Nitrate poisoning
Nitrates are generally non-toxic to ruminants, but their transformation into ammonia in the rumen produces toxic nitrites that bind with haemoglobin and prevent blood from binding with oxygen. This results in oxygen starvation of the tissues and animal death in the most severe cases (Marais, 2001). Nitrate levels ranging from 0.5 to 1% DM in the plant are considered potentially toxic to ruminants and levels higher than 1% DM are considered dangerous (Yaremcio, 1991).
In rye, nitrate accumulation in the plant can be caused by water stress, cold, diseases and high N fertilizer rates. It is thus recommended to limit access to the stand or to feed livestock with grain before it enters the sward. Ruminants can get used to high-nitrate forage provided that they are progressively introduced to such forage. If rye forage is offered as green chop, silage or hay, it should be mixed with another feed in order to decrease the overall nitrate content of the diet (Yaremcio, 1991).
Grass tetany
Grass tetany generally affects dairy cattle, beef cattle, sheep and goats in the temperate regions of the world when they receive inadequate available magnesium (Mg) in the diet. Grass tetany symptoms are nervousness and twitching, then staggering, collapsing and convulsions that may end up in death (Dalley, 2004). Autumn rye and other lush growing pastures may have low Mg levels after intensive growth (Chelliah et al., 2008). This problem often occurs when the crop has received large amounts of water (rainy spring) and has been heavily fertilized with N and K, because Mg is leached by water, and an over-application of fertilizer. In some cases, the Mg level is nominally adequate but there is an unbalanced mineral ratio if K:(Ca+Mg) is more than 2.2, which is detrimental to Mg availability (Agdex, 2009). Livestock grazing unbalanced pasture or pasture where the Mg is low, have low Mg levels in their blood. This phenomenon is of utmost importance in high-yielding dairy cows, which require extra Mg for fat mobilization and milk production. The recommended safe level of Mg in forage is 2.0 g/kg DM for lactating or pregnant cattle (Grunes et al., 1970; NRC, 2000). However, if N and K are high, the Mg concentration should be at least 2.5 g/kg DM (Grunes et al., 1984). It is possible to prevent the problem by adding Mg fertilizer to the sward, or by feeding animals with an adequate mineral and energy supplement (Agdex, 2009).
Ergotism
Rye grain is susceptible to ergot (Claviceps spp.) infestation. Mycotoxic risk associated to ergotism is negligible when forage is pastured or harvested before flowering but increases after seed-heading (ISPB, 2011). The alkaloids produced by Claviceps purpurea have vasoconstrictor and neurotoxic properties that result in necrosis of extremities, staggering, lameness, hyperthermia and sometimes death (Bourke, 2000; Guerre, 2011).