Science

Probiotics for ruminant livestock: a brief review

Dr. Kevin Hillman (BSc Hons PhD)

Micro-gastrointestinal Ecologist and author of gutbugs.com

Formerly of the Rowett Research Institute and Scottish Agricultural College (SAC)

Ruminant animals, including cattle, sheep and goats, depend on microbial degradation of their feed rather than on direct enzyme degradation, as in nonruminants. The animal then absorbs volatile fatty acids from the rumen for glucose formation in the liver, and the protein digested in the gastric stomach (the abomasum) is largely microbial. In a normally functioning ruminant, little or none of the sugars and proteins originally present in the feed are directly incorporated in the animal: they are first processed by bacterial fermentation in the rumen.

At birth, the calf's gut is sterile and the rumen is undeveloped. Milk bypasses the rudimentary rumen, entering the abomasum directly (Quigley, 2000). The rumen develops in response to dry feed intake (Haenlein, 1998). The microbial fermentation in the rumen becomes fully active only after rumen development is complete, and increasing feed intake in early life accelerates this development (Wallace and Newbold, 1995).

Probiotic additives have been shown to be of benefit at all stages of life in the ruminant. The available literature on probiotic use in cattle, and in farm animals in general, is vast, and the short timescale of this review allows little more than an overview of the tremendous amount of work that has been done on the subject. The few papers cited here represent, quite literally, the tip of an enormous iceberg.

Abu-Tarboush et al (1996), reported improved weight gain in calves with two probiotic preparations, in comparison with a control. With a mixture of Lactobacillus acidophilus plus L. plantarum, weight gain was greatest in weeks 7-9 after weaning, while with the L. acidophilus product, weight gain was observed to increase in weeks 10-12. In this case, probiotic treatment started at weaning. By contrast, Jenny et al (1991) started Holstein calves on a variety of probiotics from two days of age, and found a positive effect on feed efficiency during weeks 1 to 4.

The excellent review by Wallace and Newbold (1995), although now dated, lists a number of studies on the application of probiotics in young calves. These are summarised in table 1.

As can be seen, different effects result from the different probiotic species applied. For this reason, a multi-strain probiotic preparation is likely to prove beneficial, to provide a full spectrum of the effects reported.

Table 1: Summary of the reported studies reviewed by Wallace and Newbold (1995) on probiotic use in pre-ruminant calves.

The principal cause of lactic acidosis appears to be Streptococcus bovis, which is capable of rapid lactic acid production when the animal is abruptly provided with a high-starch diet (Russell and Hino, 1985).

Lowering of the rumen pH, perhaps initially as a consequence of Strep. bovis activity, leads to a consequent runaway lactic acid production. The lactic acid utilisers, principally Selenomonas ruminantium and Megasphaera elsdenii, are unable to cope with the rapid formation of the acid, and are inhibited by the low pH. Strep. bovis is also inhibited by low pH, and there is some evidence that the rumen lactobacilli produce a compound inhibitory to the streptococcus (Wells et al, 1997). Unfortunately, in acidosis, this happens too late. The lowered pH has already increased the metabolism of the rumen lactate producers. It is therefore preferable to increase the numbers of Lactobacillus spp. in the rumen by artificial means, by adding more as probiotics. These will be short-lived since the rumen pH is not optimal for their growth, but the increase in numbers may reduce the activity of Strep. bovis.

Hino et al (1994) reported that M. elsdenii prefers lactate over glucose when presented with both substrates, while Kung and Hession (1995) found that the addition of M. elsdenii to rumen fluid presented with a rapidly fermentable substrate in vitro, prevented lactic acid accumulation. Unfortunately the large-scale culture of this bacterium would be prohibitively expensive, preventing its current use as a probiotic or even as a potential treatment for acidosis in cattle. It has, however, been attempted and has been shown to establish a lactate-utilising population in cattle switched to a high-grain diet (Klieve et al, 2003).

One of the principal causes of acidosis is a sudden change from a roughage diet to a high-starch, grain diet, causing a surge in activity of amylolytic bacteria, including Strep. bovis. Conversely, numbers of Strep. bovis in the rumen drop by 50% when changing from grain to roughage diets. This sounds a lot, but may only mean a change from 50 million to 25 million. Bacterial replication rates mean that this population can double in a few hours, so a 50% drop is not important. However, a sudden doubling may be important, as it would also mean a sudden doubling in that population's output of lactic acid, driving pH down too fast for the lactate degraders to keep up. So the risk of acidosis remains, in roughage-fed animals, should there be another abrupt change to a high-starch diet.

Rumen bacterial populations are in excess of 10¹⁰ per ml of rumen fluid. Adding a dose of 10⁹ Lactobacillus spp. to this will have no impact on increased lactic acid production, particularly since Strep. bovis is a starch utiliser, whereas most lactobacilli are not. In fact, adding a mixture of Enterococcus faecium, Lactobacillus plantarum and Saccharomyces cerevisiae to dairy cattle was shown to reduce ruminal acidity over the diurnal cycle (Nocek et al, 2002).

Fungal additives, primarily S. cerevisiae or A. oryzae, increase total volatile fatty acid concentrations in the rumen (Adams et al, 1981; Firkins et al, 1990; Newbold et al, 1990) yet simultaneously reduce peak lactic acid levels following grain feeding (Williams et al, 1991). The increased fatty acids are primarily acetate and propionate. Lactic acid has a lower pK value than the fatty acids (ie. it is a 'stronger' acid), so a shift towards the fatty acids is one factor that would tend to raise rumen pH.

This effect may be of particular significance where a subclinical acidosis is present, where the pH of the rumen drops below 5.5 for long periods but a full acidosis has not developed. The rumen pH should be around 6.8 for optimal production, and the use of probiotics provides a low-risk means to approach this level

In feedlot cattle, a direct-fed microbial (probiotic) supplement comprising Enterococcus faecium and a Propionibacterium species reduced the numbers of Strep. bovis in the rumen. (Ghorbani et al, 2002). This probiotic was credited with causing an increase in rumen protozoa with a corresponding increase in ammonia production, which would tend to raise pH. A reduction in amylolytic activity was also observed. Overall, the authors suggest that this preparation may decrease the risk of acidosis in cattle.

The increase in protozoa observed by Ghorbani et al (2002) is of particular interest, as it is a parameter which seems to have been overlooked by many workers in this area. Anaerobic protozoa are present in much lower numbers than bacteria but they are very much larger. If the bacterial and protozoal populations of the rumen were separated, they would be found to contain much the same weight of protein - the biomass of rumen protozoa is similar to that of bacteria. Overlooking the protozoa in a study of rumen function is therefore a serious omission.

Many of the rumen protozoa feed on bacteria, as well as on particles of plant material in the feed. If the probiotic is increasing the feeding activity of the protozoa towards bacteria, it may reduce their uptake of, for example, starch grains. Metabolism of protein-rich bacteria will increase rumen pH (less acidic) by the production of ammonia. Metabolism of starch grains, in contrast, will result in acidic (reduced pH) end products.

The uptake of added L. plantarum by rumen protozoa was shown to be very rapid (Sharp et al, 1994), showing that the influence of the protozoa on bacterial additives, and their subsequent metabolism of those additives, cannot be discounted. This is, of course, pure speculation at this stage, since there is no currently reported mechanism by which a probiotic could change the rumen protozoa's feeding preferences. However, in the absence of any definite mode of action to explain the reduction in pH by bacterial probiotics, this aspect may be worth further investigation.

Cattle can become well-adapted to high grain diets, and function normally with a reduced rumen pH. The addition of probiotics (Enterococcus faecium and yeast) to such adapted cattle made little improvement to production (Beauchemin et al, 2003). In this case, the animal's physiology has adapted to the diet so there is no problem to correct: there is really nothing for the probiotic to do. Probiotics may help animals in transition, where a roughage diet is replaced with grain, but in an adapted animal there is no need for any form of gut protection, so a probiotic designed to alleviate acidosis will have little effect in this situation.

Cattle can become adapted to high grain diets, and feedlot cattle appear to function normally with a reduced rumen pH. The addition of probiotics (Enterococcus faecium and yeast) to such adapted cattle made little improvement to production (Beauchemin et al, 2003). In this case, the animal's physiology has adapted to the diet so there is no problem to correct: there is really nothing for the probiotic to do. Probiotics may help animals in transition, where a roughage diet is replaced with grain, but in an adapted animal there is no need for any form of gut protection, so a probiotic designed to alleviate acidosis will have little effect in this situation.

Dairy cattle, however, do not show such adaptation, although clinical signs of acidosis may not appear. This subclinical acidosis can have marked effects on productivity, leading to depressed milk-fat content, reduced yield, and in extreme cases leading to laminitis, weight loss, poor body condition and unexplained abscesses (Mutsvangwa and Wright, 2003). Acidosis is most often induced by feeding a high grain or concentrates ration, with insufficient roughage, and when one animal in a herd develops full acidosis, it is highly likely that the rest of the herd are experiencing a subclinical form of the disease (Laven, 2003). In cases like this, a probiotic preparation shown to increase rumen pH may be helpful, as restoring the normal pH within the rumen will assist the re-establishment of the normal rumen microflora.

Even where no clinical signs are present within the herd, it is possible that improvements in productivity can be made with an appropriate probiotic treatment. Many of the reported improvements in milk yield in the literature may, in fact, have resulted from a stabilisation of rumen pH by the probiotic, effectively treating a subclinical acidosis that had not been recognised at the time.

Probiotics have been successfully used to alleviate transport stress in a wide range of animal species, including cattle (Wallace and Newbold, 1995). A yeast culture successfully reduced morbidity due to shipping stress in feedlot calves (Zinn et al, 1999). Morbidity was reduced by 48%, and the total days of evident illness were reduced by 44%. It seems that probiotics have a useful role to play wherever transport results in intestinal disruption.

Heat-induced stress is of particular concern in dairy cattle. The normal rectal temperature of a cow is approximately 101-103ºF (around 39ºC), and its preferred ambient temperature is in the range 40-77ºF (Linn, 1997). Above this range, it becomes difficult for such a large animal to dissipate body heat, and the resulting effects on the animal's production can be severe. Feed intake is reduced, milk yield can drop by up to 25%, milk quality is reduced and the immune system can be depressed (Linn, 1997). Fertility can be severely affected: even traditional cooling using water sprays cannot completely remove the problem (De Rensis and Scaramuzzi, 2000).

Huber et al (1986, 1994) reported a reduction in rectal temperature in heat-stressed animals fed a probiotic A. oryzae. The effect appeared to result from a direct effect on the animal's physiology rather than any effects on rumen fermentation. Viable S. cerevisiae have been recovered from the duodenum and ileum of sheep fed this yeast (Newbold et al, 1990). Although no study of this type has been carried out for A. oryzae, post-ruminal effects remain a possibility.

For heat stress, it appears that the inclusion of fungal probiotics in the diet may be beneficial. Linn (1997) recommends the inclusion of A. oryzae in the diet although the effects observed on body temperature remain unexplained. However, other measures, such as spray cooling, should not be discontinued.

Milk yield

Obviously, milk yield and quality is of the utmost importance in dairy cattle. Quality can be improved by adjusting the fatty acid profile of the rumen, which affects the triacylglycerides formed in the milk. Improvements in milk have been the subject of study for many years.

As early as 1954, Renz reported that the inclusion of live yeast in cattle feed increased milk yield by 1.1 kg/day. Since then, there have been many attempts to improve milk yield and quality. Probiotics have been used extensively, primarily those based on fungi. Responses to yeast products have been varied, ranging from no response to 17% increase in milk yield (Williams and Newbold, 1990). Overall, an increase of 5% yield can be expected with either Aspergillus oryzae or Saccharomyces cerevisiae, with no obvious advantage of one over the other. As has been mentioned, it is possible that the effects noted on improved milk production may result from the probiotic's correction of a subclinical acidosis in asymptomatic animals.

A major factor in milk production is mastitis, of which there are several forms. Antibiotics are normally used to treat the condition, although this imposes a withdrawal period on the animal so that its milk cannot be sold. Further, cases of chronic infectious mastitis can be treated but often return, so that the animal must eventually be culled from the herd.

The probiotic in this case proved less effective than the antibiotic at curing mastitis, although it is impressive that it worked at all. Probiotic function is normally preventative rather than curative: perhaps this treatment, if applied to all cows, would have reduced the incidence of mastitis in the herd.

A compound produced by L. reuteri was inhibitory to a wide range of bacteria, including Staphylococcus aureus, one of the principal bacteria involved in mastitis. (Ganzle et al, 2000) The probiotic was not tested against Strep dysgalactiae, Strep. agalactiae or Strep. uberis, although it was effective against a related bacterium, Enterococcus faecalis.

Since the use of antibiotics effectively removes a cow from milk production for up to several weeks, the search for alternative means to treat mastitis is intense. Probiotics are showing great potential in this area, although it seems they may show greater success as preventative rather than curative agents at this stage. The development of multi-strain probiotics is to be recommended in this case since mastitis is caused by a variety of bacteria, and no probiotic can be reasonably expected to inhibit all of them.

Production response

In beef cattle, the main concerns are growth and meat quality. To this end, the rapid development of the rumen in calves is highly desirable as it results in an increased rate of growth of the animal. However, beneficial effects of probiotic use have also been reported in adult animals for many years. In 1952, Beeson and Perry found an increase of 6% in daily weight gain of steers fed dried live yeast. The literature that has appeared since then is vast, and can only be touched upon here.

Huck et al. (2000) reported improved feed efficiency in finishing heifers with a Lactobacillus acidophilus additive. They also found that a probiotic based on Propionibacterium freudenreichii produced an improvement in carcass grading The authors suggested that targeting probiotics to different phases of growing/finishing cattle would result in improved yields.

An increased methionine outflow into the abomasum was observed in cows fed yeast probiotics (Wallace and Newbold, 1995). No currently postulated mode of action can account for this observation. Methionine is often the first-limiting amino acid in ruminant nutrition, so increasing the availability of this to the animal would be expected to improve performance.

A probiotic S. cerevisiae diverted hydrogen metabolism from methane production (which is wasteful of carbon) to acetogens (production of acetate) in vitro (Chaucheyras et al, 1995). Assuming the process also works in vivo, this would result in reduced methane emissions and an increased transfer of carbon from the animal's feed into its growth.

Mir and Mir (1994) found a small increase in feed intake in steers supplemented with S. cerevisiae, although their data did not reach statistical significance.

As always, the results of probiotic inclusion are varied. However, examination of the literature reveals that the rate of inclusion varies considerably, as does the basal diet and the timing of the probiotic. As Huck et al (2000) suggested, certain probiotics produce optimal results at specific points in the growth of the animal. This aspect is rarely examined, but may hold the key to more consistent results from probiotic use in all areas of animal research.

Recently, the carriage of pathogens by livestock has become an issue of public concern. In reality, zoonoses have been a fact of life since farming began, and all are preventable by normal hygiene/storage/cooking practices. However, it is now necessary to show that these pathogens can be removed from farm animals, ideally without the use of antibiotics, since these are also out of favour. Thus the probiotic concept returns to its roots, to the inhibition of harmful intestinal bacteria. Here, the probiotic's action is primarily in the hindgut, so it must be dosed at sufficient levels to survive passage through the rumen and abomasum.

Conclusions

It seems there is no aspect of cattle production which cannot be improved by the correct use of a probiotic preparation. Many scientists find little or no effect from the probiotic they use, but there are many, many successful reports in the journals. The subject of probiotic use in cattle has been active for at least fifty years, sustained by the regular reports of successful applications. However, we still understand little of how these cultures produce their effects. It is likely that the observed variability has more to do with incorrect application of the probiotic than with any inherent variability in its action.

The bacterial probiotics appear to produce the best results when targeting pre-ruminant calves or hindgut infections, while the fungal probiotics show better responses on rumen fermentation, growth and the effects of stress. Many modes of action have been postulated, but none have been definitively proved. It is possible that many different effects result from the addition of even a single probiotic to the an ecosystem as complex as the rumen. Overall, it seems that different probiotics perform different ranges of functions in the ruminant animal, so there is considerable scope for a multi-strain probiotic to produce simultaneous improvements in several aspects of production.

The bacterial probiotics appear to produce the best results when targeting pre-ruminant calves or hindgut infections, while the fungal probiotics show better responses on rumen fermentation, growth and the effects of stress. Fungi, and fungi/bacteria combinations, seem to be particularly effective in raising rumen pH in animals showing signs of subclinical acidosis. Many modes of action have been postulated, but none have been definitively proved. It is possible that many different effects result from the addition of even a single probiotic to an ecosystem as complex as the rumen. Overall, it seems that different probiotics perform different ranges of functions in the ruminant animal, so there is considerable scope for a multi-strain probiotic to produce simultaneous improvements in several aspects of production.

References available upon request