The gastrointestinal microbial consortium in dairy cattle is critical to determining the energetic status of the dairy cow from birth through her final lactation. The ruminant's microbial community can degrade a wide variety of feedstuffs, which can affect growth, as well as production rate and efficiency on the farm, but can also affect food safety, animal health, and environmental impacts of dairy production. Gut microbial diversity and density are powerful tools that can be harnessed to benefit both producers and consumers. The incentives in the United States to develop Alternatives to Antibiotics for use in food-animal production have been largely driven by the Veterinary Feed Directive and have led to an increased use of probiotic approaches to alter the gastrointestinal microbial community composition, resulting in improved heifer growth, milk production and efficiency, and animal health. However, the efficacy of direct-fed microbials or probiotics in dairy cattle has been highly variable due to specific microbial ecological factors within the host gut and its native microflora. Interactions (both synergistic and antagonistic) between the microbial ecosystem and the host animal physiology (including epithelial cells, immune system, hormones, enzyme activities, and epigenetics) are critical to understanding why some probiotics work but others do not. Increasing availability of next-generation sequencing approaches provides novel insights into how probiotic approaches change the microbial community composition in the gut that can potentially affect animal health (e.g., diarrhea or scours, gut integrity, foodborne pathogens), as well as animal performance (e.g., growth, reproduction, productivity) and fermentation parameters (e.g., pH, short-chain fatty acids, methane production, and microbial profiles) of cattle. However, it remains clear that all direct-fed microbials are not created equal and their efficacy remains highly variable and dependent on stage of production and farm environment. Collectively, data have demonstrated that probiotic effects are not limited to the simple mechanisms that have been traditionally hypothesized, but instead are part of a complex cascade of microbial ecological and host animal physiological effects that ultimately impact dairy production and profitability.

The investigation into the complementary roles of essential oils (EOs) and organic acids in enhancing rumen physiology is increasingly gaining recognition within the field of animal nutrition. Essential oils are known for their antimicrobial effects, which can specifically target certain microbial populations in the rumen, thereby impacting fermentation processes, methane output, and nutrient digestion. In addition, the integration of organic acids plays a crucial role in stabilizing rumen pH and steering the metabolic activities of bacterial populations toward propionate production, a process essential for energy metabolism in ruminants. The concurrent use of essential oils and organic acids may yield synergistic benefits that could further optimize ruminal fermentation efficiency, enhance feed conversion rates, and lower methane emissions. This systematic review used the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines. The literature search was meticulously designed to encompass parameters related to ruminant species, feed additives, essential oils, organic acids, synergistic effects, and rumen physiology. The efficacy of both organic acids and essential oils is highly dependent on their concentration and the specific combinations utilized. When certain essential oils are used in conjunction with selected organic acids, they may mitigate any potential negative effects on fermentation, thereby fostering a more favorable environment for the proliferation of beneficial microbial communities. Understanding the relationship between essential oils and organic acids is essential for the formulation of diets that enhance rumen health while concurrently reducing environmental pressures through diminished methane emissions. Future research should prioritize long-term in vivo investigations to gain more comprehensive insights into the interactions among these dietary components and identify the optimal combinations for ruminant feeding strategies.

Dry seasons pose a major nutritional constraint on ruminant livestock production in tropical regions, which justifies forage conservation to meet the dry season feed requirement. Napier grass is a tropical forage that is used for silage in South Africa. The present objective was to determine the effects of Napier grass fertilization with bio-digester slurry (BDS) and the inclusion of fermentable carbohydrate additives at ensiling on the chemical composition and ruminal degradability of Napier grass silage. Napier grass was established in 5 × 4 m plots, replicated three times in a completely randomized design, and irrigated weekly with either BDS or water. After 12 weeks, the Napier was cut and ensiled for 90 days in 1-liter glass jars in a 2 (BDS, water) × 4 (no-additive, molasses, brown sugar, and maize meal) factorial arrangement replicated three times. The nutrient composition was determined using standard protocols. The ruminal degradability of dry matter (DM) and crude protein (CP) was determined using the nylon bag technique. Fertilization with BDS increased (P < 0.05) pH and CP and reduced (P < 0.05) fat content of fresh-cut Napier. Additives increased (P < 0.01) silage DM content and reduced (P < 0.01) acid detergent fiber, neutral detergent fiber content. The BDS fertilization with molasses inclusion increased (P < 0.05) silage DM relative to the no-additive and maize meal inclusion, and decreased (P < 0.05) fat content compared to the no-fertilizer, added maize meal silage. Molasses increased silage water-soluble carbohydrate and decreased the NH₃-N content (P < 0.05) compared to the no-additive and maize meal treatments. For DM, the BDS fertilized, no additive silage had the least “a” fraction (P < 0.01), while the no BDS, no-additive silage had the least “b” fraction (P < 0.01), with least (P < 0.01) potential degradability (PD) observed for the no BDS, no-additive treatment. Fertilization increased (P < 0.01) effective degradability of DM at outflow rates k = 0.02, 0.05, 0.08, with same effect for molasses and maize meal inclusion. Relative to the control, molasses inclusion increased (P < 0.01) PD of silage CP. In conclusion, our results suggested BDS fertilization of Napier grass ensiling with added readily fermentable carbohydrate substrate, particularly from molasses, induced changes in silage chemical and fermentation characteristics likely to promote better forage preservation and ruminal microbial function.

True intestinal digestibility of dry matter (DM), rumen-undegraded dietary protein (UDP) and amino acids of cowpea, silverleaf desmodium and Brachystegia spiciformis (musasa) were estimated. The true intestinal digestibility was calculated as the difference between the content of a bag that was preincubated in the rumen for 12, 24 and 48 h and was recovered in the faeces. Significantly higher DM and nitrogen disappearance rate in the rumen, postruminally and throughout the whole tract were found for cowpea than for silverleaf desmodium and musasa. Total amino acids or true nitrogen digestibility values of silverleaf desmodium and musasa tended to be higher than in cowpea. The true digestibilities for the individual amino acids in the different undegraded protein sources showed significant differences. The true intestinal digestibility of DM, nitrogen and amino acids in the mobile bags after 24 h and 48 h of preincubation in the rumen were higher for silverleaf desmodium and musasa, than in cowpea. Musasa ...

This study examined the effects of feeding forage legumes as protein supplements to poor quality natural pasture (veld) hay offered to sheep on microbial protein yield and nitrogen metabolism. Four female Sabi cross-bred sheep were used in a 4 x 4 Latin cross-over design experiment. The sheep were randomly allocated to four dietary treatments comprising veld hay alone (V), veld hay supplemented at 25% of expected feed intake with either cowpea (VC), fine stem stylo (VF) or silverleaf desmodium (VS). Microbial protein yield was determined using the purine derivatives technique. Total nitrogen (N) intake increased with supplementation by approximately 9, 11 and 17 fold for VS, VF and VC, respectively. The microbial protein supply, the calculated microbial true protein and the digestible microbial true protein were not affected (P>0.05) by legume supplementation. It was concluded that supplementation of legume forages to poor quality veld hay increased nitrogen utilisation in the sheep. Animals on poor quality forages tended to show the most efficient microbial protein synthesis. However, feeding of veld hay failed to meet the maintenance requirements of the animal as evidenced by negative nitrogen balance.