1. A total of twenty Friesian steers were grazed on pure swards of either perennial ryegrass (Lolium perenne cv. Melle) or white clover (Trifolium repens cv. Blanca) from May to late August to examine the effect of forage species and season on nutrient digestion and supply. Within each forage species, two daily allowances of forage (i.e. 30 and 60 g dry matter (DM)/kg live weight) were given, and nutrient flow into the small intestine was measured on thirteen separate occasions (viz. seven grasses and six clovers).
2. Total nitrogen content of the grasses varied between 28 (early season), 19 (mid-) and 33 (late) g/kg DM, whilst the clovers showed a much narrower range and all values were higher (39–45 g/kg DM). In vitro organic matter (OM) digestibilities of plucked samples ranged from 742 to 809 g/kg OM (grass) and 712 to 790 g/kg OM (clover), the lowest values being noted in late June (grass) and mid-July (clover).
3. OM intakes calculated from estimated faecal OM output (using unlabelled ruthenium) and predicted OM digestibility of the consumed forage indicated mean values of 20.9 (grass) and 26.0 (clover) g/kg live weight ( P <0.001), whilst the higher forage allowances increased OM intake by approximately 8% on both grass and clover (P < 0.01) compared with the low allowance. OM intake was significantly ( P < 0.01) lower on the two primary growths of grass examined in May (mean 16.8g/kg live weight) than the other five grass diets.
4. Rumen molar propionate levels declined with season but were at all times higher on the grass than on the clover diets, whilst acetate levels were unaffected by forage species. Apart from the late-season grass, mean rumen ammonia concentration was less than 100 mg NH3-N/I on all grass diets, whilst values on the clover diets ranged from 200–350 mg/l.
5. Daily duodenal OM flows in relation to animal live weight were approximately 20% higher on the clover than on the grass diets (grass 9.2, clover 11.2 g/kg live weight, P < 0.001), but estimates of the proportion of digestible OM apparently digested in the rumen were not significantly influenced by forage species (grass 0.69, clover 0.71).
6. Non-NH3-N (NAN) flow to the small intestine varied from 0.41 to 0.76 g/kg live weight on the grass diets in response to season and forage allowance compared with values of 0.6W.94 g/kg on the clover diets, with the overall forage species effect being statistically (P < 0.001) significant (grass 0.60, clover 0.76 g/kg live weight). In relation to estimated N intakes, however, these findings revealed a considerable loss of N between mouth and duodenum on the clover diets equivalent to approximately 35% of N intake.
7. A significant regression of NAN flow/unit N intake (NI) (g/g) on N content in the forage OM (g/kg) was obtained for all forages examined: NAN/NI = 1.507–0.0185 [N/OM], residual SD 0.007, r 0.929, indicating that efficiency of utilization of the N in fresh forages in the rumen was more closely related to forage N content than forage species per se.