The role of grass stems as structural foraging deterrents and their effects on the foraging behaviour of cattle

Little quantitative information is available about the role of stems as structural foraging deterrents for large grazers and the actual mechanisms by which such deterrents affect foraging behaviour. We measured bite size, bite rate and the rate of forage intake of cattle foraging on artificial micro-swards of the tropical, broad-leaved guinea grass. These micro-swards varied in total forage mass density, in forage quality, defined as the proportion of high-quality plant parts (leaves) and of foraging deterrents (stems), and in the spatial pattern of plant parts. We hypothesized that stems interfere with the process of grasping of leaves and predicted that decreasing forage quality, by reducing bite size and bite rate, depresses the slope and the asymptotic maximum of the functional response curve. Further we hypothesized that increasing cluster size of leaves increases the accessibility of leaves to cattle, thus alleviating the negative effects of decreasing forage quality, and predicted that increasing leaf cluster size has positive effects on bite size and on the rate of forage intake.The slope of the functional response curve decreased with decreasing forage quality, mainly because of depressed bite size. These effects were also found when the amount of leaves in the grass sward was kept constant while only the amount of stems increased. Thus, the observed effects are not merely the result of decreased availability of leaves, but at least in part caused by the increasing interference of stems with the grasping of leaves. Leaf cluster size had a positive effect on bite size. However, this effect did not show in the rate of forage intake, because for small leaf clusters high bite rates compensated for decreased bite sizes. Instead of the familiar negative relationship of bite rate with bite size, we found a positive relationship. We speculate that this effect is the result of decreased chewing times for small bites and increased time needed to grasp these bites, effectively changing the limitation of bite rate from chewing time to a limitation by the time needed to grasp bites.