Effect of sward surface height on intake and grazing behaviour by lactating Holstein Friesian cows

The effect of sward surface height (SSH) on grazing behaviour and intake by lactating Holstein Friesian cows on continuously stocked grass pastures maintained at mean heights of 5, 7 and 9 cm was studied during the growing season. Intake rate was estimated over periods of 1 h by weighing animals before and after grazing, with a correction made for insensible weight loss. Grazing behaviour during that hour and over 24 h was recorded automatically using sensors to measure jaw movements. Although maintained at the overall mean SSH, swards had a patchy appearance with short, frequently grazed areas interspersed with taller, infrequently grazed areas, which is typical of pastures continuously stocked with cattle. Daily organic matter (OM) intake, calculated as the product of daily grazing time and intake rate, was greater at a SSH of 7 cm than at 5 or 9 cm (14.1 vs. 10.5 and 12.1 kg respectively). On the 5-cm sward, OM intake per grazing jaw movement (GJM) was reduced compared with that on the 7-cm sward (0.182 vs. 0.264 g respectively), and because cows were unable either to increase significantly GJM rate (95.8 vs. 90.1 GJM min(-1)) or the proportion of GJM that were bites (0.80 vs. 0.81) OM intake rate was reduced (16.9 vs. 23.5 g min(-1)). Cows were unable to increase their grazing time significantly (628 vs. 604 min d(-1)) to compensate for the reduction in intake rate, and as a result daily intakes were lower. Cows grazing the 9-cm sward also incurred a reduction in OM intake GJM(-1) compared with those on the 7-cm sward (0.237 vs. 0.264 g respectively) and therefore there was a reduction in OM intake rate (21.6 vs. 23.5 g min(-1)). These animals did not compensate by increasing the time spent grazing (581 min d(-1)), probably owing to an increased ruminating requirement per kg of herbage ingested compared with those on 7 cm SSH (2264 vs. 1780 ruminating jaw movements respectively). The results show that SSH can significantly influence intake rate, but, while the cow's only effective strategy to compensate for any reduction in intake rate is to increase grazing time, this may be limited by the requirement for ruminating and non-grazing, non-ruminating activities, which is influenced by qualitative and quantitative aspects of the herbage ingested, whereas the cows' only effective strategy to compensate for any reduction in intake rate is to increase grazing time.