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• Multispecies had a higher dry matter degradability than grass/grass-clover forages. • Rumen pH was below 6.0 for longer when cows consumed multispecies. • Perennial ryegrass-white clover resulted in higher rumen ammonia throughout the day. • Multispecies can support milk production but may result in lower milk fat. An experiment was conducted to compare the rumen fermentation and degradability of three forage treatments. A perennial ryegrass (PRG; Lolium perenne), perennial ryegrass and white clover (Trifolium pratense) (PRGWC) and multispecies forage (MULTI), containing two grass species; perennial ryegrass and timothy (Phleum pratense); two clover species; white clover and red clover (Trifolium repens); and two herb species; chicory (Cichorium intybus) and plantain (Plantago lanceolata), were compared. Six rumen fistulated lactating dairy cows were used in a replicated 3 × 3 Latin square experiment consisting of 3 periods lasting 21-days. Cows were allowed ad libitum access to fresh forage and offered 2.71 kg DM concentrate each day. During days 1 – 14 of each period, cows were acclimatized to dietary treatments before entering a 7 – day sampling period. Indwelling rumen pH probes were inserted into the rumen of each cow on day 15 and 16. Rumen fluid samples were taken on day 18 every 2 h for rumen volatile fatty acid and ammonia nitrogen (NH 3 N) analysis. Forage dry matter (DM) and crude protein (CP) degradability was assessed via in sacco incubation beginning on day 19 of each period. Treatment had no effect on dry matter intake (DMI) or milk yield. There was no difference in mean rumen pH between treatments. The rumen pH of cows offered MULTI was below pH 6.0 for a higher percentage of time than the other treatments, however the concentration of acetic acid in the rumen of cows was similar between treatments. Effective degradability (ED) of DM was higher (P = 0.003) for MULTI (0.77) compared to PRG (0.68) and PRGWC (0.66). The soluble CP fraction of MULTI and PRGWC was significantly lower than that of PRG (P = 0.007), though PRGWC had the highest rumen ammonia concentration of all treatments (P = 0.002). MULTI had the highest ED and rate of DM degradation in the rumen, and lowest soluble CP of all treatments. Multispecies forages can support milk production in lactating dairy cows; however, care must be taken to ensure adequate dietary NDF due to the low fiber concentration and highly degradable nature of the forage. [ABSTRACT FROM AUTHOR]