Preference for wheat straw by lambs conditioned with intraruminal infusions of starch

Abstract

We hypothesized that feed preference depends on the interplay between flavour and postingestive effects, and we tested two predictions based on this hypothesis: (1) lambs acquire preferences for poorly nutritious feeds paired with starch; and (2) preferences persist when starch is no longer administered. Twenty lambs were randomly allocated to two groups and conditioned as follows: on odd-numbered days, lambs in group 1 received onion-flavoured wheat straw and lambs in group 2 received oregano-flavoured wheat straw. On even-numbered days, the flavours were switched and starch (2-5-9.4% of the digestible energy received/d) was infused into the rumen of aU animals during straw consumption. Four periods of 8 d of conditioning were performed and on the 9th day of each period all animals were offered a choice between onion-and oregano-flavoured straw. After conditioning, starch administration was suspended and lambs were offered onion-and oreganoflavoured straw at weekly intervals for 8 weeks (extinction). Lambs strongly preferred the flavoured straw paired with starch, and this preference persisted during extinction. Thus, these results suggest that the postingestive effects of energy play an important role in the development of feed preferences of ruminants.Palatability: Starch Feed preference: Hedonic-shift: Flavour It is not completely understood why ruminants manifest preferences for feeds. On the one hand, preferences have been attributed to flavour, which provides hedonic sensations associated with eating (reviewed by G r o w , 1988). On the other hand, nutrients have been proposed as a reason for feed selection (Westoby, 1978), and toxins as a reason for feed avoidance (Freeland 8z Janzen, 1974). According to these hypotheses, flavour and the postingestive effects of nutrients and toxins are autonomous phenomena, and the former is usually considered to influence preference more than the latter. Nevertheless, the two phenomena may be related: postingestive feedback from nutrients and toxins may influence flavour preferences (Provenza, 1995a).The postingestive effects of energy cause non-ruminants to acquire preferences for non-nutritive flavours (Sclafani & Nissenbaum, 1988;Perez et al. 1995), and the preferences persist when energy is no longer administered. Thus, animals apparently acquire a liking for flavours previously associated with energy (Mehiel, 1991; Sclafani, 1991). The energy content of the feed may also be important in the development of preferences in ruminants because energy is vital for survival, growth, and productivity (Church, 1988). For example, sheep prefer non-nutritive flavours paired with glucose to non-nutritive flavours paired with saccharin or water (Burritt & Provenza, 1992;Ralphs et al. 1995). In these studies glucose provided 26-46 % of the total daily energy intake of the animals. Thus, it is not known if sheep form preferences for feeds containing little energy or if these preferences persist aRer conditioning.

Self-medication and homeostatic behaviour in herbivores: learning about the benefits of nature’s pharmacy

“…Viewing foraging behaviour as the quest for substances in the external environment that provide homeostatic utility to the 'internal milieu' leads us to the conclusion that selfmedication is the consequence of the same mechanism that allows animals to form preferences for nutrients (Villalba and Provenza, 1996, 1997a, b, and c, 1999 and develop aversions to toxins : Behaviour by consequences. Thus, while nutrients, PSMs and medicines are labels scientists use to better comprehend foraging complexity, for the animal in a dynamic homeostatic quest, ingesting nutrients, PSMs and medicines are means to the same end -staying well.…”

“…In this case, behaviours are geared towards limiting food intake. For instance, sheep are reluctant to eat poorly nutritious foods such as straw as straw does not meet needs for energy or protein, but their intake and preference for straw increase when a nutritious food (Greenhalgh and Reid, 1971) or starch Provenza, 1997a and2000) is infused into the rumen immediately after a meal of straw. Ruminants also avoid flavours associated with excesses of nitrogen (Kyriazakis and Oldham, 1993;Villalba and Provenza, 1997a) or excesses of volatile fatty acids (Villalba and Provenza, 1997b) and limit intakes of otherwise nutritious foods that contain plant secondary metabolites (PSMs) Provenza, 1996;Dearing et al, 2005).…”

Diet learning through post-ingestive consequences in sheep: the case of starch and casein variously combined in the same foods

“…Information coming from both the literature and complementary trials led us to expect that our animals would be able to perceive the rewards we presented. Doses of starch were between 0.7 and 3.4 g/kg LW and those of casein were between 0.5 and 1.6 g/kg LW, and so were in the range that induced preferences in previous work (Villalba and Provenza, 1997a;Arsenos and Kyriazakis, 1999;Villalba et al, 1999;Arsenos et al, 2000b). Also, in our methodological trial, dose rates were between 1.2 and 3.7 g/kg LW for starch and between 0.8 and 2.3 g/kg LW for casein and induced clear preferences for the hay associated with each of the stimuli.…”

“…Ruminants generally succeed in selecting an appropriate diet relative to their nutrient requirements, and macronutrients (e.g. energy and protein) are considered as playing a primary role in their food preferences 1997a). They seem able to discriminate the effects of energy and protein (Wang and Provenza, 1996a), and faced with a choice between foods varying in both, sheep select diets with relatively constant protein/energy ratios (Kyriazakis and Oldham, 1993;Provenza et al, 1996).…”

“…Many experiments have since been reported in which food sensory characteristics (artificially added flavours) have been variously associated with post-ingestive consequences (intra-ruminally added substances), rich in either energy (Burritt and Provenza, 1992;Villalba and Provenza, 1996, 1997aand 1997b or protein (Villalba and Provenza, 1997c). These studies have shown that animals are able to perceive post-ingestive signals and their intensities (Arsenos and Kyriazakis, 1999), and modify their diet selection.…”