Milk fat represents a large part of the economic and nutritional value of milk; therefore, decreases in milk fat concentration have serious consequences for a dairy’s bottom line. With that said, researchers have done a good job outlining the factors that cause low milk fat.
Often milk fat depression is somehow linked to diet fermentability and polyunsaturated fat load. The mechanisms around these links are regularly discussed, but one thing not discussed as often is the time line for recovery from a low milk fat situation.
Before diving into the time line for recovery from low milk fat, one needs to determine if the drop in milk fat is diet-induced or just a seasonal response. Every herd is different because of weather, forage changes and herd dynamics. When monitoring records from my clients, I generally see milk fat hit its peak shortly after the New Year, and then it reaches a low point around the Fourth of July.
This is repeatable year over year, and this seasonal component can easily be 0.3 percentage units. So when trying to chase butterfat concentration during the summer, one needs to be careful because they could be giving up total milk yield trying to get a higher fat percentage.
The ruminal biohydrogenation theory has become widely accepted to explain diet-induced milk fat depression. Simply put, this theory states the rumen biohydrogenates (transforms) the fatty acids found in forages, oil seeds, byproducts and fat supplements to get rid of double bonds and create more saturated fatty acids. Biohydrogenation is a multistep process through which intermediate fatty acids can be produced.
Certain intermediate fatty acids resulting from this biohydrogenation process are what cause milk fat depression. The amount of unsaturated fat substrate available to the rumen bugs will dictate how quickly milk fat drops.
But sometimes because of the resilience of the rumen microbial community, it can take around a week before these biohydrogenation intermediates create the low milk fat condition. Knowing the root mechanism of milk fat depression helps explain the time line for recovery of milk fat.
So to fix low milk fat, one needs to reduce the amount of biohydrogenation intermediates leaving the rumen. One option is to slow passage rate by adding fiber, but that can sacrifice total milk volume.
Many times the fix requires removing unsaturated fat and/or shifting the microbial community within the rumen, which takes days. Depending on how upset the microbial balance gets, this can take a couple of weeks to return to a desired profile.
The data is limited, but scientists have shown at the cellular level, when milk fat depression-causing intermediates pass from the rumen, the enzymes in the mammary gland responsible for milk fat synthesis become downregulated.
For milk fat production to recover, those enzymatic systems need to be upregulated back to normal as well. Therefore, two things need to be fixed: the rumen microbial community and the cellular mechanisms in the mammary gland.
Cornell and Penn State researchers have shown under controlled conditions if the problematic risk factor (or factors) is removed, it will take at least 10 days to rescue milk fat. In one controlled study, when milk fat depression was induced, it took 21 days for milk fat percentage to return to the original level.
So once you have made a ration correction and have the fat-dropping trend reversed, be somewhat patient with getting milk fat back to the original level. Fat is not expected to rebound in a day or two. If ration changes are aggressive and unsystematic, then one risks losing milk and still not fixing the fat concentration concern.
Milk fat depression is an old story, but it does manifest on dairies at inconvenient times. When brought in to help correct a low milk fat case, below is the checklist I go through with the client to help determine the root of the problem. This is the order of how we examine the ration together and start finding the solution:
1. Dietary polyunsaturated fat load
Unsaturated fatty acid intake should ideally be under 700 grams per day.
2. Carbohydrate type and degree of processing
3. TMR particle size
4. Particle size sorting of the TMR
(Caused by moisture level, poor mixing, forage length)
5. Rumensin/ionophore level
6. Mold or mycotoxins from forages or wet byproducts
7. Rate of fiber digestibility
8. Physical exposure of liquid fat versus oilseed fat
9. Unknown liquid fat specs and subsequent polyunsaturated fat load
10. Effective neutral detergent fiber (NDF) of the ration and actual chewing time
11. Feeding management
(Time of delivery, amount delivered, forage selection)
12. Acid load of the TMR (pH) and amount of ensiled low-pH feed being fed
Consider very rapid carbohydrate fermentation rates, which all the other factors will not overcome even if balanced (a common problem with lush grass, wheat grain and high-moisture corn)
13. High water sulfates
14. Accuracy and consistency of the actual formulation mixing, including dry matter adjustments
15. Mixer wagon scale accuracy
16. Moisture monitoring for humid and dry weather fluctuation
17. Fermentation profile of the silage 
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Chad Mullins
- Nutritionist
- Dairystrong Consulting LLC
- GPS Dairy Consulting LLC
- Email Chad Mullins
