Intake and behaviour responses by sheep, in different physiological states, when grazing monocultures of grass or white clover

Abstract

Monocultures of ryegrass (G; Lolium perenne cultivar ‘S23’) and white clover (C; Trifolium repens cultivar ‘Huia’) were maintained at sward surface heights (SSH) of 3 or 6 cm by continuous variable stocking with core groups of sheep ( Ovis aries L.) in different physiological states (physiology). Each group comprised four lactating ewes (L), and their twin lambs, plus two non-lactating, non-pregnant ewes (D). There were two plots each of G6 and C6 plus one plot each of G3 and C3. Each plot was 0.33 ha and was irrigated to maintain soil moisture deficit at <30 mm. Treatment C received no N fertilizer, whereas treatment G received 420 kg N ha −1. The experiment ran from May to October. All lambs were weaned at 15 weeks of age on 8 August and removed from the experiment. Prior to weaning (spring/summer) measurements of ingestive behaviour were made on five occasions, using an automatic system to record jaw movements and by weighing animals before and after a 1 h period of grazing, to estimate herbage intake rate. One measurement of ingestive behaviour was made after weaning (autumn) on previously-lactating ewes and D ewes. Effects of physiology and herbage species on grazing behaviour were compared by regressing mean values for the groups of ewes on SSH. Generally ewes had higher intake rates of clover than grass and CL ewes had greater daily DM intakes than GL ewes (+ 0.56 kg). This gave rise to higher lamb growth rates (366, 312, 284 and 252 g day −1, for treatments CL6, GL6, CL3 and GL3, respectively). It is suggested that sheep can eat clover faster than grass because less time is required to prehend and masticate a bite of a given mass (handling time) for clover than grass. However, ewes markedly altered their grazing time in relation to their physiological state and intake rate of herbage, and thus relationships between sward state and bite mass and intake rate, cannot be used to predict daily intake without an understanding of factors that control grazing time.