Gossypol
Gossypol is a toxic yellow pigment found commonly in cotton (Gossypium genus) and other species of the Malvacae family. In the cotton plant, gossypol is mostly contained in special glands localized in the seed kernel, though other parts of the plant, including the seed hulls, leaves, stems and roots also contain gossypol. Gossypol plays an important role in plant protection against predators (mice, insects), diseases (bacteria and viruses) and parasites (root knot nematode) (Morgan, 1989; Blasi et al., 2002).
Gossypol is a polyphenolic compound that exists in free and bound forms: the free form (soluble in 70-30 v/v aqueous acetone) is physiologically active and toxic whereas the protein-bound form is non-toxic (Morgan, 1989). Total gossypol content in cotton seeds is highly variable and ranges from 0 to 9% (when including wild Gossypium species) (Benbouza et al., 2002). Gossypol content depends on species, cultivars, fertilizer rates, growing conditions, and insect pressure (Blasi et al., 2002; Randel et al., 1992; Carter et al., 1966). The two main cotton species, Gossypium hirsutum and Gossypium barbedense, both contain variable amounts of gossypol, G. barbadense being the richer (OGTR, 2008). In Gossypium hirsutum seeds, gossypol concentrations ranging from 0.6 to 1.15% DM have been reported (OGTR, 2008). In the kernels, gossypol content could range from 0.4 to 1.7% DM (European Commission, 2003). Cotton varieties that produce little or no gossypol have existed for many years and glandless varieties have been developed since the 1950s, with the idea of using the cottonseed products more widely for food and feed. However, such varieties tend to be more sensitive to pests and less productive and have thus been found less economically viable. Investigations on glandless cotton are still ongoing, either using traditional selection methods (Rodman, 2006) or gene-silencing techniques (Bourzac, 2006).
Free gossypol is the most important form of gossypol in unprocessed seeds: free gossypol content was reported to be in the range of 0.05 to 0.7% DM in Gossypium hirsutum seeds (OGTR, 2008; EFSA, 2008). In addition to genetics, growing conditions and environmental conditions, the free gossypol content of cotton products may depend on processing: heating, for instance, increases gossypol binding and reduces toxicity, whereas fine grinding breaks the gossypol glands, releasing gossypol in the product. Free gossypol causes moderate acute toxicity in animals. The LD50 are 2400-3340 mg/kg for rats, 500-950 mg/kg for mice, 350-600 mg/kg for rabbits, 550 mg/kg for pigs and 280-300 mg/kg for guinea pigs (EFSA, 2008). Signs of acute gossypol toxicity include constipation, dyspnoea, anorexia and loss of weight. Death usually results from circulatory failure. Although acute toxicity is uncommon, ingestion of small amounts over a prolonged period can be lethal (European Commission, 2003). In rats and humans, repeated exposure to lower doses of gossypol mostly affects the testis in males (reduced sperm motility, inhibited spermatogenesis and depressed sperm counts) and reproductive organs and embryo development in females (EFSA, 2008).
The effects of gossypol on animal health and performance depend on the type of animal. In mature ruminants, breakdown and maceration by chewing and exposure of free gossypol to rumen microorganisms causes deactivation, binding to amino nitrogen or to metal ions such as iron, and degradation that renders the gossypol unavailable to the animal, thus protecting it from the toxic effects of gossypol. However, young ruminants with an immature rumen do not benefit from this protection and are thus more sensitive to gossypol (Blasi et al., 2002 ; Poore et al., 1998). Monogastrics are also particularly sensitive to gossypol, which limits the use of cottonseed products in these species unless they have been properly detoxified. In pigs and poultry, it is possible to alleviate gossypol toxicity through the addition of iron salts to the diet. Iron salts, such as ferrous sulphate, are effective in blocking the toxic effect of dietary gossypol, possibly by forming a strong complex between iron and gossypol and thus preventing gossypol absorption (EFSA, 2008).
