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Two independent studies were conducted to assess the effects of feeding condensed tannin (CT)-containing forages to continuous cultures to evaluate their impacts on ruminal fermentation and digestibility, with an emphasis on methane (CH4) and ammonia-N (NH3-N) production. In Chapter 3, treatments consisted of: 1) 100% alfalfa hay (AH), 2) 50% AH and 50% birdsfoot trefoil hay (dry matter (DM) basis; AHBFTH), and 3) 100% birdsfoot trefoil hay (BFTH). Three replicated runs lasted 8 d each, with the first 5 d allowed for microbial adaptation to the diets, and 3 d for data collection and sampling. Methane, neutral detergent fiber (NDF) degradability, and total volatile fatty acid (VFA) production and profiles were not affected by the dietary treatments. However, NH3-N concentration and flow were reduced when replacing the AH with the BFTH. It was concluded that feeding a CT-containing diet (BFTH) increased the in vitro N utilization. In Chapter 4, five total mixed ration (TMR) diets were evaluated, each containing a different forage source as hay: 1) alfalfa (AHT), 2) cicer milkvetch (CMVHT), 3) Norcen birdsfoot trefoil (NBFTHT), 4) Oberhaunstadter birdsfoot trefoil (OBFTHT), and 5) sainfoin (SFHT). Total VFA production and acetate molar proportion were not influenced by treatments. However, molar proportion of propionate increased by the CMVHT, and decreased by the SFHT. Acetate to propionate ratio decreased with the CMVHT and the OBFTHT, but increased with the SFHT when compared to the AHT. Digestibilities of DM and NDF were not influenced by dietary treatments. Total CH4 production decreased by all treatments when compared to the AHT, whereas CH4 production expressed as mM per g of NDF digested was reduced only by the CMVHT and the OBFTHT. Ammonia-N concentration decreased when feeding the CMVHT and the SFHT. Feeding the CMVHT decreased ruminal concentration of C18:0, while ruminal C18:1 cis and trans isomers were greatest with the CMVHT. Because these effective tannin-free and tanniniferous legumes did not have negative impacts on ruminal fermentation, they can maintain potential productive performance of lactating dairy cows when fed as main forage sources in TMR diets.