Probiotics For Livestock

Probiotics used for livestock often are referred to as direct-fed microbials. Their use is based on concepts which have been set forth during the past century for potential benefits of lactic acid bacteria in humans. There is currently great interest in the use of probiotics by the livestock industry. A major reason for this interest is that probiotics offer a potential replacement for subtherapeutic levels of antibiotics in livestock diets. The microorganisms involved in this group of probiotics are the same species as used for humans plus yeast cells.

Much of the early research reported in evaluating probiotics for livestock was poorly done. Often the research reports did not provide information concerning the culture used, nor did they indicate the number of viable organisms in the product at the time of use. Furthermore, no basis was provided for having selected the particular organisms used. In the early marketing of such probiotic products for livestock, many products contained very low numbers, if any, of the organisms indicated on the product label (Gilliland, 1981).

As with humans, one strain of one species of a probiotic organism should not be expected to provide all of the possible benefits for all species of livestock. Even though the idea of using probiotics as livestock feed supplements is not new, there is much to be learned about the proper selection of strains of microbial species for use as probiotics to produce the desired effect. A mixture of bacterial species or strains may be required to yield such desired effects as improved growth and performance.

Much of the research which has been published concerning the potential of certain probiotic bacteria to control intestinal pathogens has been done using animal models. Thus, it is reasonable to expect that probiotics could function in helping control these undesirable bacteria in livestock. It is important to find means of controlling these intestinal pathogens in livestock, since they can find their way into the food supply at slaughter or through the use of waste or runoff water from livestock operations to fertilize fruits and vegetables.

There has been more research reported on the use of probiotics to control intestinal pathogens in poultry than in any other animal species. With germ-free chicks as an animal model, for example, it was demonstrated that Lb. acidophilus exerted control over Salmonella species and Escherichia coli in this animal (Watkins and Miller, 1983; Watkins et al., 1982). Using conventional chicks (1-day-old), successful use of Lb. salivarius to prevent colonization of chicks with Salmonella Enteritidis also has been shown (Pascual et al. 1999).

Some have suggested the use of a probiotic-like product made up of intestinal flora of healthy chickens to inoculate baby chicks. This has been named the competitive exclusion concept. It was accomplished by administering the mixed intestinal microorganisms from healthy adult chickens to newly hatched chicks (Nurmi et al., 1992). The idea behind this was that once established, the flora from healthy chickens could exclude infection by salmonellae. This approach is currently being advocated in the poultry industry in the United States. A problem associated with this approach, however, is the lack of control over composition of the mixed culture used to inoculate the chicks.

Perhaps the greatest interest in the livestock industry for the use of probiotics is to obtain improved growth and feed efficiency. This likely involves more than just control of undesirable microorganisms in the animal's digestive system. The mechanisms whereby such improvement could be obtained are presently unknown. Probiotics could provide some specific nutrients that enhance growth or increase appetite so the animal consumes more feed. Feeding of a probiotic product containing a mixture of four species of lactobacilli resulted in increased growth and improved the feed-to-gain ratio of broilers (Jin et al., 1998). Lactoba-cilli as feed supplements also improved feed intake and weight gain in lambs (Wallace and Newbold, 1993). Inclusion of viable yeast cells in animal feeds can provide a benefit in several livestock species; for example, both meat and milk production have thus been increased in cattle (Wallace and Newbold, 1993).

The stress of weaning young animals in most livestock species results in development of scours. Probiotics containing lactobacilli such as Lb. acidophilus can reduce or eliminate this problem in calves as well as in pigs and lambs (Jons-son and Conway, 1992; Wallace and Newbold, 1993).

The possible mechanisms of action in livestock are probably similar to those reported for humans. Control of intestinal pathogens, for instance, could involve direct inhibitory action by the probiotic bacteria or could result from stimulation of the immune system. Improved growth and performance are more difficult to explain. Although it may be that this, in part, results from control of undesirable microorganisms, it is likely that it involves far more. Some microorganisms in a probiotic mixture could provide an enzyme in a manner similar to that involved in improvement of lactose digestion in humans. In a preliminary study in our laboratories, for instance, we have shown that a strain of Lb. acido-philus having a high level of amylase activity increased growth and feed efficiency in newly weaned pigs on a starch-based diet (unpublished data). Although several studies have noted improvement in growth and performance of livestock given various probiotics, we need to determine the mechanism whereby this improvement occurs. Then we will be better able to select appropriate probiotic organisms for use in livestock feeds.

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