Fat Evaluation Index (FEI) in milk from dairy cows: What factors increase the FEI?

What is the fat evaluation index (FEI) in milk?

Milk fat composition

Milk fat is part of the milk solids part of milk that farmers get paid for. The average milk fat % for New Zealand cows is 4.74% fat based on the most recent LIC Dairy Statistics released. But many things determine what the average milk fat of your cows is compared to the NZ average. Cow breed has a large influence, as well as stage of lactation and what’s the diet is composed of. 

• For every 1kg of milk fat, the individual fatty acid composition of that 1kg of fat is extremely variable. The milk fat is made up of a large range of different short, medium and long chain fatty acids (FA). Most are ‘saturated fats’ but others can contain unsaturated fatty acids. Many things change the composition of fatty acids that make up a kg of milk fat.
• Stage of lactation and/or if cows are gaining or losing body condition is an important modifier of milk FA composition; short and medium chain FA are LOW as a percentage of milk fat earlier in lactation (since there are more long chain FA coming from back fat mobilisation when cows are losing a bit of body condition). The proportional % of short and medium chain FA increase again once cows are gaining more weight and no longer stripping body condition.
• DIET is another major factor that moves fatty acid composition around. And this is where the FEI can be ‘bounced’ around – on the basis of the diet the cows are eating.

The Fat Evaluation Index itself

• Backfat. When cows are mobilising fat off their backs, these become available for milk fat - these are almost all long chain fatty acids (mostly C18).
• Feed: Half of the FA in milk come directly from the fat contained in feed that the cows eat. These pre-made fatty acids end up directly in the milk. That means if we feed supplementary feeds that contain a heap of medium chain FA, the FEI goes up. Palm Kernel contains a high level of C12 FA (that’s one of the medium chain fatty acids).. So…. PKE lifts the FEI by adding a heap of medium chain FAs into the milk. That’s the first thing that contributes to a higher FEI in milk. Note other protein meals, specifically, copra meal also contain high levels of C12 (around 50% of coconut oil is C12, just like PKE). BUT, copra meal stopped being fed to dairy cows many years ago due to risk of aflatoxin transfer into milk. 
• Manufactured by the udder: The other half of the FA in milk are made by the udder. The scientists call this “de novo” synthesis of FA. The ‘building blocks’ that the udder needs to make these udder-manufactured FA come from the diet. Acetate and butyrate are volatile fatty acids in the rumen that enter the blood and reach the udder to be converted into milk FA. Most of these de novo FA are the short to medium chain FA. Remember the FEI is a measure of medium chain FA as a percentage of total FA.

Rumen Acidosis and milk fatty acids

• Low total milk fat % with acidosis. Most of us know that total milk fat % in milk can go down / end up lower when we have issues of clinical or sub-clinical rumen acidosis (too much acid in the rumen). So this might be from too much grain, grain that’s been over processed, high starch by-products like Tapioca and when cows first transition onto e.g. bulb crops like turnips or fodder beet.
• Relatively more medium chain FA with rumen acidosis. Although rumen acidosis reduces TOTAL milk fat in milk, it also changes the relative proportions of medium to long chain FA. The milk fat % is lower but of that milk fat, there’s relatively more medium chain FA relative to long chain FA in each kg of milk fat.
• If you’ve got a bit of an issue of some lower rumen pHs in your herd due to very soft, lush grass (e.g. mid October right now) AND you’re feeding cereal grains, tapioca or other feeds that increase risk of rumen acidosis, that MAY explain why your FEI is higher than normal even if you’re not feeding PKE.
• If you are feeding PKE AND you happen to have some short term issues of rumen acidosis, e.g. during the first few days transitioning cows onto summer bulb turnips – you’ll get a ‘double whammy’ of risk of a high FEI.

Summary

• PKE: The most common cause of high FEI is preformed medium chain FA in PKE. These are mostly C12 medium chain fatty acids. The answer is obvious – drop PKE feeding rates until the FEI stabilises at a lower value.
• Rumen acidosis: This is the second most likely cause of a high FEI. If you’re getting stuck with some elevated FEI in milk and you’re not feeding PKE, the next thing to look for is what might be upsetting rumen function in the cows. “Hot” forms of starch based feeds are the more likely culprit, e.g. over processed wheat grain, tapioca etc, especially when combined with soft very high quality pasture / not enough peNDF. These diets are likely to slightly drop your overall milk fat % (‘milk fat depression) yet of the milk fat produced, there will be a slightly increased proportion of medium chain FA as a percentage of total FA. Work on reducing risk of rumen acidosis to reduce FEI if it's higher than ideal when PKE is not being fed.