This is the time of year when we see a drop in the percentage of butterfat in milk. It is also the time of year when we hear about ways of preventing butterfat from falling. A number of feed sales companies, even those belonging to farmer owned co-ops, are pushing the inclusion of yeasts and buffers in dairy rations. In one video, the sales rep says that including yeast buffers in dairy rations will “avoid and solve milk fat depression”.

These bacteria improve the overall efficiency of the rumen and increase the amount of volatile fatty acids being produced, such as acetate

Finding scientific evidence to support this claim in grass-based systems is hard to come by. The science behind the inclusion of yeasts in the diet of dairy cows is that they increase rumen pH, which improves the environment for bacteria in the rumen. These bacteria improve the overall efficiency of the rumen and increase the amount of volatile fatty acids being produced, such as acetate. Acetate is a key volatile fatty acid as it is from this that milk butterfat production is derived.

However, the vast majority of the peer reviewed research on yeasts has taken place in fully housed systems.

Most of the Irish research on yeast buffers in dairy cow diets has taken place at UCD Lyons Estate

In a meta-analysis of 31 studies on the inclusion of yeast cultures in mostly housed dairy cow diets, milk butterfat levels increased from an average of 3.73% where there was no yeast included, to 3.74% where yeast was included. However, milk yield also increased, so overall butterfat production rose by 0.04kg/cow/day.

Most of the Irish research on yeast buffers in dairy cow diets has taken place at UCD Lyons Estate. Again, the results at Lyons show an increase in butterfat percentage when freshly calved cows are fed yeasts. One study found butterfat levels increased from 4.21% to 4.36% when yeasts were included in the diet for up to 10 weeks post-calving.

Environment

However, this trial was not conducted on grazing animals. It is well known that the rumen environment is very different between cows grazing grass and those eating preserved forages. Comparing the rumen environment and dungs of cows fed indoors and cows grazing is like comparing chalk with cheese. Incidentally, there was one study at Lyons looking at the inclusion of yeasts in freshly calved cows at grass, but there was no significant difference between the treatments and the results were never reported.

In a recent study, slight improvements in butterfat were observed when high yielding cows were fed acid buffers in 8kg of meal at grass.

The drop in butterfat at this time of year is a big issue and it does cost farmers money. But unless there’s scientific evidence that something works then spending money on that is only going to cost you more money.

The inclusion of yeasts in dairy rations will increase the cost by about €11/t. In some cases, the specification of the ration changes also, so you need to pay even more for extra ingredients. In one mill, the basic dairy ration costs €30/t less than the ration with yeast included.

Other farmers give cows’ access to small quantities of straw or other high-fibre feeds at milking time, such as soya hulls or palm kernel. Most people who do this say that it works well, but there’s no real evidence for or against it. However, it’s a relatively low-cost way of doing something.

In my view, butterfat dropping at this time of year is a natural occurrence and not an indicator of health problems. Look at what happens in the wild: grass is always scarce at the end of winter and just after animals give birth, growth rates take off, producing young leafy grass that’s high in protein and low in fibre.

The milk produced by mammals that graze grass allows their young to grow fast.