Starch, Propionate, and Glucose – “But, hey, what about Lactic acid?”

Most nutritionists and dairymen are driven by the need to supply our dairy cows with more and more starch as a fuel for milk production.

In essence, what we are doing is giving the rumen population more substrate from which to produce propionic acid. Propionic acid is then absorbed into the blood and by metabolic processes is converted to glucose.

Glucose is of course synthesised by a process called glucogenesis and utilised by the cells of the body in a process called glycolysis. Glycolysis produces the cellular fuel called ATP. In the lactating ruminant though things are a little different due to a naturally occurring condition known as insulin resistance which diverts most of the glucose to the mammary gland.

In the cells of the mammary gland, two moles of glucose are combined to produce lactose the driver of milk volume. Lactose is the main osmolite in milk; i.e. water is drawn into the milk to maintain the concentration of lactose.

That is why you never see much difference in milk lactose concentrations.

What about lactic acid though?

It is accepted that acute ruminal acidosis is due to a build-up of lactic acid in the rumen.

This occurs when the rate of production exceeds the rate at which specific rumen microbes are able to convert it into propionic acid and transport it across the rumen epithelium. Lactic acid is an intermediate between starch and propionate (Figure 1).

Figure 1 shows how propionate is derived from lactic acid. This reaction requires an abundance of CO2.  It has been shown that sodium bicarbonate is an effective source of carbon and will influence the synthesis of propionate. An anaerobic environment rich in CO2 can be promoted by the use of yeasts which scavenge oxygen. 

The Conversion of Lactate to Propionate.jpg
Figure 1. The conversion of lactate to propionate

In situations where rumen pH begins to decline these rumen microbes become less efficient and the rumen enters into a vicious circle.

Low pH favours starch fermentation to lactic acid but the regulators cannot keep up and the rumen falls into a state of acidosis that reduces fibre digestion and intakes.

Although I fully accept that lactic acid is the causable factor in fresh cow acidosis, it is less difficult to accept that it is responsible for acidosis in established lactation since variation in the concentration of lactic acid in the rumen is rarely reported.

Interestingly the difference between acute and the more common form of acidosis, sub-acute ruminal acidosis (SARA) is that the concentration of lactic acid in the rumen remains relatively low (<1mM) in cows with SARA. In these cases SARA is caused by a build-up of the volatile fatty acids, acetate, propionic and butyrate, and a mismatch between production and absorption.

There are ways in which we can help the rumen microbes keep pace with the rate of lactic acid production.

These include yeasts which compete for starch and so reduce lactic acid production, making cereals less fermentable through less processing or treating with sodium hydroxide or buffering the rumen using broad spectrum buffers or nutritionally improved straw.

One other approach is to feed less rumen degradable sources of starch.

However, it is worth remembering that starch digestion and absorption is less efficient lower down the gastro intestinal tract.

Feeding a lower percentage of concentrates is also an option as a greater percentage of forage in the diet will raise rumen pH.

However, high levels of fermentable energy are necessary to increase production on poor quality forages. Be warned well processed, wet, acidic forages fed had high rates can induce acidosis. High forage inclusions are not always a guarantee of good rumen health.

There is of course another way of going about it.

Quite simply reduce the starch level in the diet in line with the quantity of lactic acid being fed.

Table 1 below shows the potential contribution of grass silage to the rumen pool of lactic acid. Grass silage with a lactic acid content of 10% or 100g/kg at a DM intake of 12kg of DM will provide 1200g of lactic acid.

Milk Production & Latic Acid - Farmer and Cows

Starch is converted into lactic acid at an efficiency level of around 85%. This means that a 20% starch diet will produce 4080g of lactic acid.

If we subtract the 1200g from the silage that leaves us with a requirement of 2880g of LA equivalent to supplying 3388g of starch. To achieve this our diet would need only to be 14% starch. In this example reducing starch level would promote DMI’s and rumen health.

Reducing the starch level to 14% may sound like madness but it will work. It works because if we are aiming for high starch intakes and not accounting for the lactic acid intake we are adding to the acid load on the rumen. Pressure is also being placed on the lactic to propionate conversion pathway. This causes SARA, low DMI and poor fibre digestion.

If we do the above calculation we will find that you will reduce starch intakes, increase DMI and produce more milk of better quality. Of course on dry forages and forages with low levels of, or no, lactic acid we will find that we do need to maintain starch intakes as dietary lactic acid will not contribute a significant quantity of propionate precursors.

At high starch inclusion rates we need to ensure that the starch sources have differential degradability rates and some are alkaline. This will avoid large peaks in the production of lactic acid and VFA’s and large drops in rumen pH post feeding.

Making good quality well fermented grass silage is most definitely advantageous as this will contain high levels of lactic acid. The lactic acid is produced as a product from fermentation of the sugar in the grass. The higher the concentration the more starch can be removed and the happier the rumen will be. Converting the forage sugars to LA also has another benefit.

Milk Production & Lactic Acid - Cows being fed in shed.

Fermentation of forage sugars in the rumen would favour the production of butyrate and acetate. Lactic acid on the other hand will produce propionate which will result in an increased glucose supply.

Using an effective bacterial inoculant and ensuring that grass sugar levels are high enough before cutting is essential. Sugars can be concentrated by wilting, but avoid exceeding 30% DM. When sugar levels are low it is definitely worth adding some additional sugar in the form of molasses. This will ensure there is a sufficient supply of sugars for the production of lactic acid.

Table 1. Dietary starch and lactic acid supply balance

Starch intake 4800g (20% of DMI)
Lactic acid produced (85% efficiency) 4080g
Lactic acid intake from Grass Silage (12kgDM, 10% LA) 1200g
Target LA production less LA from forage 2880g
Starch required to produce target LA 3388g (14% of DMI)


Latic Acid – Take Home Messages

  1. Once the formulated diet meets the readily fermentable carbohydrate requirements do the lactic acid calculation and adjust the starch level.
  2. Use a combination of starch sources with differential degradation rates.
  3. Yeasts will help to maximise CO2 levels in the rumen by scavenging oxygen.
  4. Sodium Bicarbonate is an excellent source of carbon.
  5. Use an effective lactic acid bacteria inoculant to maximise lactic acid production in forages.
  6. Cut in the afternoon and wilt quickly to maximise sugar levels.
  7. Add sugar in the form of molasses if sugar levels are low.

Written by Dr Huw McConochie  – Wynnstay’s Head of Dairy Technical Services
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