Brewers yeast can be used as a feedstuff in ruminant diets. Large quantities of yeast are discharged as slurry from breweries or from the alcoholic fermentation of sugarcane, and might be as useful as the vegetable meals (especially cotonseed meal) for ruminant diets (Hennessy et al., 1993). Some yeast slurries are dried, which inactivates the yeast; but the high energy costs of drying might reduce the price competitiveness of the product against other protein sources. The influence of yeast on ruminant performance, either in dried or liquid forms, is detailed below. In all these studies, it was generally concluded that yeast can be used as an alternative protein source in rations for ruminants because it does not change animal performance and it has equivalent or higher nutritional value than soybean meal.
Note on yeast as probiotic
As indicated in the introduction, the probiotic effects of live Saccharomyces cerevisiae are beyond the scope of this datasheet. Supplementation of live yeast probiotics on intake, production and rumen fermentation characteristics has been widely studied during the past decades. Quantitative reviews generally reported modest improvement in rumen fermentation patterns, feed digestibility, dry matter intake, milk yield, fat corrected milk yield and milk fat content (Desnoyers et al., 2009; Robinson et al., 2009; Poppy et al., 2012; Elghandour et al., 2015). However, the response to yeast supplementation is not constant, depending on dosages, feeding times and frequencies, and strains (Elghandour et al., 2015).
Digestibility and energy values
In a comparison of dairy cattle diets supplemented either with soybean meal or with liquid brewers yeast, the digestibility of DM, gross energy, protein and ADF of a dairy cattle diet were higher with yeast supplementation (Steckley et al., 1979). But this effect is not consistent (e.g. no effect was found by Freitas et al., 2015). In goat kids and lambs, digestibilities of DM, OM, total carbohydrates and NDF were improved with increasing levels of yeast in the diet as a substitute for soybean meal (de Lima et al., 2011; Rufino et al., 2013). Rations with dried yeast showed the highest values of total digestible nutrients. The effect appears to be quadratic, with maximal digestibility coefficients for diets with approximately 45% inactive dried yeast (Rufino et al., 2013).
Dairy cows
In dairy cattle, the inclusion of dried brewers yeast as a replacement for soybean meal at up to 20% of the total dietary DM did not affect intake, digestibility or performance (Nursoy et al., 2003; Freitas et al., 2015). Some positive effects were observed, such as a low rumen NH3-N level and a high acetic acid concentration (Nursoy et al., 2003). Thus, dried yeast seems to be a valuable protein source in dairy cow diets. Liquid forms, such as brewers yeast slurry, can be fed to dairy cattle as an alternative to soybean meal, up to 12% of the diet DM, without any detrimental effect on DM intake, milk production and milk organoleptic quality (Steckley et al., 1979). Milk yield for diets with 30% wet brewers grains was higher with added liquid brewers yeast than without it (+1 kg/cow/day, West et al., 1994).
Growing cattle
In heifer calves fed a maize silage-based diet, the addition of liquid brewers yeast, up to 9% of the total DM, led to higher DM intake (+1 kg) and daily weight gains (+ 500 g/d). These performances were similar to those obtained with soybean meal supplementation. DM intake was higher when yeast was fed free choice or mixed in the diet than sprinkled on top of the basal ration (Grieve, 1979). In steers fed native pasture hay, there was no improvement in live-weight gain, or in feed conversion efficiency, when up to 4.4 kg of yeast slurry (i.e. 423 g DM) was added to the basal diet. However, when replacing cottonseed meal and alfalfa chaff as a protein source in grain-based feedlot diets, increasing dietary levels of yeast (0 to 61% of total DM) were associated with increasing live-weight gains and carcass yield (Hennessy et al., 1993). There was a trend towards decreasing feed costs when replacing cottonseed meal and sorghum grain with yeast slurry. However, the large range in slurry DM contents (4.8-15.6%) and crude protein content (43-51% DM depending on the load, Grieve, 1979) requires close monitoring, with daily changes made to the amounts of slurry offered to maintain dietary N and protein levels. These ranges could make it difficult for commercial feed compounders to produce a diet of consistent quality (Hennessy et al., 1993).
Sheep
Nutrient intake, growth performance and meat quality of lambs fed diets containing increasing levels of inactive dried yeast (0, 33, 67 and 100% on a dry matter basis) to replace soybean meal were globally not affected. 100% inactivated dried yeast in lamb diets could even improve the carcass and meat quality by reducing the deposition of subcutaneous and intramuscular fat (Rufino et al., 2013).
Goats
In Brazil, several experiments were conducted with Saanen dairy goats (primiparous and multiparous, pre and postpartum) or goat kids (¾ Boer x ¼ Saanen or Saanen). Each time, inactivated dried yeast from sugarcane fermentation was used as an alternative protein source to soybean meal in maize silage based diets (soybean meal vs. soybean meal + yeast vs. yeast alone representing 23% of the total dietary DM). The nutritive value of the diets was unchanged and there was no effect on milk yield, live-weight gain or feed conversion efficiency (de Lima et al., 2011; de Lima et al., 2012; Gomes et al., 2012; Gomes et al., 2014; Molina et al., 2016). However, the milk production efficiency (kg of milk produced/kg of crude protein ingested) was better in goats fed the dried yeast diet (de Lima et al., 2012).
