Lactic Acids Theories
There is a strongly held belief that Lactic Acid (LA) build up is responsible for ruminal acidosis, despite the evidence that lactic acid is passively converted to propionic acid in the rumen and that the concentration of lactic in the rumen shows very little variation.
There is also a widely held belief that high levels of LA in grass silage can depress both rumen pH and feed intake.
A PhD thesis completed by Mark Augustus Cole of Leeds University in conjunction with some of the great ruminant and forage research gurus of their time including Forbes, Sutton, Wilkins and Beever in 1992 demonstrated this negative effect on Dry Matter Intake (DMI) with ruminal infusions of lactic acid. Interestingly they also demonstrated how infusions of propionic and acetic acid not lactic acid depressed rumen pH.
During the completion of his thesis he also found that it was amines rather than ammonia that are produced under the same fermentation conditions which had the largest negative effect on DMI. Unfortunately they failed to investigate how LA reduced DMI despite it having no effect on rumen pH.
However, this work appears to add more weight to my hypothesis of the important energetic value of LA to both the rumen and the cow. In more recent times it has been shown that ruminal infusions of LA increase the molar proportions of propionate in the blood.
This appears to suggest that LA via propionate causes a short term state of satiety and switches off the cows desire to eat which reduces her DMI.
The best analogy I can use to describe this effect is the relationship between eating behaviour and obesity and diet and regular coke. It is not uncommon to see over weight individuals drinking diet coke. Diet coke contains no sugar and therefore does not increase blood sugar levels and does not reduce the drive to eat.
In contrast regular coke rapidly increases blood sugar levels and reduces the need to eat. LA to the cow is like a regular coke which is why it may be important to reduce starch levels in relation to the level of LA in the diet so as not to overload the propionate absorption pathway and depress rumen pH and intake.
Back to the subject of LA and rumen pH; the great Finnish researchers Jaakola and Huhtanen (1989) looked at the effect of replacing sugar with LA on rumen microbial protein synthesis, rumen pH and Volatile Fatty Acids (VFA) profile (Table 1).
They found that sucrose depressed rumen pH significantly greater than LA but more surprisingly that the concentration of LA in the rumens of the experimental animals was lower in the animals fed LA compared to those fed sucrose. This can only be related to the effect of rumen pH on the absorption of VFA’s from the rumen and is another reason why starch and probably more importantly sugar levels should be adjusted based on the level of LA in the diet.
The pathway for conversion of LA to propionate is highly developed, but is dependent on adaptation. Counotte and Prins (1981) looked at the regulation of LA production, lactate fermentation and the role of lactate as an intermediate in rumen metabolism.
What they found was that the rumen pH had a pronounced effect on lactate metabolism. It appears that LA levels in the rumen rise as a consequence of low rumen pH rather than it being the cause of low rumen pH provided the rumen microflora is adapted to lactate metabolism.
In a poorly adapted rumen in the presence of soluble sugars, amino acids and soluble proteins rumen bacteria grow extremely fast and produce more lactic acid than can be metabolized.
This is referred to as a state of high glycolytic flux (breakdown of sugars), once the lactate fermenting bacteria catch up, the glycolytic flux is reduced and rumen lactic acid concentrations decline. This is why changing diets or even changing forages can result in a state of acidosis especially in the freshly calved cow.
In summary LA is a more rumen friendly feed than sugar and starch, provided the rumen microflora is adapted to metabolizing it. This can be achieved by feeding starch and high lactic acid forages during the dry period so long as this does not increase the energy density of the diet too high or increase the risk of milk fever.
This is why maize silage is the ideal feed for dry cows. It contains starch as well as LA which conserves the LA metabolism pathway during the dry period and result in better adaptation to the milking cow diet.