Summary
When cattle are fed grain acidotic ruminal conditions and decreased efficiency in starch utilisation can result from the rapid production and accumulation of lactic acid in the rumen. These conditions depress feed intake and the growth rate of young cattle can be constrained by limited protein availability (through decreased microbial protein synthesis). In more extreme cases clinical acidosis may occur, with symptoms such as laminitis, from the rapid production and accumulation of lactic acid in the rumen of grain-fed cattle. We have investigated the use of probiotic bacteria to reduce acidosis and improve the efficiency of ruminal starch utilisation in lotfed cattle.
From earlier work it was identified that Megasphaera elsdenii YE34 was an efficient lactic acid utiliser that rapidly established dense populations in the rumen and appeared to be a major and important species in grainfed cattle. Probiotically introduced M. elsdenii established a dense and stable population 5 to 7 days earlier than in untreated Control steers. Unfortunately, a superior starch utilising bacterium that would efficiently utilise starch without producing lactic acid was not identified during this early work but would be advantageous in developing a live microbial inoculant to increase the efficiency of ruminal starch utilisation.
The purpose of this project was to identify and isolate the major starch utilising species of bacteria that, in CRC project 2.5.4, were present in the rumen of cattle efficiently utilising grain without signs of acidosis. A probiotic drench using the starch utilising bacteria and M. elsdenii YE34, the lactic acid utiliser, was then used to determine whether this would improve the efficiency of grain utilisation and reduce the incidence of lactic acidosis in lotfed cattle. Specific objectives were to: 1) Examine samples from the CRC pen-trial and identify the dominant starch utilising species of bacteria; 2) Isolate and characterise these species for use in a probiotic drench containing a combination of lactic acid and starch utilising bacteria; and 3) Evaluate in a field trial the benefits from using the probiotic drench in terms of feed intake, liveweight gain and incidence of acidosis. Samples from the CRC pen trial were examined by denaturing gradient gel electrophoresis (DGGE) and dominant DNA bands from steers fed 75% barley were selected.
DNA sequenced from these bands identified Ruminococcus bromii as the predominant bacterium present. Four strains of this bacterium were subsequently isolated and characterised. A Real-Time PCR assay was developed to enumerate R. bromii and to confirm that in the CRC trial this bacterial species rapidly developed high density populations in all the steers soon after the introduction of grain into the diet. R. bromii YE282 was selected for inclusion in a probiotic drench along with M. elsdenii YE34.
A feedlot trial was undertaken at the Queensland Department of Primary Industries & Fisheries Brigalow Research Station. The trial utilised 80 steers in 10 pens in a randomised complete block design. An empty-pen-buffer was maintained between treated and Control groups to avoid transfer of inoculant bacteria to Controls. Following allocation into treatment groups all Control replicates were moved to pens. Treatment replicates were then inoculated with M. elsdenii YE34 and R. bromii YE282 before being moved to pens. These cattle each received 100 ml of each bacterial species by oral inoculation using standard drenching equipment. The cattle were fed for a total of 70 days with commercial, barley-based, feedlot rations. High growth rates (1.91 kg/d) were achieved throughout the experiment in both the inoculated and Control steers. Liveweight changes generally reflected similar intake and growth rates in inoculated and Control cattle. With both liveweight change and intake there was less variability between inoculated steers as compared to Control steers.
There were no marked differences between treatments in carcass weight, dressing percentage or P8 fat cover. The general lack of differences between the inoculated cattle and Control cattle and the fact that all performed very well is probably due to the rapid spread of one of the inoculant bacteria and the transient dominance of the other. The Control group acquired dense populations of M. elsdenii much earlier in the trial than expected or could be anticipated on current knowledge. This meant that the Controls, in terms of the presence of this important bacterium, were similar to the inoculated group before day 14 (and in some steers by day 3). This effectively reduced the period of time over which differences between the groups could have developed.
With hindsight, the experimental design did not take into account the possibility of aerosol spread despite considerable effort being made to ensure no direct physical contact between inoculated and Control steers occurred. This highlighted the importance of experimental design where live bacterial inoculants are used and the importance of isolating the Control group from inoculated animals. The presence of high density M. elsdenii populations appears to correlate with a stable, even reduced, Streptococcus bovis (implicated in producing acidosis) population, higher pH and markedly reduced lactic acid accumulation. R. bromii was only transiently dominant in the rumen ecosystem. Populations established at high densities within the first two weeks but then declined and were undetectable by 50 days. Coincidentally, the cattle in the CRC trial from which R. bromii was identified as the dominant starch utiliser had been on grain for just 14 days. R. bromii appears to be replaced as the dominant bacterial species by Ruminobacter spp. These species became dominant between 14 and 28 days on grain, in all the steers examined, and persisted as dominant species through to the end of the trial. The role of R. bromii in adaptation of the rumen ecosystem to a grain diet is unclear. The performance of the steers in the trial was commensurate with normal to high producing commercial feedlot cattle. They showed few signs of acidosis (none in inoculated steers). Inoculation with M. elsdenii YE34 appears to be a viable alternative to rumen modifiers such as monensin for Controlling acidosis.
We recommend further research to confirm and clarify results, particularly:
Independently determine the natural rate of increase of M. elsdenii in commercial feedlot cattle.
Repeat the experiment with an improved experimental design to re-determine the possible benefits of M. elsdenii inoculation.
Confirm the uniformity in growth performance of the steers inoculated with M. elsdenii.
The longer term benefits from the use of the technology developed in this project are likely to include a reduction in the incidence of acidosis, faster introduction to a grain based diet and improved efficiency of starch utilisation. Although it requires confirmation, a more uniform response to grain feeding resulting in a greater percentage of the mob reaching market specifications within a given time frame may also result.
An additional impact may arise from the ability to safely feed grain to cattle in the field, which will enable opportunity benefits to be gained by a larger section of the industry when cheap grain is available and not impinge on the welfare of these cattle while doing so. Chemical rumen modifiers may have a limited lifespan through lack of public acceptability, their use as trade barriers and the increase in microbial resistance to their action. Live microbial inoculants may well be able to replace these modifiers and deliver similar benefits more cheaply and be more acceptable in the longer term.
