The etiology of low rumen pH and possible tools to reduce the incidence of sub-acute rumen acidosis (SARA)

摘要

The most common rumen upset of dairy cattle is a decrease in rumen pH. Whenever accumulation of fermentation end-products is excessive, removal from the rumen through absorption or passage rate inadequate or neutralization with buffers or alkalinizersinsufficient, rumen pH decreases. The ruminant animal has mechanisms to neutralize the acid produced in the rumen. For example, saliva is rich in buffers and alkalinizers, nitrogen recycling through the rumen wall and saliva may also contribute to increase rumen pH, and cows regulate intake based on rumen pH and osmolality. Rumen acidosis is usually associated with erratic feed intake. This is mostly due to the protection mechanisms that ruminants have to control rumen pH. In addition, if rumen pH decreases some rumen bacteria sensitive to pH die. The decrease in microbial biodiversity in the rumen increases the instability of the rumen flora to maintain steady conditions and thus, the rumen becomes more susceptible to experience upsets or even worseacidosis scenarios in case of sudden nutritional changes. The most common causes of rumen acidosis are 1) consumption of large amounts of NFC, 2) poor absorption of organics acids through the rumen wall, 4) altered rumen microflora, and 5) impaired ruminal buffering capacity. Acidosis can be prevented and controlled through ration formulation, particle size and feed bunk management, and inclusion of ionophores! organic acids, probiotics, and buffers and alkalinizers in the ration! Balancing for total NDF in the ration, independently of particle size may explain about half of the variation in milk fat content. Improving particle size of forages does not consistently result in increased ruminal pH or reduced risk of SARA, especially if sorting against large particles takes place. Ionophores might be effective in preventing acute rumen acidosis caused by an accumulation of lactic acid due to sudden dietary changes, but does not seem effective in preventing the accumulation of VFA in the rumen, and thusdo not seem effective in preventing SARA. Supplementation of dicarboxylic acids such as aspartate, fumarate, and malate, stimulates the growth of S.ruminantium on lactate in the rumen and may prevent acute rumen acidosis, but the dose needed might not becost-effective. Supplementation of lactic-acid producing bacteria to ruminant rations has been proposed as a means of ensuring a basal and constant production of lactic acid in the rumen that should foster the establishment of a solid mass of lactic acid utilizing bacteria. Supplementation of lactic-acid producing bacteria has been shown to also improve DMI and subsequently milk production of pre-partum and early lactation dairy cows. Also, there are studies, that report that live yeast to control areeffective in controlling lactic acid concentrations in the rumen in rumen cannulated dairy cows and in incubations of mixed ruminal microorganisms in vitro. One of the most consistent results observed with live yeast supplementation in ruminants is an increase in the number of rumen bacterial cells. In general, when large doses of sodium bicarbonate are used, rumen pH has been reported to increase, but when doses of 1.5% of DMI or less sodium bicarbonate are used there seems to be inconsistent results,with no effect on rumen pH in some studies and positive effects in others.