In this study, the nitrogen utilization in sheep was investigated when they were given only the fresh forage diet being different in the contents of crude protein (10.6-21.4%) and crude fibre (20.0-30.3%). The fresh forage was harvested at three times (1st, 2nd and 3rd cutting ; forages A, B and C) from the same Italian ryegrass/red clover pasture. The ratio of clover in pasture plants tended to increase with a progress of cutting time (1st to 3rd).
The digestibility of organic matter was nearly the same after feedings of three forages used, though there were some variations in the level of food intake in sheep. The digestibility of crude protein was highest after forage B feeding (72%), and was lowest after forage C feeding (58%). The digestibility of crude fibre was extremely low after feeding of forage C as compared with those after feedings of forages A and B. The nitrogen balance was negative after feeding of forage A, whereas a positive nitrogen balance was observed after feedings of forages B and C. The utilizability of dietary nitrogen (retained nitrogen/digested nitrogen) was 22.1 and 10.6% after feedings of forages B and C, respectively.
The changes of chemical values in rumen fluid and blood suggested that it should be necessary to investigate in detail the relationships between the plasma protein level and the nitrogen balance in sheep receiving only roughage diets.