Understand milk fat depression and prevent it in your cows
A shift in the metabolism of dietary oils in the rumen is now recognized as the major cause of milk fat depression. The cause of this shift is related to two main factors:
- The amount of vegetable oil in the diet
- Rumen pH
Controlling these two factors will allow a dairy to improve and maximize milk fat production. Some feed additives claim to improve milk fat production and, in many situations, they do. However, they cannot overcome the dramatic impact that these two main factors have in causing milk fat depression. Trying to improve milk fat without addressing these two factors is next to impossible.
Ingredients we typically see in the upper Midwest that contain high levels of readily available vegetable oil include distillers grains, hominy feed, whole soybeans, and some bakery and candy byproducts. When the unsaturated (liquid) fatty acids contained in the vegetable oil in these ingredients enter the rumen, the rumen bacteria try to convert them to the saturated (solid) fatty acids that are found in tallow and butter. This process of fatty acid conversion is called biohydrogenation and it occurs via two pathways.
The normal pathway of fatty acid conversion has no negative effects on milk fat production. The altered pathway produces some intermediate fatty acid compounds that have a very dramatic and powerful negative effect on milk fat production if they escape the rumen and are absorbed in the gut. In fact, just a few grams of these intermediates can decrease milk fat by more than 0.5 percent units. Milk fat depression occurs when the preferred (normal) pathway is overwhelmed with more vegetable oil than it can metabolize and/or when cow’s rumen pH is lowered.
When addressing a milk fat depression challenge, the first steps often involve trying to decrease the vegetable oil in the diet, which in most cases, is relatively easy. Often feed ingredients containing the high levels of readily available vegetable oil can be minimized or removed. Reducing the amount of vegetable oil reduces the potential amounts of the intermediate fatty acid compounds.
The challenge is that – even if we do reduce or remove those feed ingredients – the base diet of corn silage, corn and other feedstuffs often contains enough vegetable oil to cause milk fat depression if rumen pH is not managed.
In my experience as a dairy nutritionist, the most challenging milk fat depression cases I have been involved with have always involved challenges related to lowered rumen pH. Rumen pH is much more difficult to address because it is influenced by many factors related to both feed ingredients and management. Understanding the causes of lowered rumen pH and how to address them will help you work with your nutritionist to improve milk fat production on your dairy.
Causes of lowered rumen pH
- A ration with too much starch
- A ration in which the rate of starch digestion in the rumen is too fast; feeds that typically have a rapid starch digestion rate are:
- Wet high moisture corn and corn silage that have been ensiled for more than six months
- Bakery and cereal byproducts
- Ground small grains (ex. wheat or barley)
- A ration that is sortable
- A cow can sort long particles out of a ration, which allows her to eat more fine particles and grain. Sorting decreases effective fiber, leading to decreased cud chewing and often increases the amount of grain and starch eaten.
- Sorting can be minimized by adequately processing forages and adding liquid feeds or water to the TMR.
- A ration without enough effective fiber to encourage adequate cud chewing from either too low of forage-to-concentrate ratio or the forage particle length is too short
- Lack of adequate buffering
- Cows buffer their rumens through sodium bicarbonate in their saliva that is produced when they chew their cuds.
- When cows eat large meals, rumen pH decreases rapidly.
- Adding sodium and potassium buffers to rations often helps maintain rumen pH.
- Slug feeding
- Cows tend to eat a large meal when they return from the parlor. Slug feeding happens when cows are unable to access feed for an extended period of time and then are given full access to feed. They then rapidly consume a lot of feed. The rumen is not as full, so it is not as well buffered and this large meal leads to a rapid drop in rumen pH.
- Slug feeding can be avoided by:
- Having feed available a minimum of 22 hours a day
- Pushing up feed on a regular schedule
- Every 30 minutes for the first two hours after feeding
- As needed after that, but at least every two hours throughout the day
- Minimizing time away from the feedbunk
- Minimizing overcrowding of the feedbunk
Contact your nutrition consultant to discuss practical steps to improve and maximize milk fat.
About the author: Stacy Nichols is a Vita Plus dairy technical specialist. He grew up in northwest Illinois and finished high school in northeast Georgia. He received his bachelor’s and master’s degrees in dairy science from the University of Georgia. Nichols was the herdsman at Mississippi State University’s Bearden Dairy Research Center from 1994 to 1996. He then returned to Georgia as the herdsman on a private 350-cow dairy. Since 1997, he has been involved in the dairy feed industry on a local, national and global level. Nichols has considerable experience in transition cow management, nutritional modeling and amino acid formulation. He and his wife, Melissa, have eight children and reside in northwest Indiana.
Milk production and components