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Means in the same row followed by different letters differ (P < 0.05). Data are from Ben Salem et al. (1996).

carbohydrates (De Kock 1983; Ben Thlija 1987). In arid and semiarid regions of North Africa, cereal crop residues and natural pastures generally do not meet the nutrient requirements of small ruminants for meat production. Cladodes can provide a cost-effective supplementation, such as for raising sheep and goats on rangelands. For instance, when diets of grazing sheep are supplemented with cladode cakes, the daily weight gain increases nearly 50% (to 145 g day-1; Tien et al. 1993); when cladodes are supplied to grazing goats that have access to alfalfa hay, the milk yield is increased by 45% (to 436 g day-1; Azocar et al. 1991). When cladodes are associated with a protein-rich feedstuff, they may replace barley grains (Ben Salem et al. 1998) or maize silage (Metral 1965) without affecting sheep and cattle daily weight gains. For instance, milk yield for lactating goats supplied with 2.2 kg alfalfa hay day-1 is actually slightly higher (1,080 g day-1) when 0.7 kg cladodes replaces an equal mass of alfalfa (Azocar et al. 1996).

When sheep are offered increasing amounts of clad-odes, their intake of the accompanying straw also increases (Table 12.4). The dry matter intake of straw plus clado-des steadily increases as the amount of cladodes supplied increases. The cladodes are highly digestible, with similar digestibilities for organic matter, crude protein, and crude fiber, ranging from 45 to 64% (Table 12.4). Digestibility of cladodes by sheep is similar to that for common forage crops. The main difference between platyopuntias and other forage crops is the timing of the degradability of nutrients in the rumen. For other forage crops, maximal degradability in the rumen requires about 48 hours, whereas nutrients in cladodes are degraded very rapidly (between 6 and 12 hours) and little additional nutrient extraction occurs after 24 hours (Ben Thlija 1987). Similarly, 80% of the total digestion of singed Great Plains prickly pear (Opuntia polyacantha) by cattle occurs during the first 16 hours (Shoop et al. 1977).

A rapid rate of digestion leads to a faster passage of the material through the digestive tract, leading to more available volume for further intake. Because of the low gut fill of cladodes, an increase of cactus in the diet does not necessarily reduce the intake of other components of the ration (Table 12.4). This is of great importance for arid zones where livestock is fed mainly with straw or cereal stubble, which are low quality coarse feeds that have poor intakes, resulting in low animal daily weight gains. In feeding trials with heifers in the United States, cladodes are more readily and more completely digested than is grass-hay (Agropyron cristatum and Bromus spp.). In South Africa, the yields (tons fresh weight ha-1 year-1) and digestible nutrients (tons fresh weight ha-1 year-1) can be 80 and 5.0 for O. ficus-indica, 25 and 4.2 for Zea mays (maize silage), and 5 and 2.5 for dried lucerne hay (Rossouw 1961), suggesting a superiority of the platyopuntia as a forage.

Cladode Consumption and Sheep Rumen Fermentation

Ben Salem et al. (1996a) studied the effect of increasing the level of spineless cladodes on rumen fermentation in sheep given straw ad libitum. The cactus improves rumen fermentation (Table 12.4), as also occurs for sheep on an Acacia cyanophylla-based diet supplemented with cladodes and urea (Ben Salem 1998). In both cases the rumen fluid pH is not affected by up to 600 g dry weight of cladodes day-1, remaining at 6.9 ± 0.1. The cladodes are rich in easily fermentable carbohydrates, and their consumption probably enhances salivation.

Compared with a cactus-free diet, the highest supply of cladodes doubles the concentration of ammonia nitrogen in the rumen of sheep fed diets based on straw (Ben Salem et al. 1996a) or acacia (Ben Salem 1998). This leads to ammonia concentrations near optimal for microbial growth and fiber digestion in the rumen (Satter and Slyter 1974; Ushida and Jouany 1985). Indeed, the protozoa counts per volume increases fourfold as the amount of cladodes ingested exceeds 300 g dry weight day-1 (Table 12.4). Adding cladodes to the diet can increase the volatile fatty acids (such as propionic acid) by up to 30%, reflecting the increased intake of soluble carbohydrates.

Ben Salem et al. (1996a) used an in sacco technique (0rskov et al. 1980) to study the effect of cladode supply on cellulolytic activity in the rumen of sheep on a straw-based diet. Nylon bags (pore size of 46 pm) are filled with specified amounts of cladodes and straw, incubated in the rumen for various times, and then removed and the dry weights of the various fractions determined. The degrad-ability of cellulose is adversely affected by increasing amounts of cladodes in the diet (Table 12.4), although the rate of degradation is not affected (Ben Salem et al. 1996a). The impairment of cellulolytic activity in the rumen as the cladode intake increases may be because the increasing ciliate protozoa have a negative effect on the number of bacteria in the rumen and thus on cellulose degradation (Demeyer and Van Nevel 1979). The high level of minerals in cladodes may also decrease microbial growth in the rumen (Komisarczuk-Bony and Durand 1991). In any case, a combination of O. ficus-indica and cereal straw is a nutritionally satisfactory solution for maintaining small ruminants in arid regions. Indeed, supplying cladodes can improve the nutritive value and intake of the poor quality roughages.

Cladodes as a Water Source

Water scarcity can depress feed intake, digestion, and therefore weight gains of sheep and goats (Wilson 1970; El-

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