Starch: is a more complex CHO than sugar as it has more of a storage mechanism role in mature plants. Cereal grains such as wheat, maize, barley and sorghum contain very high levels of total starch per kg DM. Some root vegetables are also high in starch, such as potatoes and cassava (dehydrated as tapioca).
Depending on the complexity of the starch molecules found in these feeds and the form of processing we may put it through, the starch content will have differing rates of fermentation in the rumen. Therefore in ration formulation, starch is grouped as either rumen degradable starch (RDS) or rumen escape starch (RES). Excess RDS relative to fNDF can predispose cows to SARA, but this is uncommon in New Zealand pasture based systems due to typically low levels of starch feeding. RDS is fermented primarily to propionic acid, which is a blood glucose precursor.
Maize is of high value for lactating cows due to increased levels of RES. Starch that escapes ruminal degradation is converted to glucose post ruminally and is absorbed directly into the blood stream. In combination with propionic acid from RDS, this allows for optimal flows of blood glucose to the mammary gland to support increased milk volume and protein synthesis.