The addition of yeast to intensive beef diets has increased daily live weight gain and FCE [3
In relation with the growth of cattle, Swinney-Floyd et al. [34
] reported that though bacterial DFM did not alter DM intake, ADG for the animals inoculated with the combination of Propionibacterium
and L. acidophilus
was greater than control calves. In line with this, Huck et al. [38
] reported a tendency for greater carcass-adjusted ADG in heifers that were sequentially fed a Lactobacillus
and then a Propionibacterium
or a Propionibacterium
and then a Lactobacillus
probiotics between the receiving and finishing phases compared with control heifers. But, in contrast, Rust et al. [35
] reported no difference for carcass-adjusted ADG in control steers versus steers supplemented with Lactobacillus
. However, ADG for the average of all probiotics -treated steers was increased 6.2% over control steers. In addition, feed efficiency for the entire feeding period was improved for steers receiving certain probiotics treatments compared with control steers. Galyean et al. [36
] also reported that the final BW, ADG and hot carcass weight were significantly greater for steers treated with probiotics versus the control animals. Similarly, McPeake et al. [37
] reported steers fed a probiotics had greater final weight, ADG, DM intake, hot carcass weight and carcass-adjusted ADG compared with control steers. In addition, there was also a trend for improved feed efficiency as the concentration of supplemental L. acidophilus
With regards to carcass characteristics, different studies [35
] indicated that, the addition of probiotics did not affect yield grade, quality grade, dressing percentage, marbling score. However, hot carcass weight was generally greater when probiotics were fed. On the other hand, Huck et al. [38
] reported that the percentage of carcasses graded as was greater in heifers receiving a Propionibacterium
probiotics in both the receiving and finishing phases than in control heifers. To this end, it can be seen that improvements in carcass characteristics as a result of probiotics are questionable, except for hot carcass weight. Because probiotics generally improve ADG, hot carcass weight would be increased if probiotics supplementation increased ADG and final BW [39
]. Beauchemin et al. [1
] elaborated that the positive effects of probiotics on performance might be associated with a decrease in ruminal acidosis and/or improved microbial balance in the lower gastrointestinal tract.
Milk yield and composition in dairy cows:
Researches concerning the effects of probiotics in dairy cows are limited. Even in the available studies the probiotics were fed together with other additives making it difficult to judge the effects of the probiotics themselves. In the few studies available in which probiotics were fed to dairy cows, the milk yield increased by 0.75–2.0 kg/d. In general, an increased milk yield has been a consistent response, whereas changes in milk composition have been variable. In support with this, Gomez-Basauri et al. [40
] described that cows fed lactic acid bacteria and mannan-oligosaccharide produced more milk. The authors reported that milk yields increased over time for lactic acid bacteria-and mannanoligosaccharide-fed cows, whereas control cows maintained constant milk yields. Furthermore, there are experiments that suggest probiotics fed alone or in combination with fungal cultures might be efficacious for increasing milk production by lactating dairy cows [41
]. However, more research is needed before recommendations can be made have been conducted with combinations of fungal cultures and lactic acid bacteria.
Effects of probiotics on poultry health and performance:
Addition of probiotics has shown beneficial effects on growth performance of poultry. In broilers, supplementation of a diet with probiotics has resulted in improved feed conversion rate and average live weight in comparison to the control group. Moreover, in a study by Mountzouris et al. administration of the multi-strain probiotic in the drinking water significantly increased average daily weight gain and feed efficiency and numerically reduced mortality rate in comparison with a negative control.
Effects of probiotics on pig health and performance:
Some reports have indicated that supplementation of probiotics improves performance in suckling pigs, weanling pigs, grower pigs and finishing pigs. Administration of the multi-strain probiotics tended to cause higher weight gain and feed efficiency. In support with this, Giang et al. reported that piglets fed probiotic complexes diets had higher feed intake, daily gain and better feed conversion during the 1st 2 weeks after weaning. This indicates a synergistic effect of different probiotic strains under in vivo conditions. Eventually, as a result of feeding a fermented diet, the time available for the gastrointestinal microflora to decarboxylate free amino acids present in the diet minimizes which has shown to improve performance in pigs [43
In addition to the improved average daily gain, addition of probiotics reduced mortality rates of growing and finishing pigs. In line with this, different studies revealed that the incidence and severity of diarrhea as well as mortality rate were significantly decreased as a result of feeding probiotics. Moreover, live yeast supplementation to the diet of pigs has resulted in demonstrable reductions in the quantity of pathogenic bacteria.
Application of probiotics and recommendations for use:
Although some products contain purified strains of individual organisms, most probiotics products are a combination of several species of bacteria and yeast and other fungi. Most probiotics products for calves are sold as feed additives, which are added to milk or milk replacer just prior to feeding, while others are administered as gels, pastes, or boluses. But, their application on the farm is more challenging. Adding probiotics to pelleted feeds is difficult, as temperatures and pressures used in pelleting generally kill most organisms [15
The stabilisation of the digestive or microflora in ruminants and in monogastric animals can only be effectively achieved by continuous supplementation of the feed with probiotics because the microorganisms used in animal nutrition do not permanently colonise the intestine. Increased short-term supplementation of probiotics may be useful under certain conditions but should be followed by continuous supplementation thereafter. General guidelines on the optimal dosage and the period of supplementation are not possible because factors such as stability of the probiotic in the feed and in the digestive tract, the specific mode of action of the microbes contained in the product and the status of the intestinal microflora in the host all modulate the effect of the corresponding product [9
The optimal dose must be determined individually for each product and each target species in feeding trials. The rate of inclusion given by the manufacturer, therefore, is based on information gained from efficacy studies [9
]. In general, however, it is accepted that the inclusion rate of all probiotics should be higher when the intestinal microflora is unstable and particularly when for ruminant the diet composition contain high rapid-fermentable sugar which can entail sub-acidosis. In addition, the overall consumption of probiotics by older animals will be higher because of a higher feed intake compared to younger animals. Therefore, with continuous supplementation, the inclusion rate may be reduced during the growth of the animals without the concentration of the probiotic microorganisms in the intestine dropping below the level of efficacy.
Efficacy of probiotics:
Reports of probiotics efficacy are variable but this may depend greatly on survival rate of strains, varying stabilities of strains, low probiotic doses, frequency/infrequency of administration, interactions with some medicines, health and nutritional status of the animal and the effect of age, stress, genetics and type differences of animals [14
]. To be effective, the desirable microorganism should not be harmful to the host animal, should be resistant to bile and acid, should colonise the gut efficiently, should inhibit pathogenic activity, and should be viable and stable under manufacturing and storage conditions [3
Research points to the fact that probiotics are most effective in animals during microflora development or when microflora stability is impaired. The benefit of probiotics with respect to health status and performance is expected to be highest in young animals such as piglets, newly-hatched chickens or calves, because these animals have not yet developed a stable gut microflora. Moreover, when animals undergo therapeutic treatment of diseases with antibiotics, the gut microflora is generally decimated. Therefore, administration of probiotics after antibiotic treatment assists in re-establishing a beneficial gut microflora to prevent the host from recurrent pathogenic colonisation [20
For pigs, it is suggested that the effects of probiotics appear to be more consistent and positive in piglets rather than in growing finishing pigs. Hence, in a review of the response of pigs of various ages to the administration of probiotics, it was concluded that probiotics were effective for young pigs, in which the digestive tract is still developing after weaning. However, probiotics were less effective for growing and finishing pigs, which already have a balanced population of microorganisms [3
With regard to cattle, similarly, improvements in animal performance may be limited in young, milk-fed calves. Rather, it appears that probiotics may be most useful under specific conditions whereby calves are exposed to immune or management challenges that may disrupt the intestinal environment. Under stress conditions, probiotics may reduce the risk of scours caused by an upset in the normal intestinal flora of calves [5
]. As in the neonatal calf, the response to probiotics might be greater if administered to newly weaned and/or received beef calves, which are more prone to health problems. Gill et al. [44
] suggested that extremely healthy calves and extremely sick calves might be less likely to respond to probiotics treatment.