Do endogenous seasonal cycles of food intake influence foraging behaviour and intake by grazing sheep?

Abstract

Summary1. Large herbivores living in temperate regions show different degrees of seasonal biological variation, including voluntary food intake (VFI). The decline of VFI in winter has been hypothesized to be an evolved response to lower food availability or quality, which can act as an internal constraint on food intake.2. The hypotheses were tested that (i) animals that have a greater inherent seasonal variation of VFI, measured indoors under ad libitum conditions, would also have a greater seasonal variation in intake and grazing behaviour under field conditions, and (ii) greater seasonal variation in intake and grazing behaviour under field conditions would be expressed at a higher level of food availability. 3. The intake and grazing behaviour in summer and winter, of three breeds of sheep, were compared at two levels of food availability (at pasture heights of 3·7 and 5·4 cm). The breeds were known to have contrasting degrees of seasonal variation in food intake when fed ad libitum ; the VFI of the Shetland (SH) and Scottish Blackface (BF) sheep varies greatly between seasons whereas that of the Dorset Horn (DH) is less seasonally variable. 4. All three breeds consistently increased their rates of biting and duration of grazing activity in the winter, taking many more smaller bites each day than in the summer, and both digestibility and intake were lower in winter than in summer. 5. Contrary to expectation, the DH ewes had the highest seasonal difference of dry matter intake at pasture, whereas the SH breed had the lowest variation of intake between seasons. 6. This experiment provides no evidence that differences between seasons in intake and foraging behaviour in the field vary with the animals' degree of endogenous seasonal variation in VFI. Variation between seasons was consistent at both levels of resource availability, suggesting that it resulted from seasonal changes in food quality (digestibility) rather than biomass availability. It is not easy to extrapolate from laboratory feeding studies, where animals' own physiological constraints apply, to foraging ecology in the field, where constraints imposed by the environment may be more important.

Voluntary intake and digestibility in horses: effect of forage quality with emphasis on individual variability

“…This prevents the variance due to these random variables being erroneously incorporated in the error term of the analysis. Appetite, and consequently food intake, in herbivores is known to show seasonal variations (Iason et al, 2000) partly under the control of endogenous cycles of hormonal secretions linked with the photoperiod, via melatonin, which exhibits a strong rhythmicity in horses (Piccione et al, 2005). Unfortunately, in the present dataset, the diets fed to the horses were not balanced among seasons and adding this factor in the model did not explain a significant part of the variance in intake levels.…”

Seasonality of food intake in ruminants: recent developments in understanding

“…Such experiments frequently involve maintenance of animals indoors and on artificial diets (Rhind et al 1998. However, care is required in the extrapolation from data obtained in such studies to other circumstances because different breeds or species may exhibit different responses to different conditions (Iason et al 2000). 2.…”

“…The seasonality of food intake of relatively few species or breeds has been studied intensively; most of these are domesticated or semi-domesticated and inhabit relatively high latitudes or altitudes. However, of those that have been studied, most have been found to exhibit marked seasonal variations in appetite and voluntary food intake, which are a function of physiological changes and do not simply reflect a seasonal reduction in food availability (Kay, 1979;Kay & Staines, 1981;Barry et al 1991;Iason et al 2000). This phenomenon is considered to have evolved as an adaptation to regular seasonal reductions in food availability, so that the animal's drive to search for food is reduced when supply is reduced and less energy is expended in foraging (Kay & Staines, 1981).…”

“…This phenomenon is considered to have evolved as an adaptation to regular seasonal reductions in food availability, so that the animal's drive to search for food is reduced when supply is reduced and less energy is expended in foraging (Kay & Staines, 1981). The pattern of variation is subject to modification by many peripheral factors including the energy content of the food (Webster et al 2000), the species or breed under investigation (Semiadi et al 1995;Iason et al 2000) and physiological state (pregnant, lactating, rutting, high or low body condition, etc.) (Barry et al 1991;Sibbald, 1997).…”

“…(Barry et al 1991;Sibbald, 1997). In domesticated ruminants, the variation is less marked and seasonal changes in intake may depend on the breed and food type (Iason et al 2000), but it is still a significant determinant of animal performance and therefore an important topic for research.…”