Fresh forages alter rumen characteristics, but also can improve the sustainability of the system. Unlike alfalfa, berseem clover (BC) does not cause bloat and, when it is sown with grasses, grows well during the winter and improves N utilization. The study aimed to evaluate the effect of using fresh alfalfa (ALF), fresh oat/BC (MIX) and alfalfa hay/maize silage (CON) as forage basis for TMR during the winter period on dairy cows’ rumen characteristics and metabolic N. The ALF diet was used to elucidate if either the fresh forage or the species affected the responses. Three nonlactating rumen-cannulated cows were allocated to each TMR for a 14-d period in a 3 × 3 Latin Square design. Cows were individually stalled and received 13 kg DM/d of the TMR (60:40 forage-to-concentrate). Forage samples were analyzed for DM, CP and NDF contents. Samples were collected on d 13 and 14 of each period: rumen fluid was analyzed for pH, VFA and NH₃ content; blood serum, urine and feces were analyzed for N content. Data were analyzed using linear models including TMR and period. Forages differed in DM (CON: 55.0; ALF: 19.3; MIX: 21.0; SEM 0.49 DM%; P < 0.001), CP (CON: 10.2; ALF: 23.8; MIX: 14.9; SEM 0.32 DM%; P < 0.001) and NDF (CON: 45.8; ALF: 34.8; MIX: 39.2; SEM 0.46 DM%; P < 0.001) contents. The fresh forage diets (especially ALF) had greater CP content, which increased ruminal NH₃, valeric and isoacids VFAs and ureic N serum contents (Table 1). The fresh forage diets increased the propionic and decreased the acetic acid contents. The use of fresh forages changed rumen characteristics and metabolic N, showing potential to improve production and N use efficiency. This study was funded by project FONDECYT 11160697. Table 1. Effect of different forages as basis for TMR on ruminal characteristics and metabolites

	CON	ALF	MIX	SEM	P-value
pH	6.34	6.49	6.39	0.176	NS
NH3, mmol/L	6.3	14.5	7.5	0.88	<0.001
Total VFA, mmol/L	133	118	127	12.1	NS
Acetic acid, %	71.3	68.7	68.5	0.72	<0.01
Propionic acid, %	14.2	15.2	15.5	0.55	<0.01
Butyric acid, %	9.6	10.1	10.8	0.58	NS
Valeric acid, %	1.2	1.5	1.5	0.09	<0.01
Isobutyric acid, %	1.2	1.6	1.3	0.08	<0.001
Isovaleric acid, %	1.8	2.0	1.7	0.12	<0.05
Serum ureic N, mg/dl	16.2	22.0	16.5	1.29	<0.05
Urine N, mg/dl	447	295	247	60.3	<0.05
Fecal N, %	2.60	2.85	3.00	0.240	NS