Mean retention time in digestive tract and digestion of fresh perennial ryegrass by lactating dairy cows: influence of grass maturity and comparison with a maize silage diet.

The effects of grass maturity on the mean retention time of liquid and particles, digestibility and duodenal nutrient flows were measured by feeding second cuts of perennial ryegrass to 3 fistulated lactating dairy cows over 2 successive periods (late June and mid-July, 28 and 49 d regrowth, respectively). Fresh forage was given ad libitum with 12% (dry matter (DM) basis) concentrate. The increase in maturity lowered grass nitrogen content, organic matter (OM) digestibility and non-ammoniacal nitrogen flow into the duodenum, but did not affect OM intake (13.2 kg/d). Neither the fractional outflow rate of liquid in the rumen (FOR; 17.3%/h) nor the concentrate total mean retention time (TMRT; 32.9 h) were affected. With maturity forage TMRT tended to increase (43.0-48.8 h), certainly because of the longer comminution time of coarse particles. These results were compared with data obtained 2 months earlier with the same cows fed on a maize silage diet (25% DM basis of the same concentrate) ad libitum (17.4 kg OM/d). Despite a lower total OM intake, FOR was much higher with the grass than with the maize silage diet (12.1%/h) and this may explain the lower retention time in the rumen of the small particles when grass diets were given (19.0 vs 24.3 h for maize silage diet).