Mineral imbalance
Citrus by-products, including dried citrus pulps, have an unbalanced Ca:P ratio that may cause milk fever in dairy cows (Bampidis et al., 2006).
Rumen parakeratosis
High levels of citrus pulp in ruminant diets can result in rumen parakeratosis, a digestive condition that has been widely reported in livestock fed high-concentrate, low-roughage rations (Arthington et al., 2002). Large amounts of volatile fatty acids, particularly butyric acid, cause rumen papillae to become enlarged and keratinized, restricting nutrient absorption and impairing animal performance (Brugère-Picoux, 2004). In feedlot cattle and fattening lambs concentrates mixtures containing more than 60% citrus pulp can result in rumen parakeratosis (Arthington et al., 2002; Bampidis et al., 2006; Martinez Pascual et al., 1980). Keratosis was also observed in lambs fed a concentrate with 30% dried citrus pulp without hay (Martinez Pascual et al., 1980).
Mycotoxins
Due to its hygroscopic capacity, dried citrus pulp is susceptible to moulds, particularly in humid tropical climates. These moulds produce secondary metabolites such as aflatoxins and citrinin (the latter know to cause hemorrhagic syndrome) (Oliveira et al., 2004; Melo et al., 1999; Griffiths et al., 1991). Drying and pelleting should destroy the moulds and mycotoxicosis due to citrus pulp is uncommon, but a visual inspection before feeding is recommended (Arthington et al., 2002).
Limonin
Limonin is a triterpenoid present in the seeds and skins that imparts a bitter taste to citrus pulp. If large quantities of seeds are present their limonin content may render the pulp toxic to non-ruminants at inclusion rates as low as 2.5% of the diet (Rihani, 1991; Fuller, 2004; Devendra, 1988). It has been suggested that limonin caused intestinal irritation and poor absorption of the nutriments in broilers (El Boushy et al., 2000).
Limonin should not be confounded with limonene, another terpenoid present in large quantities in citrus peels and that is responsible for the characteristic odour of citrus. Limonene is practically non-toxic to birds and mammals (EPA, 1994).
Lectins
There have been reports of a Type IV hypersensitivity reaction that caused death from haemorrhage and heart failure of several cows fed citrus pulp. This inflammatory reaction may be the result of a lectin-like haemaglutinous activity thought to be present in citrus pulp. This condition could not be reproduced in sheep and rabbits (Saunders et al., 2000; Tokarnia et al., 2001).
Other natural compounds
Compounds such as tannins, saponins, phytates, oxalates and flavonoids have been identified in citrus peels but they are below the levels reported to be toxic to livestock species (Oluremi et al., 2007).
Pesticides
Contamination with pesticide residues can occur and depends on the compound used, dosage, rainfall at the time of application, time between applications, harvest method and citrus species. In Brazil, pesticide residues in citrus pulp have decreased in the early 2000s, due to a more rational application of pesticides (Oliveira et al., 2004).
Dioxin
In 1998, 100,000 tons of citrus pulp pellets from Brazil contaminated with dioxin were imported into Germany. The source of contamination with dioxin was found to be the lime used in the drying process (European Union, 1999; Malisch, 2000).