Effects of food structure and nutritional quality and animal nutritional state on intake behaviour and food preferences of sheep

There is evidence of the independent effects of plant physical and chemical characteristics on foraging, but little has been done to determine how these traits interact to affect food selection. We determined if the nutritional state of lambs (Ovis aries) affected intake behavior and preference for foods with different ratios of protein/energy (alfalfa: high proportion protein/energy; barley: high proportion energy/protein), presented in different physical forms (whole or ground). Changes in nutritional state were induced by altering the nutritional composition of the basal diet (high [Ep] or low [eP] proportion of energy/protein). We determined preference for barley and alfalfa (whole or ground) when lambs were fed (1) a nutritionally balanced diet with a 15-h (overnight) fast (Period 1), (2) an unbalanced diet (Ep or eP) with a 15-h (overnight) fast (Period 2), and (3) an unbalanced diet without a fast (Period 3). Averaged across the three periods, all lambs preferred barley to alfalfa (P < 0.001), and they spent more time feeding on barley than alfalfa (P < 0.001). Lambs also preferred whole to ground foods (P < 0.05). Nevertheless, the basal diet strongly affected preference for barley and alfalfa. During Period 2, lambs fed basal diet Ep ate less barley (142 vs. 232 g) and more alfalfa (71 vs. 9 g) than lambs fed basal diet eP (P < 0.001), especially when alfalfa was pelleted and barley was ground or rolled. Lambs fed basal diet eP strongly preferred barley to alfalfa, even if barley was ground and alfalfa was pelleted. These differences were even more pronounced after a meal of basal diet Ep or basal diet eP in Period 3. Lambs fed basal diet Ep consumed less energy during preference tests than lambs fed basal diet eP (P < 0.001). Lambs fed the two unbalanced basal diets consumed similar amounts of protein during preference tests (P > 0.05). These differences in intake of energy and protein by lambs fed the unbalanced diets led to a steady increase in the ratio of protein/energy selected by lambs fed the balanced basal diet and diet Ep during Periods 1, 2, and 3 (7.9, 11.7, 13.9 g CP/MJ), and a constant ratio of protein/energy selected by lambs fed the balanced basal diet and diet eP during Periods 1, 2, and 3 (7.6, 8.1, 8.1 g CP/MJ; P < 0.001). Thus, lambs fed basal diet Ep selected barley and alfalfa in a way that achieved a higher ratio of protein/energy in their diet than lambs fed basal diet eP (12.7 vs. 8.2 g CP/MJ DE, P < 0.001). Collectively, these results suggest that food structure and biochemical composition interacted with a lamb's nutritional state to determine preference. When a lamb's need for a particular macronutrient was high, a food's biochemical composition was more important than its structure in determining preference. (C) 1999 Elsevier Science B.V. All rights reserved.