The objective of this study was to determine effect of formaldehyde and extrusion on in situ ruminal degradability of DM and CP of soybeans. Treatments include; untreated soybean meal (USM) treated with formaldehyde (TF) and extruded soybean (ES). The treated soybean meal was sprayed with 2.5 (w/v) formaldehyde at the rate of 0.2 l/kg. Extrusion was conducted on ground soybeans (EXP 160 KW). Extrusion temperature and residence time averaged 1550 C and 25 s respectively. Four ruminally fistulated steers (400 ± 20 kg, body weight) were used. Steers were fed 5.2 kg of alfalfa hay, 1.3 kg of corn silage and 2.6 kg of concentrate. Bags (12 × 19 cm, pore size of 48 μm) containing 5 g DM of each sample was incubated in the rumen (4 replicates per each animal) for 0.0, 2, 4, 8, 16, 24, 48, and 72 h. After removal of the bags from the rumen, they were washed using cold water and dried in a forced air oven (60 0C, 48 h), weighed to determination DM disappearance, and CP of the samples determined. Data was fitted to exponential model to calculate degradation parameters of CP and DM (Orskove et al.,1980). TF had the lowest in situ quickly degradable (a) fraction (27%, P < 0.05). No difference in (a) fraction of DM was observed between ES and USM (42 and 38% respectively, P > 0.05). In situ (a) fraction of CP was highest for USM (27%, P < 0.05) and had no difference between ES and FT (7 and 11% respectively, P > 0.05). Slowly degradable (b) fraction of DM did differ between treatments (P < 0.05). USM had the highest (b) fraction (63%) and did not differ between TF and ES (34 and 27% respectively, P > 0.05). Slowly degradable (b) fractions of CP had no difference between ES and USM (78 and 72% respectively, P > 0.05), were higher than TF (34%). Rate of degradation (c) of DM and CP were significant (P < 0.05). TF had a lower (c) fraction of DM and CP (3 and 1% respectively) compared with USM (9 and 9%) and ES (12 and 4%) compared with USM. It was concluded that TF decreased (a) and (b) fraction of DM and CP relative to USM. But, ES led to decreased (a) and increased (b) fraction.

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