Effects of diet fermentability and supplementation of 2-hydroxy-4-(methylthio)-butanoic acid and isoacids on milk fat depression

Discipline: milk fat depression; Keywords: low-forage diet, methionine analog,

branched-chain volatile fatty acids, valerate. 

Milk fat depression (MFD) can be easily induced with diet modification and it is an economic loss for dairy producers. When diet digestibility and the polyunsaturated fatty acids (PUFA) in the diet increase, severe MFD can occur because of biohydrogenation of these acids in the rumen. Therefore, the term biohydrogenation induced MFD has been used to explain severe MFD occurring when diet digestibility and PUFA content increased. A change in volatile fatty acid (VFA) profile (e.g., acetate and propionate) was earlier considered another factor for MFD, but this explains only a small part. Even, although changes in biohydrogenation of intermediate fatty acids have been considered the major factor for severe MFD, this also does not fully explain MFD occurring by dietary changes, suggesting a variety of unknown factors causing MFD. Severe MFD occurs less often on commercial farms, but mild or moderate cases do occur because of animal variation and a relatively low PUFA content in rations (commercial fat supplements are usually not high in PUFA).  Supplementation with  2-Hydroxy-4-(methylthio)-butanoic acid (HMTBa) and isoacids (IA) (e.g. isobutyric, isovaleric, and 2-methylbutyric) have often been shown to increase milk fat, milk yield and fibre degradability. Therefore, supplementing an MFD-inducing diet with HMTBa and IA was hypothesized by Drs J.E. Copelin, C. Lee and co-workers to reduce the risk of MFD, and they predicted that the combination of the two supplements will have an additive effect. The objective of their study was then to determine milk production, the fatty acid (FA) profile, ruminal bacterial diversity and nutrient digestibility in lactating cows fed MFD-inducing diets supplemented with HMTBa and IA. The results of their studies were published in two papers in the Journal of Dairy Science, Volume 104 of 2021, respectively pages 1591 to 1603 and 1604 to 1617. The respective titles were: Effects of diet fermentability and supplementation of 2-hydroxy-4-(methylthio)-butanoic acid and isoacids on milk fat depression: 1. Production, milk fatty acid profile, and nutrient digestibility. 2. Ruminal fermentation, fatty acid, and bacterial community structure.

In experiment 1, ten Holstein cows were used in a 5 × 5 Latin square design. Treatments included a high-forage control diet (HF-C), a low-forage control diet (LF-C) causing MFD by increasing starch and decreasing neutral detergent fibre (NDF), the LF-C diet supplemented with HMTBa at 0.11% (28 g per day), the LF-C diet supplemented with IA at 0.24% of feed dry matter (60 g per day), and the LF-C diet supplemented with HMTBa and IA.

Dry matter intake was greater for LF-C versus HF-C, but milk yield remained unchanged. The LF-C diet decreased milk fat yield (0.87 vs. 0.98 kg per day) but increased protein yield compared with HF-C. As a result, energy-corrected milk was lower (28.5 vs. 29.6 kg per day) for LF-C versus HF-C. Although the concentration of total synthesized FA in milk fat was not affected, some short- and medium-chain FA were lower for LF-C versus HF-C, but the concentrations of C18 trans-10 isomers were not different. Total-tract NDF apparent digestibility was somewhat lower (42.4 vs. 45.6%) for LF-C versus HF-C. The decrease in milk fat yield observed in LF-C was alleviated by supplementation of HMTBa through increasing milk yield without altering milk fat content and by IA through increasing milk fat content without altering milk yield so that HMTBa or IA increased milk fat yield within the LF diets. However, interactions for milk fat yield and ECM were observed between HMTBa and IA, suggesting no additive effect when used in combination. Minimal changes were found on milk FA profile when HMTBa was provided. However, synthesized FA increased for IA supplementation.  

In conclusion, the addition of HMTBa and IA to a low-forage and high-starch diet alleviated moderate MFD. Although the mechanism by which MFD was alleviated was different between HMTBa and IA, no additive effects of the combination were observed on milk fat yield and ECM.

In experiment 2, five ruminally cannulated cows were used, also in a 5 × 5 Latin square design with the following 5 dietary treatments (dry matter basis): a high-forage and low-starch control diet with 1.5% safflower oil (HF-C); a low-forage and high-starch control diet with 1.5% safflower oil (LF-C); the LF-C diet supplemented with HMTBa (0.11%; 28 g per day; LF-HMTBa); the LF-C diet supplemented with isoacids [(IA) 0.24%; 60 g per day; LF-IA]; and the LF-C diet supplemented with HMTBa and IA (LF-COMB). The experiment consisted of five periods with 21 days per period. Rumen samples were collected to determine fermentation characteristics, long-chain fatty acid (FA) profile and bacterial community structure by analyzing 16S gene amplicon sequences were examined.

The LF-C diet decreased ruminal pH and the ratio of acetate to propionate, with no major changes detected in ruminal FA profile compared with HF-C. The relative abundance of bacterial phyla and genera associated indirectly with fibre degradation was influenced by LF-C versus HF-C. Within the LF diets HMTBa increased the ratio of acetate to propionate and the butyrate molar proportion. Ruminal saturated FA were increased and unsaturated FA concentration were decreased by HMTBa, with minimal changes detected in bacterial diversity and community. IA supplementation increased the concentration of all branched-chain volatile FA and valerate and increased the percentage of trans-10 C18 isomers in the total FA. Changes in the abundance of bacterial phyla and genera were minimal for IA. Interactions between HMTBa and IA were observed for ruminal variables and some bacterial taxa abundances. 

In conclusion, increasing diet digestibility (LF-C vs. HF-C) influenced rumen fermentation and bacterial community structure without major changes in FA profile. Supplementation of HMTBa increased biohydrogenation capacity, and supplemental IA increased bacterial diversity, possibly alleviating MFD. The combination of HMTBa and IA had no associative effects in the rumen and need further studies to understand the interactive mechanism. 

Bottom line: There is a clear advantage to supplementation with HMTBa and IA in preventing or moderating MFD. However, explanation of the mechanisms involved will still require many investigations.