**6. Mode of action of the IPY Vs FPY inside the rumen and post-ruminal GIT**

The first mode of action of the probiotic yeast is competitive exclusion (CE) [27]. The CE is a probiotic mode of action that involves the colonization of the beneficial microbial strains to GIT tract to reduce the addition of disease-causing microbial flora [18–74]. The ability of probiotic yeast cell to fight with other useless microbial flora can improve growth and function of beneficial microbial flora. The IPY has the indigenous strain, which has the advantage that it drives from animal of interest (Cow). IPY has an environmental modification capability. The concept of co-evolution of host microbial has been seen in case of IPY mode of action. The local strain gains an advantage because of its ability to adjust/modify itself in new environment by producing the antimicrobials e.g (lactic acid) to make its less suitable for its competitors. The FPY has the foreign origin strain, which has the less environmental modification capability less, competition for available nutrients, and mucosal adhesion sites. Second mode of action of the PY is reported as a good pH stabilization. Rumen microbial flora can work under stable pH [75]. Rumen pH is highly affected by animal feed intake and its composition. Ruminants eat different types of feed, like high energy concentrate diet, fodder, and silage. These types of feed have a quick impact on rumen pH. If rumen pH is not stable, the animals may have different types of metabolic diseases [76]. Literature showed that PY has a stabilizing effect on the rumen pH [77, 78]. Some studies reported a rise rumen pH when animal was fed on diet with high energy supplemented with PY. Sometimes, the increased pH might be due to the decreased VFAs inside the rumen [3–79]. The lower pH leads to the rumen acidosis, PY can prevent the acidosis condition of the dairy animals [7]. The third proposed mechanism is that yeast cell provides the anaerobic condition inside rumen by removing the oxygen thus facilitated the useful feed digestion microbes [35, 36]. The main microbial flora are bacteria fungi and protozoa. These microbial species have a fiber digestion role by secreting the cellulase and hemicellulase enzymes. Fiber is the main part of the ruminant diet. Therefore, fiber digestion, nature blessed them with unique fibrolytic digestion bacteria (*Fibrobacter succinogenes, Ruminococcus flavefaciens, Ruminococcus albus),* fungi *(Necallimastix*) and protozoa. That complex fibrolytic microbes catalyze the rumen fiber digestion and improve feed intake. The yeast supplementation provides the useful metabolites which stimulate the growth of fiber degrading bacteria [18–47].
