The release of intracellular constituents from fresh ryegrass (Lolium perenne L.) during ingestive mastication in dairy cows: effect of intracellular constituent, season and stage of maturity

The objective of the trial was to measure the proportions of intracellular constituents (IC) released during ingestive mastication in dairy cows fed ryegrass (Lolium perenne L.) and to quantify the effect of season (spring versus autumn) and stage of maturity. In each season, four dairy cows fitted with a large ruminal cannula were fed indoors with fresh grass. The grass was cut at 23 and 57 days of regrowth, respectively, for the two periods of spring, and at 34 and 68 days of regrowth for the two periods of autumn. To measure the proportion of IC released, the rumen was emptied and ingestive boli were collected directly at the cardia during 30 min. Three subsamples were rinsed carefully with water and their IC contents compared with the IC contents of grass. The grass was at a vegetative stage and of good quality throughout the trial, with an average pepsin–cellulase organic matter digestibility of 76.6% and a NDF content of 501 g kg−1 dry matter (DM). The proportion of IC released varied strongly according to the nature of the IC considered (P<0.0001). The intracellular nitrogen was clearly less easily released than the total soluble carbohydrates (34% versus 53% of intake). Considering the intracellular subfractions, the released proportions were lowest for chloroplastic constituents such as protein nitrogen and chlorophyll (22 and 28% of intake, respectively), intermediate for fructans (42%), and highest for small vacuolar molecules such as non protein nitrogen, free sugars and potassium (57.7, 60.8 and 58.2%, respectively). The unit eating time amounted to 25 min kg−1 DM intake on average and was unaffected by season but increased with increasing stage of maturity according to the length of the pieces of grass fed to the cows. Season or stage of maturity had little effect on the proportions of IC released. When the effects were significant, the proportion of IC released generally followed the same trend as unit eating time.