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

Abstract

The effect of physiological state lactating vs. non-lactating (dry) on grazing behaviour and herbage intake by Holstein-Friesian cows was examined on grass pastures maintained at 5, 7 or 9 cm sward surface height (SSH), typical of those provided under continuous variable stocking management. Intake rates were estimated over periods of 1 h by weighing the animals before and after grazing, retaining the faeces and urine excreted, and applying a correction for insensible weight loss. Grazing behaviour during these periods and over 24 h was recorded automatically using sensors to measure jaw movements. Bite mass (BM) did not differ significantly between lactating and dry cows 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) in grazing jaw movement (GJM) rate, from 75.3 to 80.3 GJM min −1, as SSH decreased, did not compensate for reductions in bite mass, and intake rate declined linearly ( P=0.006) from 24.6 to 18.9 g OM min −1. Lactating and dry cows compensated for the reduction in intake rate, by increasing total grazing 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 (40 400 to 49 300 and 31 300 to 40 600 GJM) and total bites (31 100 to 37 900 and 24 600 to 31 200 bites, respectively). As a result, there was no significant effect of SSH on daily intake of OM, although lactating 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 with a reduction in the time spent ruminating ( P<0.001), despite similar levels of daily intake being achieved across SSH treatments. Although dry cows 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 min more idling (i.e., non-grazing, non-ruminating) behaviour ( P=0.001).