Gossypol
Cottonseed meal usually contains gossypol, unless it has been obtained from glandless (gossypol-free) seeds. Monogastric animals and young ruminants are highly susceptible to gossypol in cottonseed meal. Mature ruminant animals do not suffer from cottonseed meal consumption but it may affect their reproductive performance (European Commission, 2003). The toxicity of gossypol is described in detail in the cotton seeds datasheet.
Effects of processing on the gossypol content of cottonseed meal
When cotton seeds are extracted, gossypol glands are broken, releasing the gossypol in the cake. The effects of processing (pressure, heating, solvent, duration) on the free gossypol content are variable (Zahid et al., 2003b). In some cases, the process decreases the gossypol content, improving the nutritional value (Jimenez et al., 1989; Nikokyris et al., 1991; Mena et al., 2001; Mena et al., 2004; Guedes et al., 2010; Nunes et al., 2010). In others, however, the process may have no effect, or result in a higher free gossypol content and, therefore, higher toxicity (Morgan, 1989).
The screw press method uses heat which increases the binding of the gossypol protein, converting free (toxic) gossypol into bound (non-toxic) gossypol, and the resulting meals have the lowest amounts of free gossypol (200-500 mg/kg) (Morgan, 1989; O'Brien et al., 2005). While heating may cause loss of protein quality owing to binding of gossypol to lysine at high temperatures, the shearing effect of the screw press inactivates gossypol at temperatures which do not reduce protein quality (Tanksley, 1990).
The solvent extraction method (with no mechanical and heating step) does not require high temperatures, which limits the gossypol-protein binding, and the amount of residual free gossypol may be up to ten times higher than in screw-pressed meals (1000-5000 mg/kg). Indeed, the direct solvent extraction method may have been responsible for the rise of gossypol toxicity observed in the 1980s in the USA (Morgan, 1989).
In the pre-press solvent extraction method, the amount of free gossypol is in the 200-700 mg/kg range, comparable to that of screw-pressed meals (O'Brien et al., 2005).
New solvents such as ethanol can inactivate a greater proportion of the gossypol, resulting in safer cottonseed meals (Saxena et al., 2011).
Commercial production of cottonseed meals is now achieved routinely with 1000-2000 mg/kg remaining as free gossypol (EFSA, 2008).
Alleviating the effects of gossypol
Gossypol toxicity can be alleviated through the addition of iron salts. The following ratios of iron to free gossypol have been used to improve growth performance: 1:1 (cattle, pigs and Nile tilapias), 2:1 (broilers) and 4:1 (layers) (El-Saidy et al., 2004; European Commission, 2003).