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. The effect of the nutritional state of lambs on intake behaviour and preference for feeds with different ratios of protein to energy (lucerne: high proportion protein to energy; barley: high proportion energy to protein), presented in different physical forms (whole or ground), was investigated. Changes in nutritional state were induced by altering the nutritional composition of the basal diet (high, Ep, or low, eP, proportion of energy to protein). Preference for barley and lucerne (whole or ground) was determined when lambs were fed (1) a nutritionally balanced diet with a 15-h (overnight) starvation period (period 1), (2) an unbalanced diet (Ep or eP) with a 15-h (overnight) starvation period (period 2), and (3) an unbalanced diet without a starvation period (period 3). Averaged across the three periods, all lambs preferred barley to lucerne (P<0.001), and they spent more time feeding on barley than lucerne (P<0.001). Lambs also preferred whole to ground feeds (P<0.05). Nevertheless, the basal diet strongly affected preference for barley and lucerne. During period 2, lambs fed on Ep ate less barley (142 vs. 232 g) and more lucerne (71 vs. 9 g) than lambs fed on eP (P<0.001), especially when lucerne was pelleted and barley was ground or rolled. Lambs fed on eP strongly preferred barley to lucerne, even if barley was ground and lucerne was pelleted. These differences were even more pronounced after a meal of Ep or eP in period 3. Lambs fed on Ep consumed less energy during preference tests than lambs fed on eP (P<0.001). Lambs fed on the 2 unbalanced basal diets consumed similar amounts of protein during preference tests. These differences in intake of energy and protein by lambs fed the unbalanced diets led to a steady increase in the ratio of protein to energy selected by lambs fed the balanced basal diet and Ep during periods 1, 2 and 3 (7.9, 11.7, 13.9 g CP/MJ), and a constant ratio of protein to energy selected by lambs fed on the balanced basal diet and eP during periods 1, 2 and 3 (7.6, 8.1, 8.1 g CP/MJ; P<0.001). Thus, lambs fed on Ep selected barley and lucerne in a way that achieved a higher ratio of protein to energy in their diet than lambs fed on eP (12.7 vs. 8.2 g CP/MJ DE, P<0.001). Collectively, these results suggest that feed structure and biochemical composition interacted with nutritional state to determine preference. When the need for a particular macronutrient was high, the biochemical composition of the feed was more important than its structure in determining preference.
|Publication Title||Applied Animal Behaviour Science|
|Author Address||Department of Rangeland Resources, Utah State University, Logan, UT 84322-5230, USA.|
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