Duration of regrowth of ryegrass (Lolium perenne) effects on grazing behavior, intake, rumen fill, and fermentation of lactating dairy cows

The relative importance of duration of sward regrowth and fill and fermenta tion in the rumen on the control of grazing time and intake rate during the first grazing session of the day was studied. Four lactating dairy cows we re allowed to graze ryegrass (Lolium perenne) swards, with five different r egrowth periods after mowing (6, 9, 16, 22, and 30 d). The cows were allowe d to graze until they stopped voluntarily (cessation of grazing activity fo r at least 15 min). Before and after grazing the rumen contents were evacua ted, weighed, sampled, and returned to the animals. Samples of rumen liquid were taken immediately before rumen evacuation and approximately 30, 60, 1 20, and 240 min after the grazing session was finished. Grazing time and in take rate did not follow a significant trend with period of regrowth. Bite rate did not change significantly with duration of regrowth with cows exhib iting high rates of biting for all the sward conditions. Rumen pools sizes of dry matter, neutral detergent fiber, and volatile fatty acids (VFA), mea sured after grazing, increased significantly with days of sward regrowth, e ven though the changes over days were small. Concentration of VFA followed a significant quadratic trend with a maximum concentration observed at appr oximately 110 min after cessation of grazing. In this study, rumen fill, VF A (either total or major components), ammonia, pH, and osmotic pressure as individual variables were not correlated with grazing time or dry matter in take.