Milk fat depression (MFD) is a common issue observed in Australian dairy cows. Highly fermentable and fatty acid rich pasture in winter-spring, coupled with altered rumen function, are risk factors to MFD.

Understanding the difference between milk fat percentage and milk fat yield to determine if we have a “true” or perceived milk fat depression is an important concept. Lactose (milk sugar) determines milk volume. Dietary sugar/starch can increase milk volume whilst milk fat yield is maintained, reducing milk fat percentage. MFD as discussed here will show a reduction in milk fat percentage, and also milk fat total yield.

Following a diet correction, at least 14 days are required to identify any changes in milk fat yield and animal health.

MFD is typically a multifaceted issue, and it is influenced by dry matter intake (DMI), diet balance and animal health. There are a number of nutritional and management strategies that can reduce the impact of MFD. The three risk factors are:

  1. High intake of polyunsaturated fatty acids (PUFAs)
  2. Fast rumen throughput
  3. Altered rumen function

High intake of PUFA

Traditional grasses contain 2-3% fats. Newer grass species can contain 6-8% fat, which are correlated with higher crude protein levels (Porter et al. 2001). Not only is the proportion of fat in these grasses higher, but our newer grass species are lower in effective fibre allowing higher dry matter intakes. Thus, the total amount of fat ingested is also increased. Unsaturated fats in pastures must be detoxified by the rumen bugs and converted to saturated animal fat. This process is called biohydrogenation. When fats in the diet are very high, the rumen bugs responsible for this conversion can’t keep up, and some of that fat can undergo an altered fermentation pathway. This excess unsaturated fat gets converted to trans-10, cis-12 conjugated linoleic acids (trans-10 CLAs). These trans-10 CLAs can move across the rumen wall into the bloodstream, where they travel to the udder and interfere with milk fat synthesis. The presence of trans-10, cis-12 CLAs can drop milk fat yield by up to 50%. 

Fast rumen throughput

MFD is directly correlated with low rumen pH and fast rumen throughput (Stockdale et al. 1990). Lush pastures generally have lower fibre, higher sugars and more soluble nitrogen. This means that rumen pH can be below 6 for a lot of the day, and transit time through the rumen is faster. High potassium (K) and nitrogen (N) in highly digestible pastures can also affect rumen pH and reduce the efficiency of fibre digesting bugs. Rotation length, leaf stage and fertilizer applications need to be managed appropriately so that pasture can be efficiently digested and utilised by the cow.

Physically effective fibre can slow down the rate of passage, increase rumen contractions and create a “rumen raft”. A rumen pH of 6-6.9 is suitable for the fibre digesting bugs. Rumen contractions and saliva production enhance the buffering capacity of the rumen, and help to manage pH. Introducing some hay or straw into the diet can help to slow down rumen throughput. This will also give the rumen bugs more time to biohydrogenate the fats in the pasture. This produces less of the trans-10 CLAs that affect milk fat synthesis in the mammary gland.

Altered rumen function

  1. Grain Processing– rolled/cracked grain ferments more slowly than fine ground, thus reducing the negative impact of slug feeding on rumen pH. Slug feeding high amounts (>2-3kg at a time) of unbuffered wheat/triticale/barley can lead to rapid accumulation of volatile fatty acids (VFAs) in the rumen. High starch reduces rumen pH, increases propionate, and also increases the production of trans-10 CLA (Griinari et al. 1998). A pH decline that creates sub-acute ruminal acidosis (SARA) conditions will exacerbate a MFD issue.
  2. Maize – Replacing some fast-fermenting grain with slower-fermenting maize can help to manage fluctuations in rumen pH. Almost half the starch in maize goes to small intestine and is absorbed as glucose (Overton et al. 1995). This gives the benefit of supplying the cow with a source of glucose (energy) without reducing the rumen pH as much as with faster-fermenting cereal grains (Opatpatanakit et al. 1994). Maize helps DMI too, which is conducive to more gut fill, rumination, and more milk fat. This additional intake is exactly what we’re after in late winter/spring when we want to eat as much grass as possible and convert it into milk solids.
  3. Bufferssuch as AcidBuf can help stabilize rumen pH and help promote a more ideal environment for the microbes to biohydrogenate the fat.
  4. Salt – Sodium is used for saliva (sodium bicarbonate) production and osmolality of the rumen. Salt in the ration can therefore help the rumen environment and reduce the severity of MFD.
  5. Ionophores– such as Monensin can interfere with the microbes responsible for fat biohydrogenation, amplifying a build-up of unsaturated fat in the rumen and potential MFD issue. Ionophore inclusion in the diet needs to be weighed up against the benefits (bloat control).


Control and prevention of Milk Fat Depression needs to be approached holistically. Stage of lactation, body condition score, genetics and animal health status also need to be considered. Diet balance and rumen pH can be influenced by the physical form and composition of the grain ration, as well as pasture management. Your Reid Stockfeeds nutritionist will be able to identify these risk factors and provide tailored solutions for your farm.