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Dietary RUFAL and milk fat depression

How much dietary fat is too much? The general rule of thumb would be a 6% maximum, but the number is not as nearly as important as the types of dietary fat. Compared with saturated free fatty acids, unsaturated ones have much more harmful effects on rumen micriobial population and stronger effects on depressing milk fat production.

Below are some presentation slides derived from a webinar by Dr. Tom Jenkins at Clemson University hosted by AMTS. According to Dr. Jenkins, Rumen Unsaturated Fatty Acid Load (RUFAL, Figure 1) can be used as an indicator of risk of dietary fatty acids in causing milk fat depression. RUFAL is the sum of dietary unsaturated C18:1 (oleic), C18:2 (linoleic), and C18:3 (linolenic) fatty acids. Increasing dietary RUFAL is associated with increased production of trans 10, cis 12 fatty acid, a byproduct of ruminal fatty acid metabolism and an indicator of milk fat depression risk (Figure 2), Typically, a RUFAL score greater than 3% of the DMI may raise some concern. We can tell from a large data set from CVAS, there is a wide distribution of RUFAL in TMR samples (Figure 3).

Figure 1: RUFAL is the sum of C18:1, C18:2, and C18:3 fatty acids. Courtesy of Dr. Tom Jenkins.

Figure 2: Increasing RUFAL is associated with increased production of trans 10, cis 12 fatty acid, which is an indicator of milk fat depression risk. Typically, a RUFAL score greater than 3% of the DMI may raise some concern. Courtesy of Dr. Tom Jenkins.

Figure 3: Distribution of RUFAL in TMR samples. Courtesy of Dr. Tom Jenkins.

What are the key contributors to RUFAL? It seems like that corn silage and corn grain are important sources of RUFAL (Figure 4), and corn silages with different contents of fatty acids can have big impact on total dietary RUFAL (Figure 5), especially given that large amounts of corn silages are fed to the cows. In addition, there are large variations of fatty acid contents in corn silage (Figure 6) and corn grain (Figure 7). Given the variability, testing the fatty acid contents of these feeds is very important.

Figure 4: Fatty acid sources contribute to RUFAL. Courtesy of Dr. Tom Jenkins.

Figure 5: Corn silages with different contents of fatty acids can have big impact on dietary RUFAL. Courtesy of Dr. Tom Jenkins.

Figure 6: Distribution of total fatty acids in corn silage samples. Courtesy of Dr. Tom Jenkins.

Figure 7: Distribution of total fatty acids in corn grain samples. Courtesy of Dr. Tom Jenkins.

Furthermore, there are interactions between RUFAL and other feed or cow factors. For example, cows are less likely to develop milk fat depression if they are on a higher forage diet compared with cows on a lower forage diet despite same RUFAL content (Figure 8). It is possible that cows receiving high forage have higher rumen pH, which prevent the generation of trans 10, cis 12 fatty acid in the rumen. A recent research by Dr. Adam Lock showed the dynamic interactions between rumen pH and dietary corn oil levels on risk of milk fat depression (Figure 9). Strategies that can help maintain a healthy rumen pH and/or limit the amounts of RUFAL will help reduce the risk of milk fat depression.

Figure 8: Higher forage diets have less risk of milk fat depression, as indicated by lower levels of trans 10, cis 12 fatty acid, despite same RUFAL content. Courtesy of Dr. Tom Jenkins.

Figure 9: Interactions between rumen pH and dietary corn oil levels on risk of milk fat depression, as indicated by levels of trans 10, cis 12 fatty acid. Courtesy of Dr. Tom Jenkins.

In short, RUFAL is a useful parameter to monitor fatty acid fraction that potentially disrupts rumen fuction. RUFAL > 3% of total DMI is higher risk because of the potential to incresae trans 10, cis 12 fatty acid that causes milk fat depression. RUFAL interacts with rumen pH and forage levels of the diet to affect milk fat production.

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