A review of the literature suggested that the ingestive behaviour of grazing animals is largely determined by the structure and botanical composition of the sward, but little information is available from indigenous, temperate swards or for cattle and sheep grazing together. The following three grazing experiments were carried out to examine aspects of the responses of cattle and sheep to variations in sward conditions.
In the first, animal responses, in terms of ingestive behaviour and diet composition, to changes in the structure of sown swards were examined. In the second, the influence of the presence of dung of the same or the opposite species on grazing patterns and herbage utilisation in cattle and sheep, and hence on their complementarity of grazing was examined. In the third experiment the responses of cattle and sheep to indigenous hill grass swards of different botanical and morphological composition were studied in relation to the seasonal cycle of herbage growth, through measurements of ingestive behaviour, herbage intake, diet composition and diet digestibility.
To test the validity of the assumption that oesophageal - fistulated and non -fistulated animals selected the same diet, a small experiment was carried out, in which the botanical composition of the faeces was found not to differ significantly between the two groups.
It was found that on sown pastures with a high herbage mass and highly accessible leaf, herbage intakes estimated from measurements of intake per bite and total daily bites were very high over short periods, but that herbage intake declined as a result of a reduction in intake per bite. It was postulated that the reduction in intake per bite was under an internal control, rather than a result of a response to changing sward conditions. The cattle grazed less selectively than the sheep with the result that the swards grazed by cattle were more evenly grazed than those grazed by sheep.
Cattle rejected a herbage fouled by their own species to a greater extent than did sheep. The conclusion was drawn that under mixed grazing a greater proportion of the herbage would be available to the sheep giving them an advantage over the cattle.
On the indigenous swards the cattle and sheep selected diets of similar OMD except in the spring and autumn on short swards containing a high proportion of dead herbage, where the sheep obtained diets between 5 and 12 units of digestibility higher than those of the cattle. Intake per bite was found to be the major determinant of daily herbage intake in both species, and was influenced primarily by sward height. Where intake per bite declined, due to declining sward height, rate of biting increased. Increases in grazing time occurred where intakes per bite were particularly low, but this was not a consistent response. The cattle responded to increases in the density of the sward by increasing rate of biting; the sheep increased grazing time. Very low intakes per bite in the early spring on short swards where the digestibility of the diet selected was low led to digestible organic matter intakes by the cattle that were only barely adequate for maintenance.
Cattle consistently ate higher proportions of grass flower stems and Juncus whilst the sheep consistently ate higher proportions of dicots. To obtain these diets the cattle grazed the surface horizons whilst the sheep grazed the base of the sward. On short swards in the spring the cattle were unable to avoid eating a higher proportion of dead herbage than the sheep.
The cattle and sheep altered their ingestive behaviour in a consistent manner across the range of swards. Changes in diet selection varied to a greater extent within season than within swards. The selective ability of the sheep, particularly when herbage quality was poor, allowed them to maintain the nutrient concentration of their diets. The cattle maximised nutrient intake, particularly in the summer months. The different grazing strategies of the cattle and sheep enabled them to be complementary rather than competitive grazers in the summer months.