postingestive effects of energy played an important role in modulating rates of food intake, and our findings suggest postingestive feedback from macronutrients is a fundamental factor influenc-
ing the ingestive behaviour of herbivores. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Sheep; Intake rate; Bite rate; Starch; Feeding behavior; Foraging behaviour
1. Introduction
The process of gathering food is an essential affair for living organisms. For herbivores, behaviours directed toward finding and harvesting plants are of central
Ž .
importance Shipley et al., 1994; Ungar, 1996 . Rate of food intake emerges as a fundamental variable in foraging ecology since it determines the time herbivores spend
gathering food to obtain required nutrients, and in an indirect manner the time they have Ž
. to invest in other activities such as mating or avoiding predators Shipley et al., 1994 .
Intake rate over short periods of active grazing is determined by bite weight and bite rate Ž
. Allden and Whittaker, 1970 . Traditionally, these variables have been regarded as a
Ž function of animal morphology, behaviour and sward structure Hodgson, 1981; Black
. and Kenney, 1984; Laca et al., 1994; Wilman et al., 1996 . For instance, intake per bite
is influenced primarily by the sensitivity of bite depth to variation in sward height Ž
. Hodgson et al., 1999 , and bite rate is a function of the time spent per bite on searching
Ž .
Ž locomotion, recognition and decision and handling gathering herbage into the mouth,
. Ž
. severing the herbage, ingestive mastication and swallowing food Laca et al., 1994 .
Besides their physical characteristics, plants present a complex array of biochemicals to herbivores. Recent research has shown that postingestive effects mediated by nutrients
Ž .
and toxins strongly influence food preference Provenza, 1995a,b, 1996 . Lambs can acquire preferences and increase consumption of low-quality forages such as wheat
straw when straw is associated with intraruminal infusions of energy and nitrogen that Ž
represent as little as 2.5 of their daily requirements Villalba and Provenza, 1996, .
1997a . Thus, it is conceivable that animals consume foods at rates that are also a function of the foods’ postingestive actions.
The majority of grazing studies have concentrated on the impact of a plant’s structure on intake rate, with very little reference to the potential importance of a plant’s
Ž biochemical composition as a factor controlling intake rate Illius and Hodgson, 1996;
. Hodgson et al., 1999 . This has occurred even though heterogeneity in the nutritional
Ž .
quality of plant communities is frequent rather than peculiar Provenza et al., 1998 . The influence of nutrients on rates of food consumption has been overlooked primarily
Ž because research on physical plant characteristics
height, bulk density, structural .
strength has controlled for plant quality and because experiments were conducted under conditions where nutritional and toxicological differences among plants were not
Ž .
pronounced O’Reagain et al., 1996 . We undertook this study to determine if the postingestive actions of macronutrients
affect rates of food ingestion. We supplied lambs with intraruminal infusions of starch in association with the ingestion of a low-quality forage — wheat straw — whose physical
structure is difficult to harvest and handle.
2. Materials and methods
