The way to a healthy, high-yielding cow is through the rumen

Published: 2020-11-20

The rumen is the engine room of a cow, and to ensure a healthy, high-yielding cow, it needs a well-functioning rumen. The microorganisms found in the rumen can largely take credit for the conversion of feed, and for the strength of a cow’s immune system.

The importance of microorganism composition

Because microorganisms and the host cow live in symbiosis, the composition of the former is important to the latter. There are billions of bacteria, protozoa and various fungi in the rumen, all with different roles to play in the health of the cow. These microorganisms are collectively responsible for the microbial conversion of nutrition, which requires oxygen-free (anaerobic) conditions to work.

The rumen as a fermentation chamber

The conversion of feed in the rumen also takes place under anaerobic conditions, in which carbohydrates, such as starch and sugar, are converted into acetates, butyrates and propionates, which can later be used as sources of energy. The pH value of the rumen plays an important role in which microorganisms can survive. To get the best possible composition of microorganisms, a neutral pH value is required.

For example, a starch-rich, low structure ration based on high maize silage and corn can make it hard to achieve a neutral rumen pH, whereas a greener ration with a lot of grass silage and more structure will provide a much better base for a healthy rumen environment. Unfortunately, a high energy concentration and easily-digested carbohydrates are a precondition for achieving high yield, which is why the challenge will often be to combine the desire for a high-energy ration with it also being ‘rumen-friendly’ with sufficient structure, giving the cow a stable eating pattern throughout the day with sufficient cud-chewing activity.

When the rumen gets acidic

Rumen acidosis is a familiar problem for high-yield dairy cows. It occurs when the rumen produces more acid than can be absorbed through the rumen wall. Rumen acidosis is defined as occurring when rumen pH is under 5.5, but even under 6 (referred to as ‘subclinical rumen acidosis’) it’s a problem for the cow’s health. Rumen acidosis rarely comes alone, as it's what's called a ‘multifactorial disease’, that implies higher risk of other ailments such as ketosis, laminitis, displaced abomasum, dwindling reproduction, loss of appetite, low yield, etc. There is also often a link between low fat percentage and rumen acidosis, but because the composition of the feed ration is closely linked to the milk fat percentage, it can be difficult to diagnose rumen acidosis based on milk fat percentage. This makes the financial consequences of rumen acidosis problematic.

In herds with separate feed concentrate, such as in robot herds, there will be fluctuations in rumen pH throughout the day, and there will be higher risk of periods with a pH value under 6. Here it becomes important that the other rations in the feed trough are inviting and well-mixed, to help stabilise rumen pH.

The rumen is the engine room of a cow

Emergency help for an overloaded rumen

To prevent the occurrence of rumen acidosis, it’s important that the feed ration is uniformly mixed, which can be ensured through sufficient mixing time, and the addition of water if needed. Sudden changes in feed should also be avoided, and the ration must be balanced to ensure a high starch level is offset by a sufficiently high level of digestible cell walls/NDF and a CAB value of between 200-300 meq/kg dry matter.

A cow produces bicarbonate itself in its saliva, which helps stabilise rumen pH, as it’s basic. But the amount a cow can produce itself is not always enough to protect an overloaded rumen from rumen acidosis.

If the ration is composed to avoid overloading the rumen, e.g. with a high level of starch, low level of cell walls or a low CAB value, it can be compensated to some degree by adding a buffer, which will usually be in the form of sodium carbonate. Sodium carbonate has a high CAB value of 11.739 meq/kg. Another way of helping a troubled rumen is by adding live yeast cells, which actively ensure a better composition of rumen microorganisms, in which the number of cellulolytic (fibre-decomposing) bacteria and the lactic acid-consuming bacteria are increased.

Marden et al. 2008 described the effect of adding 150 grams of sodium bicarbonate or 5 grams of live yeast cells to a ration of pH in the rumen, compared with cows in a control group on the same basic ration, but with none of the problem-solvers added. They found that the cows receiving added sodium carbonate had the highest rumen pH of an average of 6.21, followed by the group given live yeast cells, with an average rumen pH of 6.14, compared to the control group value of 5.94. Cows given live yeast cells had a lower level of lactic acid in the rumen, and higher fibre digestion, where the sodium bicarbonate acted mostly as a chemical buffer that boosted rumen pH. Figure 1 shows the results from the trial on the effect on rumen pH.

To achieve the best effect from the addition of sodium bicarbonate, it should be added directly to the feed ration, perhaps through the minerals or feed concentrate, as the cows cannot adjust their intake of sodium carbonate to suit their needs themselves. Keunen et al., 2003 have described this based on a trial in which cows contracting rumen acidosis by increasing the amount of starch in their ration through higher amounts of feed concentrates at the expense of high-structure roughage, did not increase their intake of sodium bicarbonate when it was provided alongside their ration.

The cow can therefore not adjust the need for buffers itself, and the practice used on a number of farms under which the cows are given unrestricted access to sodium bicarbonate in bails, e.g. at each end of the feeding trough, is not the best, as the cows are unable to determine their intake according to their need. The trial also indicated that there were significant differences in the amount of sodium bicarbonate intake for individual cows.

Figure 1. The effect of sodium bicarbonate (SBD) and live yeast cells (YD) on pH in the rumen compared with a control hold (CD) Marden et al. 2008.

The significance of the switch to non-GM feeding

For most farmers, the transition to non-GM feeding has meant switching to a ration with a higher proportion of rape products at the expense of soya. This gives a much higher proportion of sulphur in the ration, which moves the CAB value in a downwards direction. Because a large part of the sulphur in rape comes under protein, it will directly take part in the formation of amino acids, which means it does not act as an active anion, and therefore does not have the same significance for rumen pH. However, the calculated CAB value will be low in the high-rape rations, but without the same physiological importance as if sulphur was added as salt, such as magnesium sulphate, for example.

The transition to non-GM feeding has had an impact on protein prices, and protein ingredients have been very expensive in 2021. Many farmers will therefore probably increase the amount of grass in the ration, to be able to save on expensive proteins bought-in. A higher grass proportion in the ration will contribute to a higher proportion of digestible cell walls, which will have a beneficial effect on rumen health. But a higher proportion of grass at the expense of maize silage in several rations will mean the need to supplement with easily-digestible starch in the form of corn, which can also put strain on the rumen environment.

Ammonium chloride can also be recommended for acidifying dry cows, but because it needs HACCP approval, adding it through a dry cow premix is recommended. The advantage of using ammonium chloride is that it does not add magnesium, which the cows will get plenty of through the use of magnesium sulphate and chloride, if the dry cow ration already adds magnesium from, for example, phosphate or oxide. When acidifying a dry cow ration, it’s important to remember that the acidifying salt tastes bitter for the cows, and it's important that they are not put off the feed, which can also give major problems when it comes to calving for the cow and the calf.

How far down the DCAD value has to go in the close-up ration to prevent milk fever can vary enormously between herds. A good indicator of a suitable level is urine pH value. If it lies between 5.5-6.5, experience shows that milk fever is rare. In some herds, getting the DCAD value down to around 0 meq per kg dry matter is sufficient, while others need to get down to -100 meq or lower to avoid milk fever in the herd.

The big dilemma

When feeding high-yield dairy cows, it will be difficult to avoid the dilemma known as: to achieve high yield, cows require a relatively concentrated feed ration, which is rich in starch and with a density that will allow the cow to absorb large amounts of energy, although such feed puts a strain on rumen health.

The objective is then to seek the right balance between a high-energy ration that also allows a ruminant to absorb and convert it, which sets even higher demands on composition, taste and ration mix. We can help the cow a lot by choosing good, healthy ingredients with a balanced level of additives, such as vitamins, yeast and sodium bicarbonate.

Unfortunately, the pursuit of high yield can end in a vicious circle if the cow contracts rumen acidosis, and it’s therefore important that high yield is achieved using the healthiest ration possible, with as few additives as possible. Only by achieving high yield from a relatively rumen-friendly feed ration good health and sound economy can be achieved in the herd.


Keunen J. E., J. C. Plaizier, I Kyriazakis, T. F. Duffield, T. M. Widowski, M. I. Lindinger, B. W. McBride: Short communication: Effects of subacute ruminal acidosis on free-choice intake of sodium bicarbonate in lactating dairy cows. J Dairy Sci. 2003 marts; 86 (3)

Marden J. P., C. Julien, V. Monteils, E. Auclair, R. Moncoulon, C. Bayorthe: How does live yeast differ from sodium bicarbonate to stabilize ruminal pH in highyielding dairy cows? J. Dairy Sci. volume 91, issue 9 september 2008, pages 3528-3535

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