The effect of physiological state (lactating or dry) and sward surface height on grazing behaviour and intake by dairy cows

The effect of physiological state lactating vs. nonlactating (dry) on grazi ng behaviour and herbage intake by Holstein-Friesian cows was examined on g rass pastures maintained at 5, 7 or 9 cm sward surface height (SSH), typica l of those provided under continous variable stocking management. Intake ra tes were estimated over periods of 1 h by weighing the animals before and a fter grazing, retaining the faeces and urine excreted, and applying a corre ction for insensible weight loss. Grazing behaviour during these periods an d over 24 h was recorded automatically using sensors to measure jaw movemen ts. Bite mass (BM) did not differ significantly between lactating and dry c ows but decreased (P < 0.001) from 0.42 to 0.30 g organic matter (OM) bite( -1) as overall mean SSH decreased from 9 to 5 cm. An increase (P = 0.040) i n grazing jaw movement (GJM) rate, from 75.3 to 70.3 GJM min(-1), as SSh de creased, did not compensate for reductions in bite mass, and intake rate de clined linearly (P = 0.006) from 24.6 to 18.9 g OM min(-1). Lactating and d ry cows compensated for the reduction in intake rate, by increasing total g razing time and total number of bites per day. As SSH decreased from 9 to 5 cm, lactating and dry cows increased total eating time (528 to 607 and 419 to 510 min), total GJM (40400 to 49300 and 31300 to 40600 GJM) and total b ites (31100 to 37900 and 24600 to 31200 bites, respectively). As a result, there was no significant effect of SSH on daily intake of OM, although lact ating cows had greater intakes than dry cows; 12.9 vs. 9.3 kg day(-1), (P < 0.001). The increased time spent grazing as SSH decreased was associated w ith a reduction in the time spent ruminating (P < 0.001), despite similar l evels of daily intake being achieved across SSH treatments. Although dry co ws had much lower daily intakes, they spent only about 30 min less each day ruminating than the lactating cows (P = 0.060), allowing them 120 to 160 m in more idling (i.e., non-grazing, non-ruminating) behaviour (P = 0.001). ( C) 1999 Elsevier Science B.V. All rights reserved.