Spineless cactus (O. ficus-indica), which is drought and erosion tolerant, is being advantageously and widely established in Tunisia and Algeria to slow and direct sand movement, to enhance the restoration of the vegetation cover, and to avoid the destruction by water of the land terraces built to reduce runoff (Fig. 12.3). In central and southern Tunisia, cactus plantations play a key role in natural resources conservation as well as provide a large amount of fodder for livestock. The strong rooting system of cacti helps stabilize land terraces. One or two rows of cladodes are planted on the inner side of the terraces (Fig. 12.3). The rooting system is enhanced by the collection of the water at the base of the terrace. Roots are widespread on the elevated part of the terrace and go deeply into the soil, ensuring stability of the terraces. As an additional benefit, cut and carried cladodes from such plantings provide sufficient feed resources during dry spells. Platyopuntias are also used in combination with cement barriers or cut palm fronds to stop wind erosion and sand movement.
Platyopuntias are an unbalanced feed nutrientwise but a cost-effective source of energy and water. Cladodes are low in crude protein, fiber, phosphorus, and sodium (Le Houerou 1992; Nefzaoui et al. 1995). Therefore, diets containing cactus should be balanced for these nutrients by appropriate supplements. The water content on a fresh-weight basis averages nearly 90% (Table 12.3). The ash content is about 20% of the dry weight. The crude protein is often below 5% but can be up to 10% of the dry weight (Table 12.3). The fiber content is also relatively low, about 10% of the dry weight. The nitrogen-free extract, which includes monomeric and polymeric sugars, is about 60% of the dry weight (Table 12.3). Cladodes' low content of phosphorous (P; about 0.03% of the dry weight) and sodium (Na; about 0.01%) requires supplementation when they are fed to animals.
The ash content of cladodes is high (Table 12.3), mainly because of the high calcium (Ca) content. High Ca levels in soils in arid and semiarid regions cause cacti to accumulate high quantities of Ca compounds (Nobel 1988). Platyopuntia cladodes also have high levels of oxalates (Ben Salem et al. 2002). For example, the oxalate amount is about 13% of the dry weight and 40% is in soluble form (the high amount of oxalates may explain the laxative effect of cladodes when fed to animals). The oxalates mostly contain Ca, making this cation less available to animals. Nevertheless, the Ca content of cladodes is generally higher than animal requirements. An excess of Ca is not a problem per se, but an unbalanced Ca:P ratio, which is generally about 35 for cladodes, needs to be corrected. Indeed, Shoop et al. (1977), working with Opuntiapolyacantha, indicated that its P content is below livestock dietary requirements. In particular, Ca levels are adequate but the Ca:P ratio of about 36 is too high for optimal livestock performance. De Kock and Aucamp (1970) reported the same problem with O. ficus-indica and suggested using licks for P and Na. These licks may be made from the following ingredients: salt, monosodium phosphate, molasses, dicalci-um phosphate, and bonemeal. According to Shoop et al. (z977), most other minerals in cladodes (e.g., manganese, copper, zinc, magnesium, and iron) have concentrations within the range generally acceptable in the diets of ruminants. An exception is sodium content, which is relatively low (0.02% for O. polyacantha).
Crude fiber content (Table 12.3) is actually a poor indicator of feed fiber quality. For instance, the fibers cellulose and lignin have low digestibility. For five platyopun-tias (Opuntia engelmannii, O. filipendula, O. fragilis, O. polyacantha, and O. versicolor), cellulose averages 11 ± 2% (mean ± 1 standard deviation) of the dry weight, hemicel-lulose averages 18 ± 3%, and lignin averages 3.9 ± 0.7% (Ben Thlija 1987); in comparison, for alfalfa these values are 22%, 15%, and 7.9%, respectively.
Although the protein content of cladodes is generally low (Table 12.3), it tends to increase after nitrogen fertilization. For instance, N and P fertilizers can increase crude protein contents of Opuntia cladodes from 4.5 to 10.5% of the dry weight (Gonzalez 1989). Also, Gregory and Felker (1992) found that some clones from Brazil have over 11% crude protein. In the WANA region, large plantations of Opuntia are utilized for fodder production in dry areas where soil quality, rainfall, and finances argue
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