The main energy source for dairy cows comes from the fermentation of carbohydrates in their rumen. These carbohydrates can be divided into two main types: structural and non-structural. Pasture is high in structural carbohydrates, while supplementary feeds such as molasses and cereal grains are high in the non-structural carbohydrates, i.e. soluble sugars and starch.
In theory, milk production is maximised when non-structural carbohydrates make up 35 percent of the diet. In a pasture-based system, these non-structural carbohydrates are present only in relatively low levels. However, it’s not cost-effective to replace the structural carbohydrates found in pasture with sugar and starch from alternative feeds.
This is because the structural carbohydrates in good-quality, leafy spring pasture are highly digestible (70 to 85 percent) and can be degraded relatively quickly in the rumen. As a result, they supply similar energy to sugars and starches, so unless the total energy of the diet is increased, feeding supplements high in sugar and starch will not improve production.
It’s true that increasing the amount of sugar and starch in the diet of grazing cows will alter milk composition to favour more protein and less fat. However, it won’t increase energy generated from the rumen, or total milksolids production – unless the total energy of the diet is increased (see Figure 1 below).
Matching ‘building blocks’
This is because plant carbohydrates are made up of the same ‘building blocks’ of sugar units (like Lego blocks). The only difference between the carbohydrates is how these blocks (or sugar units) are joined together.
Soluble sugars are made up of individual sugar units already pulled apart and ready to use in the rumen. In comparison, starches are made up of sugar units joined together by a simple bond.
The structural carbohydrates found in pastures are made up of the same sugar units, but they’re joined together by a different bond to form a more complex structure.
Dairy cow digestion
Luckily, the dairy cow has enzymes in her rumen to break apart the bonds in these carbohydrates, so all the sugar units can be used.
So, even though spring pasture may be low in sugar and starch, there are enough readily digestible carbohydrates in pasture to provide a good energy source for the lactating dairy cow.
If there is enough pasture available for your lactating cows, then pasture is enough.
Myth: Increasing the amount of sugar in my cow’s diet will improve production and profitability.
Busted: The carbohydrates in good quality ryegrass pasture are readily digested and supply the cow with the same energy as sugar or starch.
This article was originally published in Inside Dairy September 2019