Acetate and propionate are two dominant volatile fatty acids, or VFAs, produced by rumen microbes during fermentation. Both are important, but they support different outcomes in the cow.
Acetate fuels milk fat synthesis in the mammary gland. Propionate is the primary building block for glucose production in all ruminants, optimising body condition and reproductive performance. The optimal acetate:propionate balance depends on diet, stage of lactation, appetite and herd goals.
This balance can be managed strategically with pharmaceutical tools such as Monensin 200, or more naturally through supplementation of probiotics, live yeast and yeast metabolite combinations. These act as rumen support when used in the correct combination as part of wider nutrition and transition plans.
What are acetate and propionate in the rumen?
Acetate comes mainly from fibre digestion. Propionate is produced from starch and soluble carbohydrate fermentation. Therefore, the balance between the two can be heavily affected through diet formulation.
Volatile fatty acids (VFA’s) supply around 70% of the cow’s energy, so shifts in their proportions change which tissues get fuel and how the cow partitions nutrients. That means VFA balance is not just a technical rumen topic. It affects milk fat, milk volume, energy balance, appetite, body condition and reproduction.
Why does acetate matter for milk fat production?
Acetate is a direct precursor allowing for further synthesised fatty acid production used to drive the mammary gland. High-acetate diets, including fibre-rich, well-chewed pasture or conserved forages, generally support higher milk fat concentration.
If rumen fermentation is pushed too far away from acetate, for example through feeding a high grain or excessive starch diet, without adequate effective fibre, milk fat depression can follow even if overall energy supply is high.
This is an important point in pasture-based systems. Milk volume can look acceptable while milk fat percentage is slipping, and that can point back to rumen fermentation patterns. It is not always a matter of feeding more energy. Sometimes the better question is whether the rumen is producing the right type of energy.
Why does propionate matter for glucose and reproduction?
Propionate is absorbed and converted to glucose in the liver. That glucose supports lactose production, and milk volume, and it also restores body condition and is important for ovarian function as well as successful early embryonic development.
Increasing propionate production can therefore reduce negative energy balance, lower ketosis risk, and improve conception prospects when energy supply is limiting.
This is most relevant in early lactation, when cows are under heavy metabolic demand and intake may not yet match output. In that window, improving glucose supply can be useful, especially for cows at higher risk of ketosis or excessive body condition loss.
The trap is assuming more propionate is always better. It is not that simple.
An approach that relies too heavily on rumen modifiers such as monensin can upset the balance if the ration has not been formulated correctly.
Can more propionate reduce appetite?
Yes, increased propionate can, in some situations, signal satiety via hepatic vagal pathways, potentially reducing dry matter intake. The magnitude depends on diet, stage of lactation and how abruptly fermentation shifts.
That is why pushing fermentation heavily towards propionate without attention to effective fibre and rumen health can backfire, especially when cows need to eat well for milk production or recovery.
Under New Zealand law and in accordance with restrictions placed on us by export market partners, monensin, within the framework of the New Zealand dairy industry, can only legally be used to reduce the risk of ketosis. This means monensin has a tight seasonal application window on New Zealand dairy farms.
Given monensin’s propensity to reduce appetite, restricting the use of monensin to the transition and early lactation period will work best on the majority of farms.
How does Monensin change rumen fermentation?
Monensin is an ionophore, a type of antibiotic, that selectively alters rumen microbial populations by decreasing one type of microbe, allowing the opposing strain to become more dominant.
This has the effect of increasing the molar proportion of propionate at the expense of acetate and butyrate production, while also slightly reducing rumen methane production. Clinically, this can improve energy efficiency, lower the incidence of subclinical ketosis and reduce rumen methane flowing from the rumen.
Monensin 200 is formulated to decrease the risk of clinical ketosis under New Zealand grazing systems, when used according to label and veterinary guidance.
Where do probiotics and yeast-based rumen support products fit?
Yeasts, such as Saccharomyces cerevisiae, and selected direct-fed bacillus microbials can stabilise rumen pH, scavenge oxygen, encourage fibre-digesting bacteria and improve rumen function throughout lactation. They can also help cover for feed quality variation.
They do not force a fermentation shift like monensin, but instead help maintain acetate- and butyrate-producing organisms. This approach leads to better overall rumen stability, supports a more consistent feed intake, and increases milk volume without changing milk composition.
Angel Yeast is a live active yeast that helps break down cellulose, while also mopping up free oxygen in the rumen. The low oxygen environment created by the yeast is then ideal for the proliferation of anaerobic lactobacillus organisms in the rumen.
Bioyeastar 4C, a high-quality yeast metabolite, is a good example of a yeast-based rumen prebiotic, designed to improve rumen stability while actively supporting good rumen microbial proliferation.
Should monensin and probiotics be used at different stages of the season?
Consider timing and herd goals. Monensin’s propionate-shifting effect can be particularly useful in early lactation, when reducing ketosis risk and improving energy status is a priority.
Probiotic or yeast-based support is valuable during ongoing seasonal feed changes, beyond the first 30 days through to late lactation, or whenever rumen stability and milk fat maintenance are the focus.
Using both tools strategically, for example, Monensin 200 around calving and early lactation, followed by a combination of Angel Yeast and Bioyeastar 4C through the grazing season, can balance energy efficiency and provide rumen stability.
What should vets look at before making a recommendation?
Evaluate diet type, pasture versus supplements, effective fibre, starch load, body condition scores, milk fat percentage trends, the risk of ketosis, as well as mating goals.
Consider appetite patterns, manure consistency, rumination time and herd-level metabolic markers. Discuss with farmers whether the priority is reducing negative energy balance and ketosis risk; in these situations, Monensin 200 may be a good fit for that period. For stabilising rumen fermentation and driving milk fat and intake throughout lactation, yeast and probiotic supplementation may be an even better fit.
Bottom line
Acetate and propionate each serve important roles. Good management should not aim to maximise one VFA at all costs, but instead balance the acetate:propionate ratio according to the cow’s needs, stage of lactation, diet and herd objectives.
Monensin 200 and yeast/probiotic rumen supports are complementary tools that, when timed and dosed correctly, help vets and nutritionists steer fermentation toward better herd-level health, production and reproduction.
