A technique for estimating herbage intake by weighing sheep before and after grazing, with an allowance for insensible weight loss (IWL) was evaluated.Live weights were estimated by weighing each animal 200 times using an electronic balance and a microcomputer. This enabled each sheep to be weighed to ± 10 g and overcame the problem of fluctuation in weight due to movement. Mean rates of IWL in housed sheep were 2-3, 1-4, and 0-8 g min~' for walking, standing eating or idling and when lying ruminating respectively. Intake rate (IR) estimated by weighing gave a small but significant bias of -0-8 g min~' when compared with measurements of IR made using housed sheep. Estimates of IWL and IR were made with ewes grazing continuously stocked ryegrass swards maintained at surface heights of 3,6,9 and 12 cm. IWL was higher outdoors than indoors and tended to be higher on the longer swards. IR estimated by weighing gave similar results to those obtained using the chromic oxide technique. At a sward surface height of 3 cm, IR and total intake were lower than for taller swards and total time spent grazing decreased with increasing sward surface height.

A 10-week grazing experiment was conducted on a perennial ryegrass sward with lactating ewes and their twin lambs. Three paddocks were rotationally grazed with rest periods of from 4 to 5 weeks. Sward surface heights at the start of each grazing were 145, 259 and 250mni for treatments RGt, RG2 and RG3. A further four paddocks were maintained by continuous variable stocking (CS) at sward surface heights (SSHs) of about 30. 60. 90 and 120mm. Sward and animat measurements were made on the two different grazing managements as the RG swards were grazed down, giving measurements at simitar sward heights for treatments RG and CS.There was tess green leaf and the totat herbage mass present under RG was less than on CS swards at the same sward heights, demonstrating the differences in structure between rotationally and continuously grazed swardsRegression anatysis of animal factors on sward factors showed that grazing behaviour was more highty corretated with green teaf mass than SSH or any of the other sward measurements. On the RG swards, maximum intake per animat was reached at about 1500 kg green leaf mass ha"'. A SSH of 60mm attowed the CS ewes to achieve the highest intake rate, but at this height the ewes on treatments RG2 and RG3 were restricted to approximately hatf this rate. The results suggest that green teaf mass or teaf area index, rather than sward surface height, could be used as a rational basis to retate intake of Correspondente: P. D. Penning. AFRC Institute of Gras.sland und Environmental Research. North Wyke, Okehampton, Devon, EX20 2SB. UK. herbage to sward state for swards changing rapidly in leaf to stem ratio.

AbstraetThe effect of herbage allowance on the herbage intake and performanee of ewes and their twin lambs at pasture was investigated. Daily herbage allowances of 40,80.120 and 160 g organic matter (OM) kg"' ewe live weight, based on herbage mass measured to ground ievel, were offered during the first 12 weeks of lactation.The' sheep were grazed rotationally around four paddocks of a perennial ryegrass pasture for 7-day periods and herbage mass, extended tiller length and digestibility of the herbage consumed by the animals were estimated. Herbage intake by the ewes was estimated during weeks two to twelve and live weights were recorded weekly: during the last two weeks of the experiment grazing behaviour of one ewe on each treatment was recorded continuously.Herbage intakes by the ewes (164, 1-81. 2-42 and 2 68 + 0153 kg d"') and liveweight gains of the lambs (202, 245, 274 and 300±7-3 g d"') increased with increasing herbage allowances. Herbage intakes by the ewes and growth rates of their lambs increased up to a herbage allowance which was over five times the amount of herbage eaten by the ewes.As the animals reduced herbage mass and sward height, biting rates by the ewes during grazing increased by 4 (+ 0 08) bites min"' cm" ' and masticating rates decreased. Mastications reached a maximum of 90 (±3-5) min"' at a sward surface height of 9 cm.