Published December 11, 2014
Lambs Form Preferences for Nonnutritive
Flavors Paired with Glucose1v2
E. A. Burritt and F. D. Provenza
Department of Range Science, Utah State University, Logan 84322
ABSTRACTS We studied lambs' preferences for
nonnutritive flavors that were paired with a
glucose solution. On the 1st d of the experiment,
lambs were offered a saccharin solution flavored
with either orange or grape. The following day
lambs received a glucose solution containing
either orange or grape flavor. Lambs that had
received grape and saccharin on d 1 received
orange and glucose on d 2, and the reverse was
true for the remaining lambs. Conditioning lasted
10 d; odd days were like the first and even days
like the second. After conditioning, when lambs
were offered a choice between orange- or grape-
flavored water without sweeteners, lambs chose
the flavor that had been paired with glucose. We
also tested the initial hedonic response of naive
lambs to the flavor of glucose and saccharin
solutions. Lambs exhibited no initial preference.
Our results indicate that lambs preferred the
flavor that was paired with glucose (calories).
Furthermore, results of this study suggest that
ruminants may not innately recognize specific
chemical constituents in foods or select diets
based on initial hedonic value. Rather, learning
plays a key role in the formation of dietary
preference.
Key Words: Learning, Feed Preferences, Sheep
J. Anim. Sci. 1992. 70:1133-1130
Introduction
Historically, researchers have considered diet
selection a genetically fixed process and have
sought to explain it by suggesting that ruminants
prefer foods with high hedonic value or that
contain specific chemical compounds they can
innately detect. Lacking from these traditional
explanations is the possible association between
feed flavor and postingestive consequences
(Provenza and Balph, 1990). Numerous studies
have described nuninant preferences for certain
compounds (reviewed by Church, 1979) but have
ignored the postingestive consequences of these
compounds.
Negative postingestive consequences can alter
dietary preferences and diet selection in
'Utah Agric. Exp. Sta. Tech Paper No. 4096. Financial
rtssistance was provided by Utah Agric. Exp. Sta. and the
Cooperative States Res. Service.
2We gratefully acknowledge Karen Launchbaugh for help in
data collection and the US. Sheep Exp. Sta. for material
support.
Received December 17, 1990.
Accepted November 6 , 1991.
ruminants. Ruminants form aversions to feeds and
forages that cause gastrointestinal illness (Burritt
and Provenza 1989a,b;Lane et al., 1990; Provenza
et al., 1990); however, the effect that positive
postingestive consequences may have on dietary
preference is unknown.
Several lines of evidence support the hypothesis
that ruminants form preferences for feeds with
positive postingestive consequences. First, physiological changes occurring within 15 min of the
initiation of eating, such as increases of volatile
fatty acids, j3-hydroxybutyrate, and insulin in the
portal and jugular blood, may enable ruminants to
associate quickly certain feeds with positive feedback (reviewed by Provenza et al., 1991). Second,
after a single trial ruminants form aversions to
foods that produce negative postingestive consequences (Burritt and Provenza, 1989a1. Conditioned preferences may similarly result from positive feedback. Finally, ruminants learn about the
effects of a specific feed even when they ingest
several feeds at once (Burritt and Provenza, 1992).
Our study examines the ability of lambs to form
preferences for nonnutritive flavors paired with
glucose.
1133
BURRITI: AND PROVENZA
1134
Materials and Methods
All lambs used in the trials were crossbred
Finnish Landrace orphans from the US. Sheep
Experiment Station at Dubois, ID. Lambs were
reared on artificial milk replacer until 4 wk of age
and were then maintained on alfalfa pellets and
rolled barley until the start of the trials, when
lambs were 3 mo old.
Conditioning. Sixteen lambs were housed in
individual pens bedded with straw. The night
before conditioning, feed boxes were removed
from the pens. At 0800 the next morning, lambs
were offered 800 g of alfalfa pellets for 1 h. They
also had ad libitum access to plain water. At the
end of 1 h, feed boxes and water buckets were
removed and residual feed and water was measured. At 0930 lambs were offered flavored water
containing .225% sodium saccharin and either
orange or grape flavor unsweetened Kool-Aid (1 g/
L). Half the lambs received grape .and the other
half orange. Lambs had access to the flavored
water for 22.5 h/d and it was their sole water
source during that time. On d 2, the residual
flavored saccharin was measured and lambs were
offered feed and plain water as on d 1. Lambs then
received a 7% glucose solution flavored either with
orange or grape. Lambs that had received grape
and saccharin on d 1 received orange and glucose
on d 2, and the reverse was true for the other
group of lambs. Conditioning lasted 10 d; odd days
were like the first and even days were like the
second. The amount of each solution a lamb
received was adjusted daily so that lambs consumed similar amounts of each solution during the
conditioning period.
Preference Test. After conditioning, water and
alfalfa pellets were offered to lambs for ad libitum
intake for 5 d. Lambs were then fasted overnight.
The next morning they were offered a choice
between orange- or grape-flavored water without
sweetener for 2 h. Intake of each flavor was
measured and paired t-tests were used to detect
differences between treatment means,
Test of Initial Hedonic Preference. We tested the
initial hedonic responses of lambs to the flavor of
glucose (7%) and sodium saccharin (.225%) in a
water solution. None of the 65 lambs used in this
trial had been used in the conditioning and
preference study. During the trial ( < 5 min
duration) lambs chose between glucose and saccharin solutions. Lambs were deprived of water for
1 h before the test. Lambs were presented with
1,000mL of either glucose or saccharin solution in
a plastic pail; lambs then were offered the other
solution in a different plastic pail. Lambs were
allowed to drink briefly ( < 30 SI from each pail. We
alternated the solution that was presented to
lambs first. Lambs could then drink from either
pail for 2 to 4 min, or until they had consumed one
or the other of the solutions entirely. Because the
trial was brief, postingestive consequences should
not have affected the results, and thus data reflect
the initial responses of lambs to the flavors of
glucose and saccharin. A lamb was eliminated
from the trial if it spilled (a1 solution(s1 or refused
to drink one or both of the solutions. We obtained
results from 57 of the 65 lambs. We used paired ttests to detect differences between treatment
means.
Results
Conditioning. Lambs consumed a n average of
2,440 rt 93 mL/d of flavored saccharin and 2,363 k
68 mL/d of flavored glucose during conditioning.
Lambs consumed similar amounts of alfalfa pellets
(475 g/d) and plain water (1,222mL/d regardless
of the solution offered during conditioning. On the
days that lambs consumed the glucose solution, it
constituted 22 to 46% of their daily energy intake.
Preference Test. When the sweeteners were
removed, lambs exhibited a strong preference (P e
.001) for the flavor that had been paired with
glucose. Lambs consumed 840 k 111 mL of the
flavored solution that was paired with glucose but
only 32 f 17 mL of the flavored solution that was
paired was saccharin.
Test of Initial Hedonic Preference. Lambs exhibited no initial preference (P = .84)for either the
flavor of glucose or saccharin solutions. Lambs
consumed 455 f 47 mL of the glucose solution and
439 f 46 mL of the sodium saccharin solution.
Discussion
In this study, lambs learned to prefer the flavor
that was paired with glucose. Because lambs
exhibited no initial preference for either the
glucose or the saccharin solutions, this preference
can only be attributed to positive postingestive
consequences (additional energy1 experienced by
lambs, and not to the initial hedonic value of
either solution. Furthermore, these results do no
support the idea that ruminants can innately
sense nutrients because lambs did not initially
prefer the glucose solution.
The hedonic value of a food is not a fixed entity
and depends to a large degree on its postingestive
consequences. Mehiel and Bolles (1988)reported
that rats preferred nonnutritive flavors paired
with energy-containing solutions, even when these
LAMBS LEARN TO PREFER NONNUTRITIVE FLAVORS
sources had low hedonic values. Sweet and bitter
are often considered feeding attractants and deterrents, respectively (Garcia and Hankins, 1975,
19771,but preference for bitter increases when it is
paired with positive postingestive consequences
and preference for sweet decreases when paired
with negative postingestive consequences (Garcia
and Holder, 1985).Additionally, Grovum and Chapman (1988)concluded that a preference for sweet is
not innate in ruminants. They reported that
sucrose was relatively unpalatable to sheep when
the flavor of sucrose was separated from its
post ingestive consequences . Finally, BuchananSmith (1990) concluded that sheep may prefer
good-quality over poor-quality silage because of
postingestive factors rather than because of the
flavor or odor of the silage. These results do not
minimize the role taste plays in diet selection, but
suggest that a n animal's preference for certain
flavors depends on the postingestive consequences
of the food associated with that flavor.
Learning plays a vital role in diet selection
because it enables herbivores to select nutritious
feeds and avoid toxic feeds in environments where
the nutrient content and toxicity of feeds, as well
as the nutritional requirements of herbivores,
change temporally and spatially. Dietary preferences of ruminants can be shaped by many factors
(reviewed by Matthews and Kilgour, 1979;
Provenza and Balph, 1987, 1988, 1990;Provenza et
al., 1992).Ruminants form preferences for feeds as
a result of interactions with social models (Green
et al., 1984;Thorhallsdottir et al., 1990;Mirza and
Provenza, 1990) and as a result of exposure to
feeds early in life b o l d and Maller, 1977;Nolte
et al., 1990, 1992; Squibb et al., 1990). Moreover,
they quickly form aversions to feeds that cause
negative postingestive consequences (Burritt and
Provenza, 1989a,b; Lane et al., 1990; Provenza et
al., 1990). Results of this study show that
ruminants can associate positive postingestive
consequences with flavors and they modify dietary
choices accordingly.
Conclusions
This study facilitates our understanding of the
ability of ruminants to form conditioned food
preferences. Previous research with rats clearly
indicated that preferences can be enhanced when
feeds or feed flavors are paired with: 1) energy, 2)
recovery from nutritional deficiencies, and 31
recovery from postingestive distress (see review in
Provenza et al., 1992). It is not known whether
ruminants form preferences for certain feeds
under all these conditions.
1135
Implications
Lambs learned to prefer flavors that were
paired with glucose; selection of flavors was not
based on the initial flavor of the compound or on
the ability of lambs innately to sense a high
energy content. Results emphasize the key role
that learning plays in diet selection by ruminants.
A better understanding of the formation of conditioned feed preferences may enhance our ability to
modify diet selection by ruminants.
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