Bloat
Rape forage is readily degraded in the rumen. It is thus necessary to balance the ration with fibre alongside the crops, and to introduce non-hungry stock gradually (Limagrain, 2019).
Nitrate poisoning
If rape forage grows fast after rain on a soil with high N fertilizer or nitrate content, the leaves accumulate nitrates that can cause nitrate poisoning in livestock. Nitrate content was reported to be between 1.5-2% at first grazing in Australia. This condition can be detrimental to cows if they are not restricted (Fulkerson, 2008).
Sulphur induced haemolytic anemia and polioencephalomalacia
Rape forage contains high amounts of sulphur in the form of S-Methyl Cysteine Sulfoxide (SMCO). SMCO levels are higher when soil phosphate levels are low and nitrogen and sulphur levels are high. SMCO levels also increase when crops are flowering (Limagrain, 2019). The content of forage rape in sulphur is 0.5-1.3%, much higher than the recommended requirement for cattle (Johnston, 2009). The ingestion of high sulphur forage causes feed intake depression, weight loss, weakness and haemoglobinurea and risk of heamolytic anemia in cattle (Johnston). It can cause polioencephalomalacia (PEM) a condition characterized by lethargy, ataxia and blindness (hence the name "rape blindness"). (Kelly, 2013). PEM can be prevented by limiting the rape forage to 35-50% of total DM intake. In any case, flowering rape forage should not be fed to cattle.(Limagrain, 2019). In Australia, a PEM outbreak could be alleviated by intramusculary injections of thiamine (Kelly, 2013).
Goitre
Rape forage may contain elevated levels of isothiocyanate, a compound that blocks iodine uptake. Rape forage is itself low in iodine, which may increase the risk of iodine deficiency, affecting the thyroid gland and the hormones it produces (Limagrain, 2019; Johnston, 2009). Iodine deficiency induces goitre in livestock. Symptoms include growth depression, thyroid enlargement, hypothermia and tendon contractions in neonates (Johnston, 2009). To avoid those issues, it is recommended that fresh forage or silage are not fed above 35-50 % of total DM on a daily basis, with the remainder of the ration being fibrous and/or dry feeds such as straw, hay and grains (Limagrain, 2019).
Scouring
Long-term feeding of rape forage at greater than 60 % of the diet may cause scouring (Johnston, 2009).
Pesticides residues
When the rape crop was not originally intended for livestock, it is important to check grazing and feeding restrictions which may apply to crops treated or sprayed with pesticides or herbicides (Johnston, 2009).