Prickly Pear (Opuntia spp.) as an Invasive Species and a Potential Fodder Resource for Ruminant Animals

Abstract

Worldwide, the invasiveness of Opuntia spp. and its impact on various ecosystem services has been recognised especially in semi-arid areas where rainfall is erratic. The semi-arid environments are the habitats of plants which have adapted to be able to grow in severe hot and dry regions. Opuntia spp. normally thrives in conditions of high temperatures, low rainfall, saline soils and it can also adapt and survive in severely degraded soils which have a limited nutrients supply. Its positive impact includes its recognised value as livestock fodder. Opuntia’s adaptability to harsh conditions, high dry matter yield, palatability and significant levels of energy, as well as its availability at a low cost during the dry season, decreases the use of expensive supplements and conventional diets in many areas. There is a need to understand the importance of this invasive Opuntia species when incorporated in animal diets. As a part of its control measures, the use of livestock in controlling the spread of Opuntia may assist in reducing its abundance and invasiveness while at the same time providing a consistent supply of forage during the dry season. Information on its nutritive value, incorporating the species in animal diets and the means to control it must be well understood to recognise the species’ contribution to an ecosystem.

1. Introduction

Opuntia is an invasive flowering succulent plant species that grows up to 5 m tall and is a crassulacean acid metabolism (CAM) plant which belongs to the Cactaceae family. This species is native to Mexico [1] and has spread through cultivation to many regions around the world, including South Africa (Figure 1), Ethiopia, Australia, Italy, Spain, Argentina, Chile and the Mediterranean basin (Table 1). Varieties of the Opuntia species are differentiated by various features such as the presence or absence of thorns, and the size of pear produced.

Due to their adaptability to harsh conditions of less fertile soils, high temperatures and low rainfall, they are drought tolerant. Prickly pear is considered as a well- adapted species to hot temperatures in arid areas especially on low profile soils [2]. Due to its morphology, this plant is categorised as an extremely valuable source of nutrient feedstuff in areas where other species fail to survive due to extreme environmental conditions [3]. The biodiversity of Opuntia spp. is a very vital function when associated with arid and semi-arid plantations [4,5]. In past decades, studies have been conducted on a functional approach to characterize vegetation, rehabilitation programmes, biodiversity, and the adaptability of the invasive Opuntia spp. [5,6]. The importance of Opuntia spp. as a feed for ruminants has been noted globally especially in semi-arid areas where rainfall is erratic [7,8,9]. Opuntia spp. are known as profitable resources of both the arid and semi-arid areas around the world as they can be used as human food [10] and the mature cladodes can be used as an animal feed [11]. This inexpensive source of energy can decrease the use of conventional diets in many areas, and it is always ready during the dry season when there is a serious shortage of feeds [12,13], due to its high dry matter yield and its palatability [14]. Abay [8] also highlighted the importance of this species in sustainable agricultural systems in arid and semi-arid areas especially to those that do not have the means to access conventional feeds. Additionally, along with the foraging benefits, Opuntia has also been used in the restoration of degraded lands [15] and also for medical purposes [16].

Though the species can be used as forage, there are advantages and disadvantages of using Opuntia. Guevara et al. [17] reported that injuries are common to livestock feeding on this species due to the glochids of the areoles attached to the Opuntia spp. Usually this leads to a significant loss of livestock numbers in many semi-arid regions [18]. Furthermore, cultivation of the spineless Opuntia risks cross-pollination with spiny types leading to a spread of the invasive spiny species. The advantages of using Opuntia lies in a water use efficiency as influenced by the CAM photosynthetic pathway [19]. Moreover, this species is always available throughout the year for use in livestock diets. This review focuses on highlighting the usefulness of Opuntia to animal diets. There is a need to understand the importance of this invasive Opuntia spp. when incorporated in animal diets as part of the biological control of invasive species. Information on its morphological parts, nutritive value, incorporating the species in animal diets and the means to control it must be well understood to recognise the species’ contribution to an ecosystem.

Although the Opuntia species variability is well known, the potential for the use of the invasive Opuntia spp. as fodder during drought years and dry seasons has not been extensively reviewed. Furthermore, there is no consensus on the level of invasiveness, which should be studied region by region by attributing a specific level of invasiveness (e.g., casual, invasive, naturalized) [20], and which offsets its value as fodder for livestock and human use. It is therefore important to review Opuntia’s current use and impact in animal production, and its nutritional attributes. This review focuses on the literature available in South Africa, and relevant information from other areas.

Table 1. Scientific, common name, origin and usefulness of prickly pear species.

Taxon	Origin	Use	References
Opuntia leucotricha and O. rastrera	Mexico	Health benefits, domestic animal feedstuff, water supply, ornamental purposes	[21,22,23,24,25,26]
Opuntia humifusa	Canada	Human consumption, fodder crop, prevent soil erosion, pharmaceutical (emergency hydration)	[27,28,29]
Opuntia monacantha	Spain, South America	Human consumption and health and pharmaceutical benefits, chemical industrial uses	[30,31,32,33]
Opuntia engelmannii	south-central, south-western United States and northern Mexico	Cultivated as an ornamental or as live hedge and the fruit is edible, health benefits	[25,34,35]
Opuntia microdasys	central and northern Mexico	Animal feedstuff, pharmaceutical and health benefits, human consumption	[36,37]
Opuntia megacantha	North America and Mexico	Health benefits and animal feedstuff, human consumption	[38,39,40]
Opuntia stricta	Australia, central America	Health benefits and animal feedstuff, human consumption, living fences	[18,33,41,42]
Opuntia cochenillifera	South Africa	Both traditional medicinal and edible purposes	[43,44]
Opuntia ficus-indica	Ethiopia	Health benefits, animal feedstuff and fruits for humans	[45]
Opuntia ficus-indica	Ethiopia	Health benefits, animal feedstuff and fruits for humans	[45]
Opuntia ficus-indica	Argentina and Mediterranean region	Health benefits, animal feedstuff and fruits for humans	[46,47]
Opuntia dillenii	Italy	Health benefits, animal feedstuff and fruits for humans	[33]
Opuntia ficus-indica	Chile	Health benefits, animal feedstuff and fruits for humans	[33]

Table 1. Scientific, common name, origin and usefulness of prickly pear species.

Taxon	Origin	Use	References
Opuntia leucotricha and O. rastrera	Mexico	Health benefits, domestic animal feedstuff, water supply, ornamental purposes	[21,22,23,24,25,26]
Opuntia humifusa	Canada	Human consumption, fodder crop, prevent soil erosion, pharmaceutical (emergency hydration)	[27,28,29]
Opuntia monacantha	Spain, South America	Human consumption and health and pharmaceutical benefits, chemical industrial uses	[30,31,32,33]
Opuntia engelmannii	south-central, south-western United States and northern Mexico	Cultivated as an ornamental or as live hedge and the fruit is edible, health benefits	[25,34,35]
Opuntia microdasys	central and northern Mexico	Animal feedstuff, pharmaceutical and health benefits, human consumption	[36,37]
Opuntia megacantha	North America and Mexico	Health benefits and animal feedstuff, human consumption	[38,39,40]
Opuntia stricta	Australia, central America	Health benefits and animal feedstuff, human consumption, living fences	[18,33,41,42]
Opuntia cochenillifera	South Africa	Both traditional medicinal and edible purposes	[43,44]
Opuntia ficus-indica	Ethiopia	Health benefits, animal feedstuff and fruits for humans	[45]
Opuntia ficus-indica	Ethiopia	Health benefits, animal feedstuff and fruits for humans	[45]
Opuntia ficus-indica	Argentina and Mediterranean region	Health benefits, animal feedstuff and fruits for humans	[46,47]
Opuntia dillenii	Italy	Health benefits, animal feedstuff and fruits for humans	[33]
Opuntia ficus-indica	Chile	Health benefits, animal feedstuff and fruits for humans	[33]

2. Growing Conditions and Varieties of Prickly Pear

Climatic fluctuations have long been suggested as a key driving power of the diversification of the cacti family and their distribution [48]. The species of Opuntia is highly adaptive to any climatic conditions and soil and it is also resistant to drought [49]. It was reported that there is little information regarding the response of the invasive Opuntia species on climatic changes [50,51] and its effect on biodiversity.

Since Opuntia spp. grow in arid and semi-arid environmental regions, a high proportion of the species is threatened by extinction as a result of overexploitation and land use change [52,53]. Conversely, under global warming, most environments are becoming more arid [53] such that Opuntia will inhabit areas that are not suitable, with climatic changes resulting in an expansion of the geographical collection of the species in the near future [53], alongside a reduction in rainfall and drop in carbon dioxide [54]. The diversification mechanism of Opuntia could be compounded with the availability of arid areas, and climatic changes are suspected of playing a significant role [55,56].

Prickly pear is a drought-resistant and sustainable feed source for livestock [53]. The variation in climatic conditions, water shortages and an increase in the human population require plants—which are of significant value to livestock and humans—to adapt to these factors for sustainable production. In semi-arid environments, plants for livestock would be from those species that can adapt and produce in severe hot and dry regions [9]. Genus’ such as Opuntia normally thrive in conditions of high temperatures, elevated levels of carbon dioxide, and low rainfall, and are characterised by increased cladodes productivity and extensive growth of their root systems [57]. On uncultivated lands, the dry matter (DM) yield from Opuntia can surpass 2000 kg ha⁻¹ [58]. The DM content of different Opuntia spp. Is varied between 61 and 105 g kg⁻¹ DM [12].

2.1. Temperature

Temperature is one of the determinants of natural plant distribution [59]. Different plants respond to temperature sensitivity differently with leaves being known as more sensitive than stems [60]; however, Nobel and Bobich [61] reported that the roots are more sensitive than stems. In South Africa, the Opuntia species are normally found in arid and semi-arid regions with high temperatures [9]. Drennan and Nobel [62] have also stated that the normal temperature for Opuntia species root growth is between 27–30 °C and some branches can be ruined by ambient temperature (−16 °C) depending on the type of species [63]. Opuntia plants can also survive in temperatures above 65 °C [64]. Nobel and De le Barrera [65] observed the tolerance of Opuntia at the age of 10 years and above to a low temperature of 2–6 °C, which normally prevents the plant from dying during freezing times. In a study conducted by Snyman et al. [66] in South Africa, it was highlighted that freezing temperatures during spring time did not result in the death of the Opuntia plant but rather had a negative impact on the plant moisture stress, and phonological stage. Water stress reduces irradiance on the physiological responses in basal mature cladodes [67]. Nobel [68] and Valdez-Cepeda et al. [63] found difficulties when assessing the strength of severe cold and frost on Opuntia in semi-arid areas, as there was little occurrence of cold and frost in those regions, whereas in other regions, low temperatures during winter periods become the major limiting factors in the cultivation of the species. High temperatures (above 30 °C) can decrease the photosynthetic process by up to 70% and low temperatures (lower than 0 °C) create irreversible damage to the cladode tissue. High temperatures (greater than 30 °C) during the day and greater than 20 °C during the night produce a higher number of new cladodes than fruits, which is ideal for fodder production [69].

2.2. Soil and Water

In order to have sustainable agriculture production, plant species which are salt tolerant and adapt well during drought are ideal for most arid and semi-arid regions [70]. Opuntia spp. adapt well in poor soils and some varieties adapt and survive in severely degraded soils with a limited or no nutrient supply [71]. Gajender et al. [70] highlighted that some Opuntia spp. are moderately tolerant to salt while being sensitive to alkaline conditions and that they can fail to adapt when the pH is more than 9.

Apart from other nutrients, water is another limiting environmental parameter of plant growth [72] and several studies have reported the adaptability of Opuntia to water scarcity [73,74]. Opuntia is a drought tolerant species, which normally survives under moisture stress [75] and performs well in an area with limited water supply and under a rainfall range of 100–300 mm [76]. It uses a shallow and horizontally spread root system to access what little moisture is available [75]. The root adaptation of this species adds to the classical physiological and structural modification of CAM plants to tolerate severe drought periods [70,75,77]. Nobel [78] and Edvan et al. [79] indicated that this species’ adaptability to drought is also due to its ability to store water in its shoots and to fix CO₂ during the night leading to reduced transpiration due to low night temperatures compared to the daytime. Edvan et al. [79] further stated that the species’ water use efficiency is six times greater than legume plants, and three times greater than herbaceous plants.

3. The Effect of the Invasive Opuntia Species on Biodiversity and Ecosystems

The authors correctly linked the effects of Opuntia on biodiversity, vegetation and an ecosystem [80,81] and there is information in the literature regarding the invasiveness of alien species and their impact on an environment [80,81], such as the utilisation of 7% of South African water resources to the detriment of native plants. Most of the Opuntia spp. in South Africa were sourced from the USA around 1914 [82]. This species now covers most areas around the world including Ethiopia and the Mediterranean Basin, France, Egypt, Greece, Italy, Libya, Turkey and South Africa [83,84], and it is regarded invasive by several countries such as Australia, Ethiopia, Mauritius, Yemen, the United states and Madagascar [85]. In line with the latest list of invasive plant species by the Conservation of Agricultural Resources Act No 43 of 1983 in South Africa, this species was categorised in the category number 1b and is regarded as a weed, meaning it is not supposed to be planted [81,86]. The negative impact of this species is associated with a disturbance of natural vegetation through a reduction in grazing capacity and causing injuries to people and livestock [87]. Mokotjomela et al. [88] indicated that birds are some of the animals that spread the cacti seeds in the arid areas of South Africa. Jones et al. [89] and Shackleton et al. [41] highlighted that the Opuntia species are regarded as an engineer of ecosystems due to its ability to modify the indigenous plant species habitats and also because it prevents livestock movement as it forms a thicket. Shackleton et al. [41], Pyšek et al. [90] and Seebens et al. [91] stressed that the introduction of many Opuntia around the world resulted in them becoming major invaders which have social and ecological costs. This is in agreement with Githae [92] and Tesfay and Kreyling [93], who found a significant homogenisation of the plant species composition and richness, and a poor rangeland condition leading to land degradation as influenced by the presence of Opuntia ficus-indica, and this leads to some plant species suffering because of its presence.

4. Nutritional Value of Opuntia Species

Both abiotic and biotic factors can contribute to both the yield and nutritive quality of the fruits and cladodes of Opuntia spp. [72,94]. The nutritional value of Opuntia spp. fruit is mostly determined by its high levels of ascorbic acid, vitamin E, carotenoids, fibre, amino acids, glucose and fructose [95,96]. Studies report that the Opuntia spp. are highly digestible with an energy-balanced diet that is relatively high in sugars, starch, ether extract, crude protein, amino acids, and fibre while also meeting the requirements of vitamin and calcium needs [32,61]. Heba et al. [97] stated that Opuntia spp. are good beneficial sources of dietary fibres and bioactive phytochemical compounds from their cladodes. Furthermore, these plants are also consumable natural antioxidants sources and as a result, they are considered as a functional food and their antioxidant activity can guard against oxidative cellular damage [32] while the most significant source of polyphenols and flavonoids appears to be the Opuntia blossoms. Opuntia is high in phenols, flavonoids, betaxanthins, and betacyanins, all of which promote good health by acting as hypoglycaemic and hypolipidemic agents [31,98]; however, some authors have also highlighted that it is a major source of total phenolic compounds, with high quantities of betalains, total carotenoids, ß-carotene, and ascorbic acid. It was also reported that the antioxidant qualities of polyphenols, ascorbic acid, especially the flavonoid constituents (e.g., kaempferol, quercetin, and isorhamnetin), and the blend of yellow betaxanthin and red betacyanin pigments in the prickly pear fruit, contribute to the fruit’s nutritional and health benefits [99].

Table 2. Primary and secondary metabolites present in Opuntia spp. (mg g⁻¹).

Variety	Tannins	Oxalates	Flavonoids	Saponins	Phenolics	References
O. humifusa	-	-	23.5		23.0	[100]
Opuntia Strica	-	-	1.65		1.99	[101]
O. Cochenillifera	-	2.1	1.87		1.51	[101,102]
O. megacantha	42	-	25	-	71.4	[103]
O. ficus-indica (L.) Mill	29.5	-	109		265	[104]
O. ficus-indica	5.5	-	11.2	22.5	19.6	[105]

,

Table 3. Amino acid profiles of Opuntia (g 100 g⁻¹).

Amino Acids	Values	Values
Alanine	0.6	1.25
Agrenine	2.4	5.01
Asparagine	1.4	3.13
Glutamine	17.3	36.12
Glycine	0.5	-
Histidine	2.0	4.18
Isoleucine	1.9	3.97
Leucine	1.3	2.71
Lysine	2.5	5.22
Metheinine	1.4	2.92
Phenylanine	1.7	3.55
Serine	3.2	6.67
Threonine	2.0	-
Tyrocine	0.7	-
Tryptophane	0.5	-
Valine	3.7	-
References	[106]	[98]

,

Table 4. Mineral content (g kg⁻¹, unless stated) of Opuntia spp. varieties and cultivars cladodes.

Varieties	Ca	P	K	Mg	Na	Zn	Cu	Fe	Mn	References
O.humifusa (mg/100 g)	1.967	1.110	1.269	1.411	282.8	20.4	2.2	16.8	-	[100]
O. ficus-indica f. inermis (g/kg)	70.2	1.8	4.4	0.43	6.7					[107]
O. ficus-indica (L.) Mill cv Marado (g/kg)	22.5	0.7	25.0	15.0	0.8	26.50 (dpm)	5.05 dpm	55.0 dpm	547.50 (dpm)	[12]
O. ficus- indica cv Algerian (g/kg)	20.5	0.7	21.0	19.5	0.6	27.5 dpm	9.06 dpm	64.00 dpm	485.00 dpm	[12]
O. atropes (g/kg)	17.2	2.0	2.3	5.7	-	-	-	-	-	[108]
O. ficus-indica (g/kg)	34.4	4.7	33.4	7.4	-	-	-	-	-	[109]

Ca: calcium, Mg: magnesium, Na: sodium, P: phosphorous, K: potassium, Fe: iron, Cu: copper, Mn: manganese, Se: silicon; Zn: zinc.

and

Table 5. Nutritive value of Opuntia spp. varieties cladodes (g kg⁻¹ DM).

Variety	DM	OM	CP	ADF	NDF	ADL	EE	References
Malta	925	827	106	97	194	-	12	[108]
Copena	165	866	46	202	359	-	-	[110]
Miuda	118	-	57		259	-	26	[102]
Gigante	105	889	50	200	418	29	14	[111]
Halibo	82	815	39	23	186	16	6	[112]
EPP	127	915	33	173	227	108	12	[113]

DM: dry matter, OM: organic matter, CP: crude protein, ADF: acid detergent fibre, NDF: neutral detergent fibre, ADL: acid detergent lignin, EE: ether extract, EPP: erect prickly pear.

summarise the anti-nutritional factors, and nutritional composition of the cladodes.

5. Enhancing the Feeding Value of Opuntia for Ruminants

There is an increasing interest in the utilisation of legume species as an alternative source of protein to low quality Opuntia cladodes in animal nutrition, as observed in many genera of the Fabaceae family, especially in the wild species of genus Vicia [114], and Lathyrus L. [115]. Legumes provide high quality feed that yield good animal products [116]. Tesfay and Kreylin [93] reported enhanced health and reproductive characteristics in small stock ruminants when feeding on Opuntia forage with other protein rich feeds such as legume residues. Opuntia species are rich in phytochemicals and are grown for both food and feed utilisation [91]. Opuntia enhances feed palatability and has been shown to supply antimicrobial, antioxidative, immunogenic, and anticoccidial impacts when fed to Dorper sheep [117]. Opuntia species are considered as an outstanding affordable feed source that can be utilised to reduce the cost of ruminant production in dry regions [12]. Specifically, Opuntia cladodes are rich in ash, calcium and carbohydrates [7], while being low in crude protein and fibre [12]. The reported availability of nutrients in prickly pear resulted in improved feed intake and productivity of sheep and dairy cows [118,119,120].

The use of Opuntia spp. in silage is another method of conserving feed for animals [3]. Çürek and Özen [121] and Matias et al. [122] reported a pH increase and evaluated that the ensiling characteristics and variation ranges from 3.54 to 4.5% of silage from Opuntia. The study of Mokoboki et al. [123] reported lactic acid ranging from 4.9 to 10.5% of Opuntia’s fermentation characteristics. Several studies have also outlined that the ensiling material directly impacted the production of acetic acid [122,124]. Matlabe [9] stated that Opuntia must be combined with other protein sources as this will allow a synchronisation between energy and nitrogen as a result of high soluble carbohydrates as shown in

Table 6. Chemical composition (g kg⁻¹ DM, unless stated) of silages from Opuntia spp.

	OSM	OLL	Roedtan	Vas as	CAAS	CLIS
DM	393	278	913	917	440	380
CP	67	107	69	61	200	250
EE	18	24	66	61	-	-
NDF	322	337	259	248	573	634
ADF	224	234	213	198	523	574
References	[9]		[123]		[125]

DM: dry matter, CP: crude protein, EE: ether extract, NDF: neutral detergent fibre, ADF: acid detergent fibre, OSM: Opuntia Senegalia mellifera, OLL: Opuntia leucaena leucocephala, CAAS: cactus Acacia angusitissima, ClIS: cactus leucaena leucocephala.

.

6. Health Benefits of Opuntia spp. in Diets for Ruminant Animals

Besides Opuntia spp.’s contribution to human health [23,97], this species has a fundamental role in animal production [9]. Several varieties are annually pruned to stimulate the production of quality fruit and mostly the fresh cladodes are used as an alternative feed for livestock. In southern Africa, the availability of Opuntia cladodes and prickly pears in the dry season covers the drinking water and forage quality and quantity deficit, as they are rich in water and digestible fibre, thus being a significant source of nutrients for small ruminants [99]. In some developing countries, this productive plant is used for health purposes, as a nutritional conventional feed for ruminants and in cosmetic industries [1,98]. The beneficial health factors of Opuntia cladodes might depend on the chemical antioxidant scavenging ability of gallic acid, which inhibits the capacity to prevent DNA damage, buffering free radicals due to a high antioxidant activity [98,126]. Apart from the benefits to animals, extracts of Opuntia have been reported to strongly reduce the bacterial (Campylobacter spp.) effect causing food-borne gastroenteritis in humans [98]. Sanchez et al. [127] reported that Opuntia ficus-indica is rich in phenols, flavonoids, betacyanins and betaxanthins which improves health status through hypoglycaemic, antioxidant parameters and hypolipidemic movements [98]. Li et al. [128] studied mainly cultivated rat cerebral blood artery endothelial cells, showing nicotiflorin decreased cerebral ischemia damage and upregulated endothelial nitric oxide synthase. The above-mentioned authors also reported a high level of omega-6 linoleic acid in cactus oil, which is known to benefit health (cardiovascular illnesses, inflammatory problems, autoimmune disorders and diabetes).

7. The Use of Opuntia Cladodes and Prickly Pears in Ruminant Nutrition

Opuntia is utilised as fodder in several semi and arid areas around the world, including South Africa, where farmers burn off the spines and enable sheep, goats and cattle to utilise the cladodes as a source of feed during periods of drought [123,129,130]. Opuntia cladodes are a source of water, carbohydrates, vitamins and calcium in dry season ruminant diets. Todaro et al. [99] highlighted that prickly pear cladodes, in dry-land regions, combined with other feed sources, may be beneficial in reducing ruminant forage crop requirements. Opuntia cladodes are very palatable [125] and when combined with conventional roughage sources, they can maintain adult sheep during feed shortages [131]. Growth studies using varying levels of Opuntia in lamb diets attest to its positive effect on growth at inclusion levels not exceeding 50% [132]. It was also found that it had no negative effect on live weight, metabolites and reproduction in goats during the breeding season [133]. An investigation conducted by Kamble et al. [23] reported on the chemical makeup of the cladodes rather than the fruits. Opuntia cladodes can substitute pasture hay by up to 60% in ruminants [134] while elsewhere, Opuntia was used as a substitute to concentrates with dairy lactating goats and cows, with minimum effects on milk production [135,136]. Shetty et al. [129] stated that Opuntia cladodes are not marketable as a vegetable but that they are used as dairy forage as they improve the flavour and quality of milk, as well as the colour of butter, thus reducing the need to buy concentrate feeds for lactating dairy cows [136]. Due to a lower concentration in crude protein (CP) (30 < CP g kg⁻¹ DM), supplemental protein, such as non-protein nitrogen, should be added to the diet of these animals to suit their maintenance and production needs [137]. A scarcity of water can reduce the feed intake, digestibility and consequently weight improvements in livestock. The cladodes of Opuntia contain a high water content, and when they are supplied in abundance, ruminants require little or no additional water [138] and their feed intake can be improved. Costa et al. [135] concluded that Opuntia is vital for decreasing water intake in lactating dairy goats. Therefore, Opuntia cladodes can serve as an alternative to natural water for ruminants, and more especially throughout dry seasons. Vitamins, antioxidants, and different flavonoids are found in Opuntia cladodes, including quercetin 3-methyl ether, a highly effective radical scavenger [98].

In terms of meat quality and carcass traits, several studies have recommended the incorporation of Opuntia in ruminant diets and have observed its effect on ruminant meat quality [9,139]. Opuntia cladodes could be introduced in goat kids’ diets, without a negative effect on the carcass characteristics and meat quality [139]. It was also reported that Opuntia decreases the fat content and increases the muscle proportion of a carcass [140]. Opuntia resulted in an increased meat pH and had no effect on tenderness [139]; however, a linear decrease in the ash content was reported of the goat meat fed Opuntia ficus-indica meal [141].

8. Control of Opuntia Species

There are several ways to combat the spread of this invasive species and mechanical, chemical and biological means can be used to control the species [41,142]. Historically, the success of the biological control programmes of Opuntia across the globe have benefited from collaboration, the sharing of similar problems and the transfer of successful agents amongst countries [143]. Since the 1980s, the use of biological agents such as Cactoblastis cactorum, Dactylopius opuntiae have showed success in controlling the spread of Opuntia stricta in the Kruger National Park [144], while D. opuntiae has become the most disastrous pest of the Opuntia spp. across the world [145,146]. Zimmermann and Moran [147] stated that biological control is the only economically viable and sustainable control for most Opuntia spp. and livestock can be used as the biological means to control Opuntia spp. Small stock (sheep and goats) start consuming the fruit and cladodes only after the spines are burnt off and this plays a part in the control of the plant [130]; however, horses are also one of the animal species that feed on this plant. In previous decades, studies discussed that when Opuntia reached a height of 1 m, livestock could be introduced with at least one animal per hectare, such that the consumption of both grass and Opuntia occurs [148,149]. Cattle do not feed on Opuntia where the spines are not burnt, and the burning of spines controls a greater quantity of Opuntia than ones used as feed [150,151]. Other studies have also highlighted that the cultivation of spineless Opuntia species will increase in the near future as they are more acceptable by livestock for biological control [152,153]. The use of biological control has resulted in substantial savings of herbicides and labour costs while specialised hand tools have been developed to cut Opuntia as a means of control which are later disposed of. There are two herbicides (monosodium methanearsonate and glyphosate) that are recently reported as a chemical control of Opuntia species. These herbicides are usually injected into the stem of the plant as concentrated solutions [148]. Meanwhile, although chemical control is expensive, a combination of biological and chemical control in other regions is still needed [147].

9. Conclusions and Recommendations

This review focused on the impact of the Opuntia species on the environment and also on how this invasive species can contribute to ecological niches globally. Due to its nutritional composition and acceptability by livestock, the species has a potential as an alternative source of feedstuff for ruminant animals. It is, however, important to devise strategies of cooperating approaches that reflect its contribution to arresting erosion, increasing grazing capacity, assisting ruminants and the provision of other pharmaceutical benefits, while maintaining species diversity at a minimal state in arid and semi-arid regions.