Nitrate poisoning is generally caused when animals eat too much
forage that is high in nitrates not changed to protein in the plant.
Poisoning can also happen when animals eat too much urea or
nitrogen fertilizer spilled in the field or left where the animals
can find it.
Nitrate fertilizer is palatable, especially to cattle (tastes like candy to them) 
Causes of High Nitrates in Forage
All plants contain some nitrate, but excessively high levels are
likely to occur in forages grown under stress conditions, such as
when corn fertilized for high grain yield is stunted by drought
and is alternatively harvested for silage.
Other plants such as sudangrass, sorghum, pearlmillet, oats,
orchardgrass, and tall fescue can also accumulate nitrates at high
levels. Most weeds commonly found in corn also accumulate
toxic levels of nitrate; they include red root pigweed, common
lambsquarters, ragweed, velvetleaf, witchgrass, Canada thistle,
and black nighshade.
Nitrates accumulate in plants only when 1) there is a large
amount of nitrate in the soil, or 2)
some factor interferes with
normal plant growth.
High rates of nitrogen fertilization and
drought conditions are
the most important factors contributing to nitrate buildup in
plants
http://pubstorage.sdstate.edu/AgBio_Publications/articles/ExEx4015.pdf
