The implications of compound chew–bite jaw movements for bite rate in grazing cattle

Abstract

Many aspects of the management of grazing systems are directly or indirectly related to the rate of herbage intake achieved by the animal. Intake rate depends, in part, on the time budget of the process, which derives from the basic behavioural component—the jaw movement. Chewing and biting jaw movements have generally been considered mutually exclusive, but acoustic monitoring has demonstrated the existence of the compound chew–bite jaw movement. We used a simple model to examine the implications of chew–bites for the forage-intake time budget, and tested it empirically. The model defines the total number of jaw movements per bite ( α) and the number of chew actions per bite ( β) in relation to the allocation of jaw movements between chews, bites and chew–bites. We examined empirically the variation among animals in the allocation of jaw movements for cattle ( Bos taurus L.) grazing leafy, uniform swards in two separate studies. Grazing sessions were recorded on video, with the sound track originating from a forehead microphone. Each sound burst produced by a jaw movement was classified as a bite, chew or chew–bite. Jaw movements in these segments generated a virtually uninterrupted, regular succession of bite, chew and chew–bite sounds. Among individual animals, the variation in the rate of jaw movement was extremely low (Study 1: mean = 78.9 min −1, CV = 6%; Study 2: mean = 77.9 min −1, CV = 4%), but variation in the allocation of jaw movements was high (CV of proportion of jaw movements allocated to bites, chews and chew–bites = 58, 21 and 50%, respectively, for Study 1; 32, 12 and 32%, respectively, for Study 2). The proportions of chews and bites traded off directly against chew-biting. As the proportion of chew–bites increased, the number of jaw movements per bite declined and therefore the bite rate increased. Different combinations of the three types of jaw movement conformed to an isocline of a constant number of chew actions per bite (CV = 11 and 9% for Studies 1 and 2, respectively). Furthermore, this ratio was close to unity (1.27 and 1.24 chews per bite for Studies 1 and 2, respectively), and was similar for different levels of herbage mass in the range 145–255 g m −2. The possible implications of these findings for the regulation of bite weight and diet quality are discussed.