Starch is an important source of glucose and is particularly found in grains and silage maize. Glucose is the most essential source for the formation of lactose in milk. Starch in rations is partly resistant (i.e. is not broken down in the rumen) and partly unstable. Starch resistance depends on the rate of degradation and is determined by the nylon bag method. A correct ratio of resistant to non-resistant starch is important is needed for the milk yield of cows.

The non-resistant starch is fermented by rumen microbes, forming relatively large amounts of propionic acid. Propionic acid provides energy and is mainly used for the formation of lactose in milk. Lactose is a determining factor for milk production. However, too high a level of unstable starch in the ration increases the risk of rumen acidification.

The (rumen) bypass starch ends up in the small intestine, where it is enzymatically broken down into glucose. In this way, it makes a greater contribution to the glucose supply than non-resistant starch. The optimum amount of (rumen) bypass starch in the ration is between 30 and 75 g/kg dry matter. With too high a level of (rumen) bypass starch, excess starch ends up in the large intestine, where the starch provides less energy for the cow and can also lead to the formation of undesirable bacteria, such as E. Coli.

Starch in silage maize

The main supplier of starch in rations is silage maize. The starch content ranges from 250 to over 400 g/kg dry matter, depending on the variety and stage of maturity. This also means that the amount of NDF varies considerably; after all, the more starch, the less NDF, and vice versa. A high starch content generally goes together with a high Digestibility Coefficient of Organic Matter %, “Feed Unit Milk” (Dutch NEL system) and dry matter content. Starch resistance increases as the cob matures.

The new intestinal digestible protein system, introduced in 2007, showed that the passage rate of starch from corn silage into the rumen is higher than was previously assumed. The higher passage rate means that there is less time for the starch to break down in the rumen, so more starch arrives in the gut. Consequently, the (rumen) bypass starch content has been higher as of 2007.