Starch
Definition
Starch is a carbohydrate that serves as an important source of glucose for ruminants. It is particularly abundant in grains and silage maize. Glucose derived from starch is essential for the formation of lactose in milk, which directly influences milk yield.
Importance
Starch plays a critical role in energy supply and milk production. The correct balance between resistant and non-resistant starch in the ration is vital for optimal rumen function and cow performance. Resistant starch bypasses rumen fermentation and is digested in the small intestine, while non-resistant starch is fermented in the rumen.
Types of starch and their role
Non-resistant starch
Fermented by rumen microbes, producing propionic acid. This acid is a key energy source and is primarily used for lactose synthesis in milk. However, excessive non-resistant starch increases the risk of rumen acidosis.
Resistant (bypass) starch
Passes through the rumen and is enzymatically broken down into glucose in the small intestine. This provides a more efficient glucose supply. The optimal amount of bypass starch in the ration is 30–75 g/kg dry matter. Excess bypass starch can reach the large intestine, reducing energy efficiency and promoting undesirable bacteria such as E. coli.
Starch in silage maize
Silage maize is the main starch source in ruminant diets. Its starch content ranges from 250 to over 400 g/kg dry matter, depending on variety and maturity stage. Higher starch levels generally correspond with:
- Lower NDF (Neutral Detergent Fibre)
- Higher digestibility ( Digestibility Coefficient of Organic Matter %)
- Increased Feed Unit Milk (NEL system)
- Higher dry matter content
As maize cobs mature, starch resistance increases. Since 2007, the updated intestinal digestible protein system has shown that starch from silage maize passes through the rumen faster than previously assumed, resulting in higher bypass starch levels.
Key considerations
- Balance resistant and non-resistant starch for optimal milk yield.
- Avoid excessive unstable starch to prevent rumen acidification.
- Monitor bypass starch to prevent energy loss and bacterial issues in the large intestine.
- Consider crop maturity and fertilisation practices to optimise starch content.
