Les composantes de l’acidose ruminale et les effets acidogènes des rations

Abstract

Ruminal acidosis is a major concern in modern nutrition of ruminant. Effectively, the increase of production potentials has been associated with a subsequent increase of the proportion of concentrate in the diets. Therefore the reticulo rumen of the high yielding ruminants has to digest increased quantities of fermentable organic matter. The following fermentation rate is sometimes dramatically increased, therefore ruminant pH decreases and this acidosis status lead to several zootechnical drawbacks: negative digestive interactions, decrease of the milk fat content, digestive and metabolic pathologies... An acidosis status of the rumen is the consequence of a reduction of salivary flow which recycles phosphate and bicarbonate buffers. The assessment of acidosis status is performed from the pH measurement of the rumen fluid. It is generally believed that the mean pH must not be less than an approximative value of 6.25. Numerous experimentations allowed to demonstrate that various diet characteristics have an influence on the ruminal pH. Considering the threshold value of 6.25, it is possible to define the corresponding recommended values for the various diet items. A part of the allowances are expressed as minimal values. Thus it is recommended that the diets contain a minimum of 35 % of NDF (% diet Dry matter), 25 % of forage NDF, 2.5 mm of mean particle size in the diets or 40 % of the diet dry matter from particles larger than 2 mm. Other recommandations are under the form of constraints of maximum. Thus the concentrate proportion in the diet must remain below to around 45 %/DM. For starch, the corresponding value is of 25 % dry matter and for ruminal degradable starch, it is of 20 % of DM. Moreover, it is recommended that the index of mastication of the diets must be maintained to values greater than 40 min /kg DM intake. Otherwise, it is known that high levels of intake induce a shorter transit time and a lower pH in the rumen. Thus the mean pH presents values below the threshold of 6.25 when the level of DM intake is higher than about 2.5 % of live weight. Endly when the animal has a rate of intake, more than 50 g/min, pH values become less than 6.25. In case of high risk of acidosis, it is recommended to provide mineral buffers at an approximative rate of 1 to 2 % of DM intake.

In conclusion, it is very important to characterise precisely the diets which are offered to ruminants by taking into account safety parameters in addition to the classical parameters of the nutritive value. An example of empirical prediction of the pH from such criteria is given.