One of the main reason milk fat may be depressed is due to insufficient dietary energy intake, especially during early lactation phase. Cows that are "fatter" during early lactation phase have higher milk fat percentage compared to cows that are in thin condition. Most high producing cows will lose weight during this period. Hence, energy intake should be maintained as high as possible without causing the animals to go off feed. When the cows lose too much body fat too rapidly, ketosis can occur, which will affect the overall health of the cow.
Cows that are fed with high grain throughout the dry period will in general, produce more milk which is richer in fat and SNF for a few months postpartum compared to cows that are fed modest amount of grain. However, this will cause some metabolic disorders to the cows and is not economical. The recommended practice is therefore to only increase the amount of grain fed during early lactation phase where most milk is produced, hence most energy is needed.
Usually, low fat% can occur due to:
Energy Shortage - This usually happens between 90 - 150 days of lactation. Cows are usually quite thin and have a low BCS score. The milk protein % is also reduced. Generally, the ration is not balanced.
Milk Fat Depression - Can occur at any stage of lactation. The cow probably has decent BCS score. Cow is off feed and. The milk protein % is higher than fat %.
Sometimes, feeding certain dietary compounds will reduce the fat % of milk while keeping the milk production constant. Examples such as feeding high grain, low roughage rations will help increase the milk production but will depress the milk fat %. This usually happens when roughage is restricted to 30% or less of dry matter (DM) fed. Other examples of dietary compounds that will reduce milk fat % include:
Finely ground (< 1/8 inch) or palleted diets.
Certain oils or polyunsaturated fatty acids such as cod liver oil or extracted seed oils.
Flaked corn, heated starches or expanded grains
Grazing certain pasture plants such as lush spring pastures, young oats or pearl millet.
When fat % is reduced to 2% or less, this shows that there is a true milk fat depression syndrome. This is usually due to the imbalance of production of volatile fatty acids (VFA) in the rumen.
Diets that can cause increased in milk fat % include feeding protected fats. Diets with less than 5% fat in diets will inhibit the rumen bacteria. Dietary unsaturated fatty acids are saturated by the microbes. Sodium salts of fatty acids can act as a detergent and kill the microorganisms. Other diets that can help increase the milk fat % include feeding certain saturated fatty acids such as palm oil, feeding high roughage diets and feeding frequently.
Supplemental Dietary Fats
Sometimes, even for high producing cows that are fed with sufficient amount of grain can still remain in energy deficit, especially during early lactation. The energy demands of producing milk as well as dry matter intake limited by rumen can affect the milk production yield. By increasing grain concentrate in the cow's diet up to 60% or more of total dry mater will only cause other problems such as acidosis, reduced fiber digestion, lowered feed intake, off feed problems and finally, overall lower milk fat %. Therefore, a solution would be to increase the energy density of the diet. One good way to do that is to use commercial fat products such as rumen bypass fats. A good time to include supplemental fats is during early lactation phase. Normally, 3-4% of a cow's diet will be fat. However, during early lactation phase, this can be increased up to 7-8% of total diet dry matter. With that being said, over feeding supplemental fat can also depress ruminal fermentation of fiber.
Milk fat % should also be maintained or increased if the main reason it was depressed is due to over feeding grain. The milk fat % response to adding supplemental fat may vary depending on the amount of fat, type of fats, physical form of fats and fatty acid composition of the fats. Adding supplemental fats to the diet will generally decrease the milk protein %, usually up to about 0.3% units. The fatty acid composition of milk fat is also affected by the supplemental fats. In general, there will be an increase of long chain fatty acids and decrease in short or medium chain fatty acids since most supplemental fats contain long chain fatty acids.
Other nutrient changes needs to be considered when supplementing fats to the cow's diet. Sufficient fiber needs to be available in the diet in order to stimulate ruminal fermentation. Adding fats to the diet will decrease the proportion of grain in the diet, thus lowered available energy to the rumen microorganisms, and hence less microbial protein synthesised and made available to the cows. A good source of dietary protein that is of low rumen degradability will help make up for this deficit.
The type of supplemental fats added will also affect the results. In general, adding seed oils extracted from plants such as cottonseed oil, sunflower seed oil or soybean oil will have negative effects on ruminal fermentation. An "inert" fat source such as PalmFat S100 or PowerFat will not alter ruminal fermentation. Other acceptable supplemental fats include oil seeds such as whole cottonseeds, whole soybeans; tallow, hydrolysed animal-vegetable blends and prilled fats.