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a Hoffman and Walker (1912)

b Lozano (1958); De Kock (1965); Theriez (1965); Teles (1978) c Nobel (1988)

dA. Nefzaoui (unpublished observations)

against fertilizer application. Therefore, increasing the nitrogen content of cladodes through selection or hybridization is more promising. Protein deficiency can also be solved through appropriate supplementation and/or feed combinations. Wehren (1976) showed that the amino acid composition of cladodes of O. ficus-indica var. iner-mis is quite satisfactory for animals; the proteins of three cultivars contained an average of 10.6% aspartic acid, 13.0% glutamic acid, 4.3% threonine, 4.3% serine, 6.5% proline, 4.8% glycine, 7.8% alanine, 0.8% cystine, 7.0% valine, 2.1% methionine, 5.2% isoleucine, 8.3% leucine, 4.1% tyrosine, 5.1% phenylalanine, 5.9% lysine, 2.3% his-tidine, and 5.5% arginine.

The nutritive quality of Opuntia cladodes depends on plant type (species, varieties), cladode age, season, and agronomic conditions (e.g., soil type, climate, and growing conditions). In Tunisia, the percentage dry weight is highest and crude protein content is lowest during the summer season; the ash content tends to be highest in the spring and crude fiber content highest in the winter (Ben Salem et al. 1996b). The percentage dry weight increases as cladodes age. Also, when cladode age increases from 1 to 5 years, crude protein content of O. ficus-indica var. inermins decreases (from 5 to 3% of dry weight) and crude fiber increases (from 9 to 20%; A. Nefzaoui, unpublished observations). This trend is similar to that for other fodder sources, where nutrients decrease with plant age resulting from the relative increase in fiber content (Nefzaoui and Ben Salem 1998).

Intake and Digestibility

Animals can consume large amounts of cladodes. For instance, Jersey cows fed on cladodes and supplemented with 1 kg day-1 of concentrated feed consume 51 kg fresh weight day-1 of the cladodes (Woodward et al. 1915). Metral (1965) similarly found a voluntary intake of 60 kg day-1 when cladodes alone are fed to cows. Viana (1965) reported an average voluntary intake of 77 kg day-1 for cows. For sheep, Monjauze and Le Houerou (1965) reported cladode intakes of 2.5 to 9 kg day-1. Valdez and Flores (1967) observed higher intake with O. ficus-indica (11 kg day-1) than with O. robusta (6.5 kg day-1). Intakes are higher when the water content of the cladodes is high (A. Nefzaoui and H. Ben Salem, unpublished observations). Cladode consumption can have a laxative effect on sheep, leading to a more rapid passage of the food through the animal's digestive tract. This leads to poorer digestion, especially when the cladodes constitute more than 60% of the dry matter intake; supplementing with fibrous feed (e.g., straw or hay) can alleviate such laxative effects.

Sheep fed straw consume up to 600 g dry weight of cladodes without any digestive disturbance (Cordier 1947). Supplementation of poor quality roughages with molasses increased their palatability (Preston and Leng 1987; Rangnekar 1988), so cladodes with their high carbohydrate contents may have similar effects as molasses. The energy content of cladodes is 3,500 to 4,000 kcal kg-1 dry weight, just over half of which is digestible and comes mainly from

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