DairyNZ senior scientist Jane Kay busts the myth around pasture not being a complete fuel source for a cow's engine.
The modern dairy cow needs more than pasture can offer.
Good quality pasture is a well-balanced feed for the dairy cow, supplying energy, protein and fat.
Although additional minerals and vitamins are sometimes needed, pasture is a highly digestible feed and the modern-day dairy cow will “run” perfectly well (production and reproduction) on pasture alone.
Pasture versus a balanced ration
Many recommendations for dairy cow nutrition are derived from cows fed a total mixed ration. It is true that cows fed a total mixed ration can produce more milk; however, most of this difference in production is due to system-related differences, such as increased feed availability and reduced activity. Very little of the difference is due to the nutritional components in the diet.
Sugar and starch
It is still often claimed that supplements are needed to increase the amount of sugar or starch in a pasture diet. Even though pasture contains less sugar than supplements such as molasses, and less starch than many concentrates (e.g. maize grain or barley), the fibre in good quality, leafy pasture is highly digestible and is degraded relatively quickly, supplying a similar amount of energy as a high sugar or starch supplement.
This is because the building blocks of carbohydrates (sugars, starches and fibre) are essentially the same with the only difference being the chemical bond that joins them together. Luckily, the rumen bugs can break all these bonds, and replacing pasture with supplements high in sugar or starch does not improve the energy generated from the rumen bugs, unless the total amount of energy supplied is increased.
Another component often deemed as inadequate in pasture is fibre; however, there is no benefit to rumen function or milk production when fibre (straw or hay) is added to a pasture diet. In fact, it can actually reduce energy intake and production as a low quality feed (e.g. straw) replaces a high quality feed (e.g. pasture).
Typically, spring pasture contains more protein than the cow requires and although the protein in pasture is highly degradable (80-90%), the fast rumen passage rate means sufficient high quality protein is available to the cow.
Therefore, there is no need to balance a pasture diet with supplements. High quality, leafy pasture is a highly digestible feed, providing energy and necessary nutrients for the modernday dairy cow. Manage your pastures and you will look after your cows.
This article was originally published in Inside Dairy October 2015