Postingestive Feedback from Starch Influences the Digestive Behavior of Sheep Consuming Wheat Straw

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Applied Animal Behavior Science

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Plant species present a complex array of biochemicals to herbivores that in conjunction with a plant's physical structure influence intake. Our objective was to determine the role of postingestive feedback from macronutrients on the ingestion of a low-quality forage by sheep (Ovis aries). We compared the ingestive behaviour of two groups of lambs conditioned with intraruminal infusions of starch (100 g/lamb/day; Group 1) or water (250 ml/lamb/day; Group 2) while they grazed wheat straw (held erect in wooden frames; 30 cm height; Trials 1, 2 and 3) or consumed wheat straw from food boxes (1–2 cm particle size; Trial 4). Lambs were conditioned as individuals during Trials 1 and 4, in pairs during Trial 2, and as two groups (Groups 1 and 2) during Trial 3. Ingestive behaviour – intake, intake rate, number of bites, bite size, bite rate — was assessed during individual tests that lasted 5 min/lamb. After conditioning and testing, all animals had restricted access (80% of their daily energy requirements) to a basal diet of alfalfa pellets. No differences between groups were detected during initial tests (before infusing starch) or during Trial 1 (P>0.05). Lambs conditioned with starch in Trial 2 consumed more straw (8 vs. 2 g; P<0.05) at higher rates (2.3 vs. 1.0 g/min; P<0.1), and took more (13 vs. 5; P<0.1) and larger (0.59 vs. 0.31 g/bite; P<0.05) bites than lambs conditioned with water. This pattern was maintained during Trial 3: Lambs infused with starch showed higher intake (24 vs. 5 g; P<0.01), intake rate (5.4 vs. 1.4 g/min; P<0.001), bites (29 vs. 10; P<0.01), bite rate (6.7 vs. 2.6 bites/min; P<0.01) and bite size (0.86 vs. 0.54 g/bite; P<0.05) than lambs infused with water. In Trial 4, lambs conditioned with starch consumed more straw (21 vs. 9 g; P<0.1), at higher rates (5.4 vs. 2.6 g/min; P<0.1), than lambs conditioned with water. Thus, the 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 influencing the ingestive behaviour of herbivores.

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