Ž .
Ž .
Fig. 6. Experiment 2 Trial 5 . Mean SEM intake of pomace–starch and pomace before and after LiCl Ž
. administration 300 mgrkg BW to two groups of lambs. The Milo–LiCl group received milo before LiCl
infusions whereas the Control group only LiCl. Intake of pomace–starch did not differ between groups Ž
. P 0.05 .
. respectively; P 0.05; SEM s 5 . When exposure increased to 8 hrday, intake of
pomace was higher than intake of the pomace–starch mix on the first 2 days, but this Ž
pattern reversed from day 3 to the end of the trial food by day interaction P - 0.001; .
Fig. 4 . Average intake of the pomace–starch mix was higher than intake of grape Ž
. pomace 252 g vs. 191 g; P - 0.1; SEM s 22 .
When we compared the last 20 min preference test of Period 1 with the 20 min preference test performed after Period 2, intake of the pomace–starch mix increased
Ž .
Ž .
P - 0.001 , whereas intake of pomace decreased P - 0.05 after Period 2, as reflected Ž
. in a significant test food by day interaction P - 0.001; Fig. 5 .
4.2.5. Trial 5 The objective of this trial was to determine if lambs generalized an aversion from a
Ž .
Ž .
familiar grain milo to a novel food pomace that contained starch. Intake of the Ž
. pomace–starch mix decreased after LiCl administrations from 104 g to 70 g , but intake
Ž .
of pomace did not change across days 33 g to 28 g . There were significant differences Ž
. Ž
. due to day P - 0.05 and food by day P - 0.05 , but no differences between groups,
Ž .
group by food, group by day, or group by food by day interactions P 0.05; Fig. 6 .
5. Discussion
5.1. Energy supplement We hypothesized that lambs first fed milo and then fed straw might acquire a
preference for straw because the post-ingestive effects of milo would be experienced Ž
while the lambs ate the straw. This temporal ordering between stimuli backward .
conditioning typically leads to a lower level of conditioning than the reverse procedure
Ž .
e.g., short-delay conditioning; Mazur, 1994 , but it may facilitate conditioning when flavors are subsequently associated with the post-ingestive effects of nutrients, particu-
larly if the post-ingestive effects of absorbed calories occur during straw ingestion Ž
. Boakes and Lubart, 1988 . Nevertheless, only lambs in the experienced group in Trial 2
Ž .
Ž .
Experiment 1 developed a weak preference for straw. That occurred only after we Ž
. restricted the amount of digestible energy in their basal ration Fig. 1 , which is
consistent with studies showing that energy restriction enhances preferences for energy Ž
. Capaldi, 1990; Mehiel, 1991 . Conversely, lambs acquired a preference for straw when
Ž .
milo was infused in the rumen before straw ingestion in Trial 3 Experiment 1; Fig. 1 . Ž
The much stronger preference for straw when lambs were infused with milo than when .
they ate milo suggests that oral experience with milo was needed for lambs to quickly discriminate between the specific flavor-post-ingestive effects of milo and straw. The
strong preferences for flavored straw acquired after milo infusions indicates that the low levels of preference and intake typically displayed for this forage are mainly due to a
lack of nutrient feedback from the gut and not to the forage’s appearance or to its physical structure.
Rats acquire preferences for solutions consumed shortly after glucose ingestion Ž
. Ž
Boakes and Lubart, 1988 . Nevertheless, the preferences are weak e.g., Capaldi et al., .
Ž .
1987 and in some cases animals fail to display a preference e.g., Simbayi et al., 1986 because rats more strongly associate the flavor of glucose, rather than the non-nutritive
Ž .
flavor, with the calories supplied by glucose Elizalde and Sclafani, 1988 . Oral
Ž .
experience influences preference Rolls, 1986; Swithers and Hall, 1994 , and the
combination of oral and post-ingestive effects are more important than either alone in Ž
. food choices Perez et al., 1996 .
´
Lambs discriminated between milo and straw even with brief exposures on a limited Ž
. number of occasions. Rapid post-ingestive effects may help ruminants Provenza, 1995
Ž .
and rats Melcer and Alberts, 1989 discriminate between foods eaten in close temporal association. Feedback occurs quickly when nutritious foods are ingested. For ruminants,
levels of portal and jugular blood metabolites increase within minutes of beginning to Ž
. eat Evans et al., 1975; Chase et al., 1977; deJong, 1981; Van Soest, 1994 , and
responses of chemoreceptors in the gastrointestinal tract occur on the order of seconds Ž
. Leek, 1977; Cottrell and Iggo, 1984; Mei, 1985 . Conversely, a lack of feedback as an
animal begins to eat quickly signals that the food is poorly nutritious. 5.2. Amount ingested
Our results suggest that lambs must ingest a threshold amount of novel food for post-ingestive effects to condition a preference. On the first day of exposure to
Ž .
pomace–starch and pomace–cellulose Experiment 2; Trial 1 , lambs consumed from 18 g to 40 g of starch. Intraruminal infusions of these amounts of starch condition
Ž .
preferences for poor-quality foods in lambs Villalba and Provenza, 1997a . Conversely, lambs did not acquire preferences for flavors paired with intraruminal infusions of
Ž .
Ž cellulose Experiment 2; Trial 3 , and the maximum amount of cellulose ingested 20 g
. in Experiment 2; Trials 1 and 2 was not sufficient to condition a preference, even when
Ž .
the alternative was grape pomace Experiment 2; Trial 3 , as opposed to pomace–starch Ž
. Experiment 2; Trial 1 .
Ž The amount of food eaten in a meal affects aversions to novel foods Provenza et al.,
. 1998 . For instance, goats eating current season’s and older growth twigs from the shrub
blackbrush display less preference for the novel plant part — current season’s growth or Ž
. older growth — eaten in the greatest amount prior to toxicosis Provenza et al., 1994 .
After drinking solutions of different flavors and experiencing toxicosis, rats also acquire Ž
. aversions to the flavor consumed in the largest amount Bond and DiGuisto, 1975 .
Ž .
Results from Trial 4 Experiment 2 also suggest that lambs must eat a threshold amount of food for post-ingestive effects to influence preference. When lambs were
offered a choice between novel foods — grape pomace and grape pomace–starch — for 20 minrday, they did not show a preference until day 9, when they actually began to
Ž .
prefer grape pomace, the food of lower nutritional quality Fig. 4 . The maximum amount of starch consumed on any day was only 14 g, an amount too low to condition a
Ž .
preference for pomace–starch Villalba and Provenza, 1997a . However, when the Ž
. amount of exposure increased from 20 minrday to 8 hrday in Trial 4 Experiment 2 ,
Ž .
lambs increased preference for the pomace–starch mix Fig. 4 , and when the trial Ž
. ended, they preferred pomace–starch over pomace alone Fig. 5 . The amounts of starch
consumed during this period ranged from 59 g to 88 g, amounts that condition strong Ž
. food preferences in lambs Villalba and Provenza, 1997a .
Collectively, these results suggest that lambs must ingest a threshold amount of food for post-ingestive effects to change preference, and that the availability of alternative
foods influences if and when lambs will ingest a threshold amount of a novel food. Thus, our results suggests that preference is determined not only by nutrient concentra-
Ž .
tions in food Belovsky, 1981 , but also by the amount of the food ingested. It is likely that only plant species consumed in significant amounts foster post-ingestive effects
sufficient to be relevant for herbivores to discriminate. Plant species consumed in low Ž
amounts, such as those reported in the low-rank order of diet composition tables van .
Wieren, 1996, Watson, 1997 , probably provide nutrient input that is indistinguishable from the physiological background, and are of little importance in changing preference.
This may be particularly relevant for foods of medium to low quality, where many bites may be required to reach a minimum threshold.
5.3. Generalization Animals generalize food preferences and aversions based on past experiences. Sheep
and goats prefer hays sprayed with extracts obtained from highly preferred foods Ž
. high-grain concentrates; Dohi and Yamada, 1997 . Lambs made averse to cinnamon-
flavored rice — by experiencing toxicosis after eating cinnamon-flavored rice — Ž
. generalize the aversion to cinnamon-flavored wheat Launchbaugh and Provenza, 1993 .
Our results suggest that lambs discriminated among novel foods by generalizing Ž
. flavor cues from familiar to novel foods. During Trial 1 Experiment 2 , lambs preferred
Ž .
the novel food with starch from the first day of exposure Fig. 2 , which is consistent with the notion that they generalized a preference from grains high in starch to the
Ž .
pomace–starch food. During Trial 2 Experiment 2 , lambs that ate milo and then Ž
received LiCl, consumed much less of the starch–pomace mix than did Controls Fig. .
3 , which is also consistent with the notion that lambs generalized preferences based on
sensory cues — probably starch — in milo and in the pomace–starch mix. Lambs showed a similar pattern of preference when we increased the proportion of starch in the
mixture in the next preference test, though the differences between groups — Milo–LiCl and Control — were not significant, evidently due to variation among lambs offered a
novel food combination. However, the reduction in pomace–starch intake shown by
Ž .
both groups of lambs after infusions of LiCl during Trial 2 Experiment 2; Fig. 3 , and the similar decrease in preference for pomace–starch by both groups of lambs —
Ž Milo–LiCl and Control — after administration of LiCl during Trial 5 Experiment 2;
. Fig. 6 , argue against a very specific or robust generalization of the aversion in
starch-containing foods. Collectively, these results suggest that generalization based on the starch content of familiar foods partially explains the high degree of discrimination
shown by the lambs of this study.
6. Conclusions
