Annual greenhouse gas emissions from livestock (CO₂ equivalent) are estimated at approximately 7.1 billion tons, accounting for 14.5% of global emissions, with beef and dairy cattle production contributing 41% and 20% of total emissions, respectively. Greenhouse gases released by ruminants not only lead to feed energy loss but also result in environmental degradation. Therefore, reducing greenhouse gas emissions from ruminants is crucial for the sustainable development of the ruminant industry. The primary greenhouse gases produced by ruminants include nitrous oxide from ruminant manure storage and methane generated in the rumen via the action of methanogenic archaea. Tannins, a class of polyphenolic compounds present in many plants, play a significant role in animal feed. Recent studies have shown that incorporating certain tannins and their metabolic products into diets can modulate protein metabolism and the ruminal microbiome, thereby regulating greenhouse gas emissions from ruminants. This review summarizes the types and properties of dietary tannins, as well as the latest advancements in understanding the impacts of tannins and their metabolites on cattle nutrient digestion and greenhouse gas emissions, concluding that dietary tannin supplementation can reduce greenhouse gas emissions from ruminants. Future research should focus on identifying the optimal concentrations of different tannins and their metabolites in diets to minimize ruminant greenhouse gas production while maintaining animal performance and health.

The present experiment aimed to investigate the effects of harvest stages on the fermentation quality and nutritional value of sorghum stalk silage. Sorghum stalks were harvested at the three stages (milk, dough, and ripe), chopped, and ensiled for 60 d. Each treatment had five replicates, and the silages were evaluated for fermentation quality, nutritional composition, in vitro rumen fermentation characteristics, and bacterial community profiles. The results showed that the crude protein, neutral detergent fiber, and acid detergent fiber contents decreased significantly with harvest maturity (p < 0.05). Consequently, silage from the ripe stage possessed the highest dry matter, relative feed value, and total digestible nutrients (p < 0.05). In vitro rumen fermentation indicated that the ripe stage silage exhibited the greatest gas production, and the lowest concentrations of ruminal ammonia–nitrogen (p < 0.05). Microbial analysis revealed a shift from dominant epiphytic Proteobacteria to fermentative Firmicutes post-ensiling, with the ripe stage community co-dominated by Lactobacillus and Leuconostoc, in contrast to the milk stage’s enrichment with Klebsiella. In conclusion, harvesting sorghum at the ripe stage is the optimal strategy as it establishes a beneficial microbial community, resulting in silage with superior nutritional value and rumen fermentation efficiency.