Using brewing yeast cultures as feed additives primarily optimizes the nutritional value of feed (Wed-meller et al., 1987; Hamson et al., 1988; Shaver et al., 1997). Yeast cultures are a pure, natural feed ingredient that enhances feed palatability and digestibility. Improved palatability maintains consistent feed intake; better feed digestibility allows animals to absorb more nutrients to boost productivity.
Recent studies have shown that yeast cultures can stimulate the proliferation of ruminal cellulolytic bacteria and lactic acid bacteria, alter ruminal fermentation patterns, reduce ruminal ammonia concentration, and increase the yield of ruminal microbial protein and feed digestibility. Researchers have investigated the effects of yeast cultures on the milk production and milk composition of Holstein cows under domestic feeding conditions, with an addition of 60g per head. The results indicate that milk production increased by 0.9kg per day, reaching a peak of 1.77kg per day by the fifth day, and significantly increased the content of milk fat and lactose. Adding 60g per head of yeast cultures to the daily ration of lactating cows in the early stages of lactation has a notable effect on milk production, increasing the average daily milk yield by 0.48-1.74kg, and the average daily increase in milk production by 1.13kg. However, the effects of yeast cultures on cows are influenced by the nutritional level of the diet and the animal's physiological status. Harris et al. (1988) reported that the effect of yeast cultures on early lactation is more significant than on late lactation; researchers believe that the milk-increasing effect of yeast cultures tends to be better with higher parity; Cuiuonz et al. (1988) reported that the effect is not significant when cows are fed on fresh grass and wheat.
Yeast culture exhibits significant milk-enhancing effects in cows due to its metabolic byproducts, which promote ruminal fermentation. This leads to the proliferation and growth of ammonia bacteria, protein-synthesizing bacteria, and fiber bacteria, thereby enhancing the digestibility of roughage in feed and improving the efficiency of bacteria utilizing non-protein nitrogen (NPN) to synthesize bacterial body proteins.

The application of yeast culture in monogastric animals has demonstrated effects on improving feed digestibility (Glade, 1991) and reproductive performance (Wanani, 1991). Whink et al. (1990) found in trials with pigs and chickens that adding yeast culture to high-fiber diets not only increased growth rates and feed conversion but also enhanced the digestibility of certain fiber components in energy-rich feeds. Korenegy (1995) observed that adding 8% peanut or soy hulls to growing rations reduced daily weight gain and feed intake in pigs, while supplementing with yeast culture mitigated the negative effects of crude fiber and improved daily weight gain. Chapple (1979) discovered that adding yeast culture to low-phosphorus diets increased pig daily weight gain by 10% and also improved feed intake. Experimental studies have shown that adding 2% yeast culture to broiler diets can promote growth, while adding 4% significantly slows down growth rates, a finding consistent with reports by Guevara (1977) and Savape et al. (1990). Yeast culture supplementation in poultry feeds can alleviate stress responses and enhance feed efficiency (Teeter, 1993).