By Bill Mahanna, PhD, Dipl. ACAN - Global Nutritional Sciences Manager
It is my impression that nutritionists spend considerable time troubleshooting milk fat depression (MFD), and I am always interested in listening to them retrospectively analyze what they think precipitated the onset.
Thinking back on all the possible causes I have heard over the years, ranging from poorly fermented silage to variability in oil content of distillers grains, I found myself nodding in agreement when reading the following recommendation in a recent paper authored by Adam Lock (2009): "You can be on the 'edge' for MFD and be pushed over by one small diet change. Don't misinterpret that diet change as evil."
Given that dairy producers can ill afford reductions in income from MFD and knowing that in many parts of the country, some atypical feeds are fed due to late-planted and stressed crops (hail, frost, etc.), I decided to review MFD in anticipation of even more MFD issues rearing their ugly head during the feeding season.
Evolution of MFD Theories
Mammary tissue produces fat from two approximately equal sources: (1) short-chain volatile fatty acids (primarily acetate and butyrate) from rumen microbial fiber digestion and (2) long-chain fatty acids from dietary fat sources, bypass microbial fat and metabolized body tissue (Varga and Ishler, 2007).
MFD was historically linked to lowered ruminal acetate concentrations associated with poor forage digestion. Later research showed that a lower ruminal acetate concentration was the result of increased propionate production and that adequate acetate levels existed to fuel mammary tissue requirements for milk fat production (Oetzel, personal communication).
To fully understand the current theory of MFD, it might be helpful to briefly review fatty acid chemistry. Fatty acids are the basic subunit of lipids and consist of molecules containing two or more carbon atoms connected by single or double bonds. Most fatty acids contain an even number of carbons atoms. In nature, short-chain fatty acids (two to six carbons) are relatively rare, medium-chain fatty acids (8-12 carbons) are more common — approximately 4-10% of the fatty acids in feedstuffs - and long-chain fatty acids (more than 16 carbons) predominate.
Fatty acids exist in either saturated or unsaturated states. All of the carbon atoms in the fatty acid chain have the capacity to attach four other atoms. Fatty acids are termed fully saturated when hydrogen atoms have replaced all of the double bonds between carbons. Animals tend to store fat as saturated fatty acid chains. Highly saturated fats (beef tallow) tend to be more solid at room temperature (compared to hog lard) because dietary lipids have been bio-hydrogenated by rumen microbial populations.
Unsaturated fatty acids contain at least one double bond in the chain, and polyunsaturated fatty acids have two or more double bonds in the carbon chain. The greater the number of double bonds, the greater the likelihood that the fat exists as a liquid (oil). Plants tend to store energy in an unsaturated form as oils.
Rumen microbes saturate 80-90% of the unsaturated fatty acids presented to the rumen from the fats found in forage and grain. Most of these naturally occurring unsaturated fatty acids exist in the cis -configuration, meaning the hydrogen atoms are on the same side of the double bonds of the carbon chain.
However, during the bio-hydrogenation process, partial hydrogenation reconfigures some of the double bonds so that the hydrogen atoms end up on different sides of the chain. These are termed trans -intermediate fatty acids and can pass out of the rumen to be absorbed in the intestines. Only unsaturated fats can be termed trans-fats because fully saturated fatty acids have no double bonds and are incapable of a transconfiguration.
The current reigning "trans-fatty acid or bio-hydrogenation" theory of MFD suggests that specific intermediates of ruminal fatty acid — notably trans -10, cis - 12 conjugated linoleic acid (CLA) — will bio-hydrogenate, escape the rumen and signal a decrease in lipoganic enzymes, causing a reduction in mammary gland milk fat synthesis (Figure). The production of as little as 1-2 g per day of trans -10, cis -12 CLA leaving the rumen has been shown to reduce milk fat by as much as four to six points.
The literature also discusses trans -10 18:1, and although it is not thought to inhibit milk fat synthesis, the relative ease with which it can be analyzed in the laboratory makes it a suitable marker for the presence of the undesirable trans -10, cis -12 CLA (Lock, 2009).
The challenge facing nutritionists is identifying not a single causative factor but sorting through the additive effects of multiple diet or management factors responsible for altered rumen conditions that lead to increased production of these undesirable bio-hydrogenated trans-fatty acid intermediates (Lock, 2009).
MFD Risk Factors
According to current thinking, there are three predominant ways to predispose herds to MFD: (1) increase the total ration content of 18-carbon unsaturated fatty acids, (2) alter the rumen environment and bio-hydrogenation pathways or (3) change the rate of bio-hydrogenate production or passage from the rumen (Lock, 2009).
Fluctuations in rumen pH, as well as additives such as monensin, can alter rumen populations and the potential for increasing rumen outflow of bio-hydrogenation intermediates. Bifi dobacterium, Propionibacterium, Lactococcus, Streptococcus and Lactobacillus isolates from the rumen and other environments have been reported to be capable of producing trans -10, cis -12 CLA (Lock, 2009). Many of these strains tend to predominate in low-pH environments.
Nutritionists should probably start troubleshooting MFD by reviewing the current ration and feed delivery status of the herd for issues that have the greatest potential to alter rumen pH. These include:
As an example of the effect of overlooking just one of these factors, I recently became involved in an MFD situation when a Midwest dairy switched from old-crop to new-crop corn silage. The consulting nutritionist adjusted the ration for the increase in starch content in the new-crop silage (10 points higher) but did not observe that the producer had purchased a roller mill for his chopper.
The kernel processing score on the new crop showed that 78% of the starch passed the 4.75 mm screen, compared to only 32% in the old-crop silage. The differences in rate of starch digestion appeared to alter the rumen environment enough to cause MFD, yet observations of the cows showed that they had adequate cud-chewing and acceptable manure consistency. When the ration starch level was reduced, the herd's milk fat percentage returned to reasonable levels.
It should be noted that several studies have reported MFD with no change in rumen pH or obvious signs of subacute acidosis such as off-feed/gorging episodes or inconsistent manure. The fact that low rumen pH is not always a prerequisite for MFD episodes reinforces the difficulty of pinpointing causative factors when very subtle changes in the rumen environment can lead to minute levels of problematic trans-fatty acids (Lock, 2009). CORN silage is rapidly becoming the major forage in many dairy rations and may cause herds to be more susceptible to MFD unless careful consideration is directed at the potential of a corn silage-based total mixed ration to alter rumen pH.
Corn silage is unique in that it contributes high levels of digestible fiber from the stover with highly digestible starch and approximately 55% unsaturated linoleic acid in the grain. This makes it difficult to separate the effect of starch on rumen pH versus the contribution of additional linoleic acid when diagnosing the causes of MFD in high-corn silage rations (Van Amburgh et al., 2008).
Similar challenges exist in sorting out cause and effect in studies (Onetti et al., 2003) where replacing corn silage with alfalfa improved milk fat percentages in rations containing 2% tallow. This is because the starch in fermented corn silage was replaced in the alfalfa treatment group with dry grain (less ruminally available).
The fiber digestibility of the alfalfa (while not reported) was very likely lower than for the corn silage, contributing to improved buffering capacity from the reported increased chewing time. Also, the chop length in the treatment corn silage was 12 mm, much shorter than the typical 17-19 mm used on most commercial dairies.
Nevertheless, research has shown that the combination of elevated levels of high-starch corn silage with supplemental oil-based energy sources (such as cottonseed or distillers grains) and the inclusion of an additive such as monensin can initiate a negative stepwise effect on MFD, demonstrating the need to focus on the additive effect of various ration ingredients (Van Amburgh et al., 2008).
Researchers have also questioned if abnormally fermented (e.g., high acetic acid) silages might exhibit an altered fatty acid profile and contribute to MFD. Florida researchers (Amaral et al., 2009) have reported changes in fatty acid profiles with abnormal silages, but further research is needed to determine if undesirable trans-intermediates are actually being produced during silage fermentation. Field experiences have also led some nutritionists to correlate high-yeast silages with MFD.
While these may be contributing factors, more research is required to determine specific causes and effects such as the specific genus/species/strain at fault, given the recent molecular research showing the complexity of microbial populations involved in the silage ecosystem (Ahmad et al., 2009 and 2009a).
It may be premature to get overly focused on this one dietary/management aspect given the limited capabilities for silage microbial/fungi identification and enumeration currently available to consulting nutritionists, especially if aerobically challenged silages become the MFD scapegoat while other factors are not seriously investigated.
However, for dairies desiring improved silage biosecurity and inhibition of yeast growth, some Lactobacillus buchneri silage inoculants on the market today have been shown to significantly reduce silage mold and yeast growth (Mahanna, 2007; Adesogan et al., 2005).
Some of the newer ration software makes it easier for nutritionists to monitor the rumen load of unsaturated fatty acids, particularly linoleic (18:2) acid, delivered in the total ration. Linoleic acid is the primary unsaturated fatty acid in cottonseed, soybeans, corn and corn byproducts such as distillers grains.
I have heard various nutritionists voice thumb rules for upper levels of unsaturated fatty acids in the ration, typically having concerns over possible MFD when levels exceed 500-600 g per day. Researchers in this area are more reluctant to assign a specific threshold level of total rumen unsaturated fatty acids because it is such a variable, moving target that is highly dependent on other dietary components and interactions (Lock, personal communication).
Attention should also be focused on the unsaturated fatty acid loads coming from various "ruminally inert" commercial fat sources in addition to the variability in fat content (5-15% of dry matter) commonly observed with distillers grains.
The Bottom Line
MFD is a complex issue that frustrates nutritionists and costs dairy producers significant revenue. The current theory of MFD involves the production of bio-hydrogenated, trans-fat intermediates — most notably trans-10, cis-12 CLA — that escape the rumen and signal a decrease in lipoganic enzymes, reducing mammary tissue fat synthesis.
Nutritionists should monitor the total ration unsaturated fatty acid level and investigate ration or management factors that might contribute to lowering ruminal pH, despite the fact that MFD can occur in herds with seemingly stable rumen pH. Research is clear that no single dietary factor is the causative agent, but, rather, MFD is the result of the interaction between various dietary components that conspire to allow for increased rumen outflow of undesirable trans-fatty acids.
This article was originally published in October 2009 Feedstuffs issue, and is reproduced with their permission.