Cereals are commonly sown in vineyards during the colder month of the year to increase the organic matter in the soil, as erosion control, relative vigour of the vineyard etc.

Cereals can be sown as monoculture or in a mix with legumes, brassicas, herbs and forbs.

The most commonly grown cereals are:

Ryecorn
Typically used when sowing is delayed as it's the most cold tolerant cereal. Ryecorn is also the most drought tolerant cereal therefore best on sandy and rocky soils. It grows rapidly, forms a dense tall stand with an extensive root system. 

Black Oats
More tolerant to wet and heavy soils with moderate fertility, drainage and low ph than other cereals but tolerantes wide range of soil. Great biomass and soil organic matter builder - often paired with a legume in a two species sowing.

Barley
Fast growing cereal with more tillers than Ryecorn or Oats, perfectly suited to dry or salty soils.