Changes in ingestive behaviour of yearling dairy heifers due to changes in sward state during grazing down of rotationally stocked ryegrass or white clover pastures

A group of 12 Holstein x Friesian heifers were rotationally stocked in sequence on uninterrupted spring growths and regrowths following cutting of perennial ryegrass and then on regrowths of white clover. Measurements of ingestive behaviour and plant factors were made every 2-3 days during four grazing periods, which lasted between 12 and 20 days. The sward surface height in the ryegrass paddocks at the start ranged between 20 and 3 8 cm, and contained 2.7-2.9 t DM ha(-1) of green leaf mass. At the end of the grazing periods, the swards had a surface height of 8-9 cm, most of the leaf had been consumed and bite mass was considerably reduced compared with levels measured at the start. There was no change in eating time but ruminating time increased as the leaf:stem ratios declined. Total jaw movement rate during grazing declined, possibly because the heifers spent more time searching for leaves. Correlations between bite mass and plant factors suggested that total herbage mass was less useful for assessing likely levels of intake, than green leaf mass or sward surface height when managing grazed ryegrass swards, but total mass may have a place in predicting intake of white clover. Further, more detailed plant measurements indicated that the lengths of the 2nd and 3rd oldest fully expanded grass live leaves were significantly correlated with bite mass and this, along with the role of sheath tube lengths in defining the grazed horizon in ryegrass swards, warrants further investigation. (C) 2004 Elsevier B.V. All rights reserved.