The effect of forage species and stage of harvest on the processes of digestion occurring in the rumen of cattle.

1. Pure swards of perennial ryegrass (Lolium perenne L. cv. Melle) or white clover (Trifolium repens L. cv. Blanca) were harvested daily as either primary growth (May-June) or mid- (July) and late- (August-September) season 4-week regrowths and offered to Friesian steers at two levels of feed allowance (18 and 24 g dry matter (DM)/kg live weight), to examine the effect of forage species and stage of harvest on nutrient digestion and supply. 2. The early- and mid-season grasses had low nitrogen (23 g/kg DM) and high water-soluble carbohydrate (169 g/kg) contents whilst the late-season grass had a higher N content (28 g/kg). All clover diets had high N (average 45 g/kg) and low water-soluble carbohydrate (89 g/kg) contents, and DM digestibility on all diets ranged from 0.77 to 0.83 (mean of two feeding levels). 3. Mean total rumen volatile fatty acid concentrations were significantly (P less than 0.001) higher on the clover diets, whilst on the grass diets molar proportions of propionate showed a slight but not significant decline with advancing season and tended to be higher than those on the clover diets. Mean rumen ammonia concentrations were significantly (P less than 0.001) lower on the early- and mid-season grasses (59 mg NH3-N/l) than the late-season grass (242 mg/l) and early-season clover (283 mg/l) which were all significantly (P less than 0.01) lower than the mid- and late-season clovers (372 and 590 mg/l) respectively. 4. Rates of organic matter (OM) and N digestion in the rumen were estimated using established nylon-bag techniques and found to be high on all diets, but significant effects due to forage species (clover greater than grass; P less than 0.001) were detected, whilst overall potential degradability in the rumen exceeded 0.89 for both OM and N on all diets. 5. Significantly (P less than 0.001) more OM entered the small intestine of calves fed on white clover (10.2 g/kg live weight) than those fed on ryegrass (8.33 g/kg) and similar effects due to level of feeding (g/kg; low 7.9, high 10.6; P less than 0.05) and stage of harvest (g/kg; early 8.3 v. mid 10.0, late 9.50; P less than 0.05) were also detected.(ABSTRACT TRUNCATED AT 400 WORDS)