In the last issue we began a discussion of direct-fed microbial products and the impact they might have on production in beef cattle. In this issue we need to examine the use of one of the most commonly used products of this nature � Yeast cultures. Numerous varieties of yeast products exist and there is some difference in the actual manner in which the different yeasts function. We will examine these differences a bit as well.

Introduction

Yeast usage has found applications in many areas. One particular area of interest is in cattle grazing fescue pastures. Much of the eastern and southern United States has endophyte-infected fescue as the main source of forage protein and energy. While new lines of endophyte-free fescue exist, it is unlikely that there will be wide-spread replanting of fescue areas. There is a renewed interest in year-round or extended grazing to reduce the feed cost of cow-calf production programs. Yeast products may assist in digestion of forages. The yeast Saccharomyces cerevisiae is produced by fermenting selected liquid and cereal grain raw ingredients with bakers yeast. Yeast delivery by means of a mineral supplement to improve animal performance would be less labor intensive than replanting a vast acreage of pastures.

Yeast cultures have been shown to positively affect animal performance and mineral consumption. Studies in Florida and California resulted in improved feed intake, production, and reduced rectal temperatures during summer heat stress in dairy. Other research trials have shown that yeast cultures have also increased rumen bacteria numbers and improved the digestion of feedstuffs in both beef and dairy animals. Both mineral consumption and absorption have been positively affected by the addition of yeast culture to free-choice mineral mixes. Finally one 1986 study showed improved weight gains in yeast culture fed cattle grazing fescue pasture.

As mentioned previously, much of the eastern and southern United States has endophyte-infected fescue as the main source of forage protein and energy. Yeast products, such as Saccharomyces cerevisiae, may assist in digestion of forages. In some studies pregnant heifers appeared to gain slightly more weight if they had access to a free-choice mineral supplement containing yeast when compared to a control mineral. In this particular study, however, the control heifers lost less weight during the interval of calving and peak lactation although this may have some relation to milk production differences. Cow-calf pairs consuming yeast-mineral mixes resulted in increased weaning weights. This is obviously a benefit.

In a study by Boyles and Co-workers at Ohio State University gestating heifers appeared to gain slightly more weight if they had access to a free-choice mineral supplement containing yeast than a control mineral (Table 1). There also appeared to be slightly more body-weight gain for the yeast- supplemented heifers compared to controls during early-spring grass growth.

The period of calving, peak lactation, and rebreeding is a very critical time in the production stage of beef cattle. In the same study, body condition was critically evaluated during the months of April, May, and June. Body condition based upon pounds to inches in height was found to be similar between control and yeast-supplemented cattle (Table 2). All heifers had hip height measurements of approximately 50 inches in April. Milk production in May was 15.4 and 15.6 lb per day for control and yeast-supplemented heifers, respectively (Table 2). Milk production for heifers consuming the yeast-mineral mix appeared to be greater. Weaning weights and weight per day of age appeared to be improved by availability of a yeast-mineral mix.

Finally, this study showed that yeast inclusion increased total mineral supplemental intake. Total supplemental mineral intake was 0.23 and 0.40 lb per day for the control and yeast-mineral, respectively. The yeast-mineral intake was 4.8 ounces per day, and the total yeast consumption per day was 1.2 ounces per day. The difference in total supplement intake between treatments was 0.19 � 0.072 lb per day.

From a different perspective, use of yeast has been shown to have a positive influence on intake in newly received stocker and feedlot cattle. Yeast appears to be useful in reducing stress effects in these cattle and has been shown to be of benefit in getting fresh cattle started on feed somewhat faster.

As you can see, evidence exists that use of yeast cultures does have a positive influence on cattle performance.

Practical Considerations for DFM

In general, most would agree that DFM based on bacteria must be �live.� In light of this, they must survive processing, storage and the gut environment. In contrast, the need to provide a high number of �live� yeast (Saccharomyces cerevisiae) has been the subject of many debates. As previously mentioned, some products guarantee live yeast cells (e.g., 1 � 109 cfu per g) and are fed at low inclusion rates (only 10-20 grams per day) but other products suggest that live organisms are not required for beneficial effects. The metabolites present in the culture extracts have been suggested to be the �active� ingredients. One study reported that heating (such as in the pelleting process), but not irradiation, decreased the ability of an Aspergillus oryzae extract to stimulate rumen bacterial growth and activity. Another trial reported that the stimulatory effect of yeast on numbers of rumen cellulolytic bacteria was diminished when yeasts were heated. The debate on the need for live yeasts will continue unless more definitive studies addressing this issue are conducted.

Direct-fed microbial products are available in a variety of forms including powders, pastes, boluses, and capsules. In some applications, DFM may be mixed with feed or administered in the drinking water. However, use of DFM in the latter manner must be managed closely since interactions with chlorine, water temperature, minerals, flow rate, and antibiotics can affect the viability of many organisms. Non-hydroscopic whey is often used as a carrier for bacterial DFM and is a good medium to initiate growth. Bacterial DFM pastes are formulated with vegetable oil and inert gelling ingredients. Some fungal products are formulated with grain by-products as carriers. Some DFM are designed for one-time dosing while other products are designed for feeding on a daily basis. However, there is little information comparing the efficacy of administering a DFM in a single massive dose compared to continuous daily dosing. The need for a bacterial DFM to actually attach and colonize gut surfaces in order to have a beneficial effect is also questionable. However, in certain applications, the argument could be made that a DFM organism need only produce its active component (without colonization) to be beneficial. Additionally, dose levels of bacterial DFM have varied. Studies can be found where L. acidophilus have been fed at levels ranging from 106 to 1010 colony forming units (cfu) per animal per day. A 1980 study suggested that feeding more than 107 cfu per head per day may cause lower nutrient absorption due to overpopulation of the gut.

Tolerance of DFM microorganisms to heat is important since many feeds are pelleted. In general, most yeast, Lactobacillus, Bifidobacterium, and Streptococcus are destroyed by heat during pelleting. In contrast, bacilli form stable endospores when conditions for growth are unfavorable and are very resistant to heat, pH, moisture and disinfectants. Thus, bacilli are currently used in many applications that require pelleting. Over-blending can sometimes compensate for microbial loss during pelleting, but this is not an acceptable routine practice. Future improvements in strain development may allow use of heat-sensitive organisms in pelleted feeds. Bacterial products may or may not be compatible with use of traditional antibiotics and thus care should be taken when formulations contain both types of additives. For example, some species of bacilli are sensitive to virginiamycin, and lactobacilli are sensitive to chlortetracycline and penicillin. Information on DFM and antibiotic compatibility should be available from the manufacturer.

Viability of DFM products has improved over the past several years but it is highly advisable to adhere to storage recommendations. For example, products should be kept away from moisture, excess heat, and light. Future research on new DFM products will need to address viability if oxygen sensitive microorganisms are to be developed for commercial purposes.

Conclusions

The use of microbial products has been shown to have merit. Obviously research is still required to better grasp application and how cattle respond to microbial product feeding in different situations.