Direct-fed microbials (DFMs) are feed additives containing live naturally existing microbes that can benefit animals’ health and production performance. Due to the banned or strictly limited prophylactic and growth promoting usage of antibiotics, DFMs have been considered as one of antimicrobial alternatives in livestock industry. Microorganisms used as DFMs for ruminants usually consist of bacteria including lactic acid producing bacteria, lactic acid utilizing bacteria and other bacterial groups, and fungi containing Saccharomyces and Aspergillus. To date, the available DFMs for ruminants have been largely based on their effects on improving the feed efficiency and ruminant productivity through enhancing the rumen function such as stabilizing ruminal pH, promoting ruminal fermentation and feed digestion. Recent research has shown emerging evidence that the DFMs may improve performance and health in young ruminants, however, these positive outcomes were not consistent among studies and the modes of action have not been clearly defined. This review summarizes the DFM studies conducted in ruminants in the last decade, aiming to provide the new knowledge on DFM supplementation strategies for various ruminant production stages, and to identify what are the potential barriers and challenges for current ruminant industry to adopt the DFMs. Overall literature research indicates that DFMs have the potential to mitigate ruminal acidosis, improve immune response and gut health, increase productivity (growth and milk production), and reduce methane emissions or fecal shedding of pathogens. More research is needed to explore the mode of action of specific DFMs in the gut of ruminants, and the optimal supplementation strategies to promote the development and efficiency of DFM products for ruminants.

中文翻译：

---

直接饲喂微生物补充剂对改善反刍动物生产和健康的影响和挑战


直接饲喂微生物 (DFM) 是含有天然存在的活微生物的饲料添加剂，可以有益于动物的健康和生产性能。由于抗生素的预防性和促生长用途被禁止或严格限制，DFM 已被认为是畜牧业抗菌替代品之一。用作反刍动物DFM的微生物通常由细菌组成，包括产乳酸菌、乳酸利用菌和其他菌群，以及含有酵母菌和曲霉的真菌。迄今为止，反刍动物可用的 DFM 主要基于其通过增强瘤胃功能（例如稳定瘤胃 pH、促进瘤胃发酵和饲料消化）来提高饲料效率和反刍动物生产力的效果。最近的研究表明，新的证据表明 DFM 可以改善幼年反刍动物的表现和健康，然而，这些积极的结果在研究中并不一致，而且作用模式尚未明确定义。本综述总结了过去十年在反刍动物中进行的 DFM 研究，旨在为反刍动物各个生产阶段的 DFM 补充策略提供新的知识，并找出当前反刍动物行业采用 DFM 的潜在障碍和挑战。总体文献研究表明，DFM 具有减轻瘤胃酸中毒、改善免疫反应和肠道健康、提高生产力（生长和产奶量）以及减少甲烷排放或病原体粪便排出的潜力。 需要更多的研究来探索特定DFM在反刍动物肠道中的作用模式，以及促进反刍动物DFM产品开发和功效的最佳补充策略。