2. Materials and methods
We conducted four trials at the Green Canyon Ecology Center, located at Utah State University in Logan. The objective of these trials was to determine if lambs modify their
rate of straw consumption when they received intraruminal infusions of starch. As the trials progressed, we increased the degree of exposure to straw and we modified the
temporal association between food ingestion and postingestive feedback in order to enhance the association between straw and starch. Herbivores constantly manifest
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dynamic and adaptive interactions with their environment Provenza et al., 1998 . We Ž
show the emergence of some of these interactions e.g., ingestive behaviour–postinges- .
tive feedback in response to the evolving experimental conditions of the study. The same animals and straw–starch or straw–water associations were maintained
Ž throughout the study. During each trial, lambs Finn–Polypay–Suffolk crossbreds of
. both sexes had free access to salt blocks and fresh water. Alfalfa pellets were the basal
diet. Lambs were approximately 3 months of age at the beginning of the study. 2.1. Before conditioning
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Eighteen lambs 25 kg BW were penned as a group in a 14 by 10 m plot. In order to familiarize the animals with the upcoming experimental procedures, every day at 0800 h
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pairs of lambs were selected at random and transferred to a testing pen 3 by 4.5 m . Lambs were then offered hand-clipped plants of alfalfa held erect with clamps between
Ž w x
w x
w x. the major faces of the two pieces of lumber 13.5 cm H = 120 cm W = 3.5 cm D
that formed a wooden frame. Ž
. Lambs were exposed in pairs during 2 consecutive days 5 minrpairrday and
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individually 5 minrlambrday for the ensuing 3 days. After individual exposure, all lambs were consuming alfalfa from the wooden frames. Alfalfa plants were then
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replaced by wheat straw 100 g; 30 cm height placed along the central 30 cm of the wooden frame. During 4 consecutive days, pairs of lambs selected at random were
exposed to straw in the testing pen for a period of 5 minrpairrday. After exposure to straw, lambs were penned as a group and received an amount of
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alfalfa pellets 1270 grlambrday , which supplied about 80 of their digestible energy Ž
. requirements NRC, 1985 .
2.2. Initial indiÕidual tests The day after exposure to straw ended, lambs were tested individually between 0730
and 1000 h. Each lamb in the testing pen was offered a previously weighed amount of Ž
. straw about 100 g secured in the wooden frame. We recorded the time each lamb spent
eating straw during 5 min. Lambs were observed individually from a distance of approximately 3 m while they foraged. We excluded bouts of inactivity from calcula-
tions of active feeding time. The total number of bites was recorded with a hand-counter while observing the lambs. We calculated intake by the difference in straw weight
immediately before and after each lamb was tested. We used these data to calculate
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intake rate gramsrminute , bite rate bitesrmin , and bite size gramsrbite .
2.3. Trial 1 2.3.1. IndiÕidual tests
After the initial determination of straw intake, lambs were sorted in decreasing order by initial rate of straw intake and sets of two lambs were randomly assigned to two
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groups Groups 1 and 2; nine lambsrgroup . Thus, differences between groups due to initial recordings of intake rate were balanced.
Ž Each lamb was offered a previously weighed amount of straw about 100 g; 30 cm
. height — secured in a wooden frame — during a 5-min period. Immediately after
being tested for straw consumption, lambs in Group 1 were given intraruminal infusions of a starch suspension by oral intubation. The starch suspension was prepared by mixing
100 g of starch with 250 ml tap water at room temperature. Lambs in Group 2 received infusions of 250 ml tap water after consuming straw. Lambs from Groups 1 and 2 were
tested in an alternate and consecutive sequence. The order in which individual lambs were tested was reversed each day. Tests were conducted between 0730 and 1000 h
daily during 3 consecutive days. We calculated intake, intake rate, number of bites, bite rate and bite size during 5-min periods, as described previously. After the daily test,
lambs were penned as a group and at noon they received alfalfa pellets, as described previously.
2.4. Trial 2 Individual lambs were reluctant to eat straw alone in the testing pen, which reduced
the active feeding time and their exposure to straw. Consequently, during Trial 2 we attempted to increase exposure to straw — and thus enhance the association of straw
with starch — by conditioning animals in pairs.
2.4.1. Conditioning — pairs Four pairs of lambs from Group 1 and four pairs of lambs from Group 2 were formed
at random. Each pair of lambs was transferred to the testing pen and offered wheat straw Ž
. 100 g; 30 cm height secured in a wooden frame for 5 min. Pairs of lambs from Groups
1 and 2 were tested in an alternate and consecutive order. The order of testing pairs of lambs was reversed each day. After testing, lambs in the pair received intraruminal
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infusions of starch 100 grlambrday; Group 1 or water 250 mlrlambrday; Group 2 . Lambs were penned as a group and at noon they received alfalfa pellets, as described for
Trial 1. During 4 consecutive days of conditioning, we calculated intake, intake rate, number
of bites, bite rate and bite size for the 5 min period, as described for Trial 1. Pairs of lambs were the experimental units.
2.4.2. IndiÕidual tests After conditioning in pairs, all lambs were tested as individuals and received
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intraruminal infusions of starch Group 1 or water Group 2 as described for Trial 1. Testing was performed during 1 day and we calculated intake, intake rate, number of
bites, bite rate and bite size during 5 min, as described for Trial 1.
2.5. Trial 3 During Trial 3 we attempted to enhance the association between straw and starch.
First, we increased the exposure to straw by increasing the amount of time lambs were allowed to consume straw. We also allowed lambs to consume straw while they were
with their peers. We also modified the temporal ordering between straw offer and starch infusion by
first infusing starch and then offering straw. This order may facilitate conditioning Ž
. because the postingestive effects of starch i.e., the effects of absorbed calories occur
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during rather than after straw ingestion Boakes and Lubart, 1988 . 2.5.1. Conditioning — groups
At 0730 h, on the first day of conditioning lambs from Group 1 received intraruminal Ž
. infusions of starch 100 grlambrday by oral intubation. Lambs were then transferred
to the testing pen as a group and offered straw held erect in wooden frames. Condition- ing lasted 40 minrday and every 2 min we recorded the number of animals that were
consuming straw. After conditioning Group 1, lambs from Group 2 received intrarumi-
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nal infusions of water 250 mlrlambrday and were offered straw in the same fashion described for Group 1. The order in which groups were conditioned was reversed each
day. Four days of conditioning formed one conditioning period and two conditioning periods were performed. After the daily tests, lambs were penned as a group and at noon
they received alfalfa pellets, as described for Trial 1.
The experimental area in the testing pen was prepared daily before each group of Ž
. lambs was conditioned by placing about 2.5 kg of straw 30 cm height in three wooden
frames of the same dimensions and characteristics described before. 2.5.2. IndiÕidual tests
After each 4-day conditioning period, all lambs were tested individually, as described Ž
. Ž
for Trial 1. Lambs received intraruminal infusions of starch Group 1 or water Group .
2 immediately before being offered straw secured in a wooden frame. Testing was performed during 1 day and we calculated intake, intake rate, number of bites, bite rate
and bite size during a 5-min period, as described for Trial 1.
2.5.3. IndiÕidual tests without infusions Lambs were tested as described before but without starch or water administrations.
2.6. Trial 4. 2.6.1. Conditioning
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The day after Trial 3 ended, lambs 33 kg BW were penned individually in adjacent Ž
pens of 1.6 = 2.2 m. Lambs from Group 1 received intraruminal infusions of starch 100 .
Ž grlambrday and lambs from Group 2 received intraruminal infusions of water 250
. mlrlambrday . Immediately after the infusions, all lambs were offered 100 g of
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chopped wheat straw 1–2 cm particle size for 40 min. Refusals were collected and weighed. Conditioning was conducted between 0800 and 0900 h daily and was repeated
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during 9 consecutive days. At 1200 all lambs received alfalfa pellets 1400 grlambrday Ž
that provided approximately 80 of their daily digestible energy requirements NRC, .
1985 . 2.6.2. IndiÕidual tests
After conditioning, and immediately before being tested for straw consumption, Ž
. lambs in Group 1 were given intraruminal infusions of a starch 100 grlambrday and
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lambs in Group 2 received infusions of water 250 mlrlambrday . We recorded the time each lamb spent eating straw during 5 min. Lambs were observed individually and
bouts of inactivity were excluded from calculations of feeding time. We determined the amount of straw ingested as the difference between the weight of food offered and that
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remaining after 5 min. These data were used to calculate intake rate grmin . At noon all lambs were offered alfalfa pellets, as described during conditioning.
2.6.3. IndiÕidual tests without infusions Lambs were tested as described before but without starch or water administrations.
2.7. Statistical analyses Straw intake, intake rate, number of bites, bite rate, bite size and active feeding time
Ž were analyzed as a split-plot design with lambs nested within groups. Group 1: starch
. or 2: no starch was the between-lamb factor. Pairs of lambs were the between-subject
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factor when lambs were conditioned as a pair Conditioning; Trial 2 . When tests were Ž
. repeated on different days — Trial 1: 3 days; Trial 2: 4 days Conditioning — Pairs ;
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Trial 3: 2 days Individual Tests after Periods 1 and 2 ; Trial 4: 9 days Conditioning — day was the repeated measure in the analysis. Means were compared using the LSD
test.
3. Results
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