Starch levels are often seen as a measure of milk potential and rumen stability in diets, thus used as a benchmark when formulating diets.
In reality, starch levels in diets do not matter, because in isolation they don’t tell you a great deal. Starch is only one of four energy sources to the rumen and furthermore, the degradability of starch sources greatly influences rumen dynamics.
Rumen microflora feed on all rumen degradable carbohydrates, converting them into Lactic acid, which is then converted into volatile fatty acids (VFA). These transported across the rumen wall, leaving microflora and other parts to be washed out of the rumen and digested in the small intestine.
The type of ingredients and fibre provision dictates the rate of this process, the acid loading on the rumen and therefore, the pH in which the rumen operates. There are, however, several other factors contributing to this at a much greater level then the ration’s starch value. Many influential factors that cause acidosis or sub-acute acidosis (SARA) are environmental, such as ration sorting, feed space allocation, heat stress and feed bunk management.
Ration factors are secondary, of which starch levels paint only part of the picture. With UK diets (especially in the West), where the forage base is predominantly grass silage made from high sugar ryegrasses, lactic acid loading presents a risk to rumen health if not managed correctly. Although lactic acid is a valuable energy source to the rumen and an intermediate step in all carbohydrate fermentation, overloading can happen. Feed ingredients such as whey and molasses also provide rapid energy source to the rumen that, along with lactic acid, is not factored into starch calculations.
The other great influence on rumen health is fibre provision. A ration with high uNDF demands a higher supply of rumen fermentable energy (often as starch), than one that has lower uNDF and higher levels of digestible NDF. This is due the digestibility of the NDF having a significant influence of availability of carbohydrates and influencing rumen passage rate.
Starch sources supplied in a ration will have a significant impact on the economics of feeding a product (assuming rumen stability can be attained). This is because not all starch is equal, with some rapidly fermentable (e.g. bread) and some slower (e.g. cracked maize) – irrespective of the total amount of starch supplied by 1kg of the ingredient.
In essence, measuring starch on its own is a poor evaluation of a ration, both in terms of rumen health and yield potential. Better metrics to use include: 7hr starch rate (evaluating the starch degradation over a 7hr period) and NFC (non-forage carbohydrates), that better capture the effects of all rapidly degrading energy sources provided to the rumen and MP E production which is often a limiting factor within a ration formulation.
More importantly, starch measure does not take into consideration fibre degradation, which can have as big an effect upon rumen function and output as any other factor. This can often be seen in practice, with herds showing evidence of acidosis but can have low starch levels (15%), and some healthy, productive herds can run with starch levels of 28%. Similarly, some herds show impressive yields at low starch levels and some struggle to reach expectation with a high starch ration. If there is as much variation on farm as this, using starch levels to evaluate rations can be misleading and counterproductive.
Dairy Technical Specialist (Midlands)
m: 07876 824314