysaccharide that hardens following mechanical injuries to the cuticle; and h) possession of epigynous flowers (flowers wholly above the ovaries) (Bloke 1980, Cronquist 1981, Benson 1982).
The pricklypears (genus Opuntia, subgenus Opuntia) are distinguished by flattened stem joints that are called cladophylls, cladodes, phylloclads, joints, or pads. Most pricklypear species in Texas have spines, although four species within the State may or may not have spines (Correll and Johnston 1970). In contrast, tasajillo and cholla (genus Opuntia, subgenus Cylindropuntia) have terete (round) stems. The spines of tasajillo and cholla are covered by an epidermal sheath, whereas pricklypear spines are not. The glochids of pricklypear are barbed and larger than those of tasajillo and cholla.
Pricklypear taxonomy
Hybridization is common among the pricklypears and this has made their classification into distinct species somewhat nebulous. Hybridization may result in several kinds of populations, including: hybrid swarms originating from outcrossing among two or more parental species; segregating introgressive populations, resulting from repeated backcrossing of hybrids with parental species to incorporate new alleles into the gene pool; uniform microclonal species of hybrid derivation, made possible through vegetative reproduction; or combinations of two or more of the above (Grant and Grant 1980). Many species of Opuntia, consequently, are not true species in the populational sense, because they occur sympatrically and hybridize freely (Grant and Grant 1979).
About 28 species of pricklypear (Opuntia subgenus) occur in the United States and Canada, including the native and introduced species (Benson 1982). Only about 13 to 20 species of pricklypear occur in Texas, depending upon whose taxonomy you follow (Correll and Johnston 1970, Weniger 1984). The pricklypears of most importance on Texas rangelands include the larger-growing species Lindheimer pricklypear (Opuntia lindheimeri), Engelmann pricklypear (O. discata), O. phaeacantha var. major and O. stricta and the more prostrate species grassland pricklypear (O. macrorhiza), plains pricklypear (O. polyacantha), and O. edwardsii.
Pricklypear physiology
Most grasses, forbs, shrubs, and trees fix atmospheric carbon dioxide through either the Calvin Benson (C3) cycle or through the dicarboxylic acid (C4) pathway, whereas the pathway in pricklypears and many other desert succulents involves Crassulacean acid metabolism (also called CAM) (Moore 1977). CAM involves two primary processes, acidification and deacidification. Acidification generally occurs at night with atmospheric and respiratory carbon dioxide being converted within the cladophylls to organic acids, primarily malic acid, which is stored in the vacuoles. Deacidification occurs during the following day when the carbon dioxide is released within the cladophyll, then used in photosynthesis. The CAM pathway allows pricklypear to take up carbon dioxide during the cool nights when relative humidity is normally greatest, and then to carry on daytime photosynthesis with the stomata closed, thereby reducing water loss through transpiration.
Pricklypears have the ability to photosynthesize yearlong, even when soil water availability is extremely low. Common beavertail pricklypear (O. basilaris) exhibited its greatest photosynthetic rates during winter when soils were moist and differences between daytime and nighttime temperatures were greatest (Szarek and Ting 1974a, 1974b). The major period of replenishment of total nonstructural carbohydrates in Lindheimer pricklypear (O. lindheimeri) in west-central Texas was during August through March (Potter et al. 1986), indicating that photosynthesis occurred during autumn through winter.
Photosynthetic rates and the replenishment of carbohydrates in the pads, crowns and roots of pricklypear are diminished when water contents of the soil and pricklypear pads are low (Szarek et al. 1973, Potter et al. 1986). Pricklypears may not open their stomata to take in carbon dioxide when under severe moisture stress, but there is evidence that they continue to photosynthesize, using endogenous gases (respiratory carbon dioxide) even when under severe water stress (Szarek et al. 1973).
The carbohydrate reserves in Lindheimer pricklypear pads, basal crowns, and roots were greatest just prior to bud break (late March to late April) in a study in west-central Texas (Potter et al. 1986). These reserves were depleted rapidly as the new pads, flowers, and fruits appeared and matured, and reached minimal levels when the fruits and new pads grew to full size. The mature pads and basal crowns of Lindheimer pricklypear appear to be the major storage organs for total nonstructural carbohydrates.
Ecological adaptations of pricklypear
Pricklypears have many adaptations which give them an ecological advantage over most other plant species in our rangelands and deserts. They can reproduce both vegetatively and sexually, have extremely effective methods for water conservation, are protected from herbivores, and they can photosynthesize essentially year round.
Pricklypears are prolific seed producers with fruit and seed characteristics that ensure dissemination of the seeds and recruitment of seedlings. Each mature pad is capable of producing one to several fruits, and each fruit contains a large number of seeds. Lindheimer pricklypear fruits collected in Crockett County, Texas in 1988 contained an average of 288 seeds per fruit (Steve Whisenant, Texas A&M University, pers. commun.). After the fruits ripen in early to mid-August, they contain about 50% total nonstructural carbohydrates (Potter et al. 1986). Consequentially, they are sweet, succulent, and very palatable to cattle, horses, sheep, goats, deer, coyotes, raccoons and many other wildlife species, regardless of the quantity, quality or succulence of other forages.
Pricklypear seed coats are very hard, impermeable to water, and may contain secondary plant compounds that inhibit germination (Potter et al. 1984). The seeds of some species may have after-ripening requirements. Germination of untreated seeds of O. edwardsii, O. lindheimeri, and O. discata was 1, 2, and 36%, respectively, in 28-day germination trials at San Angelo (Potter et al. 1984). The optimal temperature for germination for these species, 86 degrees F, indicates that most germination occurs during late spring through early autumn. Maximum germination usually does not occur for two weeks or longer, suggesting that surface soil water contents may have to remain high for about 2 weeks when temperatures are relatively high for optimal germination to occur. These characteristics ensure that all the seeds produced during the year or preceding years do not germinate at the same time or following minor rainfall events. This strategy minimizes the risk of depletion of the soil seed bank of pricklypear seeds and maximizes the probability of survival of new pricklypear seedlings.
Livestock and wildlife play an important role in the dispersal and germination of pricklypear seeds. Seeds of Lindheimer pricklypear that had been manually removed from ripened fruits during the autumn exhibited greater germination than those that remained within intact, dry fruits over winter (Potter et al. 1984). Germination of pricklypear seeds that had been ingested and defecated by cattle and jackrabbits was about 1.5 times greater than that of non-ingested seeds (Timmons 1941, Potter et al. 1984). Ingested seeds undergo mechanical scarification as the seeds are chewed, as well as acid scarification and/or leaching while in the digestive tracts of herbivores. Soaking seeds of Engelmann pricklypear, O. edwardsii, and Lindheimer pricklypear in concentrated sulfuric acid for 30 minutes increased their subsequent germination percentages 2.3, 10, and 17 fold, respectively, in laboratory experiments (Potter et al. 1984). Leaching uningested seeds of Engelmann pricklypear for 12 hours in water significantly increased germination, compared to seeds that were not leached, at constant germination temperatures of 25, 30, and 35 degrees F (Potter et al. 1984). However, leaching did not increase the germination of pricklypear seeds that had been digested by cattle.
Little is known about the duration of viability of pricklypear seeds in the soil or upon the soil surface. However, the impermeable nature and apparent presence of chemical germination inhibitors in the seed coats suggest that pricklypear seeds may remain dormant in the soil for considerable periods of time. During the autumn of 1996, we counted almost 100 juvenile pricklypear plants per acre in a pasture in Crockett County where the original pricklypear population had been essentially eradicated with the sequential application of prescribed fire and aerial application of picloram about 8 years previously. Thus, it is obvious that pricklypear will reinfest rangelands from seeds where mature stands have been controlled. The reinfestation may arise from pricklypear seeds already within the soil seed bank or from seeds disseminated by wild or domestic animals. Because of its ability to propagate vegetatively, pricklypear would not likely decline in abundance even if seed production could be totally eliminated over vast areas. Every pricklypear pad, or major part of a pad, that is dislodged from the parent plant and that falls upon mineral soil has the potential to become a new pricklypear plant or colony. Roots may rapidly emerge from any areoles (modified axillary buds) that come in contact with the soil surface. These roots begin to supply water and nutrients to the pad within a few weeks. The wounds on pads that are broken or partially eaten by livestock or wildlife are rapidly sealed by the polysaccharide secreted by mucilage-producing cells. This conserves water within the pads and prevents infection of wounds by disease organisms, thus enabling even damaged pads to take root. Other areoles on these dislodged pads are capable of giving rise to new pads and to flowers and fruits. Pricklypear pads are commonly dislodged and scattered by both domestic and wild grazing animals and by mechanical brush control methods, such as chaining, tree dozing and root plowing. Some pricklypear species even have areoles and glochids on their roots.
In a field experiment at San Angelo, I found that, on average, a single 3/4 lb pad of Lindheimer pricklypear placed upon bare ground grew into a 69-pad plant that weighed 50 lbs in 7 years (D.N. Ueckert, unpubl. data). If each of these new pads were dislodged, scattered, and then successfully rooted on the surrounding acre of rangeland during year 7, they could potentially result in 69 pricklypear plants, with almost 5,000 pads, weighing a total of 3,500 lb, and covering 1.14% of the acre by year 14. If all these pads were dislodged, scattered, and successfully rooted within the acre during year 14, then, by the 21st year after the original pad rooted, there could potentially be 5,000 pricklypear colonies on the acre, supporting 328,000 pads, tipping the scales at almost 240,000 lb/acre, and covering almost 80% of the soil surface. This scenario is purely hypothetical, but it provides insight into the potential for pricklypears to reproduce vegetatively or asexually. Also, it demonstrates that pricklypear can be quickly and easily introduced or reintroduced into rangelands if this strategy is deemed useful for improving the wildlife habitat value of selected range sites.
Several of the morphological characteristics of pricklypear give it ecological advantages for survival in harsh, arid environments. It is extremely well equipped to thrive in areas with limited rainfall or soil moisture. Pricklypear leaves, which are very small and round in cross section, appear only on the areoles of new pads and the floral tubes for a brief period during the spring, then they are shed. As the new pads mature, they develop a thick cuticle and a thick layer of wax on their surfaces. The stomata of pricklypear open for atmospheric gas exchange only during the nighttime or during daytime periods when relative humidities are high. These features reduce water loss from pricklypear to the atmosphere.
Pricklypear pads are capable of absorbing large quantities of water and are able to store and utilize this water to carry on photosynthesis during extended dry periods. The water contents of Lindheimer pricklypear varied from about 80% following wet periods to about 60% following dry periods in a west-central Texas study (Potter et al. 1986).
Pricklypears have shallow, spreading root systems that enable them to efficiently utilize moisture from small rainfall events. The vertically oriented pads of pricklypear intercept rainfall and the waxy pad surfaces minimize the amount of rainfall intercepted by the pads and returned to the atmosphere through evaporation. The waxy, vertical pads “funnel” essentially all the water downward to the soil where the pricklypear roots are most heavily concentrated. Desiccated pricklypear plants can absorb enough water to become fully re-hydrated within a few hours after a rainfall event.
The pads of most pricklypear species are somewhat protected from grazing animals by spines. Hence, pricklypear is utilized only lightly or periodically, and it usually remains in good vigor, with well-developed root systems and an abundance of carbohydrate reserves. In contrast, the palatable grasses, forbs, and browse plants are regularly defoliated, especially under heavy stocking rates and continuous yearlong grazing. The vigor of these plants decline because their root systems die back and their carbohydrate reserves are depleted. These plants loose their capability to utilize rainfall, sunlight, and soil nutrients. Under long-term over utilization, many of the palatable, herbaceous species decrease in abundance or totally disappear from the rangeland. Pricklypear gains an ecological advantage because of this “differential palatability” factor.
The spines on pricklypears are occasionally ignored by hungry grazing animals when the quantity and/or quality of other forage is very low. Hungry cattle will readily eat pricklypear after the spines have been singed with propane burners. The vigor of pricklypear can be diminished and it can even be killed if it is grazed too closely or too frequently (Maltsberger 1989). The spineless form of Indian fig (Opuntia ficus-indica) is cultivated as a fodder for livestock in some areas, and is often used as an ornamental in urban landscapes. In my opinion, the spineless pricklypear would be an asset on most Texas rangelands, even those grazed by sheep and goats. My attempts to establish the spineless pricklypear in the Edwards Plateau and Trans-Pecos areas have routinely failed because of heavy grazing on the pads by jackrabbits or winter freeze damage.
Rangeland plants that are evergreen, such as pricklypear and juniper (cedar) have another ecological advantage over the warm-season, herbaceous forage species in that they are capable of photosynthesizing throughout most of the year. Warm-season herbaceous forage plants go dormant during the winter and when soil water contents drop below critical levels during the growing season. Pricklypears and the junipers continue to utilize the resources (sunlight, carbon dioxide, water, and soil nutrients) essentially yearlong in the warmer regions of their geographical ranges.
Natural enemies of pricklypear
The insect enemies of pricklypear have been widely studied in attempts to: l) biologically control infestations of the plants on rangelands; 2) cultivate the plants commercially as a source of livestock feed; and 3) understand the ecology of the plants in rangeland ecosystems. Those insects which are primarily cactus feeders, almost without exception, do not attack other plants, and insects which are general feeders rarely feed on the Cactaceae. Several lists of insects that attack the pricklypears have been compiled (Mann 1969, Lavigne 1976). Mann (1969) listed about 160 species that were considered to be definitely restricted to Cactaceae, and provided information on the life histories, habits, and distribution of the various arthropods. A review of the literature on pricklypear insects has been published by Watts et al. (1989).
The most widely known insect enemy of pricklypear is the cactus moth, Cactoblastis cactorum (Lepidoptera: Pyralidae), which was credited with the successful biological control of pricklypear in Australia. The cactus moth was introduced from Argentina into Australia in 1925, where it was free from its own natural parasites and predators. Cactus moth larvae consume the pulpy tissues within the pads, which are then rapidly invaded by soft rot bacteria and fungi. This results in a rapid and virtually complete collapse of the plant (Mann 1970). The moths avoid nutritionally deficient pricklypear plants. Fertilization was necessary to get successful control of pricklypear with the cactus moths on soils with low fertility levels. This insect does not occur in North America and has not been introduced because of the conflict of interest between those who value pricklypear for various reasons and those who primarily regard it as a pest weed.
Other arthropods that were introduced into Australia prior to 1925, and which were providing partial suppression of pricklypear, included: the spider mite Tetranychus opuntiae (Acari: Tetranychidae); the cactus bugs Chelinidea tabulata and C. vittiger (Hemiptera: Coreidae); and the cochineals Dactylopius opuntiae, D. tomentosus, and D. austrinus (Homoptera: Dactylopiidae) (Dodd 1940). The cactus moth and/or one of the cochineal insects have been utilized for biological control of various pricklypear species in numerous countries, with varying degrees of success.
Insects recognized as pests of pricklypear in the United States which occupy niches similar to that of Cactoblastis cactorum include the banded cactus borers, Olycella junctolineela and Ol. subumbrella, and the blue cactus borers, Melitara dentata, M. prodeniales, and M. fernaldialis. Two cochineal insects, Dactylopius confusus and D. opuntiae, are indiginous to the United States and attack pricklypear. The cochineals are small, red-bodied insects similar to mealybugs in appearance that suck the juices from pricklypear pads. After the female cochineal nymph settles and inserts her proboscis into a pricklypear pad, she covers her body with a floculent mass of white, waxy plates that resemble cotton, and remains stationary for life. Heavy and prolonged infestations result in chlorosis, pad abscission, necrosis, and death of the plant parts affected.
Cactus bugs (Chelinidea spp.) resemble squash bugs in appearance. They also suck juices from pricklypear pads and fruits. Their feeding punctures cause circular, chlorotic lesions and the pads turn yellow, but their impact is most often negligible. Coreids in the genus Narnia suck juices primarily from the fruits, but they rarely have a major impact on seed production.
Adults of long-horned beetles in the genus Moneilema chew on edges of new pricklypear pads, sometimes causing pad abscission. Their larvae bore into the older pads and basal stems, making wide galleries that become blackened and that are accompanied by the exudation of a very noticeable black sap from the pads. Several weevils and flies also attack pricklypear. The red spider mite Tetranychus opuntiae attacks Lindheimer pricklypear. Its feeding causes a greyish layer of corky epidermis, initially around the areoles, but completely covering the pad surfaces when infestations are heavy.
Populations of the native insect enemies of pricklypear in Texas rarely have a significant impact on pricklypear populations, except in isolated instances and for brief periods, because their abundance is regulated by their own complex of parasites and predators. Some of the native insects increase in abundance for a year or two following grassland fires and appear to augment the direct effects of fire in suppressing or killing pricklypear. However, the duration and extent of the fire-insect interaction is rarely sufficient to totally control pricklypear. Some insect enemies of pricklypear, including the blue cactus borer, cactus bugs, and cochineal insects, are reported to increase in abundance in the Central Great Plains during periods of warm moist weather which result in increased standing crops of grasses around pricklypear plants (Cook 1942, Bugbee and Reigel 1945).
Impact of fire on pricklypear
Most plant ecologists agree that fire played a major role in limiting the abundance of pricklypear in grasslands of the Southern Great Plains and Southwest prior to establishment of the domestic livestock industry by Europeans (Wright and Bailey 1982). The frequency of these fires is unknown, but they likely occurred every 5 to 20 years. They were probably started by lightning during convection thunderstorms in the dry summer months, and by aborigines during late winter to attract game animals to their hunting grounds. These fires were likely very intense because of high temperatures, low humidities, and the presence of an abundance of dry, standing vegetation and mulch.
Prescribed burning during the winter has had a substantial effect on pricklypear where it has been used in Texas. Bunting et al. (1980) reported gradual increases in pricklypear mortality during the 3-year period following winter fires. They attributed the progressive increase in mortality to fire weakening the plants, thus increasing their susceptibility to attack by insects, rodents and rabbits. Several insect enemies of pricklypear, including the cochineal Dactylopius confusus, the cactus bug Chelinidea vittiger, and the banded cactus borer Olycella subumbrella have been reported to increase in abundance following prescribed winter fires (Sickerman and Wangberg 1982, Gilreath 1985). Single summer fires have recently been shown to essentially eliminate pricklypear in both mesquite-tobosagrass communities and common curlymesquite grass-juniper-live oak communities (D.N. Ueckert, unpubl. data; C.A. Taylor, pers. commun.).
Winter fires in some tobosagrass-dominated rangelands can reduce live pricklypear cover 80 to 95% within 3 years after the fire, and can often provide acceptable pricklypear control for about 10 years (Ueckert et al. 1988). In lighter fine fuel types, such as Texas wintergrass, sideoats grama, and buffalograss, winter fires usually reduce live pricklypear cover 70 to 80% within 1 to 2 years after the fire, but the pricklypear recovers much more quickly than in tobosagrass-dominated communities. Winter fires are quite effective in killing, or at least top-killing the low-growing pricklypear species such as O. edwardsii. However, the taller growth forms, such asEngelmann pricklypear, are often only singed around their perimeter because of the sparsity or absence of fine fuels (dry grasses and mulch) within their canopies.
Prescribed fires installed on most of our rangelands today are undoubtedly much less damaging to pricklypears and the associated woody plants than were the “natural” fires that occurred prior to establishment of the domestic livestock industry. Continuous grazing by domestic livestock at excessive stocking rates has killed out many of the more productive forage species that formerly provided an abundance of fine fuel to support intense fires. In many cases, the forage plants have been replaced by high densities of woody species or succulents which further suppress the productive grasses and forbs.
Many of our rangelands currently do not have the potential to grow enough grass to support intense fires because of this change in species composition and because topsoil has been lost to erosion. In many cases, ranchers will have to utilize mechanical or chemical brush control methods initially in order to be able to grow enough fine fuel (primarily grasses) to utilize fire effectively. Burning can be a very useful and inexpensive tool for improving rangelands for livestock and wildlife production, but it requires a high level of grazing management expertise and a commitment to long-term management planning. Vegetation serves a dual role as forage for grazing animals and as fuel for fires. The manager must balance the amount of forage that is grazed with the amount that is used as fuel for effective fires (Kothmann et al. 1997).
Cattle and other livestock can be used to enhance the effects of fire on pricklypear. Cattle will readily eat pricklypear pads that have been recently singed by grass fires. After pricklypear pads killed by fire dry and become crisp, they are also readily eaten by cattle. Cattle will also readily consume the tender, new pads that sprout on pricklypear crowns not killed by the direct effects of fire. Heavy browsing of the resprouts would undoubtedly reduce the vigor of pricklypear and reduce its post-fire growth rate by depleting the carbohydrate reserves in its crowns and roots. Post-fire grazing should be managed very carefully so that the desirable herbaceous plants are not damaged along with the pricklypears. Grazing burned pastures at high stocking rates for a 2 or 3 week period when the new pads reach a length of about 2 or 3 inches , then resting the pasture until about June 15, is a safe strategy in the Edwards Plateau and Rolling Plains.
Pricklypear is very vulnerable to two or more treatments or injuries within a period of a few months. The sequential application of prescribed winter fires and aerial application of herbicide picloram has been shown to be highly effective for pricklypear control (Ueckert et al. 1988). In numerous experiments during 1981 and 1983, fire, applied as a single treatment, reduced live pricklypear cover by an average of 65%. Broadcast sprays of picloram at 0.25 and 0.5 lb active ingredient/acre, applied as single treatments, reduced live pricklypear cover by an average of 73 and 88%, respectively. The sequential application of winter fires plus aerial application of picloram at 0.13 lb/acre the following spring (April 15 to May 1) reduced live pricklypear cover 98%. There is some evidence that a very high level of pricklypear mortality can also be achieved by a prescribed winter fire within about a year after an aerial application of picloram at 0.25 lb/acre (Steuter 1978), but this sequencing has not been thoroughly tested. Extremely cold temperatures during the winter following summer or autumn aerial applications of picloram frequently result in a very high level of pricklypear mortality and deterioration of the colonies by the following spring. In contrast, the effects of aerial sprays of picloram on pricklypear are usually not fully manifested for about 3 years if the winters are mild.
Pricklypear interactions with other plants and with animals
Plant ecologists and rangeland managers generally agree that high densities of pricklypear interfere with the production of herbaceous forages and/or the ability of most grazing animals to utilize the herbaceous forage (Dodd 1940, Bement 1969, Price et al. 1985). The larger growing pricklypear species or hybrids that grow into dense, upright colonies very noticeably exclude herbaceous plants, primarily due to competition for space and sunlight. Dense stands of the large-growing pricklypears commonly eliminate herbaceous plant production on 25% or more of the surface area of some pastures. The low-growing pricklypears have not been consistently shown to reduce production of associated herbaceous species, but it is generally accepted that they reduce the availability of herbaceous plants to livestock by about 25% within their colonies. This interference with forage utilization may not be acceptable if the additional 25% could be used without over utilizing the forage plants. However, pricklypear may be the only thing preventing severe overgrazing and the demise of some very desirable herbaceous species on rangelands that are continuously overgrazed.
On some rangelands, the micro-environment within low-growing pricklypear colonies and the protection afforded by the spines has facilitated the establishment and survival of desirable cool-season grasses. For example, Texas wintergrass only occurs within pricklypear colonies in some areas of the Edwards Plateau dominated by common curlymesquite grass. The presence of Texas wintergrass in these areas is significant in that it increases the plant diversity and provides relatively high quality forage for livestock and wildlife during the winter.
Some woody plants appear to facilitate the establishment of pricklypear. We recently documented that pricklypear was more abundant beneath mature redberry junipers than in the interspaces between mature junipers in a study in the northern Edwards Plateau (Dye et al. 1995). The establishment of pricklypear seedlings beneath the juniper canopies likely resulted from birds dropping pricklypear seeds while they were perched or feeding in the juniper canopies. Pricklypear is apparently well adapted to withstand the drought conditions that prevail beneath juniper canopies (Thurow and Hester 1997).
There is some evidence that pricklypear may improve soil conditions. Its canopies and roots may help protect the soil from erosion on hilly terrain or where the herbaceous plants have been destroyed. However, pricklypear should not be considered equivalent to good stand of bunchgrasses for protecting the soil from erosion or for facilitating rainfall infiltration. One study showed that soil water contents were consistently greater under pricklypear colonies than under grass cover (Clapp 1969), but this only reflects the efficiency of native grasses in utilizing available soil moisture.
In addition to providing emergency fodder for livestock (Hanselka and Paschal 1989), pricklypear provides shelter and food for a wide variety of wildlife species. Many birds, reptiles, and small mammals make their nests or dens in or beneath pricklypear plants. The fruits, seeds, and pads provide food for over 40 species of wild animals (Martin et al. 1951). Pricklypear is a staple food of white-tailed deer and of collared peccary in the Rio Grande Plains (Arnold and Drawe 1979, Everett and Gonzales 1979, Everett et al. 1981). It is interesting, however, that white-tailed deer did not eat pricklypear on rangeland in either excellent or poor range condition during a 1-year study in the Edwards Plateau (Bryant et al. 1981). Pricklypear has been rated as an important food and cover plant for bobwhite quail in South Texas (Lehmann 1984). Slater (1996) recently presented indirect evidence that pricklypear colonies may serve as a predator deterrent for bobwhite quail in areas where bunchgrasses are rare or overgrazed.
Management implications
Pricklypear is a natural component of many of the rangelands in Texas and in other regions, but its density on many rangelands is excessive in relation to the management objectives of landowners and resource managers. Pricklypear control or management programs can positively or negatively impact wildlife habitat depending upon the control method used, the pattern of application, the plant species present in the habitat and their densities, and the size of areas treated. The technology is available to achieve very high levels of pricklypear control (Ueckert et al. 1988), but it would not seem either ecologically nor economically sound to totally eliminate pricklypear over vast acreages, especially where wildlife production is an important management objective. Prescribed fire is an excellent tool to reduce the abundance of pricklypear, and the degree of pricklypear control achieved with fire can be manipulated by carefully selecting the environmental parameters for the fire (air temperature, wind speed, relative humidity) or by utilizing livestock and grazing management to adjust fine fuel loads. Areas of pricklypear deemed to be critical for wildlife could be protected from fire by installing fire guards (dozed lines) around the perimeter of these areas. Broadcast sprays of picloram herbicide can be applied in strips that alternate with untreated strips to take out or leave as much pricklypear as is desired. The Brush Busters “How to Take Care of Pricklypear and Other Cacti” program (Ueckert and McGinty 1997) offers more labor-intensive, but highly selective methods which have minimal impact upon associated desirable vegetation. The Brush Busters methods facilitate the thinning of pricklypear to the density desired to meet management objectives. They also make it possible to completely eliminate pricklypear from selected patches and to leave dense pricklypear stands in adjacent patches to maximize the “edge” effect.
The rangeland resource manager should review his (her) long-term goals before making decisions about controlling pricklypear. The decision on whether to control pricklypear, which method(s) to use, and how much to control will vary depending upon whether the objective for managing pricklypear is to improve the rangeland for livestock production, to improve the rangeland for both livestock and wildlife, or solely to improve the rangeland for wildlife. Pricklypear’s strong potential to reproduce vegetatively is a great asset to the wildlife habitat manager if pricklypear is not sufficiently abundant. Mature pads can be collected from the growth form(s) of pricklypear that best fits the needs of the wildlife species of interest from within about 100 miles of the ranch being “sculpted”. These pads should be air dried in full sunlight for a week or two, then they may be transplanted at the appropriate density in the selected areas. Cultural practices for establishing and producing pricklypear have been reviewed by Felker et al. (1989).
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Comments: Dale Rollins, Professor and Extension Wildlife SpecialistUpdated: Mar. 18, 1997